Von Schiller Daniel

Thibault

Marce Rafael

García-Baquero Gonzalo

Odriozola Iñaki

Mendoza-Lera Clara

Arnon Shai

Ryan M Burrows

De Girolamo

M Orcid

Figueroa Ricardo

Gómez-Gener Lluís

Langhans Simone

Leigh Catherine

Mcintosh Angus

Mlambo Musa

Morais Manuela

Negus Peter

Rolls Robert

Zak Dominik

Daniel Von Schiller
email: d.vonschiller@ehu.eus

T Datry

R Corti

A Foulquier

K Tockner

R Marcé

G García-Baquero

I Odriozola

B Obrador

A Elosegi

C Mendoza-Lera

M O Gessner

R Stubbington

R Albariño

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F Altermatt

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A Bruder

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A Dehedin

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Duerdoth

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M A S Graça

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S Kubheka

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C Leigh

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A Savić

O Shumilova

A Steward

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R Vander Vorste

N Waltham

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A Zoppini

Sediment Respiration Pulses in Intermittent Rivers and Ephemeral Streams

 Sediment respiration in intermittent rivers and ephemeral streams increases substantially in response to rewetting  Respiration pulses are driven by sediment properties, which, in turn, are influenced by climate and catchment characteristics  Effects of wetting-drying cycles on respiration and CO 2 emissions in stream networks need consideration in upscaling and modeling efforts

Introduction

Most streams are heterotrophic ecosystems that act as net mineralizers of organic carbon (OC) and emit large quantities (0.56-1.8 Pg C yr -1 ) of carbon dioxide (CO 2 ) to the atmosphere [START_REF] Aufdenkampe | Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere[END_REF][START_REF] Raymond | Global carbon dioxide emissions from inland waters[END_REF]. A major limitation of current estimates of the influence of stream networks on global C cycling is their exclusion of intermittent rivers and ephemeral streams (IRES), which cease to flow and dry at some points in space and time [START_REF] Acuña | Why should we care about temporary waterways?[END_REF][START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF]. IRES may represent over half of the global stream network length [START_REF] Acuña | Why should we care about temporary waterways?[END_REF] and are increasing in extent due to the combined effects of climate change, water abstraction, and land use change [START_REF] Döll | How is the impact of climate change on river flow regimes related to the impact on mean annual runoff? A global-scale analysis[END_REF][START_REF] Pumo | Climate change effects on the hydrological regime of small non-perennial river basins[END_REF]. Despite their prevalence, the role of IRES in C budgets of stream networks, including their contribution to respiration and CO 2 emission, is largely unknown [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF].

The hydrological regimes of IRES are characterized by alternating dry and wet phases, which exert a strong influence on C cycling [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]. Recent studies indicate that the effects of drying and rewetting on C mineralization in IRES sediments are similar to those in soils [START_REF] Arce | A conceptual framework for understanding the biogeochemistry of dry riverbeds through the lens of soil science[END_REF][START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]. Rewetting of dry IRES sediments through rain, groundwater upwelling or surface flow resumption may represent a respiration 'hot moment' [START_REF] Mcclain | Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ecosystems[END_REF] or 'control point' [START_REF] Bernhardt | Control points in ecosystems: moving beyond the hot spot hot moment concept[END_REF] similar to the 'Birch effect' described in soils, resulting in large pulses of CO 2 compared to both the preceding dry phase and the subsequent extended flowing phase [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]. Accordingly, a study addressing the effect of rewetting on CO 2 emissions from IRES sediments of the semiarid southwestern USA reported some of the largest flux increases ever observed in rewetting experiments [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. Such CO 2 pulses could have considerable implications for stream network C budgets [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]. However, available data are restricted to few sites, limiting our ability to determine the variability and drivers of respiration in IRES and thus to upscale CO 2 emissions and include IRES in global biogeochemical models.

Here, we present a global research collaboration that collected and analyzed sediments from 200 dry IRES reaches across 29 countries, encompassing a wide range of conditions found in IRES worldwide (Figure 1; Table 1; Data file S1). We assessed the immediate effect of rewetting on sediment respiration using standardized assays and estimated its potential influence on CO 2 emissions from stream networks. We predicted that, analogous to soils, dry IRES sediments would experience substantial increases in respiration upon rewetting. We further predicted that the magnitude of the effect would be a direct function of sediment characteristics such as texture and OC and nutrient content (proximal drivers), which in turn would depend on catchment characteristics such as climate, land use, and riparian plant cover (distal drivers).

Materials and Methods

Experimental Design

This coordinated experiment was conducted by contributors to the "1000 Intermittent Rivers Project" global research network (https://1000_intermittent_rivers_project.irstea.fr/). The collaborating research teams followed a standardized protocol (available on the website) to collect sediments and measure complementary environmental variables from a total of 200 IRES reaches during dry phases in the years 2015 and 2016. Despite the unparalleled global coverage of IRES in our experiment, gaps in spatial coverage exist due to a lack of contributors from some areas (e.g., large parts of Asia) or scarcity of IRES (e.g., in the boreal biome). Contributors were drawn from professional relationships and research networks and by responses to invitations posted on Twitter, Facebook and websites of professional organizations.

Sediment Sampling

The length of each sampled stream reach was defined as 10 times the average active channel width, to cover a representative area and to ensure consistent sampling effort [START_REF] Leopold | Channel and Hillslope Processes in a Semiarid Area[END_REF]. The active channel was defined as the area of inundated and exposed streambed sediments between established edges of terrestrial vegetation, abrupt changes in slope, or both [START_REF] Gordon | Stream Hydrology: an Introduction for Ecologists[END_REF]. Within each reach, 5% of the streambed was randomly sampled within 1 m 2 quadrats (for example, five quadrats in a 100 m 2 reach). Streambed sediment samples were collected from each quadrat using a spoon or shovel to a depth of 10 cm and pooled into one composite sample of ~3 L in total across the sampled stream reach. In the laboratory, sediments were sieved (2 mm) and airdried for one week. A homogenized subsample of ~160 g was packed airtight in plastic containers and sent to the University of the Basque Country (Bilbao, Spain) for analysis. Upon arrival, samples were immediately stored at room temperature in the dark.

Environmental Variables

The active channel width (m) was determined by establishing 5-10 transects along each sampled stream reach. Riparian plant cover (%) was estimated by averaging the measurements obtained with a spherical densiometer or by visual assessment at each of the transects. Latitude and longitude (WGS 84 datum) of the sampling sites were determined with a global positioning system in the field or with a geographic information system (GIS). The proportion of the catchment area covered by agricultural and urban land uses (%) was determined using GIS based on the most updated national land cover maps. Results from GIS analyses were verified and corrected, if necessary, using ground-based surveys. Mean annual temperature (ºC) and precipitation (mm) were estimated based on the worldclim 2.0. database (http://www.worldclim.org), which gives 1-km 2 spatial resolution climate surfaces for global land areas over the period 1970-2000.

Sediment Characteristics

The OC and total nitrogen (N) content (%) of the sediments were determined using an elemental analyzer (TruSpec Micro CHNS, Leco Corp., St. Joseph, MI, USA) after grinding and acidification with 2M HCl. Analyses were run in duplicate. The C/N ratio is reported as the molar ratio of OC to N content. Clay content (% sediment particles <2 μm), a measure of sediment texture, was determined with a laser light diffraction instrument (Coulter LS 230, Beckman-Coulter, USA) after removing organic matter with H 2 O 2 [START_REF] Arriaga | A fast method for determining soil particle size distribution using a laser instrument[END_REF]. Clay content is the main contributor to sediment permeability and sediment-specific area [START_REF] Bear | Dynamics of fluids in porous media[END_REF] which determine the area available for microbial colonization [START_REF] Mendoza-Lera | Importance of advective mass transfer and sediment surface area for streambed microbial communities[END_REF]. Sediment water content, determined by weighing a subsample before and after oven drying (60ºC, 72 h), was low (mean ± SE = 1.7 ± 0.1%, median = 1.1, range = 0.1-15) and uncorrelated with sediment respiration change upon rewetting (Code S1).

Sediment Respiration Assays

Sediment respiration was measured in the laboratory under standardized conditions, which enabled us to compare intrinsic differences among sediments. Two different methods were used to ensure robust results. In dry conditions, we measured respiration using the commercial MicroResp TM device (Macaulay Scientific Consulting Ltd., UK), whereas in wet conditions, we measured respiration both with the MicroResp TM system and by determining the decline of dissolved oxygen (DO) concentrations in sealed incubation bottles.

For MicroResp TM measurements, 0.5 g of sediment was weighed into a deep well of an autoclaved 96 well microtiter plate. Samples were acclimated for 24 h at the temperature used later for the measurements (20ºC), and the incubation chamber was gently flushed with air to ensure that the partial pressure of CO 2 in the headspace of the wells was initially close to the atmospheric value (ca. 400 ppmv). For each sample, three analytical replicates were left in dry conditions and three were rewetted with 50 µL of air-saturated Volvic ® mineral water immediately before covering and sealing the microtiter plate with a second microplate containing a CO 2 detection gel in the wells. We included controls consisting of empty wells and wells filled with 0.5 g of combusted and acid-washed glass beads (both with and without 50 µL of water added). Replicates and controls were randomized in the plates. The plates were incubated for 6 h at 20ºC, and the CO 2 molar fraction in each well's headspace was recorded immediately before and after the incubation by reading the absorbance of the detection gel in the microplate at 570 nm using a spectrophotometric microplate reader (BioTek EPOCH, Winooski, VT, USA). Because we used 26 different MicroResp TM plates for the whole experiment, we tested for any plate effects by duplicating the measurements of 18 samples in two different plates and found no significant differences. The mean error was 1.05 nmol CO 2 g -1 dry mass h -1 for dry samples and 7.15 nmol CO 2 g -1 dry mass h -1 for rewetted samples. Moreover, a linear mixed model of the whole dataset for CO 2 production in the wells including plate as a random factor allocated zero variability to the plate factor. Finally, the percentage change of CO 2 in the headspace was converted to a respiration rate (nmol CO 2 g -1 dry mass h -1 ) considering the incubation time and temperature, gas constant, headspace volume and sediment mass, as described in the MicroResp TM technical manual. The mean values of the analytical replicates were used in further data analyses.

