Felix De Tombeur
email: felix.detombeur@cefe.cnrs.fr@

John A Raven

Aurèle Toussaint

Hans Lambers

Julia Cooke

Sue E Hartley

Scott Johnson

Sylvain Coq

Ofir Katz

Jörg Schaller

Cyrille Violle

Why do plants silicify?

Keywords: costs, ecosystem functioning, fitness, functional trait, silicification, grassland

Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies are focused on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organisation, and remains hard to capture. We show that the costs of Si accumulation are greater than currently acknowledged, and we discuss potential links between Si and fitness components (growth, survival, reproduction), environment and ecosystem functioning. We propose that Si is more important in trait-based ecology than currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and expansion of specific biomes.

Silicon in plant ecology

, many aspects of Si in plant ecology remain puzzling. Why does the variation in foliar Si concentrations comprise several orders of magnitude in terrestrial plants, ranging from virtually none to very high concentrations greatly exceeding those of macronutrients (up to 10% dry weight) ? Does silicification have adaptative value, and does it contribute, or has it contributed, to the success of clades and the expansion of

specific biomes (e.g., grasslands [START_REF] Strömberg | Evolution of grasses and grassland ecosystems[END_REF])? What are the costs and benefits of Si, and can trade-offs with other functional traits be identified?

Although the compelling questions raised above remain unanswered, our understanding of Si in plant biology has progressed significantly in recent years. Si transporters have been identified in several taxa and a plant's capacity to accumulate Si is thought to be both heritable and inducible, interspecific variation is well characterised, and major functions have been identified for Si in plant tissues [START_REF] Debona | Silicon's role in abiotic and biotic plant stresses[END_REF][START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][START_REF] Deshmukh | New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants[END_REF][START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF]. In particular, when supplied to plants, Si often increases their resistance to biotic (herbivores and microbial pathogens) and abiotic stresses (metal toxicity, salt and water stress, wind and other physical forces, UV, and nutrient deficiency), augments mechanical strength of plant organs, and, as a result, promotes plant growth and crop yields [START_REF] Coskun | The controversies of silicon's role in plant biology[END_REF][START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][START_REF] Cooke | Consistent alleviation of abiotic stress with silicon addition: a meta-analysis[END_REF][START_REF] Datnoff | Silicon in agriculture[END_REF]. Several mechanisms may contribute to these benefits depending on the nature of the stress [START_REF] Thorne | Is silicon a panacea for alleviating drought and salt stress in crops? Front[END_REF], though a recent model has attributed most Sirelated functions to the deposition of silica in the apoplast, through the process of silicification [START_REF] Coskun | The controversies of silicon's role in plant biology[END_REF].

The well-established functions of Si have attracted interest in its role in plant ecology [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][START_REF] Cooke | Is plant ecology more siliceous than we realise?[END_REF][START_REF] Katz | Silicon content is a plant functional trait: implications in a changing world[END_REF][15]. The role of Si as a plant defence is better understood, and Si is increasingly seen as an important functional trait that impacts plant fitness [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][START_REF] Cooke | Is plant ecology more siliceous than we realise?[END_REF][START_REF] Katz | Silicon content is a plant functional trait: implications in a changing world[END_REF][START_REF] Massey | Physical defences wear you down : progressive and irreversible impacts of silica on insect herbivores[END_REF]. However, the potential for Si to enhance fitness remains speculative, as does the importance of Si in ecological functions other than defence against herbivores and pathogens. The element is still under-appreciated by many ecologists, especially when compared with macronutrients like nitrogen (N) and phosphorus (P). In fact, significant gaps in elementary knowledge on Sisuch as its costs of accumulation, and potential links with fitness components and environmentprevent us from better understanding variation in Si concentration and discerning its significance in plant ecology. For instance, trait-based approaches have proven to be helpful to study trade-offs in ecological strategies and constraints on evolution [START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF][START_REF] Züst | Trade-offs between plant growth and defense against insect herbivory: An emerging mechanistic synthesis[END_REF], but Si is underrepresented in traitbased ecology.

Here, we review the significance of Si in plant ecology at different levels of organisation. We considered Si concentration in leaves (hereafter leaf [Si], in mg g-1 dry weight), because we have significantly more information for this organ. We discuss [START_REF] He | Physiological and ecological significance of biomineralization in plants[END_REF] leaf [Si] variation among terrestrial plant species and its inclusion into trait-based ecology, [START_REF] Coskun | The controversies of silicon's role in plant biology[END_REF] adaptive values of leaf silicification, (3) Si effect on ecosystem processes and ( 4) the costs and benefits of leaf Si accumulation.

Silicification: evolution, constraints, and trade-offs

Phylogenetic approaches are useful to understand interspecific variation in leaf [Si] [START_REF] Deshmukh | New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants[END_REF][20][21][22].

Poales (e.g., Poaceae, Cyperaceae, Juncaceae) accumulate more Si than plants in other orders, but substantial Si accumulation has been demonstrated for other orders as well (e.g., Cucurbitales, Fabales) [START_REF] Deshmukh | New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants[END_REF][23][24][25]. Interestingly, there is no single stimulus or function that explains the multiple emergences of this trait [START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF]22]. Phylogenetic analyses show that Si transporters in all land plants evolved from a small clade of aquaporins that do not differ substantially from the known Si transporters [21]. The potential capacity to take up and accumulate Si is probably ancestral to all land plants, and fully evolved only in the clades in which all necessary Si transporters evolved, possibly under specific evolutionary pressures [START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF]22,26,27].

In a pioneering work conducted in the Serengeti National Park, McNaughton et al. [28] showed that plants native to the more heavily grazed grasslands accumulate more Si than plants from less heavily grazed sites. This work supported the hypothesis that silicification has adaptive value in response to increased herbivore pressure when open grasslands expanded (e.g., mid-Cenozoic) [reviewed in 5]. However, silicification might also have adaptive value in habitats characterised by seasonal aridity, low atmospheric carbon dioxide concentrations, low-nutrient availability, or strong wind exposure, and the adaptive origin of silicification may not have been the defence against mammalian grazers [29][30][31][32][33][34]. In fact, paradigms that associate grassland evolution with any single environmental factor (e.g., grazing) are not well supported by chronologyat least at the global scale [START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF]35] and various environmental factors may have contributed to grassland evolution in various parts of the world [35], suggesting that silicification is more likely an exaptation, rather than an adaptation to grazing.

In addition, environmental factors, including soil properties (e.g., degree of weathering, mineral composition), might also have impacted soil Si availability which might explain the evolution of silicification [36], but soil Si availability is barely considered in evolutionary studies. For instance, overgrazing enhances the expression of P-mobilising traits in the rhizosphere (e.g., root exudates) [37], which, in turn, increase chemical weathering and soil Si mobilisation [38]. Implementing long-term experimental evolution studies would be particularly relevant for Si, to understand its adaptive value according to different environmental parameters [39].

Leverage trait-based approaches might be useful to better understand constraints on the evolution of Si accumulation in plants and potential trade-offs or linkages with other traits having similar functions. The global spectrum of plant form and function (GSPFF) describes a dimensional phenotypic space made of six major traits critical to growth, survival and reproduction [START_REF] Díaz | The global spectrum of plant form and function[END_REF]. One major axis of variation reflects a plant size gradient (height, seed mass, specific stem density), and the other balances more conservative species with high leaf mass per area (LMA) versus more acquisitive fast-growing species with high leaf N concentration [START_REF] Díaz | The global spectrum of plant form and function[END_REF]. The association of leaf [Si] with the plant size axis of the GSPFF (Box 1) reflects the greater Si accumulation in leaves of non-woody compared with woody species (Box 1 and Figures S1 & S2 in Supplementary Material). Leaf silicification might play, or have played, a role in physical support and plant defence mechanisms mostly for non-woody plants, and possibly at the expense of C-based compounds such as cellulose and lignin (Box 2). As discussed above, the evolutionary history of grasslands is the most widespread argument to explain the higher levels of leaf Si in non-woody species, but silicification could also represent significant constraints for taller woody species.

Another major trait-based framework is the leaf economics spectrum (LES), that describes a major axis of cross-species leaf physiology comprising key traits such as LMA, leaf lifespan, N and P concentrations, and photosynthetic (A mass ) and respiration (R mass ) rates [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]. The spectrum runs from fast-growing species with rapid resource acquisition to slow-growing species with conservative strategies [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]. Evaluating if and how Si aligns with the LES is challenging because leaf [Si] might be associated with fast growth and return on C investment given its role as a growth-promoting beneficial element [40], but prominent ecological theories predict more investment in anti-herbivore defences for slow-growing species [START_REF] Züst | Trade-offs between plant growth and defense against insect herbivory: An emerging mechanistic synthesis[END_REF]41]. Cooke et al. [40] showed that species with shorter leaf lifespan generally exhibit higher leaf [Si], and concluded that Si could be a metabolically cheaper alternative to C in shorter-lived leaves and that leaf Si might be associated with the fast end of the LES. Here, despite significant relationships between leaf [Si] and the LES traits (Box 1 and Figure S3), leaf [Si] seems independent of the LES overall (Box 1), at least globally and at the interspecific level.

It is possible that the implementation of the LES in Si research is most promising at the intraspecific level, or at least in a phylogenetically controlled set of related species, to reduce the strong phylogenetic signal of Si accumulation [20]. We also suggest combining trait-based approaches with metabolomics (i.e. tens of thousands of metabolites that are the substrates and products of enzymatic reactions) in Si research which has the potential to provide a better mechanistic understanding of trade-offs and ecological strategies [42].

For non-woody species, the question remains as to how high silicification affects leaf density, which has long been postulated [START_REF] Raven | The transport and function of silicon in plants[END_REF]. Species that accumulate more Si in their leaves do, indeed, have denser leaves (greater leaf dry mass content; LDMC), because of the high density of silica compared with C-based compounds (Box 1). Leaf thickness decreases with increasing silicification (Box 1), and we suggest a trade-off between thick leaves and high degree of silicification as different solutions to minimise the impact of some biotic and abiotic stresses (e.g., water stress, herbivory) and improving leaf mechanical properties [43,44].

Overall, silicification seems to be a neglected driver of the LDMC of non-woody species allowing leaf mechanical protection and/or water stress avoidance, especially for species with thinner leaves, and future studies on leaf physical strength (e.g., force to punch, force to tear) should integrate Si in their framework. Leaf [Si] is also positively related with leaf mass per area (LMA), albeit with a weak R² (Box 1). This is possibly because silicification increases LDMC but high Si-accumulating species have thinner leaves, making the relationship with LMA unclear. Overall, we argue that phylogenetic analyses must be coupled with trait-based approaches to better identify potential trade-offs and constraints on Si evolution.

Si and plant fitness: to grow or to survive?

Positive effects of silicification on plant fitness have long been postulated [START_REF] Cooke | Is plant ecology more siliceous than we realise?[END_REF], but convincing evidence is lacking so far. The numerous stresses mitigated by Si addition to the growth medium that can stimulate plant growth [START_REF] Coskun | The controversies of silicon's role in plant biology[END_REF][START_REF] Debona | Silicon's role in abiotic and biotic plant stresses[END_REF][START_REF] Cooke | Consistent alleviation of abiotic stress with silicon addition: a meta-analysis[END_REF] led to the idea that Si is a growthpromoting, beneficial nutrient overall. This is particularly true since studies have demonstrated positive effects of Si on growth of unstressed plants too [45][46][47], although this remains contentious [START_REF] Coskun | The controversies of silicon's role in plant biology[END_REF]. However, information on the effect of Si on plant growth is based mainly on experiments in controlled conditions in which Si is manipulated in the growth medium, and there are no studies considering leaf [Si] in relation to fitness and the three fitness components, i.e. growth, survival, reproduction. This gap in the literature prevents associating Si with fitness (Figure 1).

Identifying links between leaf [Si] and fitness components is challenging because of antagonistic processes (Figure 1). For instance, well-established trade-offs exist between growth and survival [48], because investment in defence or resistance traits reduces resources available for growth (i.e. growth-defence trade-off) [START_REF] Züst | Trade-offs between plant growth and defense against insect herbivory: An emerging mechanistic synthesis[END_REF]. High leaf [Si] might be linked to higher survival rates because it mitigates biotic and abiotic stresses, but also to growth, because of its links with plant architecture, light capture, photosynthesis, and eventually competition (Figure 1 and Box 2). The absence of clear links between leaf [Si] and the LES, as discussed above, prevents us from associating silicification with growth or survival, at least at the interspecific level. Besides, although Si supply increases grain yield in crops [45,46], no links between Si and seed production are known for natural systems (Figure 1).

Towards a siliceous growth-defence trade-off in grasses?

Despite the lack of information on fitness-leaf [Si] relationships (Figure 1), recent comparative studies show that higher leaf [Si] is associated with slower growth rates in grasses [49][50][51][52]. In particular, Massey et al. [50] showed that leaf [Si] is negatively correlated with relative growth rate (RGR) and N concentration among 18 grass species. Similarly, Simpson et al. [49] found that higher leaf

[Si] is associated with slower RGR among eight cultivated species. Also, Thorne et al. [51] showed a strong negative correlation between RGR and leaf [Si] among 19 rice genotypes.

Although few, these studies consistently suggest (1) the existence of ignored direct costs associated with leaf Si accumulation, (2) that silicification might be positively associated with survival, rather than growth, and (3) the existence of a Si-mediated trade-off between these two fitness components. In addition to direct costs, ecological costs of Si accumulationthat emerge from interactions with other organisms (e.g., plant-plant or plant-herbivore interactions) [START_REF] Züst | Trade-offs between plant growth and defense against insect herbivory: An emerging mechanistic synthesis[END_REF] have never been considered and should also be estimated in future studies.

Potential relationships between fitness and leaf [Si] could be tested through comparative studies involving species and/or genotypes with inherently different [Si], or by using mutants with contrasting expression of Si transporters [53,54]. Different methods for estimating fitness have been suggested, ranging from time-consuming quantitative common garden experiments to easier but less rigorous population-level monitoring [48]. The use of Si-accumulating model species (e.g., Brachypodium distachyon, Setaria viridis) might be valuable here, although Si functions have also been reported for non-accumulating model species, such as Arabidopsis thaliana [55] and thus should be also studied. More generally, Si-related functions are not restricted to accumulating species [24,56], and future fitness-Si studies should embrace different plant families and orders.

In addition to its potential link with fitness, a plant's capacity to accumulate Si in leaves is assumed to be heritable, given the evidence for genetic control of transporters [START_REF] Deshmukh | New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants[END_REF], but highly plastic in response to environment. In particular, leaf [Si] shows plasticity in response to soil water availability [57], herbivory [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][58][59][60], N and P limitation [31,61] and CO 2 concentrations and temperature [52,62,63]. However, the mechanisms underlying Si plasticity to environmental changes are mostly unclear, especially regarding the relative contribution of passive and active Si accumulation [58,64]. In fact, it is very likely that multiple abiotic and biotic factors thought to directly increase plant Si accumulation also affect soil Si availability, transpiration and growth rates which will, in turn, affect [Si] in plant tissues. For instance, CO 2 and temperature might affect transpiration or soil water content, which would, in turn, affect the passive mode of Si uptake [57,62,63]. Similarly, increased Si accumulation with N or P limitation might be physiologically controlled through activation of Si transporters [65], but also due to the same Si uptake but in more or less plant biomass (i.e. dilution/concentration effect [66,67]). We suggest that future studies on plasticity in [Si] pay specific attention to underlying mechanisms whenever possible. In particular, determining changes in stomatal conductance and other physiological parameters would help to estimate the contribution of passive versus active Si uptake, and information on total dry matter production and total Si content to understand potential dilution/concentration effects [66].

Si, environmental factors and ecosystem functioning

Response-effect trait frameworks are commonly used in trait-based ecology to clarify the mechanistic links between environmental factors, species' traits and ecosystem functioning [68]. Metrics that consider the relative abundance of species in a community, such as the community-weighted mean (CWM), are effective and widely used to scale up from species to higher organisational levels [69][70][71]. Assigning more importance to dominant species and their traits is useful for better capturing the responses to environmental gradients and for understanding which effects can be expected on ecosystem processes, and with what magnitude [69,72]. This approach is starting to be used for Si [23,25,[73][74][75] but still only rarely, although it might be particularly relevant because leaf [Si] is both a response and effect trait, in that it responds to environmental gradients and has an effect on ecosystem functioning

[68] (Figure 2).