For the measurements of DO decline over time, we used two analytical replicates of 5 g aliquots per sediment sample and three controls without sediment for each of 10 successive runs comprising 25-50 samples. Samples and controls were introduced in acid-washed 250 mL glass incubation bottles filled with air-saturated Volvic ® mineral water and sealed airtight using a 3.2-mm-thick silicon-PTFE septum and a cut-out open-top cap. Care was taken to ensure that air bubbles were excluded. Samples were incubated for 24 h at 20°C in an incubation chamber with gentle shaking (100 rpm, Multitron standard, INFORS HT, Bottmingen, Switzerland). DO concentrations were measured at the end of the incubation with a DO microsensor (Microx 4 DO meter with a needle-type microsensor, PreSens, Regensburg, Germany) using a standalone, portable, fiberoptic DO meter (Microx 4 trace, PreSens). The incubation bottles were gently agitated before each measurement to ensure homogeneous DO concentrations. The DO decline (computed as the DO concentration difference between the control mean and the sample at the end of the incubation) was converted to CO 2 production (nmol CO 2 g -1 dry mass h -1 ) based on a respiratory quotient of one. The mean values of analytical replicates were used in further data analyses.

To examine the response of sediment respiration to rewetting, we subtracted the respiration values in dry conditions (MicroResp TM dry) from those of the MicroResp TM and bottle incubations in wet conditions.

Potential Contribution to CO 2 Emissions from the Global Stream Network

To obtain an estimate of the potential contribution of respiration in IRES sediments upon rewetting to CO 2 emissions from the global stream network, we scaled up our results from the bottle incubations, assuming that i) all CO 2 produced and released by sediment respiration was emitted to the atmosphere, ii) sediment density averaged 1.6 g cm -3 across all sampled stream reaches [START_REF] Hillel | Fundamentals of Soil Physics[END_REF], and iii) the sediment depth potentially affected by a rewetting event was 30 cm, following common definitions of homogenous topsoils [START_REF] Pistocchi | A simplified parameterization of the monthly topsoil water budget[END_REF]. The obtained areal release rate was multiplied by the global annual accumulated dry area of IRES (84,461 km 2 ) estimated by [START_REF] Raymond | Global carbon dioxide emissions from inland waters[END_REF]. We considered one rewetting event with an effect duration of 5 days, following the mean duration of increased CO 2 flux in soils after rewetting estimated by [START_REF] Kim | Effects of soil rewetting and thawing on soil gas fluxes: A review of current literature and suggestions for future research[END_REF].

Statistical Analyses

All statistical analyses were conducted in R 3.4.0 (R Core Team, 2016). First, we tested for differences among the three measures of sediment respiration (MicroResp TM dry, MicroResp TM wet, and bottle incubations). The null hypothesis of no effect of rewetting on sediment respiration was tested with a randomized complete block ANOVA and Tukey's HSD post hoc test, using site as blocking factor and method as fixed explanatory factor (Code S1). The analysis was conducted on rank-transformed data to deal with heteroscedasticity in the residuals. Second, we tested the relationship between sediment respiration changes upon rewetting measured by the MicroResp TM and the bottle incubation method by fitting a Gaussian linear model using the function gls() in the R package nlme [START_REF] Pinheiro | nlme: Linear and Nonlinear Mixed Effects Models[END_REF]. Adequate homoscedasticity in regression residuals was achieved by square-root transformation of both variables. The function gls() argument for modelling residual spatial autocorrelation included an exponential variogram model of the X (latitude) + Y (longitude) form, which was empirically shown to be appropriate to fulfil the model assumption of independence (Code S1). To assess whether the bottle incubation method overestimated sediment respiration change upon rewetting compared to the MicroResp TM method, we also tested the null hypothesis that β, the slope of the regression line, is β = 1 (rather than the null hypothesis that β  0).

To model the causal relationships between environmental drivers and sediment respiration change upon rewetting, we used structural equation modelling (SEM) following the guidelines proposed in [START_REF] Grace | Guidelines for a graph-theoretic implementation of structural equation modeling[END_REF], which allows the study of complete causal networks. The first step of the SEM approach was to devise a metamodel (Figure 2) [START_REF] Grace | Guidelines for a graph-theoretic implementation of structural equation modeling[END_REF], defined based on a priori theoretical knowledge and insights from the exploratory data analysis. . In this metamodel, we considered sediment respiration upon rewetting to be directly controlled by proximal drivers associated with sediment characteristics, which in turn depend on distal drivers linked to catchment characteristics. For proximal drivers, we selected clay content as a measure of sediment texture, and OC content and the C/N ratio to indicate organic matter quantity and quality, respectively (Figure 2). Based on results for soils, we predicted a positive effect of OC content [START_REF] Canarini | Soil carbon loss regulated by drought intensity and available substrate: a meta-analysis[END_REF] and a negative effect of the C/N ratio [START_REF] Ramirez | Consistent effects of nitrogen amendments on soil microbial communities and processes across biomes[END_REF] and clay content [START_REF] Borken | Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils[END_REF] on the response of respiration to rewetting. We also predicted positive effects of clay on OC content and of OC content on the C/N ratio [START_REF] Rice | Organic matter and nutrient dynamics[END_REF]. For distal drivers, we selected mean annual temperature and precipitation to describe climatic conditions, catchment land use (i.e., the percentage of agricultural plus urban areas) as a proxy of anthropogenic influence, and riparian plant cover and channel width to characterize stream features (Figure 2). We predicted that temperature, precipitation, land use and riparian cover indirectly affect the response of respiration to rewetting through their effects on OC content and the C/N ratio [START_REF] Colman | Drivers of microbial respiration and net N mineralization at the continental scale[END_REF][START_REF] Raich | Global patterns of carbon dioxide emissions from soils[END_REF]. We also predicted a negative effect of land use and channel width on riparian cover [START_REF] Naiman | Riparia: Ecology, Conservation, and Management of Streamside Communities[END_REF].

In the second step of the SEM approach, we used the maximum likelihood method to obtain a global estimation [START_REF] Grace | Guidelines for a graph-theoretic implementation of structural equation modeling[END_REF]. Exploratory data analysis suggested the need to square-root transform or ln-transform the response variables (including respiration rates) and five explanatory variables (Code S1), to ensure linearity of relationships and, hence, the suitability of the global estimation method. The metamodel was fitted and tested by means of the function sem() in the R package lavaan [START_REF] Rosseel | Lavaan: An R package for structural equation modeling[END_REF]. The user-friendly or minimalist approach was used because it provides an iterative process to confront all our initial assumptions with data, and because, after prior transformations of variables, non-standard models were not required (Code S1). Data to model discrepancy was evaluated by means of the function modindices() in the lavaan package (Code S1). This is an iterative process in which data to model consistency is assessed with a chi-square test comparing the tested models to a saturated model. The iterative process ended when the modindices() output indicated that no further meaningful modifications were possible and the null hypothesis of model consistency could not be rejected. Two goodness-of-fit measures, the comparative fit index (CFI) and the root mean square error of approximation (RMSEA), were also calculated. The final models were accepted when the p-value associated with the corresponding chisquare test was > 0.05, CFI > 0.95 and RMSE < 0.05 (Code S1).

Because changes in sediment respiration upon rewetting were measured with two methods (MicroResp TM and bottle incubation), the above SEM process was applied independently for the two response variables. Upon acceptance of a final SEM, the presence of residual spatial autocorrelation was tested using spatial correlograms (Moran's I statistic) with Holm's correction for multiple testing [START_REF] Legendre | Numerical ecology[END_REF]) (Code S1). Because no spatial autocorrelation was found in the residuals once the SEM processes ended, special spatial structures were not introduced. Finally, we explored partial effects fitted in the SEM using regressions between variables and model residuals for the main relationships.

Results and Discussion

Magnitude of Sediment Respiration

Sediment respiration in dry conditions (MicroResp TM method) ranged from 0.01 to 14.1 nmol CO 2 g -1 dry mass h -1 (mean ± SE = 1.1 ± 0.1, median = 0.8; Figure 3a; Table 1). These low respiration rates are similar to those reported from other ex situ dry stream sediments in earlier studies (0.2-4.5 nmol CO 2 g -1 dry mass h -1 ; measured with the MicroResp TM method in dry conditions) [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF], and overall support results from soil studies showing a reduction in, but not full suppression of, respiration after drying [START_REF] Schimel | Life in dry soils: effects of drought on soil microbial communities and processes[END_REF]. This indicates that dry IRES sediments, like soils, support a moderately active microbial community.

Sediment respiration increased upon rewetting, ranging from 0.01 to 147 nmol CO 2 g - 1 dry mass h -1 (mean ± SE = 34.9 ± 4.7, median = 27.0) for the MicroResp TM method and from 0 to 411 nmol CO 2 g -1 dry mass h -1 (mean ± SE = 72.0 ± 4.7, median = 54.3) for the bottle incubations method (Figure 3a; Table 1). These values are in the upper range of respiration rates reported from perennial stream sediments (range = 0-356 nmol CO 2 g -1 dry mass h -1 , median = 19; Table S1), suggesting that rewetting events after dry phases in IRES are associated with rapid recovery of metabolic activity by heterotrophic organisms present in the sediments [START_REF] Schimel | Life in dry soils: effects of drought on soil microbial communities and processes[END_REF]. Comparison of our respiration rates with estimates of soil respiration are hampered by the different methods used to measure respiration in soils, and by the reporting of most rates on an areal basis [START_REF] Kim | Effects of soil rewetting and thawing on soil gas fluxes: A review of current literature and suggestions for future research[END_REF]. Nonetheless, a cross-European study of basal soil respiration measured with the MicroResp TM method at 60% water holding capacity reported similar values to ours (41.6-225 nmol CO 2 g -1 dry mass h -1 , n = 81) [START_REF] Creamer | Measuring respiration profiles of soil microbial communities across Europe using MicroResp TM method[END_REF], suggesting that respiration rates in IRES sediments upon rewetting are similar to those in mesic soils.

The much higher sediment respiration upon rewetting, with a mean 32-fold (MicroResp TM ) or 66-fold (bottle incubation) increase in wet compared to dry conditions (Figure 3a; Table 1), is in the upper range of increases reported after rewetting from both streams and soils. The CO 2 flux from soils can increase 0.4-130-fold (mean = 12) after rewetting, with the highest increases typically reported from deserts [START_REF] Kim | Effects of soil rewetting and thawing on soil gas fluxes: A review of current literature and suggestions for future research[END_REF]. Similarly, in dry IRES of the semiarid southwestern USA, the CO 2 flux increased 6-33-fold (mean = 19) immediately following experimental rewetting [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. These findings point to the 'Birch effect' in IRES sediments, which is likely to result from microbial activity being stimulated by the rapid mobilization of nutrients and OC that accumulated during the dry phase, supplemented by newly available OC released during the disintegration of sediment aggregates and microbial cell lysis in response to osmotic stress upon rewetting [START_REF] Borken | Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils[END_REF][START_REF] Kim | Effects of soil rewetting and thawing on soil gas fluxes: A review of current literature and suggestions for future research[END_REF][START_REF] Schimel | Life in dry soils: effects of drought on soil microbial communities and processes[END_REF].

The higher rates measured by bottle incubations compared to the MicroResp TM method (Figure 3a andb) were expected, because the bottle incubation assay simulates a typical reinundation event, whereas MicroResp TM simulates more moderate rewetting such as that caused by light rainfall. Nonetheless, the rates estimated with both methods were positively related to one another (Figure 3b), indicating they were interchangeable with respect to the mechanistic analyses of drivers of respiration response.

Drivers of Sediment Respiration

The SEM of sediment respiration change upon rewetting determined in bottle incubations supported the metamodel (Figure 4; Table S2). However, the final diagram depicting causal relationships (i.e., the paths linking the considered variables; Figure 4a) included just 10 paths and was thus more parsimonious than the metamodel. The final fitted model (Figure 4a) confirmed some of the predicted relationships (e.g., between riparian cover and land use) but not others (e.g., between land use and C/N ratio). In addition, we identified two unpredicted causal paths (between precipitation and riparian cover, and between clay content and the C/N ratio), increasing the overall model-to-data fit. In the final model output, 45% of the variance in sediment respiration change upon rewetting was explained by sediment OC content, the C/N ratio and clay content. In turn, OC content (R 2 = 30%) and the C/N ratio (R 2 = 13%) were explained by different combinations of proximal drivers (clay content) and distal drivers (riparian cover and temperature), and riparian cover (R 2 = 5%) was explained by land use and precipitation (Figure 4a). The partial relationships fitted in the SEM were significant (Figure 4b, c andd). The same structure was obtained when respiration change upon rewetting determined with the MicroResp TM method was modelled, with the only major difference that overall R 2 = 21% instead of 45% (Table S3). The absolute β estimate values corresponding to the partial relationships also changed, but not their direction (Table S3). These results demonstrate how distal drivers (i.e., climate, land use and riparian cover) modulate proximal drivers (i.e., sediment texture, organic matter quantity and quality) to control sediment respiration upon rewetting in IRES.

The positive relationship between OC content and the respiration response to rewetting (Figure 4a) aligns with results from studies in rewetted stream sediments [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF] and soils [START_REF] Kim | Effects of soil rewetting and thawing on soil gas fluxes: A review of current literature and suggestions for future research[END_REF], in which OC content and respiration or CO 2 release are typically positively related. This relationship held despite the low OC content of the sampled IRES sediments, which ranged from 0.1 to 8.5% (mean ± SE = 1.0 ± 0.1%, median = 0.5; Table 1). In a compilation of sediment chemistry data from perennial streams in the USA [START_REF] Horowitz | The effects of land use on fluvial sediment chemistry for the conterminous U.S. -Results from the first cycle of the NAWQA Program: Trace and major elements, phosphorus, carbon, and sulfur[END_REF], the OC content ranged from 0.01 to 28.7% (mean ± SE = 3.8 ± 0.1%, median = 2.7, n = 949), indicating that OC content in IRES sediments may be lower than in perennial streams. The dynamic hydrologic regime of IRES, which includes frequent flushing of accumulated material during periods of flow, may account for this difference [START_REF] Arce | A conceptual framework for understanding the biogeochemistry of dry riverbeds through the lens of soil science[END_REF]. In an extensive survey of European topsoils [START_REF] Tóth | The LUCAS topsoil database and derived information on the regional variability of cropland topsoil properties in the European Union[END_REF], OC contents ranged from 0.1 to 58.7% (mean ± SE = 4.9 ± 0.06%, median = 2.1, n = 19,969), which is also higher than in the studied IRES sediments, corroborating two previous comparisons of IRES sediments and soils within the same catchment [START_REF] Boix-Fayos | Sediment flow paths and associated organic carbon dynamics across a Mediterranean catchment[END_REF][START_REF] Gómez-Gener | When water vanishes: magnitude and regulation of carbon dioxide emissions from dry temporary streams[END_REF]. The lower OC content in dry IRES sediments compared to soils is likely to reflect geomorphological processes that form streambed sediments from eroded soils as well as the greater hydrological variability and lower biomass or lack of plants in IRES [START_REF] Arce | A conceptual framework for understanding the biogeochemistry of dry riverbeds through the lens of soil science[END_REF].

The negative relationship between the C/N ratio and sediment respiration upon rewetting (Figure 4a) matches previous observations in soils and could reflect the selection of microorganisms with copiotrophic life strategies [START_REF] Fierer | Toward an ecological classification of soil bacteria[END_REF] under relatively nutrient-rich conditions (i.e., low C/N ratio) [START_REF] Ramirez | Consistent effects of nitrogen amendments on soil microbial communities and processes across biomes[END_REF]. These fast-growing organisms have high requirements for labile OC and nutrients [START_REF] Fierer | Toward an ecological classification of soil bacteria[END_REF] and are expected to be less resistant but more resilient to environmental stresses such as rewetting [START_REF] De Vries | Controls on soil microbial community stability under climate change[END_REF]. Copiotrophic microorganisms may be responsible for the rapid reactivation of respiration observed upon rewetting, as they also thrive on labile OC and nutrients released by cell lysis upon rewetting [START_REF] Schimel | Microbial stress-response physiology and its implications for ecosystem function[END_REF]. Notably, the sediment C/N molar ratio measured in the sampled IRES was highly variable (mean ± SE = 26 ± 2.2, range = 2.4-211; Table 1), spanning, for instance, a wider range than samples in the European topsoil database (mean ± SE = 11.8 ± 0.04; range = 0.6-168, n = 19,952, data excluded where N content = 0%) [START_REF] Tóth | The LUCAS topsoil database and derived information on the regional variability of cropland topsoil properties in the European Union[END_REF].

The negative relationship between clay content and respiration upon rewetting (Figure 4a) indicated greater respiration in coarser sediments. This result contrasts with findings in perennial streams, in which increases in sediment respiration with clay content have been linked to the increased surface area [START_REF] Mendoza-Lera | Importance of advective mass transfer and sediment surface area for streambed microbial communities[END_REF]. In soils, however, high clay content favors compaction during drying and can delay OC mineralization by isolating microorganisms and adsorbing OC, which thus becomes less bioavailable upon rewetting [START_REF] Borken | Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils[END_REF]. Moreover, the indirect positive effect of clay content on respiration mediated by a decrease in the C/N ratio (Figure 4a) might be because higher clay content favors cation exchange capacity and nutrient retention [START_REF] Bach | Soil texture affects soil microbial and structural recovery during grassland restoration[END_REF]. These results collectively suggest that upon rewetting, the influence of sediment texture on respiration is similar to that observed in soils. Notably, these patterns emerged despite the lower, less variable clay content of sampled IRES sediments (mean ± SE = 5.9 ± 0.4%, range = 0-32; Table 1) compared to soils (mean ± SE = 18 ± 0.09%, range = 0-79, n = 19,969) [START_REF] Tóth | The LUCAS topsoil database and derived information on the regional variability of cropland topsoil properties in the European Union[END_REF], the former being mainly formed by deposition and sorting processes during transport and characterized by a lack of stabilizing structures (e.g., biocrusts, vascular plants) [START_REF] Arce | A conceptual framework for understanding the biogeochemistry of dry riverbeds through the lens of soil science[END_REF][START_REF] Boix-Fayos | Sediment flow paths and associated organic carbon dynamics across a Mediterranean catchment[END_REF].

SEM results also indicated that the effect of OC content on respiration was partially regulated by distal drivers (Figure 4a). Specifically, OC content was related negatively to temperature and positively to riparian cover. The latter, in turn, was negatively related to land use and positively to precipitation. The negative relationship between sediment OC content and temperature may reflect reduced C mineralization rates in both soils and stream sediments in colder regions, favoring OC accumulation in dry IRES sediments [START_REF] Conant | Temperature and soil organic matter decomposition rates -synthesis of current knowledge and a way forward[END_REF]. The positive relationship between OC content and riparian cover indicates a key role of riparian plant litter as an OC source in IRES sediments [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF], with land use and precipitation only indirectly related to sediment OC content via their effects on riparian cover. Thus, streams in catchments with lower anthropogenic influence and higher precipitation tended to have greater riparian cover, leading to higher sediment OC content and respiration. Channel width had no effect on sediment respiration either directly or indirectly via riparian cover (i.e., reduced OC inputs from riparian vegetation in wider streams), suggesting that respiration upon rewetting is independent of channel width and only partially depends on riparian vegetation. Other sources of OC in dry IRES sediments may include organic matter imported from upland and upstream as well as autochthonous sources (e.g., macrophyte remnants, periphyton) (von [START_REF] Schiller | Nutrient and Organic Matter Dynamics in Intermittent Rivers and Ephemeral Streams[END_REF]. However, these results are inconclusive, because the studied IRES reaches were mostly located in low-order streams with narrow channels and high riparian cover (Table 1).