Si as a response trait

In natural ecosystems, leaf [Si] is significantly affected by soil Si and water availability [32,76,77], herbivory [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][78][79][80][81], wind [34], nutrient limitation [23,30], and elevation [25] (Figure 2). Although some community-level trends with environment are clear (e.g., Nakamura et al. [25] with elevation, and de Tombeur et al. [23] with soil nutrients), responses of [Si] to environmental conditions are mostly studied at the intraspecific level and mainly in grasses. Therefore, more interspecific studies along environmental gradients are needed to understand how [Si] is expressed in different ecosystems and plant communities.

Another challenge of community-level studies is to take into account intraspecific variation (Figure 2 Si as an effect trait First, there is evidence that leaf [Si] can play a significant role in nutrient cycling [86,87] (Figure 2). For instance, silicified trichomes can slow down leaf decomposition of tropical trees by soil meso-and macrofauna [86], and Si accumulation can affect leaf nutrient stoichiometry and concentrations of C-based defence compounds in grasses (e.g., cellulose, phenols, lignin) [88,89]. Second, leaf silicification can be induced in response to increased herbivore density [80], thereby influencing herbivore growth rates, reproduction and richness in grasslands [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF]75]. Changes in wild herbivore populations affect several ecosystem processes, such as nutrient cycling, C storage or primary productivity [90].

Third, the positive effect of Si fertilisation on crop productivity suggests that community-level

[Si] might be linked to annual net primary productivity (ANPP). Long-term field Si fertilisation showing increased aboveground biomass of grasses provides support for this [91].

We also note that over half of the terrestrial ANPP is attributed to actively Si-accumulating vegetation (33 Gton C yr

Beyond a "Si-C trade-off": rethinking the energetic costs of Si

Understanding the costs and benefits of trait values is at the very foundation of plant ecology [98]. It is paramount to understand trait variation as dependent on environment, trade-offs between traits, and more generally plant ecological strategies [99]. Since Raven's paper [START_REF] Raven | The transport and function of silicon in plants[END_REF], plants trading "expensive carbon (C)" for "cheap Si" has become a pervasive idea in the recent literature [23,40,61,100], assuming apparent trade-offs between Si-and C-based components (Box 2), and that accumulation of Si incurs lower energetic costs than synthesising C-based defence compounds [START_REF] Raven | The transport and function of silicon in plants[END_REF]. However, the validity of this hypothesis requires work at the cell and organ levels to better understand Si roles compared with C roles [100], and more accurate quantification of the costs of Si accumulation. If Si is effective and less costly than C-based compounds, why do not all plants invest in it, rather than in C-based compounds? Clearly, identifying the costs of plant silicification is imperative to explain the wide variation with phylogeny and environment, and why trade-offs with other traits exist.

Here, we aim to update the costs of Si accumulation, by separating costs that are independent of soil Si availability, such as movement from the root cytosol to the xylem, from costs that increase with decreasing soil Si availability [101] (Box 3).

The costs related to Si movement from the root symplasm are estimated to be 2 mol ATP, well below the costs of synthesis of lignin and carbohydrates (around 13 and 7 times higher for lignin and carbohydrates, respectively) (Box 3). However, this estimation is derived for Oryza sativa and costs can differ among species and distribution of the Si transporters Lsi1 and Lsi2 (Box 3). Morevoer, significant gaps and controversies remain in our understanding of Si(OH) 4 transport in plants [102,103]. For instance, the structure of the Lsi2 putative Si(OH) 4 :H + antiporter is still not well-characterized [104].

The costs related to soil Si mobilisation prior to plant uptake have been ignored so far, and doing so may challenge current thinking of Si as a cheap resource [38] (Box 3). We suggest that silicification is costlier than currently acknowledged, and that this depends primarily on soil Si availability that is limiting in many soils [105] (Box 3). Better estimating these costs would require more research at the rhizosphere level, to fill the gap between Si forms that are poorly available for plants (i.e. poorly-reactive minerals) and Si transportersroot exudates, mycorrhizal associations, silicate-solubilising bacteriato better estimate Si costs.

Overall, we suggest that active Si accumulation should be considered through the lens of costbenefit analyses, incorporating soil Si availability and Si demand for plant functions (Figure III). As discussed above, Si demand might increase in stressful conditions, or more competitive environments. Therefore, for a given soil and Si availability, a lower demand for Si would decrease the benefits relative to the costs. Eventually, greater silicification for defence or leaf construction might become less advantageous than C-based compounds, and cost-benefit analyses are mandatory to go beyond a simple economic Si-C trade-off. Beyond that, potential disadvantages of Si have been postulated (e.g., toxicity, high density of Si, poorer biomechanical properties compared with C-based compounds, less effective defence against some types of herbivores) to explain why Si is not universally used by plants [START_REF] Raven | The transport and function of silicon in plants[END_REF][START_REF] Cooke | Is plant ecology more siliceous than we realise?[END_REF]40,106]. We specifically provide evidence for higher leaf density associated with silicification for non-woody species (Box 1), but other potential drawbacks mentioned above

are not yet understood.

Concluding remarks

Despite a tremendous increase in Si research in plant biology in recent years, many aspects of siliceous ecology remain puzzling. Here, we reviewed the significance of Si in plant ecology at different levels of organisation and raised several questions and perspectives for future research (see Outstanding questions). We show that silicification varies between and within species, with environmental variation and other functional traits, and most likely influences plant fitness, interactions with other organisms and ecosystem processes. Si quantification in plant organs has become increasingly fast and easy in the last decade [e.g., 107,108], thus paving the way towards a full integration of Si at different organisational levels of terrestrial ecology. We also provide a datafile with Si concentration in leaves of about 1800 species in the online version of this article, to encourage future studies to incorporate Si in their framework. A better comprehension of Si-related ecological processes could ultimately help us to develop more sustainable and diversified agroecosystems, in which Si might have a more central role than currently appreciated [109].

Figures

Figure 1 -Theoretical relationships between the functions conferred to Si, fitness components, and fitness. Silicification is thought to impact fitness positively [START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF][START_REF] Cooke | Is plant ecology more siliceous than we realise?[END_REF], but experimental evidence is lacking, especially because fitness components are most of the time considered separately. Both growth (and perhaps competitive ability; [67]) and resistance functions have been ascribed to Si accumulation. Si accumulation could be linked to increased survival rates but slower growth rates (red scenario), or the opposite (green scenario). As discussed here, the red scenario is more likely (thicker lines in the plot), at least for grasses.

The relationship between Si concentration and reproduction might be positive, since Si addition can increase crop grain yields, but field studies on Si and seed production for noncrop species are lacking. Overall, these gaps prevent us from understanding if Si accumulation is linked to population growth rates and the success of this trait. Adapted from Laughlin et al. [48]. not only in leaves but also in stems, wood and roots. Intraspecific [Si] variation can be driven by phenotypic plasticity, ontogeny, and/or local adaptation. Leaf [Si] shows plasticity in response to herbivory [START_REF] Hartley | The ecology of herbivore-induced silicon defences in grasses[END_REF][58][59][60], N and P limitations [31,61] and CO 2 concentration and temperature [52,62,63], but underlying mechanisms remain unclear. Potential changes of leaf [Si] with ontogeny have been largely ignored, but leaf [Si] increases with increasing leaf age, because Si continually accumulates and is not remobilised [85], and Si can be diluted/concentrated in more or less plant biomass (i.e. dilution/concentration effect, [66]).

Finally, local acclimation and adaptation through changes in the expression of Si transporters (e.g., Lsi1,Lsi2,Lsi3,Lsi6;[110,111]) can also be postulated. This figure was created using BioRender (https://biorender.com/).