The magnitude and drivers of sediment respiration pulses upon rewetting reported in this study should be viewed with caution, because respiration was measured in small samples, disconnected from their structural matrix, and under standardized laboratory conditions. For instance, the incubation temperature and the nutrient concentration in the water used for rewetting may have differed from those found at ambient conditions at the sampling sites. Nonetheless, our rates were obtained using two alternative methods, which we suggest effectively determined the effect of rewetting and allowed us to compare responses among sediments with different intrinsic properties. A substantial proportion of the variance in sediment respiration change upon rewetting remained unexplained in our SEM, suggesting some important drivers were not characterized. These may include sediment properties such as phosphorus content or microbial biomass and distal drivers such as the time since the last rewetting event. We encourage researchers to conduct more in situ rewetting experiments across multiple IRES and to measure these additional variables to corroborate and expand our observations.

Potential Contribution to CO 2 Emissions from the Global Stream Network

Our results suggest that the high sediment respiration rates in IRES upon rewetting may significantly contribute to CO 2 emissions from the global stream network. We estimated a mean (range) release rate from rewetted sediments of 10.0 (0.0-56.9) g C m -2 day -1 . These rates greatly exceed the release rates from rewetted leaf litter collected from the same IRES sites (mean = 0.24 g C m -2 day -1 , range = 0-3.7) [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF], suggesting that sediments are key contributors to CO 2 emissions. A global upscaling of this release rate resulted in a mean (range) global CO 2 flux from a single rewetting in IRES sediments of 0.0045 (0.000-0.025) Pg C yr -1 , representing 4% (0-21%) of the global CO 2 emissions from dry IRES (0.124 Pg C year -1 according to [START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]) and between 0.2% (0.0-1.4%) and 0.8% (0.0-4.5%) of the global CO 2 emissions from perennial streams (0.56-1.8 Pg C yr -1 according to [START_REF] Aufdenkampe | Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere[END_REF] and [START_REF] Raymond | Global carbon dioxide emissions from inland waters[END_REF], respectively). Thus, a single respiration pulse resulting from rewetting of IRES sediments could increase annual CO 2 emissions from the global stream network, including IRES and perennial streams, by on average 0.2-0.7%.

The estimated contribution of respiration in IRES sediments upon rewetting to CO 2 emissions from the global stream network reported here may initially seem small. However, our estimate is conservative because the IRES surface area on which our calculations are based is likely an underestimate [START_REF] Benstead | An expanded role for river networks[END_REF][START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF], IRES are often subject to multiple rewetting events (i.e., due to rain and flow reconnection) per year [START_REF] Corti | Invertebrates and sestonic matter in an advancing wetted front travelling down a dry river bed (Albarine, France)[END_REF][START_REF] Jacobson | Variation in material transport and water chemistry along a large ephemeral river in the Namib Desert[END_REF][START_REF] Schiller | Nutrient and Organic Matter Dynamics in Intermittent Rivers and Ephemeral Streams[END_REF], and other processes also contribute to CO 2 emissions from IRES. For example, processes not recognized in our estimates include respiration in other stream compartments, such as plant litter and deeper sediments [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF], and abiotic processes such as physical displacement and carbonate weathering [START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF].

Although it is uncertain how well our laboratory-derived respiration rates scale to the natural environment, our results suggest that emissions from IRES during rewetting episodes may be a dominant term in the annual CO 2 balance in many stream networks where IRES and rewetting episodes are frequent. In any case, we are far from producing a robust global estimate of CO 2 emissions from IRES during rewetting events, because our calculations rely on several assumptions that need to be considered with caution: sediment densities are highly spatially variable [START_REF] Boix-Fayos | Sediment flow paths and associated organic carbon dynamics across a Mediterranean catchment[END_REF], respiration rates may change with sediment depth [START_REF] Fang | The variation of soil microbial respiration with depth in relation to soil carbon composition[END_REF], and the number of rewetting events varies greatly in space and time [START_REF] Schiller | Nutrient and Organic Matter Dynamics in Intermittent Rivers and Ephemeral Streams[END_REF]. Future research is needed to clarify the relevance of these uncertainties and processes, including the influence of antecedent flow conditions and type of rewetting (i.e., fed by groundwater, surface water or rainwater) on respiration response to rewetting.

Conclusions

Our global study, spanning 200 IRES reaches across six continents and covering a wide range of environmental conditions, enabled us to assess the magnitude and environmental drivers of respiration pulses in IRES sediments upon rewetting. Our data indicate that rewetting greatly increases sediment respiration, supporting the view of IRES as coupled aquatic-terrestrial ecosystems that function as 'punctuated biogeochemical reactors' in response to spatiotemporal fluctuations in drying and rewetting [START_REF] Larned | Emerging concepts in temporary-river ecology[END_REF]. The results also demonstrate that key sediment properties drive the response of respiration to rewetting, and, in turn, are influenced by climate and catchment conditions. Specifically, we found that organic-rich, low C/N and coarse sediments experience a larger respiration pulse upon rewetting, with greater riparian cover in more natural and humid catchments leading to higher respiration pulses by increasing the sediment OC content. These results expand our understanding of metabolism and C cycling in stream networks with implications for largescale modelling efforts [START_REF] Bernhardt | The metabolic regimes of flowing waters[END_REF]. Furthermore, our findings support research demonstrating that rewetting events represent 'hot moments' [START_REF] Mcclain | Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ecosystems[END_REF] or 'control points' [START_REF] Bernhardt | Control points in ecosystems: moving beyond the hot spot hot moment concept[END_REF] of CO 2 release in IRES, that is, short periods of high biogeochemical activity that may contribute significantly to the emissions of CO 2 from the global stream network. An update of respiration and CO 2 emissions in the global stream network is therefore needed, especially because the spatial extent of IRES and the frequency of wetting-drying cycles is increasing due to climate change and other anthropogenic pressures. ). The grey area indicates the 95% confidence interval for the regression line. The slope of the regression line is significantly different from the 1:1 line (dashed line; t 199 = 12.9, p < 0.001), indicating that the bottle incubation method tends to produce higher sediment respiration rates than the MicroResp TM method. Note the square root transformation of both axes.

Figure 1 .

 1 Figure 1. Global distribution of the sampling sites and photos of selected sites. The 200 sampled dry stream reaches were in 29 countries on six continents and encompassed a wide range of environmental conditions. The inset illustrates the spatial distribution within the most densely sampled area. Photos are shown for a high mountain stream in Ecuador (1), a tropical stream in Colombia (2), a desert stream in Namibia (3), a semiarid stream in Australia (4), and for temperate forested streams in Serbia (5), Switzerland (6) and the USA (7). Photo credits: 1: S. Cauvy-Fraunié, IRSTEA, France. 2: J. F. Blanco-Libreros, Universidad de Antioquia, Colombia. 3: M. Moleón, University of Granada, Spain. 4: P. Negus, Queensland Government, Australia. 5: A. Savić, University of Niš, Serbia. 6: A. Bruder, University of Applied Sciences and Arts of Southern Switzerland, Switzerland. 7: D. Niyogi, Missouri University of Science and Technology, USA.

Figure 2 .

 2 Figure 2. Metamodel showing all predicted connections among variables potentially driving sediment respiration change upon rewetting. Dark-blue frames and light-blue frames indicate distal and proximal drivers, respectively. The white frame represents the response variable. Hexagons and rectangles indicate exogenous and endogenous variables, respectively.

Figure 3 .

 3 Figure 3. Sediment respiration in dry and rewetted conditions. (a) Mean + SE of sediment respiration in three types of standardized assays. Differences between types are significant (ANOVA, F 2, 398 = 748.6, p < 0.001). Upper case letters denote significant differences as determined by Tukey's post hoc comparisons (p < 0.001). (b) Relationship between changes in respiration upon sediment rewetting as measured with the MicroResp TM and bottle incubation methods (F 1,199 = 1993.3, p < 0.001, R 2 = 48.5%). The grey area indicates the 95% confidence interval for the regression line. The slope of the regression line is significantly different from the 1:1 line (dashed line; t 199 = 12.9, p < 0.001), indicating that the bottle incubation method tends to produce higher sediment respiration rates than the MicroResp TM method. Note the square root transformation of both axes.

Figure 4 .

 4 Figure 4. Drivers of sediment respiration change upon rewetting. (a) Final accepted structural equation model (SEM) showing all significant connections supported by the bottle incubation data (χ 2 (12, n = 200) = 13.3; p = 0.35, CFI = 0.99, RMSEA = 0.023). The white frame represents the response variable. Hexagons and rectangles indicate exogenous and endogenous variables, respectively. Solid arrows and dashed arrows indicate positive and negative relationships, respectively. Numbers adjacent to arrows are the standardized effect sizes of the relationship (unstandardized coefficients are shown in Table S2). Arrow width is proportional to the strength of the effect size, and R 2 values denote the percentage of variance explained. Asterisks indicate relationship not included in the metamodel. (b, c and d) Linear regressions between variables and model residuals for the main relationships in the final SEM are indicated by matching letters in b (z = 4.3, p < 0.001 for c; z = 5.4, p < 0.001 for d; z = 12.5, p < 0.001 for e). Note the square root or ln transformation of some axes.

Acknowledgments and Data

We thank Y. Etxeberria, L. Sánchez, C. Gutiérrez, G. LeGoff and B. Launay for laboratory support. DvS was supported by a Short-Term Scientific Mission of the COST Action CA15113 (SMIRES, Science and Management of Intermittent Rivers and Ephemeral Streams, www.smires.eu), supported by COST (European Cooperation in Science and Technology) and received additional funding from the EU's 7 th Framework Programme for research, technological development and demonstration under grant agreement No. 603629 (GLOBAQUA) and a Grant for Research Groups of the Basque University System (IT-951-16) funded by the Basque Government. RM and BO were supported by the Spanish Ministry of Science, Innovation and Universities through project C-HYDROCHANGE (CGL2017-86788-C3-2-P and CGL2017-86788-C3-3-P). FA was funded by the Swiss National Science Foundation grants No. PP00P3_150698 and PP00P3_179089. NC was supported by the EU project LIFE+ TRivers (LIFE13 ENV/ES/000341). SDL received funding from the EU's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 748625. PP and MP were supported by INTER-COST project LTC17017. The authors declare that they have no competing interests. The dataset (Data File S1; DOI: 10.6084/m9.figshare.8863721) and the R code used to generate the results (Code S1; DOI: 10.6084/m9.figshare.8863655), including step by step explanations of the statistical tests, have been deposited in Figshare Digital Repository (https://figshare.com/projects/Sediment_Respiration_Pulses_in_Intermittent_Rivers_and_Ep hemeral_Streams/66104).