Boxes (3) BOX 1. Si: an overlooked trait in trait-based ecology

Studying leaf [Si] in relation with other key plant ecophysiological traits is useful to elucidate constraints on evolution, and identify trade-offs and potential links with plant strategy theories (e.g., the leaf economics spectrum, global spectrum of plant form and function, Grime's C-S-R strategy) [START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF]. In spite of some exceptions [73,112], Si remains poorly considered in trait-based ecology. We found that leaf [Si] is well represented in the global spectrum of plant form and function [START_REF] Díaz | The global spectrum of plant form and function[END_REF] (Figure IA, and Table S1 for PCA results; see Supplementary Material), and aligned with the first dimension of the spectrum, i.e. the plant size axis (plant height, stem density, and seed mass). Cooke et al. [40] aimed to include leaf [Si] in the leaf economics spectrum (LES; [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]). They showed that species with shorter leaf lifespan generally exhibit higher leaf [Si], but did not identify correlations between leaf [Si] and other LES traits [40]. With more species, we found that leaf [Si] was significantly positively related with leaf [P], A mass and A area and significantly negatively related with leaf lifespan and LMA (Figure S3). However, leaf [Si] was independent of the LES (Figure IB), because of weak R² (Figure S3) compared with the LES framework [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]. A fairly good relationship with A mass was identified, however (R² = 0.19; n = 454).

For non-woody species that have higher leaf [Si] (Figures S1 andS2), the question remains how strong silicification affects the density of leaf tissues and leaf morphological traits [START_REF] Raven | The transport and function of silicon in plants[END_REF].

We found that leaf [Si] is significantly positively related with the leaf dry mass content in non-woody species, reflecting the presence of silica in plant tissues (Figure IC). The relationship coefficient increases when leaf Si is expressed on an area-basis (R² = 0.18; n = 458). Leaf [Si] is also negatively related with leaf thickness and positively with LMA although the relation was less clear than for LDMC.

Figure I (BOX 1)

-Leaf [Si] and functional trait spaces. Leaf [Si] in the global spectrum of plant form and function [START_REF] Díaz | The global spectrum of plant form and function[END_REF] with 258 species in (A), and in relation to the most-represented traits of the leaf economics spectrum [START_REF] Wright | The worldwide leaf economics spectrum[END_REF] in our database (LMA, leaf N and leaf P) with 843 species in (B). In (C), bivariate relationships between leaf [Si] and LMA, leaf dry matter content (LMDC) and leaf thickness for non-woody species. These plots were obtained by compiling information on species-level leaf [Si] from 14 studies [START_REF] Deshmukh | New evidence defining the evolutionary path of aquaporins regulating silicon uptake in land plants[END_REF]15,20,21,23,25,34,74,[113][114][115][116][117][118]] that we crossed with two plant trait databases to extract major ecophysiological traits [119,120] (see Supplementary Material for details). In (A) and (B), the plots show the projection of species (dots) on the plane defined by the principal component (PC) axes 1 and 2. Open squares and filled circles represent non-woody and woody species, respectively. Solid arrows indicate direction and weighting of vectors representing the traits considered, and the colour gradients indicate regions of highest (red) to lowest (white) occurrence probability of species in the trait space, with contour lines indicating 0.5, 0.6, 0.7, 0.8, 0.9 and 0.99 quantiles. See also Supplementary Figures S1, S2 In grasses, Si has been linked to different plant architectural traits such as decreasing leaf insertion angle, increasing leaf straightness [44,[121][122][123] and increasing plant height [46,67,123,124], as shown in Figure IIA. This has been attributed to the hardness of silica, which strengthens plant tissues, especially through silica deposits in cell walls [125,126]. In grasses, veins located on the abaxial epidermis made of fully silicified cells may also play a role in leaf erectness, as exemplified in sugarcane in Figure IIB [76]. Negative relationships between leaf [Si] and concentrations of C-based compounds have also been reported (e.g., lignin, cellulose) which has reinforced the contention of a mechanical role of silicification, and has led to suggest "trade-offs" between Si and C components in leaves [START_REF] Raven | The transport and function of silicon in plants[END_REF]44,100,127].

We found a significant negative relationship between leaf concentrations of Si and C of 838 species and driven by plant woodiness (R² = 0.24; Figure S3), suggesting that non-woody species invest relatively more in Si than in C for leaf construction/defence compounds compared with woody species. After correcting leaf [C] for silica content, the relation is weaker but still highly significant (R² = 0.10; p < 0.0001), suggesting that dilution is not the An effect of Si on plant strength may contribute to avoiding plant lodging and protection against strong winds in grasses [128], and eventually to greater photosynthetic rates due to greater light interception (Figure S3). In fact, some authors suggested that high silicification could have adaptive significance against physical forces, including wind and waves, rather than the more frequently discussed herbivore pressure [15,34]. In addition, Si-induced modifications of plant architecture may play a significant role in light capture and competition for this resource [67]. Plant height is often associated with strong competitive abilities [129], and leaf insertion angle and erectness may also affect competition for light [121]. The role of silicification on plant architecture may affect plant-plant interactions and explain the selection of this trait, but this requires further investigation [START_REF] Strömberg | Functions of phytoliths in vascular plants: an evolutionary perspective[END_REF]15,67]. moved from the root medium to the xylem, the energy cost of producing a mol of the monomers of cell wall structural compounds, lignin and polysaccharide, is 13-fold and 7-fold, respectively, of that of the inorganic substitute, SiO 2 [START_REF] Raven | The transport and function of silicon in plants[END_REF]. This equivalence assumes that there is no energy cost of unloading Si(OH) 4 from xylem, or organic compounds associated with SiO 2 deposits, and that SiO 2 /lignin or polysaccharide monomers are structurally equivalent.

Costs dependent on soil Si availability Previous calculations [START_REF] Raven | The transport and function of silicon in plants[END_REF] have assumed assumed an inexhaustible supply of Si(OH) 4 which is not the case in many soils worldwide [105]. Si mobilisation from poorly-available forms, and plant Si uptake are significantly increased by release of root exudates (e.g., carboxylates) and arbuscular mycorrhizal associations [38,109,138]. These nutrient-acquisition strategies are costly [101], and, therefore, Si accumulation involving rhizosphere processes would be significantly more expensive than currently acknowledged [38], as is the case for P and Fe [101,139]. We argue that costs of Si accumulation would largely depend on soil Si availability (Figure III). However, whether plants increase carboxylate secretion or their association with fungiand are therefore spending energyin responses to Si deficiency itself is unknown. The possibility of changes in root morphology, mycorrhizal symbionts, and root and mycorrhizal secretions related to deficiency of Si, rather than P or some other element, deserves further investigation [38,101].

Moreover, costs of carboxylates and other Si-mobilizing root exudates may be negligible if they have already been covered by other processes (e.g., to mobilise soil P) or if they result from a C surplus under conditions of growth being limited by resources other that carbon supplied in photosynthesis [140,141] (Figure III). We call for more research at the rhizosphere level to answer these compelling questions. of Si-mobilizing root exudates (e.g., carboxylates, phytosiderophores) have already been incurred to mobilize other nutrients (e.g., P or Fe) [38,101,139], or result from 'free' surplus carbon [140,141], the benefits would increase relative to the costs.

Glossary

Active Si accumulation: movement of Si(OH) 4 into plants from a lower to a higher Si(OH) 4 concentration catalysed by specific transporters energised directly (primary active transport) or indirectly (secondary active transport) by metabolism Biomineralization: process by which organisms form minerals Diatom: single-celled photosynthesising alga that has a silica-containing wall (frustule) and is found in almost every aquatic environment Direct cost: a decrease in growth and/or reproduction associated with greater stress resistance (e.g., growth rates, changes in phenology), not mediated by interactions with other species Ecological cost: a decrease in growth and/or reproduction associated with greater stress resistance that manifest itself only through interactions with other organisms

  )[82]. So far, studies have reported both convergence and divergence between intraspecific and community-level[Si] responses to environment[23,25]. In addition to plasticity, as discussed above, intraspecific variation can also be driven by ontogeny[83] and local adaptation [84] (Figure 2). Although few studies are available on [Si] variation with ontogeny, we know that leaf [Si] increases with leaf age because Si continually accumulates and it is not remobilised [85], and Si can be diluted/concentrated in more or less plant biomass, just like N [83]. Examples of local adaptation with determined genetic basis are not available.

  -1 ) [92]. Finally, Si cycling in terrestrial ecosystems affects the global Si and C cycles [92-95]. On geological time scales, plant-induced weathering of silicates consumes atmospheric CO 2 , thereby affecting Earth's climate [96]. On biological time scales, Si recycling by vegetation strongly impacts soil-plant Si cycling [95,97] and affects Si transfer from land to oceans, where it is used by siliceous marine diatoms that contribute up to half of marine ANPP [93].

Figure 2 -

 2 Figure 2 -Silicon as both a response and an effect trait. Leaf [Si] varies across environments (with wind [34], herbivory [28], soil type and properties such as N, P and Si availability [23,32], elevation [25] and precipitation [30]), but the relative contribution of intra-and interspecific variation is still unclear. Intraspecific [Si] responses to environment can be contrasted among species (species a, b and c), thereby following or not communitylevel trends (community-weighted mean, i.e. CWM) [23,25]. [Si] affects several ecosystem processes (annual net primary productivity, litter decomposition, and nutrient cycling and the global C cycle at different time scales), the magnitude of which depends on the CWM [Si],

  and S3. In (C), standardised major axis (SMA) regression lines and statistics (R²; sample size and p-values) are given for each relationship.

BOX 2 .

 2 The overlooked influence of silicification on plant architecture, resistance to physical forces, and competition for light.

  only factor explaining the relationship between Si and C [30]. Nevertheless, the hypothesis of a Si-C trade-off requires more work at the cell and organ levels to better understand Si roles compared with C roles [100]. Future studies on Si and C-based components should pay specific attention to mechanisms underpinning the apparent trade-off [100].

FigureBOX 3 .

 3 Figure II (Box 2). Influence of Si fertilisation on rice architecture modified from Zanão Júnior et al. [123] in (A), and silica deposits (white structures above, yellow signal below) on sugarcane abaxial surface leaf from de Tombeur et al. [76] in (B). Images in (B) show silica deposits along veins of about 20-70 μm wide which were formed by two to three rows of short broad epidermal cells fully silicified that could play an important role in leaf straightness.

Figure III (

 III Figure III (Box 3). Schematic representation of the costs and benefits of active Si accumulation in plant organs as a function of soil Si availability. For a given soil and associated soil Si availability, a lower demand for Si, as, for instance, in less stressful and/or competitive environments, would decrease the benefits relative to the costs. Similarly, if costs

Acknowledgments

This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 101021641 (project SiliConomic granted to FdT), from the European Research Council (ERC) Starting grant 'Ecophysiological and biophysical constraints on domestication in crop plants' (grant ERC-StG-2014-639706-CONSTRAINTS) awarded to CV, and is supported by the TRY (http://www.try-db.org) and AusTraits initiatives on plant traits (https://austraits.org/).

Ecosystem process: a process impacting the flow of energy and matter between the biotic and abiotic components of an ecosystem; it includes primary production, trophic transfer between plants and animals, or nutrient and C cycling Exaptation: any adaptation that performs a function different from the function that it originally held Fitness components: individual performance including survival, growth, and reproduction; estimating fitness components yields an estimate of fitness; also called vital rates at the population level (rates of birth, death, and growth of individuals)

Functional trait: a morpho-physio-phenological trait that impacts fitness indirectly via its effects on growth, reproduction and survival Model species: extensively studied plant species chosen for the ease of investigating particular biological phenomena  Trait-based analyses show that Si accumulates most in leaves of small non-woody species, and is linked to key functional traits suggesting trade-offs and physiological constraints on evolution of Si accumulation in plants.

 Plant Si-related functions affect fitness components (growth, survival, reproduction), but a sound understanding of Si-fitness relationships is missing. In grasses, there is evidence for a Si-mediated trade-off between growth and defence.

 Si accumulation is more expensive than currently acknowledged, and cost-benefit analyses are pivotal to understand variation as dependent on environment and tradeoffs with other traits.

 Si plays a significant role in many aspects of plant ecology, but major gaps prevent us from understanding why this trait has been selected for and contributed to the success of specific clades. We provide a datafile with leaf Si concentrations of about 1800 species.

Outstanding questions

Can the large variation of [Si] in land plants be explained by evolutionary constraints, and what are the trade-offs with other functional traits? For instance, does Si play a role in physical support and defence mostly in small non-woody species, but not in tall woody species due to physiological constraints? Or can silicification be limited by the increase in leaf density? Elucidating these questions would require the combination phylogenetic and traitbased approaches in future research, and consider Si in relation with plant strategy theories (e.g., the leaf economics spectrum, Grime's C-S-R strategy).

Has Si contributed to the success of some clades and expansion of specific biomes such as grasslands? In particular, what are the links between [Si] and fitness components (growth, survival, reproduction) and population growth rates? Species and/or cultivars with inherently different [Si] should be studied to consider if Si accumulation comes with costs (either direct or ecological) and if and how Si is linked to fitness. In particular, the Si-mediated growthdefence trade-off in grasses should receive more attention, but future works should also embrace different plant families and orders.

How does [Si] vary in diverse plant communities and ecosystems when indices are weighted by species abundance (e.g., community-weighted mean), and how does it affect ecosystem processes, such as nutrient cycling, herbivore populations, primary productivity, and C cycling?

What are the costs and benefits of Si accumulation compared to the costs of C-based compounds that could play similar functions (e.g., phenols, lignin), and do they depend primarily upon soil Si availability and Si demand? Cost-benefit analyses in different environments and for different species are needed to understand the impact of soil Si availability and stress and competition intensity on the costs of Si accumulation in relation to C-based compounds.


Abdelaziz Jouaiti

Lavinia Ballerini

Hsiang-Ling Shen

Ronan Viel

Federico Polo

Nathalie Kyritsakas

Stefan Haacke

Yu-Ting Huang

Chin-Wei Lu

Christophe Gourlaouen

Hai-Ching Su

Matteo Mauro

Binuclear Copper(I) Complexes for Near-Infrared Light-Emitting Electrochemical Cells

Keywords: near-infrared emitters, light-emitting electrochemical cells, binuclear complexes, phosphorescence

Two binuclear heteroleptic Cu I complexes, namely Cu-NIR1 and Cu-NIR2, bearing rigid chelating diphosphines and -conjugated 2,5-di(pyridin-2-yl)thiazolo [5,4-d]thiazole as the bis-bidentate ligand are presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1-M2) and enables shifting luminescence into the nearinfrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal-to-ligand charge transfer ( 3 MLCT) character as demonstrated by in-depth photophysical and computational investigation. Noteworthy, X-ray analysis of the binuclear complexes unravels two interligand --stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the Cu I centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1-M2. These findings prompt the successful use of Cu-NIR1 and Cu-NIR2 in NIR light-emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43%. Their suitability for the fabrication of white-emitting LEC is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earthabundant Cu I emitters Keywords: copper complexes •

Introduction

Light-emitting electrochemical cells (LECs) are appealing optoelectronic devices due to their potential application in low-cost, large-area display technology and disposable devices. [1] Ionic transition metal complexes (iTMCs) are typical employed as electroluminescent materials, particularly cyclometalated Ir III -based iTMCs. [2] More recently, earth-abundant metals are currently the subject of intensive research, [3] being d 10 Cu I -based emitters considered an attractive alternative. [4] To date, Cu I complexes with either mononuclear, [5] binuclear, [6] or multinuclear [7] scaffolds have been investigated, typically displaying luminescence spanning from the blue to orange region. Designing complexes that efficiently emits into the deep-red to near-infrared (NIR) region is still highly challenging and examples of Cu I derivatives are still very rare. [8] The difficulty stems from a three-fold reason: i) the smaller spin orbit coupling (SOC) constant of copper compared to iridium and platinum (ζ Cu = 857 cm -1 ; ζ Ir = 3909 cm -1 ; ζ Pt = 4481 cm -1 ) yields complexes with slower radiative rate constant (kr); [9] ii) given the third-power dependency with emission energy, as by Einstein's theory of spontaneous emission, red to NIR emitters possess intrinsically smaller kr; iii) efficient non-radiative vibronic coupling between T1 and ground state is at play (energy gap law). NIR emitters are very appealing for the fabrication of light-emitting devices for night vision display, optical telecommunication technology, biomedical devices for phototherapy. [10] Costa and coworkers reported on deep-red emitting mononuclear heteroleptic Cu I complexes [Cu(P^P)(N^N)]PF6 bearing a 4,4'-diethylester-2,2'-biquinoline ligand as the N^N ligand, and mono-and diphosphines.