Author contributions: TD, AF, RC, DvS, and KT were responsible for project planning and project coordination. All authors collected sediments in their countries and processed this material. DvS, RM, BO and AE were responsible for laboratory analyses. RC, CML and DvS carried out the data compilation and database management. GGB, IO and DvS performed the data analyses. DvS led the writing of the manuscript with notable contributions by TD, AF, KT, RM, GGB, IO, BO, AE, CML, MOG and RS. All the other authors commented on and contributed to revising draft versions.


Lise Comte
email: lcgcomte@gmail.com

Jonathan Lenoir

CLIMATE CHANGE ECOLOGY Decoupled land-sea biodiversity trends

Summary

A global analysis of biodiversity time-series across temperate zones shows contrasting fingerprints of contemporary climate warming on species assemblages over land and sea. A net increase in the number of species is evident in the warmest temperate oceans but no systematic biodiversity trend is detected in the terrestrial realm.

Main text

As climate change unfolds, rapid changes in population dynamics and species distributions have been documented from polar to tropical regions [START_REF] Poloczanska | Global imprint of climate change on marine life[END_REF][START_REF] Chen | Rapid range shifts of species associated with high levels of climate warming[END_REF] . These changes are thought to result from the sensitivity of organisms to climate exposure and from intrinsic characteristics associated with the capacity to adapt to these changes in the environment [START_REF] Foden | Identifying the world's most climate change vulnerable species: a systematic trait-based assessment of all birds, amphibians and corals[END_REF] . It takes substantial efforts to understand the redistribution of a single species, so anticipating climate-induced effects on species assemblages is a challenging task. A key question is how predictable is the climate-induced redistribution of biodiversity on Earth. Writing in Nature Ecology and Evolution, Antão and collaborators provide a detailed analysis of the extent of climate effects on biodiversity in both marine and terrestrial species assemblages, using the largest database of biodiversity time series available to date, which spans plant, invertebrate and vertebrate groups (bioTIME [START_REF] Dornelas | BioTIME: A database of biodiversity time series for the Anthropocene[END_REF] ). Antão et al. quantified two aspects of changes in species assemblages, namely trends in species richness and total community abundance. They then related these biodiversity trends to mean temperature trends observed over the same period of time, ranging from a few years to 97 years for the longest time series (Figure 1). The authors hypothesized that warm temperate regions that border the species-rich tropics would experience the greatest influx of climate migrants, resulting from the convergence of higher temperature sensitivity of tropical species and tropical areas being hotspots of biodiversity. They also predicted that the higher sensitivities to warming and faster rates of colonization in the marine realm would lead to a tighter coupling between temperature and biodiversity changes at sea than on land.

In accordance with their expectations, Antão et al. find evidence of a global fingerprint of climate warming on marine species assemblages but no systematic temperature-induced biodiversity reshuffling on land, despite the rate of temperature increases being nearly twice faster on land than in the ocean. Importantly, the authors show that the number of species has increased in species assemblages occurring in the warmest temperate oceans, following rises in sea surface temperature, but that the same amount of warming resulted in net biodiversity losses in the coldest temperate oceans. While the results suggest a lower degree of predictability of climate-induced changes in the terrestrial realm, they also indicate that even in the ocean, climate exposure alone is not sufficient to explain the redistribution of biodiversity.

The real-world patterns revealed by Antão et al. open a new line of inquiry about how the

mechanisms that promote temperature-induced biodiversity reshuffling may differ between land and sea. Given that terrestrial and marine organisms occur, and have evolved, in fundamentally dissimilar environments, the fact that the consequences of climate change on biodiversity differ in important ways as well is not entirely surprising. Physiological experiments suggest that the spatial scale of climate change responses may differ radically between the marine and terrestrial realms 5 : large-scale species range shifts may be the dominant response at sea, whereas local thermal refugia may be key for terrestrial species to adapt to the rapid pace of contemporary climate change. To further our understanding of the mechanisms underlying biodiversity redistribution on land, we must thus study them at the appropriate scale [START_REF] Potter | Microclimatic challenges in global change biology[END_REF] . In this regard, recent developments in remote sensing technology may provide new opportunities to accurately map microclimates and associated biological responses to climate change [START_REF] Zellweger | Advances in microclimate ecology arising from remote sensing[END_REF] . Understanding how these pieces connect across scales is an ongoing but fascinating challenge facing ecologists.

Interestingly, Antão et al. show that in the ocean the increase in the number of species in the most rapidly warming areas of the temperate zone results from species gains currently outpacing species losses. This is consistent with climate-induced range expansions being five times faster than range extirpations in the ocean [START_REF] Poloczanska | Global imprint of climate change on marine life[END_REF] . However, the concomitant decrease in total abundance observed across the warmest temperate oceans raises questions about the permanence of the reported biodiversity increase at these latitudes. Should one expect delayed extinction of the most sensitive species within these assemblages to offset or even outpace species gains in the future? As climate warms, direct risk of thermal thresholds being exceeded (when temperatures rise above thermal tolerance) is increasingly likely to challenge the persistence of organisms in the warmer parts of their distribution range. Indirect biotic interactions in which climate immigrants exert new competitive or predation pressures on the recipient assemblages could also accelerate local extinction events. These transient dynamics in a time of rapid environmental change are poorly understood, and deserve formal analysis and experiment.

Even though Antão et al. do not directly address the question of how intrinsic species characteristics mediate community responses, they show that the initial temperature regime experienced by communities is key to understanding the redistribution of biodiversity, at least in the ocean. A clear next step is thus to get a better mechanistic understanding of the reported patterns. For instance, is the increase in species richness in the biogeographic transition zones bordering the tropics associated with an increased representation of warm-water species over time -a process referred to as the 'tropicalization' of species assemblages? Conversely, can species losses in the coldest ocean waters of the temperate zone be explained by the interplay with human activities such as marine fishery exploitation 8 ? The fairly consistent responses documented across taxonomic groups also calls for more detailed analysis of how different thermal physiologies (cold versus warm-blooded) and life history strategies (slow versus fast life cycle) may be sensitive, and respond to climate change. Although the questions raised by the findings of Antão et al. cannot all be resolved at once, the study should inspire future research on the contrasting biodiversity responses to climate change between marine and terrestrial realms.

Figure caption Figure 1 |

 caption1 Figure caption


Francesco Muniz-Miranda

Pierpaolo Minei

Luca Contiero

Fredeŕic Labat

Ilaria Ciofini

Carlo Adamo
email: carlo.adamo@chimie-paristech.fr

Fabio Bellina

Andrea Pucci
email: andrea.pucci@unipi.it

Frédéric Labat

Prof Carlo Adamo

Prof Andrea Pucci

Aggregation Effects on Pigment Coatings: Pigment Red 179 as a Case Study

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

Nowadays, it is widely accepted that human activities strongly contribute to the urban heat island (UHI) effect, with metropolitan areas significantly warmer than surrounding country areas. [START_REF] Muscio | The solar reflectance index as a tool to forecast the heat released to the urban environment: Potentiality and assessment issues[END_REF][START_REF] Sarrat | Impact of urban heat island on regional atmospheric pollution[END_REF][START_REF] Chakraborty | A simplified urban-extent algorithm to characterize surface urban heat islands on a global scale and examine vegetation control on their spatiotemporal variability[END_REF] Urban areas are densely populated thus contributing to average and peak energy demands especially during summer and straining energy resources mainly based on fossil fuel still today.