LECs fabricated with these emitters provided electroluminescence peaking at λ EL = 675 nm. [11] Yet, Cu I complexes that emit beyond 700 nm that were also employed as active materials in light-emitting devices is unprecedented, to the best of our knowledge.

Recently, bi-and multi-nuclear complexes appeared to be a valuable strategy for the preparation of deep-red to NIR emitters. [12] Herein, we describe a dinuclearization strategy in heteroleptic copper complexes to achieve NIR emission. The compounds bear a π-extended thiazolo [5,4-d]thiazole (TzTz) scaffold that acts as the bridging bis-bidentate chromophoric ligand and P^P is a chelating diphosphine. Remarkably, NIR electroluminescence (EL) is demonstrated for the first time with λEL maximum up to 756 nm when the investigated binuclear complexes are employed as emitters in LEC devices.

Results and Discussion

Synthesis and characterization

The chemical structure of the investigated binuclear Cu I complexes and their mononuclear counterparts is depicted in Figure 1. Full details on the synthetic procedures along with the chemical characterization of all the intermediates and final compounds are provided in the Supporting Information (see Scheme S1-S2, Figure S1-S21 and Supplementary procedure). Complex Cu-NIR1 and Cu-NIR2 possess general formula [Cu(P^P)(L1)Cu(P^P)](PF6)2, where L1 is a 2,5-di(pyridin-2yl)thiazolo [5,4d]thiazole (TzTz) scaffold acting as bridging bis-bidentate chromophoric ligand, and P^P is the chelating bis[ (2-diphenylphosphino)phenyl]ether (DPEPhos) and 4,5-bis(diphenylphosphino)-9,9dimethylxanthene (XantPhos), respectively. The mononuclear complexes have general formula [Cu(P^P)(L2)]PF6, namely M1 and M2 for P^P = DPEPhos and XantPhos, respectively, where ligand L2 is 2-(5-(3,5-di-tert-butylphenyl)pyridin-2-yl)benzo [d]thiazole. The synthesis of these latter follows an adapted version of a procedure reported elsewhere. [8a] Complexation reaction afforded the target compounds in high yields. The structure of ligand L1 as well as all the four investigated complexes have been established by single crystal X-ray diffractometric analysis and the ORTEP diagrams of Cu-NIR2 and 4, M1, M2 and Cu-NIR1 are displayed in Figure 1 and S22-S25, respectively (see Table S1-S5 for crystallographic parameters). [START_REF]Deposition numbers[END_REF] The structure of the complexes agrees with those reported previously for related compounds, showing the coordination of the Cu I ion in a distorted tetrahedral environment as expected for a d 10 formed by the N^N and P^P bidentate ligands, and for binuclear Cu-NIR1 and Cu-NIR2 derivatives confirmed their bimetallic nature. [5,14] The four coordination sites of each Cu I ion are occupied by the chelating P^P ligand and two nitrogen atoms of the 2,5-di(pyridin-2-yl)thiazolo [5,4-d]thiazole, this latter acting as the bis-bidentate scaffold.

Minimizing quenching processes is of paramount importance especially for NIR emitters that possess intrinsically lower kr and high knr values. Remarkably, two -stacking interactions are present between one of the phenyl groups of each of the chelating phosphines and the -extended ligand L1, where interplanar distance is ca. 3.57 and 3.62 Å, for Cu-NIR1 and Cu-NIR2, respectively (see Figure 2 andS25). The presence of these interactions gives rise to a doubly locked structure that helps mitigate quenching processes such as pseudo-Jahn-Teller distortions and structural flattening typically occurring in the excited state of Cu I complexes. [5,14,15] Indeed, it has been recently highlighted the fundamental role played by such π-stacking interaction in preserving the emissive properties of mononuclear [Cu(P^P)(N^N)] + derivatives. [15] Interligand -stacking interactions are absent in the monometallic derivatives M1 and M2 instead (see Figure S23-S24). The compounds possess remarkably high thermal stability under air with T5%, i.e. the temperature at which 5% of the initial weight loss has occurred, ≥300°C (see thermogravimetric analysis in Figure S26-S27). Voltammetric investigations in the full width potential window CH 2 Cl 2 /0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF 6 ) are shown in Figure 3 and S28, while a summary of data is reported in Table 1. In the negative-going scan, either two or three reversible and irreversible reduction processes Ri,n (where i denotes the investigated binuclear or mononuclear compounds and n the process number) occurred in the potential range from -1.2 V to -2.2 V. In particular, the process Ri,1, which is common to both bimetallic complexes, shows a clear reversibility at all the investigated scan rate (0.05-0.5 V/s). This process is more likely associated to the reduction of the -accepting ligand L1 coordinated onto the two Cu I centres. [8a] L1 was also investigated and showed indeed two reduction processes, a reversible R L1,1 at -2.02 V and an irreversible R L2,2 at -2.38 V vs Fc (see Figure S28). Whereas, R i,2-3 are irreversible, although R Cu-NIR2,2 seems to show some reversibility at scan rate higher than 0.2 V s - 1 .

In the positive-going scan, the binuclear complexes showed a very weak process around 0.5 V vs Fc and a main irreversible oxidation process, Oi,1, whose peak potential 𝐸𝑝,# i,1 is at about +1.07 V vs Fc for Cu-NIR1, whereas it is not well defined, but above 1.1 V vs Fc for Cu-NIR2. The latter one also showed an irreversible cathodic peak at about 0.08 V vs Fc, which appeared only when scanning the potential toward very positive potential (ca. 1.5 V) before reverting the scan direction and it is most likely due to irreversible oxidation products. L1 did not show any process in the positive-going scan. Therefore, we can attribute the Oi,1-2 to the oxidation of the metal center. It is also worth mentioning that limited redox reversibility may represent a drawback for their application in LECs, although one should keep in mind that redox behavior in solid state may differ sharply from that observed in dilute solution. The parent mononuclear complexes, namely M1 and M2, were also investigated to compare the electrochemical pattern of the derived binuclear ones (Figure S28). Both M1 and M2 showed a reversible process Ri,1 in the negative-going scan at -1.77 and -1.73 V vs Fc, respectively. Similarly, in the positive-going scan another reversible process Oi,1 occurs at 0.94 and 1.00 V vs Fc, respectively, as shown in Figure S28. It is also worth noting that the peak-to-peak separation for all compound is in the range 80-130 mV (see Table 1), which is larger than expected for an ideal Nernstian behavior (59 mV). [START_REF] Bard | Electrochemical Methods, Fundamentals and Applications[END_REF] However, the behavior of the redox couple ferrocene/ferricenium (Fc + |Fc 0 ), used as internal standard, showed the same trend. This effect can be attributed to the ohmic drop of the system, despite the feedback correction was applied, as previously observed for aprotic media. [START_REF] Omer | [END_REF] Furthermore, by taking into account the diffusion coefficient of Cu-NIR1 and Cu-NIR2 (see supplementary discussion #1 and Figure S29 in the Supporting Information)

and by comparing the current intensities of their R1,n and O1,n processes with the oxidation of Fc, one can confidently state that the reduction processes are monoelectronic, whereas oxidations might involve more than one electron.

Photophysical investigation

Firstly, the optical properties of complex Cu-NIR1 and Cu-NIR2 were investigated in dilute (3x10 -5 M) CH2Cl2 solution at room temperature in both air-equilibrated and degassed condition. The corresponding photophysical data are listed in Table 2 and the absorption and emission spectra are displayed in Figure 4. Figure S30 displays the spectra of pro-ligand L1.

As far as the bimetallic complexes Cu-NIR1 and Cu-NIR2 are concerned, their electronic absorption spectrum displays rather similar features that are characterized by three main bands. On the higher energy side, an intense band (λabs = 280 nm, ɛ = 4.1x10 4 M -1 cm -1 ) is attributable to electronic transitions with singlet-manifold intraligand ( 1 IL) nature involving both the arylphosphines and ligand L1. The intense and narrow band observed at λabs = 407 nm (ɛ = 5.8x10 4 M -1 cm -1 for compound Cu-NIR1) can be ascribed with confidence to a metal-perturbed 1 π-π ( 1 IL) absorption mainly localized onto the ligand L1 scaffold, in agreement with the data of this latter. On the lower energy side, the much less intense (ɛ = 5.8x10 4 M - 1 cm -1 for compound Cu-NIR1) and broader band can be ascribed to a transition with admixed singlet metal-to-ligand charge transfer ( 1 MLCT) and metal-perturbed ligand-to-ligand charge transfer ( 1 LLCT) with dπ (Cu) π *TzTz and π (aryl-P) π *TzTz character, respectively. Such band is shifted hypsochromically by about 3690 cm -1 (λabs = 425 nm, ɛ = 3.3x10 3 M -1 cm -1 ) in the corresponding mononuclear derivative M1, whereas it is absent in the spectra of ligand L1, supporting this attribution. Overall, these assignments are made by comparison with the spectrum of L1 (cf. Figure S30) as well as related derivatives reported previously, [5,8,15] and are in agreement with computational data (see below).

Upon excitation in the range 400-500 nm, solution samples of both binuclear complexes display broad and featureless NIR photoluminescence at room temperature, with emission centered at λem = 746 and 732 nm for derivative Cu-NIR1 and Cu-NIR2, respectively, and photoluminescence quantum yield of 0.13% and 0.06% in degassed condition. The steady-state emission appeared to be wavelengthindependent, but it displayed a slight bathochromic shift in air-equilibrated condition. Upon pulsed pslaser excitation, excited-state decay traces display a bi-exponential decay with τ1 = 81.6 (64%) and τ2 = 38.9 (36%) ns, for derivative Cu-NIR1, and even shorter lifetimes are recorded for the analogues Cu-NIR2 (see Table 2). De-excitation decays were little dependent on the presence of the triplet dioxygen, as expected for a diffusion-controlled quenching process kinetically competing with such a short-living excited state.

Conversely, mononuclear parental complexes M1 and M2 display hypsochromically shifted emission with maximum at λem = 700 nm, owing to the much shorter π -conjugation length of the chromophoric ligand, along with a prolonged excited state and higher PLQY, being the values τ = 319 ns and 0.5%, respectively, for M1 in degassed condition, in agreement with the energy gap law. Overall, the bi-exponential decay and the oxygen-dependency observed in the photoluminescence of the binuclear compounds point towards the presence of two energetically close-lying excited states, yet not fully equilibrated on the time scale of the overall radiative processes. Within the first nanosecond, a significant decay by more than 90% of the emission is observed at all wavelengths (Figure 5A). This is in line with the low PLQY of the complex in solution (Table 2). The decay is almost unresolved for wavelengths shorter than 600 nm, but it shows a resolved decay at longer wavelengths on the time scale of a few tens of ps. The remaining weak emission can be better detected at longer time scales when the number of photons per time interval is increased by a factor of 50 or 500, respectively. The data show a strong dependence of the decay times as a function of wavelength. In the 50 ns time range (Figure 5B), one observes a decay of the emission on a time scale of a few ns at <600 nm, while the emission at >600 nm hardly decays within 50 ns. The full decay of the long wavelength emission is captured on a 500 ns time range (Figure 5C). Figure 5 shows exemplary kinetics traces for the three different time ranges and at the emission wavelengths indicated as dotted lines in Figure 5, together with non-linear fits as sum of exponentials convolved by a Gaussian which represents the temporal instrument response function (IRF). The results of the fit give four main time constants, as summarised in Table S6.

These four main decay times are τ1 <10 ps, τ2 = 40 ps, τ3 = 4.0 ns and τ4= 65 ns (exact values with error bars). The value of τ4 agrees well with the longest lifetime found for this compound by timecorrelated single photon counting (TCSPC) technique (see Table 2). Since the amplitudes of the decay times are strongly wavelength-dependent, we performed a global fit, assuming wavelength-independent decay times. This provides the decay associated emission spectra (DAES) for τ2 to τ4 (and a longer component τ5 of negligible amplitude) plotted in Figure S32 of the Supporting information. In addition to the unresolved τ1, found to occur only at λ <600nm, we observe that these three lifetimes appear at selected ranges of wavelengths, which guides our assignments as follows. The longest lifetime τ4 is with confidence assigned to phosphorescence from an excited triplet state with 3 MCLT character. This component largely dominates the steady-state emission spectrum and is detected by TCSPC and its associated DAES agrees well the former (cf. Figure 4 andS32). The fact that it slightly shortens in degassed solution samples confirms the triplet assignment. On the other hand, τ3 is only observed at shorter wavelengths. Indeed, the DAES associated with 3 and 4 are clearly separated, indicating that they are related to two different transitions. The 3.5 ns value of 3 may suggest that it is due to a highly quenched state that lies at higher energy compared to the 3 MCLT manifold. Comparison of the emission spectra observed at 77 K with the DAES associate to 3 allow us to rule out the 3 LC nature of this latter, which is instead tentatively assigned to a 1 LC state (see computational section). Note that this <600 nm component does not show up in the time-averaged fluorescence spectrum due to its short lifetime as compared to τ4. The lifetime τ2 is a nonradiative quenching process, most likely related to a flattening of the Cu complex, as usually observed for Cu(I) under tetrahedral coordination. [18] τ1, not included in the global fit, is mainly observed in the shortest wavelength range, where the fluorescence emission takes place. It is therefore assigned to intersystem crossing (ISC) process populating the triplet manifolds. But since it is not resolved by the streak camera, it does not show up as a rise time of the phosphorescence.

At 77 K in frozen matrix in CH2Cl2 emission shifts hypsochromically by ca. 2570 cm -1 and the appearance of a clear vibronic progression for the binuclear complexes is observed with spacing of ca.

1375 cm -1 attributable to the intramolecular C=C and C=N vibrational modes based on related data on thiazolo[5,4-d]thiazole organic emitters (Figure S33 and Table 2). [19] The shift is accompanied by a prolongation of the excited-state lifetime that reach a value as long as τ1 = 320.6 μs (72%) and τ2 = 81.6 μs (28%) (τaverage = 299.0 μs) for Cu-NIR1. Similar findings are observed for Cu-NIR2. Overall, this effect is indicative of the nature of the emissive excited state that becomes mainly 3 LC and is located on the πconjugated ligand L1. In stark contrast, mononuclear derivatives M1 and M2 retain the broad and featureless emission profile at low temperature pointing towards a still large 3 MLCT character admixed with 3 LLCT component of the emitting excited state. Such temperature-dependent data seems to rule out the presence of thermally-activated delayed fluorescence (TADF) processes in dilute solution for these dimetallic derivatives. [8a,15,20] For all the complexes, emission spectra of solid-state samples as neat powder and thin film blends are displayed in Figure S34-S36 of the Supporting Information and the corresponding photophysical data are summarized in Table S7-S8. For both mono-and dinuclear complexes, powder samples show broad and featureless emission profile that is hypsochromically shifted (by ca. 780-1270 cm -1 ) when compared to fluid solution due to the large CT nature of the emissive excited state and the reduced geometrical relaxation and tendency toward flattening occurring in a more rigid environment such as inthe solid state, as previously observed in related mononuclear [Cu(P^P)(N^N)] + derivatives (cf.

data in Table 2 and S7-S8). [5,14,15] Interestingly, larger shifts are obtained for mononuclear derivatives M1 and M2 compared to binuclear Cu-NIR1 and Cu-NIR2, indicative of the fact that for the latter geometrical distortion due to environmental effect play a smaller role. Additionally, this spectral shift is accompanied by a prolongation of the excited-state lifetimes that become as long as 3.0-3.5 μs and a remarkable enhancement of PLQY values, which are among the highest for Cu I emitters in this region. energetically close minima at the S0 potential energy surface (PES). This conformational freedom is expected to be suppressed in solid state restoring and hypsochromically shifting the emission.