In EU countries, buildings account for 40% of final energy demand and 32% of CO 2 emission, including residential, industrial and service sectors. [START_REF]Energy efficiency in Europe -Overview of policies and good practices[END_REF] With the stringent need for increasing energy efficiency and reducing greenhouse gas emission, improving the energy performance of Europe's building stock is mandatory. Nevertheless, EU renovation rate (1.2% in EU) is slower than expected and Kyoto and Paris agreement targets reasonably appear out of reach as also mentioned in the 2010/31/EU Directive, which states that each new building should be made "nearly zero energy" from 2020 onwards. An accessible solution to comply with the EU regulation and to mitigate the UHI effect is mimicking the NIR transparent and reflective features of plants and leaves by coating the building surfaces with cool pigments. [START_REF] Llado | Cool solution for the coatings industry: improving performance by evaluating reflectance and pigment chemistry[END_REF][START_REF] Akbari | Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas[END_REF] When the sunlight shines onto a material, part of the radiation is absorbed thus heating its surface. This is caused by the 52% of the electromagnetic spectrum of light composed by the Near Infrared (NIR) window (i.e., from 700 to 2500 nm,) that promotes chemical bonds vibration, thus provoking temperature increase. [START_REF] Jose | Recent advances in infrared reflective inorganic pigments[END_REF] Heat is then transported by conduction into the material, elevating the temperature of the internal side by convection. It is already reported that cool pigments and coatings are able to reduce the heating effect of sunlight with corresponding temperature differences > 10 °C, therefore reducing the electricity demand and saving energy required to downgrade the UHI phenomenon. [START_REF] Synnefa | On the development, optical properties and thermal performance of cool colored coatings for the urban environment[END_REF][START_REF] Yaghoobian | Effect of reflective pavements on building energy use[END_REF] Cool surfaces are measured considering the amount of NIR radiation they reflect (solar reflectance) and how rapidly and efficiently they dissipate heat (thermal emittance). [START_REF] Kaur | Near-infrared reflective properties of perylene derivatives[END_REF] NIR reflective pigments are categorized as cool pigments, which are nowadays dominated by inorganic compounds such as chromium and rare-earth oxides, thanks to their higher refractive indexes, but affected by evident toxicity issues. [START_REF] Synnefa | On the development, optical properties and thermal performance of cool colored coatings for the urban environment[END_REF] Titanium dioxide is also reported as an effective reflector of VIS-NIR light but it strongly absorbs UV light thus also promoting potential degradation of the organic coating. Therefore, organic cool pigments are object of a growing interest owing to their safe properties, low cost and very good processability and dispersibility in most of polymeric formulations. Among all the available organic pigments, perylene bis-imide derivatives represent the most investigated cool components of organic coatings thanks to their light and thermal stability, dispersion properties in polymers and color modulation in the visible region thanks to the different functionalization at the peripheral positions of the perylene nucleus. [START_REF] Mazhar | Synthesis, characterization and near infra-red properties of perylenebisimide derivatives[END_REF][START_REF] Würthner | Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures[END_REF][START_REF] Carlotti | Thermochromic polyethylene films doped with perylene chromophores: Experimental evidence and methods for characterization of their phase behaviour[END_REF][START_REF] Donati | Modulation of the optical response of polyethylene films containing luminescent perylene chromophores[END_REF][START_REF] Raj | Influence of imidesubstituents on the H-type aggregates of perylene diimides bearing cetyloxy side-chains at bay positions[END_REF][START_REF] Kaur | Interpreting the near-infrared reflectance of a series of perylene pigments[END_REF] The NIR transparent and potential reflective properties of perylenes have been investigated in the literature and attributed to different parameters such as pigment particle size, crystallinity, and dipole moment of the chromophoric supramolecular structure. [START_REF] Meymand | Investigation of substituent effect on cool activity of perylene bisimide pigments[END_REF][START_REF] Wang | Aggregation-induced visible light absorption makes reactant 1,2-diisocyanoarenes act as photosensitizers in double radical isocyanide insertions[END_REF] Nevertheless, certain cool perylene bisimides pigments still lack a full investigation aimed to determine the influence of their aggregation extent on the VIS-NIR optical features of the derived polymer coatings. These properties are thus of paramount relevance for current and prospective applications, and are deeply rooted into the molecular geometry, environment, and electronic structure of ground and excited states. Color, in particular, may depend in general on the molecular environments around the chromophores and, as it has been shown, on their aggregation state. [START_REF] Wang | Aggregation-induced visible light absorption makes reactant 1,2-diisocyanoarenes act as photosensitizers in double radical isocyanide insertions[END_REF] Therefore, a deep understanding of the effects induced by the aggregation on the opto-electronic properties of dyes is needed to fine-tune them for application where light-matter interactions are relevant. In this regard, the computational modeling at the quantum level (QM) can give unique insights on the mechanism behind aggregation-induced optical processes, both qualitative and quantitative. [START_REF] Presti | Understanding Aggregation-Induced Emission in Molecular Crystals: Insights from Theory[END_REF][START_REF] Wilbraham | Electrostatic Embedding To Model the Impact of Environment on Photophysical Properties of Molecular Crystals: A Self-Consistent Charge Adjustment Procedure[END_REF] However, most investigations of periodic molecular crystals performed at QM level are limited to the simulation of the electronic ground state, while excited states are often simulated using a user defined cluster of molecules extracted from the solid state structure and treated often at the time dependent (TD) density functional theory level (DFT). This approach, while at times successful, does not consider the effect of an extended periodic environment properly, thus resulting in limited predictive and explanatory power. In order to properly include short and long range electrostatic effects, recently some of us developed a charge embedding technique enabling to take into account the effect of the exact Madelung crystal field potential on a small region of interest treated at the quantum mechanical (QM) level, by a finite array of fitted point charges. This computationally affordable model has been proven to correctly simulate the environmental effects in absorption and emission spectra of molecular crystals.

In this work, we have combined these theoretical tools with experimental investigations to understand the optical features upon aggregation of the N,N'-bis-(methyl)perylene-3,4,9,10tetracarboxyldiimide dye (hereafter called Pigment Red 179 or PR179, see Figure 1), a perylenederivative dye of significant current and prospective relevance. Different coatings were prepared as a function of the pigment content and their VIS-NIR optical features studied in terms of their supramolecular structure. We have organized the paper as follows. In the EXPERIMENTAL DETAILS section we report the experimental optical setup used to probe of PR179 in both solution and solid state. In the COMPUTATIONAL DETAILS section we describe the adopted DFT and TD-DFT procedure.

In the RESULTS AND DISCUSSIONS section we present the collected chemical and physical data analyzing and commenting them. Finally, in the CONCLUDING REMARKS section we discuss the main findings of this paper and how they can be relevant for the field of light-matter interaction.

EXPERIMENTAL DETAILS

Materials

Coating preparation

To a desired amount of Pigment Red 179, Disperbyk 2015 (1:1 by weight with respect to the pigment) and 2 g of the Polidisp™ acrylic dispersion polymer were added and mechanically mixed at room temperature until an homogeneous mixture was obtained. After the addition with 500 μL of deionized water, the resultant paint was applied to Leneta ® checkerboard charts or to cellulose acetate films (3M AF4300, Write-on Transparency Film) using a ZUA 2000 Universal Applicator (Zehntner Testing Instruments) in order to obtain 60-70-μm-thick films after complete water evaporation. In case of coatings based on linear low-density polyethylene (LLDPE), a desired amount of Pigment Red 179 was dispersed under vigorous stirring in a 5 wt.% LLDPE solution in toluene. After evaporation of the solvent, the blend was compression molded at 180 °C for 5 min into thin film of about 200 μm of thickness.

Methods UV-Vis-NIR reflectance measurements of black and white coated surfaces were recorded over the wavelength range of 300-2500 nm by using an Agilent Cary 5000 spectrophotometer equipped with a 150 mm integration sphere and according to ASTM E903-12 (Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres). NIST standards were utilized as primary standards for the calibration of instrument.

X-ray powder diffraction (XRD) was performed using a Bruker D2 Phaser diffractometer (30 kV, 10 mA) operating in Bragg-Brentano geometry (θ-θ scan mode) and equipped with a 1dimensional Lynxeye detector. Ni-filtered Cu Kα radiation was used. Data were collected in the scan range 4-70° in 2θ, with scan step of 0.018° and counting times of 0.5 s/step. Data were processed through the software Diffrac.Eva (Bruker AXS).

The thickness of the coating layer was measured with a CM1S dial indicator (Borletti, Milan, Italy) with ruby movement bearing.

A commercially available 100W IR lamp (Kerbl, Buchbach, Germany) was used for the heating tests. The specimens were irradiated at a distance of 20 cm for 15 minutes and then the temperature measured by means of a FLIR™ E6 infrared thermo-camera (FLIR, Wilsonville, OR, USA).

The optical microscope analysis was accomplished on a Reichert Polyvar optical microscope with crossed polarizers. Particle analysis was performed using the public domain ImageJ 1.52k software version image analyzer program developed by the National Institutes of Health (USA) and available on Internet at https://imagej.nih.gov/ij/index.html.

COMPUTATIONAL DETAILS

Non-embedded calculations PR179 has been studied here by means of DFT structural groundstate optimization in vacuo with the B3LYP [START_REF] Lee | Development of the Colle-Salvetti Correlation-energy Formula into a Functional of the Electron Density[END_REF][START_REF] Becke | Density-functional Thermochemistry. III. The Role of Exact Exchange[END_REF] functional using the Gaussian software (development version: ES64L-GDVRevI.12+ 23 ). To model dispersion forces, we adopted Grimme's D3-BJ correction [START_REF] Grimme | Effect of the damping function in dispersion corrected density functional theory[END_REF] (keyword GD3BJ in the Gaussian software). All relaxed structures are actual potential energy minima as checked by harmonic frequency calculations (i.e. all normal modes have a positive frequency). Using the relaxed ground-state structure, vertical excitations have been computed at TD-DFT level, with the PBE0 [START_REF] Adamo | Toward reliable density functional methods without adjustable parameters: The PBE0 model[END_REF][START_REF] Ernzerhof | Assessment of the Perdew-Burke-Ernzerhof exchangecorrelation functional[END_REF] , LC-PBE [START_REF] Henderson | Can short-range hybrids describe long-range-dependent properties?[END_REF] and CAM-B3LYP [START_REF] Yanai | A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP)[END_REF] functionals: these exchange-correlation functionals have been chosen to span both global hybrid (PBE0) and range-separated hybrid ones (LC-PBE, CAM-B3LYP), which often provide more reliable description of through space charge-transfer excitations. Calculations at the PBE0 and LC-PBE level are reported in Figures S2 andS3 of the Supporting Information.

These calculations were performed both in vacuum and simulating interactions with the nonprotic polar solvent (acetonitrile, ACN), using an implicit solvent model (polarizable continuum model, the Gaussian default IEFPCM). [START_REF] Tomasi | Quantum mechanical continuum solvation models[END_REF] The results of solvent calculations are reported in Figure S2 of the Supporting Information.

Periodic and Embedded TDDFT calculations All periodic calculations have been carried out with the Crystal 17 30 code at the B3LYP-D3 level, as done for the isolated species, along with a modified all-electron 6-31+G(d,p) basis set omitting the most diffuse functions. Numerical DFT integration has been performed considering 75 radial points and 974 angular points, ensuring an error on the integrated electron density to an accuracy of 10 -5 e per unit cell. The Coulomb and exchange series were truncated with threshold values of 10 -7 , 10 -7 , 10 -7 , 10 -7 and 10 -21 . A tolerance of 10 -7 au has been used for the convergence of the self-consistent field procedure. Cell parameters and atomic positions have been relaxed during geometry optimization. Convergence was determined from the root-mean-square and absolute value of the largest component of the forces and displacements, considering default values (0.00045 and 0.00067 a.u. for the forces, and 0.0018 and 0.0027 a.u. for the displacements, respectively). The crystalline structure of PR179 belongs to the monoclinic P2 1 /c space group, with flat perylenetetracarboxylicdiimide portions packed in stacks through - interactions, with experimental lattice parameters of a=3.874(1) Å, b=15.580(2) Å, c=14.597(2) Å, β=97.65(2). The optimized lattice parameters obtained are of a=3.715 Å, b=15.535 Å, c=14.193 Å and β=100.50. In addition, the flat portions of neighboring molecules are found 3.75 Å apart, with respect to an experimental value of 3.87 Å. In order to create a more cubic-like box, we adopted a 4 x 1 x 1 monoclinic supercell, resulting in a simulation box of volume ~ 14.9 x 15.5 x 14.2 Å 3 , shown in Figure S1 of the Supporting Information.

Starting from the optimized crystal structure obtained, a large supercell was built and partitioned into three zones, according to the SC-Ewald approach. [START_REF] Klintenberg | Accurate Crystal Fields for Embedded Cluster Calculations[END_REF] As the inner first zone treated at the QM level, four different clusters with either one, two, three or four PR179 molecules have been considered. The intermediate second zone consisted of point charges fixed at the value of the Mulliken atomic charges of the periodic calculation, while the point charges values of the outer third zone have been adjusted in order to impose the exact Madelung potential of the infinite periodic system on the first two zones sites. Further details are available in Ref. 20 as well as in previous works of some of us . [START_REF] Wilbraham | Revealing the origins of mechanically-induced fluorescence changes in organic molecular crystals[END_REF][START_REF] Presti | Computational protocol for modeling thermochromic molecular crystals: salicylidene aniline as a case study[END_REF][START_REF] Presti | Modeling Emission Features of Salicylidene Aniline Molecular Crystals: a QM/QM' Approach[END_REF][START_REF] Presti | Optical properties of the dibenzothiazolylphenol molecular crystals through ONIOM calculations: the effect of the electrostatic embedding scheme[END_REF][START_REF] Wilbraham | Modelling photophysical properties of metalorganic frameworks: A Density Functional Theory based approach[END_REF] Vertical TD-DFT calculations with the aforementioned models (embedded monomer, dimer, trimer, and tetramer of PR179) have been carried out, thus including the effects of the crystal environment. The embedded dimer model was also used to test the dependence of computed spectra on the inter-molecular distance. All calculations (both embedded and non-embedded) have been carried out using the Pople's 6-31+G(d,p) basis set and dispersion forces were modeled employing Grimme's D3-BJ correction [START_REF] Grimme | Effect of the damping function in dispersion corrected density functional theory[END_REF] .

RESULTS AND DISCUSSIONS

Experimental investigations of Pigment Red 179 (PR179) dispersions

Properties of PR179 dispersions on opaque substrates VIS-NIR reflectance spectra recorded on PR179 dispersion over the white portion of Leneta ® (L) checkerboard chart (Figure 2a) display a reflectance pattern affected by pigment content, i.e.

particularly pronounced up to 1200 nm, and in agreement with the absorption characteristics of the chromophoric unit. [START_REF] Würthner | Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures[END_REF] Notably, the 10% of reflectance reduction due to the pigment coating was more pronounced by increasing the pigment content from 2.5 wt.% to 5 wt.%. Conversely, no evident variations actually occurred for the coating containing the highest PR179 content (10 wt.%), thus suggesting that a sufficient coverage of the white checkboard was reached with the Moreover, reflectance values as high as 80% recorded from white substrates suggested that Pigment Red 179 is relatively NIR transparent. [START_REF] Meymand | Investigation of substituent effect on cool activity of perylene bisimide pigments[END_REF][START_REF] Mazhar | Synthesis, characterization and near infra-red properties of perylenebisimide derivatives[END_REF] This characteristics was confirmed by inspecting the reflectance behavior of pigment coatings over black substrates (Figure 2b).

Relatively low reflectances were recorded in agreement with the NIR transparent features of Pigment Red 179. Nevertheless, a worthwhile increasing of the overall reflectance in the 700-1600 nm wavelengths window occurred with pigment concentration. Notably, on passing from 2.5 wt.% to 10 wt.% of pigment concentration, a 75% of light reflection was recovered. This behavior suggests that Pigment Red 179 could also behave as a NIR reflective molecule, in agreement with certain symmetrically substituted perylene derivatives. It was worth noting that the maximum temperature reached by the uncoated black surface (Figure 3a) was recorded as the highest at 61.5 °C, i.e. 2.7 °C and 4.9 °C higher than those coated by the 5 wt.% (Figure 3b) and the 10 wt.% (Figure 3c) Pigment Red 179 dispersions, respectively. This performance, while on one hand appears less pronounced than those of ultimate paints with chilling effects higher than 10-15 °C, on the other one confirms the cooling characteristics of the investigated symmetrically substituted perylene bisimide derivative.

Aimed at determining the origin of the cool features of Pigment Red 179, the dispersions were analyzed by XRD investigations at room temperature (Figure 4). XRD pattern of the pigment dispersions displayed sharp reflections attributed to the crystalline content that are dominated by an evident elevation caused by the amorphous phase contribution, which appears predominant at lower theta region and mostly attributed to the acrylic amorphous polymer coating. Notably, the diffraction profiles in the higher theta regions (theta > 25°)

showed crystalline contributions that are attributed to self-assembled and well-ordered microstructures promoted by the formations of intermolecular π-π interactions. [START_REF] Kaur | Interpreting the near-infrared reflectance of a series of perylene pigments[END_REF][START_REF] Presti | Understanding Aggregation-Induced Emission in Molecular Crystals: Insights from Theory[END_REF] It is worth noting that a considerable fraction of crystalline Pigment Red 179 is present at all investigated concentrations with an evident and progressive increase of the reflection intensities particularly evidenced on passing from the 2.5 wt.% to the 5 wt.% pigment concentration. Unfortunately, the presence of the strong amorphous contribution provided by the polymer covered most of the crystalline peaks of the pigment at theta region lower than 22°. This issue flanked by experimental systematic errors made a quantitative assessment of the crystalline degree

Properties of PR179 dispersions on transparent substrates

Pigment Red 179 dispersion was also deposited on transparent cellulose acetate polymer films aimed at determining the morphology of the pigment aggregates as well as the spectroscopic features in the visible region. Optical micrographs reported in figure 5 show the presence of a broad distribution of micro-sized pigment particles and large aggregates. The average diameter calculated on passing from 2.5 wt.% to 10 wt.% solid dispersions remained almost unchanged (about 3-5 micrometers), whereas the variation of light intensity and sample color appeared evident for pigment concentrations higher than 5 wt.%. This behavior agreed with the results gathered from reflectance experiments of pigment dispersions over white substrates (Figure 2a), whose coverage resulted mostly completed by using the dispersion with 5-10 wt.% of Pigment Red 179 concentration. It is also well reported that diffuse reflection intensity depends on the number of particle boundaries. [START_REF] Liu | Investigation of near infrared reflectance by tuning the shape of SnO 2 nanoparticles[END_REF] By increasing pigment concentration, the coating layer contains a large number of small micrometer-sized particles with high surface area that possibly contribute to the higher NIR reflectance due to the enhanced number of boundaries.

The color of the Pigment Red 179 is mostly addressed to its planar conjugated π-electron system, which causes strong absorption in the visible region. The methyl substituent at the periphery of the molecule is reported to be responsible for the energetic position of the absorption maximum. [START_REF] Hasche | Coherent phonons in quasi-onedimensional organic crystals[END_REF] Figure 6. Absorption spectra of Pigment Red 179 dispersions over cellulose acetate transparent films as a function of pigment content and 5 wt.% Pigment Red 179/LLDPE film (yellow line) Perylene bisimides exhibit the electronic 0-0, 0-1 vibrational peaks and high energy 0-2 featureless shoulder peaks in the region of 410-450 nm, which are usually fused together in a wavelength (nm) 2.5 wt.% 5 wt.% 10 wt.% 5 wt.% in LLDPE tested here, with only a shift in energy in the computed spectra due to their different Hartree-Fock exchange content (up to 100% and ~63% for long inter-electronic distances in LC-PBE and CAM-B3LYP, respectively, and 25% for the global hybrid PBE0). To model the effect of π-π inter-molecular interactions, a minimal model represented by a dimer of PR179 molecules (refer to COMPUTATIONAL METHODS section for details) was firstly considered and the results are reported in Figure 7 (right side). As can be seen in Figure 7, bottom right panel, the nature of the main transitions changes (I of the HOMO→LUMO type, and II of the HOMO-1→LUMO+1 type) but retaining their localized character, without any significant charge transfer, neither intra nor inter-molecular. To have an estimate of the charge-transfer distance associated to these transitions, we employed the D ct index [START_REF] Bahers | A Qualitative Index of Spatial Extent in Charge-Transfer Excitations[END_REF] , which is basically a measure of the hole-electron distance due to the photo-excitation. We found a D ct value of 0.06 and 0.09 Å for excitations I and II, respectively: these small values, much smaller than an average covalent bond distance, are fully consistent with the notion of a localized transition (i.e. a transition with no chargetransfer involved). CAM-B3LYP vertical excitations show an absorption in the blue-cyan region of the visible spectrum (for isolated pigment and dimer model), while the absorption energies computed at the PBE0 level of theory are significantly red-shifted with optical-gaps in the orange region of the visible spectrum, as expected it is due to the reduced amount of Hartree-Fock exchange. [START_REF] Muniz-Miranda | Assessment of Exchange-Correlation Functionals in Reproducing the Structure and Optical Gap of Organic-Protected Gold Nanoclusters[END_REF][START_REF] Muniz-Miranda | DFT and TD-DFT Assessment of the Structural and Optoelectronic Properties of an Organic-Ag 14 Nanocluster[END_REF][START_REF] Muniz-Miranda | Electronic and optical properties of the Au 22[END_REF][START_REF] Muniz-Miranda | Benchmarking TD-DFT against Vibrationally Resolved Absorption Spectra at Room Temperature: 7-Aminocoumarins as Test Cases[END_REF][START_REF] Muniz-Miranda | On the opto-electronic properties of phosphine and thiolate-protected undecagold nanoclusters[END_REF] The spectrum of the isolated dimer computed at CAM-B3LYP and LC-PBE level show no qualitative difference with respect to that of the monomer but for a slight red-shift of around 50 nm. The PBE0 spectra show larger differences.

Nonetheless, to correctly simulate the crystalline environment effect on the optical spectra, as mentioned in the introduction, long-range electrostatic effects have to be included. To this end, embedded TD-DFT calculations were performed and the corresponding simulated spectrum at CAM-B3LYP level is reported in Figure 8 in the case of the dimer model, the minimal one to check the existence of intermolecular CT transitions. The orbital component of the first (I) and the second (II) transition are reported in Figure 8. Contrary to the calculations performed in the absence of the crystalline potential, here we can clearly see the presence of a transition (I at low energies in red region when using the CAM-B3LYP functional), which shows a clear intermolecular charge transfer character (the D ct index [START_REF] Bahers | A Qualitative Index of Spatial Extent in Charge-Transfer Excitations[END_REF] estimates a charge transfer distance of ~3.91 Å, consistent with the intermolecular distance between PR179 molecules). This transition involved the HOMO and LUMO orbitals localized on different molecules, whereas the second one (II) is of the HOMO →LUMO+1 type and does not show any charge transfer character. The predicted transition energies (at TD-CAM-B3LYP level of theory) are λ I = 605 nm and λ II = 448 nm, corresponding to energies of 2.04 and 2.77 eV. These absorption energies are in good agreement with the experimental ones (2.13 and 2.69 eV, respectively, see Figure 6). Calculations performed on embedded trimer and tetramer clusters confirm the inter-molecular charge transfer nature of the 1st transition: the orbital involved in it belong to two adjacent molecules, without the creation of an extended exciton: these are reported in the Supporting Information (Figure S4 andS5). In order to investigate how both energies and intensity of the two bands are affected by packing, we decided to monitor the variation in oscillator strength and transition energies of the two bands as a function of the intermolecular distance in the presence of the embedding potential. Changing the intermolecular distance significantly affects the oscillator strength of transition I, as reported in Figure 9. Here it has be pinpointed that distances much smaller than the experimental equilibrium value (of about ~3.86 Å) might have little physical meaning, for at least two reasons:

(1) it is experimentally unfeasible to compress the crystal system so far as to reach such short inter-molecular contacts; (2) for very short inter-molecular distances, the orbital character of the S 0 → S 1 and S 0 → S 2 transitions may change drastically, giving rise to a discontinuous behavior of both energy and oscillator strength; considering this caveat, we report them anyway in Figure 9 (from 2.5 to 4 Å) for sake of completeness. When the distance decreases of about 15% with respect to the equilibrium one, the UV-Vis peak due to transition I matches the absorption of transition II. To match the relative height of the two peaks, as observed experimentally in Figure 6, a shrinkage of the intermolecular distance of about 6% is needed. The oscillator strength of a transition is in fact proportional to its probability of occurrence and, consequently, the calculations show that the intensity of the intermolecular charge transfer increases when the intermolecular distance decreases. While the energy of transition I remains remarkably stable when passing from intermolecular distances between ~3.1 to ~4.0 Å (corresponding to energy values of 1.85 and 2.06 eV, respectively), the energy of transition II is instead more affected by the change in this parameter, and in particular has an abrupt change between 3.2 and 3.1 Å. In fact, the energy of transition II decreases significantly with compression of the embedded dimer model, with values spanning the interval between 2.37 eV (d = 3.1 Å) and 2.76 eV (d = 4.0 Å). This can be qualitatively related to the experimentally observed red-shift of the band ascribed to transition II from ~460 nm (2.69 eV) to ~490 nm (2.53 eV) when increasing in the pigment concentration, as shown in Figure 6. The combined rise of transition I and redshift of transition II make the pigment darker when the concentration is increased corresponding to a locally more dense (closer) packing of the dimers. The results gathered from the computational investigations strictly evidence how the optical features of Pigment Red 179 are strongly affected by its aggregation extent. As far as concentration increases in the supporting polymer matrix, a compact and crystalline-like structure governed by π-π stacking interactions grows thus inducing the progressive emersion of the low energy absorption CT band that in turn contributes to the overall color of the pigment.

CONCLUDING REMARKS

In this work we investigate, by means of a combined approach of density functional theory calculations and VIS-NIR experiments, the effects due to aggregation on the optical properties of Pigment Red 179, particularly on its color and NIR reflective features. Experimental spectra show that the pigment coverage and NIR reflective features are strongly dependent on the chromophore content. Notably, on passing from 2.5 wt.% to 10 wt.% of pigment concentration, a 75% of light reflection was recovered in the 700-1600 nm wavelengths window, which in turn produces a cooling effect of about 5°C of the black coated surface. Calculations addressed this evidence as due to an intermolecular charge transfer excitation, which becomes more probable as the molecules get closer in agreement with their X-ray diffraction spectra. This work provides new insights on the aggregation-induced absorption and on the development of modern paints that offer a colored coverage to the different substrates without contributing on their heating during light exposure.

For Table of Contents Only

Figure 1 .

 1 Figure 1. Chemical structure of N,N'-bis-(methyl)perylene-3,4,9,10-tetracarboxyldiimide (Pigment Red 179)

Figure 2 .

 2 Figure 2. VIS-NIR reflectance spectra of Pigment Red 179 dispersions over white (a) and black (b) Leneta ® (L) checkerboard chart

11

 11

Figure 3 .

 3 Figure 3. Thermographs of the a) uncoated, b) 5 wt.% and c) 10 wt.% of Pigment Red 179 coated black checkboards with maximum temperature reached after irradiation with a 100W IR lamp for 15 min.

Figure 4 .

 4 Figure 4. XRD spectra of the Pigment Red 179 dispersions at different dye concentration.

  inaccurate. Nevertheless, the favorable morphology and crystallinity of π-π pigment stacked structures possibly explain enhancement in the NIR reflectivity.

Figure 5 .

 5 Figure 5. Optical micrographs of Pigment Red 179 dispersions over cellulose acetate films at different pigment content. Scale bar = 350 μm. a) 2.5 wt.%; b) 5 wt.%; c) 10 wt.% of pigment

Figure 7 .

 7 Figure 7. Computed UV-Vis spectra of PR179 monomer (left) and isolated dimer (right). Vertical red spikes represent computed vertical excitations transitions, while blue lines are spectra obtained by Gaussian convolution using a full width half height of 0.33 eV. The orbitals involved in transitions I and II are reported for CAM-B3LYP. No energy/wavelength scaling has been employed.

Figure 8 .

 8 Figure 8. Computed UV-Vis spectrum of PR179 dimer embedded in the electrostatic field of the crystal. Red spikes represent actual transitions, while blue line is the spectrum obtained by Gaussian convolution using a full width half height of 0.33 eV. The orbitals involved in transitions I and II are reported for CAM-B3LYP. No energy/wavelength scaling has been employed.

Figure 9 .

 9 Figure 9. Oscillator strength (CAM-B3LYP level of theory) and energies of transitions I and II, as a function of the intermolecular distance.

Acknowledgements

This work was supported by the Tuscany region POR FESR 2014-2020 COOLSUN -New NIRreflective "cool" organic pigments. FM-M gratefully acknowledge Ms. Federica Maschietto's preliminary calculations on the perylene systems. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program (grant agreement n o 648558 STRIGES CoG).

broadened absorption band in chromophoric aggregates in solution or in the solid state. In the case of Pigment Red 179 dispersions over cellulose acetate films, this broad absorption band is located at about 480-490 nm and flanked by more growth of shoulder peaks at the longer wavelengths of 570-580 nm. This feature is in agreement with the formation of the typical faceto-face chromophoric arrangements promoted by the strong intermolecular π-π interactions as analogously reported in the main literature. [START_REF] Wuerthner | Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials[END_REF][START_REF] Goerl | Molecular Aggregates of Perylene Bisimide Dyes in Water[END_REF][START_REF] Chen | Self-assembled π-stacks of functional dyes in solution: structural and thermodynamic features[END_REF][START_REF] Wuerthner | Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures[END_REF] Notably, the ratio between the absorbances of these two contributions did not seem affected by pigment concentration, thus suggesting that the formation of large aggregates already occurred at the lowest 2.5 wt.% content, as also evidenced by optical microscopy in figure 5a. The band at lower energies did not prevail over the molecular one even in a polyethylene matrix (Figure 6, yellow curve), which is considered the less compatible with organic pigments and its apolar and aliphatic nature is well known to strongly promote the formation of large-scale chromophoric aggregates. [START_REF] Prampolini | Computational design, synthesis, and mechanochromic properties of new thiophene-based π-conjugated chromophores[END_REF][START_REF] Ciardelli | Dye-containing polymers: Methods for preparation of mechanochromic materials[END_REF][START_REF] Pucci | Mechanochromic polymer blends[END_REF][START_REF] Pucci | Polymer composites with smart optical properties[END_REF][START_REF] Pucci | Green-blue luminescence dichroism of cyano-containing poly[(m-phenylene ethynylene)-alt-(p-phenylene ethynylene)] aggregates dispersed in oriented polyethylene[END_REF]

Computational analysis of Pigment Red 179 (PR179) assemblies

Aimed at understanding the effect of the aggregation extent of Pigment Red 179 molecules on their optical characteristics, a computational analysis was carried out. We report in Figure 7 (left) the computed absorption spectrum for an isolated PR179 molecule. Effect of solvent on band shape profile and transition character is negligible, as clearly shown by comparison with calculations in ACN reported in Figure S2 of the Supporting Information.

The spectrum is dominated by two main transitions, all of localized character with negligible intra-molecular charge-transfer character. Both transitions are basically π-π* excitations corresponding to a HOMO→LUMO excitation for transition I and to a HOMO-1→LUMO for II (see bottom left side of Figure 7). This assignment is consistent among the three levels of theory