Computational investigation

Geometrical optimization of pro-ligand L1 and complexes

For both complexes the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are two combinations of π* with quinoidal character and located on ligand L1 and resemble those of pro-ligand L1 (cf. Figure 7 and Figure S38-S39). On the other hand, the highest occupied molecular orbital (HOMO)

and HOMO-1 consist of two almost degenerate antibonding Cu-P orbitals with dπ(Cu) character. The HOMO-2 is delocalized over the whole system of the ligand L1. 2 and Table S11). The second band between 250 to 350 nm is due to the convolution of several singlet-manifold excitations, the most intense being S0 S13 computed at 267 nm. This electronic transition displays intraligand charge transfer (ICT) character with For both complexes Cu-NIR1 and Cu-NIR2, a first weak band is computed around 525 nm corresponding to the S0 S1 transition with a 1 MLCT dπ(Cu)-π*TzTz character (S1 in Table S14) and symmetrically involving both [Cu(N^N)(P^P)] + fragment at Franck-Condon (FC) geometry (Figure 8). At higher energy, both complexes present a very intense band between 400 and 450 nm in the experimental spectrum (see Figure 4), which possesses slightly different character for complex Cu-NIR1 and Cu-NIR2.

For the former, this band is computed at 433 nm and associated to the S0 S3 with mainly 1 IL character involving ligand L1 and very small 1 MLCT contribution (Figure 8 and S43). For Cu-NIR2, the band is the convolution of three energetically close electronic transitions at 449 nm (S0 S5, πTzTz π*TzTz ), 427 nm (S0 S6, 1 MLTzTzCT) and 413 nm (S0 S7, 1 MLTzTzCT) with highly mixed character (Figure S44). The higher energy band computed between 300 to 325 nm in both complexes is due to several transitions with mixed character involving significant contribution of the diphosphine. These electronic processes can be described as ligand-to-ligand charge transfer ( 1 LLCT) with πPPh π * TzTz character. showing negligible contribution of the [Cu(P^P)] + fragment (Figure 9). The geometries of the lower-lying excited states retain a highly distorted structure like that found for the GS (Tables S9 andS10). It is worth to notice that for T1a minima, flattening is observed around the Cu I where excitation is localized by ca. 15° as defined by the angle between the plane containing (Cu, P, P) and (Cu, N, N). The emission arising from both the T1a and T1b state are computed in the NIR domain and the values are listed in Table S15. It is worth to notice that on a model system Cu-NIR1-Me, in which the tert-Bu moieties were replaced by Me groups, optimization of the 1 LC state within Ci symmetry yields an emission wavelength of 528 nm. This finding strongly suggests that the transitory emission experimentally observed at 550 nm might arise from a higher-lying 1 LC state.

Emission arising from the T1a state with 3 MLCT character is computed at 938 and 954 nm for derivative Cu-NIR1 and Cu-NIR2, respectively, and such state lies lower in energy than T1b. For this latter manifold, emission is computed at shorter wavelength being the value 805 and 839 nm for Cu-NIR1 and Cu-NIR2, respectively. The shift between the theoretical values and the experimental ones can be due to the large geometrical flexibility that characterize these complexes and their strong tendency to adopt a distorted geometry to maximize interligand -stacking interactions. As abovementioned and demonstrated for proligand L1 (see Supplementary Information), these large geometrical distortions might sizably affect the computed emission values making one static picture alone not enough to obtain quantitative agreement.

Hence, it requires an in-depth conformational analysis and Boltzmann averaged distribution of emission energies that will be reported in due course. Besides, it should be pointed out that the computed energy difference between the T1a and T1b

states is small, especially for Cu-NIR2 for which the difference is found to be only 35 meV, a value that is comparable to thermal energy (25 meV). Despite these difference in computed emission wavelengths, theoretical results show that the lowest triplet PES possesses close-lying minima ascribed to state with either 3 IL or 3 MLCT character. This finding is consistent with the experimental steady-state emission profile that is described as arising from a 3 MLCT state. A more rigid environment, such as either solid state or glassy matrix, destabilizes energetically the 3 MLCT state and constrains the structure by limiting large distortions. This rigidochromic effect yields to a switch of the nature of the lowest-lying excited state that becomes now the 3 IL manifold in agreement with the hypsochromic shift observed in emission spectra at low temperature (see photophysical section). Similarly, rigidification of the environment in the solid state establishes interligand -interactions and restore the NIR luminescence.

Electroluminescent characteristics of LECs

The LECs employing complexes Cu-NIR1 and Cu-NIR2 were fabricated and measured to test their EL properties. The EL characteristics of these LECs are summarized in Table S16. The EL spectra of the LECs based on complexes Cu-NIR1 and Cu-NIR2 are depicted in Figure S45. Both complexes showed similar thickness-dependent evolution in EL spectra, despite the fact that complex Cu-NIR1 shows slightly redshifted EL emission. The thinnest LECs (ca. 70-80 nm) exhibited deep-red EL spectra (λmax >650 nm). With thickened emissive layer (ca. 150-160 nm), the EL spectra shifted to the NIR spectral region (λmax >730 nm). Further increasing the emissive-layer thickness (ca. 270-280 nm) shifted the EL spectra back to the shorter-wavelength spectral region. Thickness-dependent EL spectra of these LECs result from the modified microcavity effect upon adjusting the device optical structure. [21] These data reveal that adjusting the device thickness is essential in obtaining more NIR EL emission from these LECs. To the best of our knowledge, this work demonstrates the first NIR LECs based on copper complexes.

The device current density, light output power, and external quantum efficiency (EQE) of the LECs based on complex Cu-NIR1 and Cu-NIR2 are shown in Figure S46 and S47, respectively. The bias voltage for each device with different thickness was chosen to obtain the optimized device efficiency. After a bias was applied, the mobile ions in the emissive layer of the LEC drifted toward electrodes and lead to gradually formed electrochemically p-and n-type doped layers. These doped layers facilitated carrier injection and thus the device current increased with time. The device current decreased rapidly after reaching the peak value possibly due to the increased resistance of degraded material. The light output power initially followed the increasing device current. However, the light output power decreased faster than the device current. In addition to the material degradation, exciton quenching due to the extending doped layers toward the emission zone was responsible for reducing light output power before the device current was decreasing. [22] Owing to the improved device carrier balance induced by the growing doped layers, the EQE increased rapidly after a bias was applied. After reaching the peak value, the EQE decreased gradually due to the same reasons responsible for the reduction in the light output power with time.

Among the LECs based on complex Cu-NIR1 under test, the optimized emissive-layer thickness was 151 nm and the peak EQE reached 0.32%. LECs based on complex Cu-NIR2 showed similar temporal evolution trend in the EL characteristics, yet with lower peak EQE (Figure S47). These EL characteristics confirm that the LECs employing the proposed copper complexes can offer NIR EL emission with moderate device efficiency. For comparison, the LECs based on complexes M1 and M2 were also fabricated. The EL characteristics of these LECs are summarized in Table S16. The EL spectra of the LECs based on complexes M1 and M2 are shown in Figure S48. The EL emission peaks of the LECs based on M1 and M2 were 666-716 and 664 nm, respectively. Compared to the EL spectra of the LECs based on the binuclear complexes with similar thicknesses (Figure S45), the mononuclear complexes showed hypsochromic shifts in the EL spectra due to shorter -conjugation length of the chromophoric ligands. It is noted that complex M1

showed time-dependent EL spectrum since the moving recombination zone in the LEC altered the microcavity effect, moving deep-red emission toward NIR region. [21] The device current density, light output power, and EQE of the LECs based on complex M1 and M2 are shown in Figure S49 and S50, respectively. The temporal evolution trends of the EL characteristics of the LECs based on mononuclear and binuclear complexes were similar. However, lower voltages were required for the LECs based on mononuclear complexes to reach similar or even higher light output power.

Since the mononuclear complexes exhibit higher energy gaps than the binuclear complexes, lower driving voltages of the LECs based on mononuclear complexes may be attributed to higher carrier mobilities of thin films of mononuclear complexes. Furthermore, the mononuclear complexes showed higher device EQEs than the binuclear complexes (Table S16). These results are consistent with the higher thin-film PLQYs of mononuclear complexes (Table S8). Without the influence of microcavity effect, both mononuclear complexes show deep-red EL emissions and thus were not used in the host-guest approach (see below) for further improving device efficiency. To further enhance the device efficiency, both complexes were mixed with the yellow complex YIr [23] to improve the device carrier balance (see Figure S51 for chemical structure). The bias voltage for the device with a given doping concentration of complex YIr was chosen to reach the optimal device efficiency. The EL characteristics of the LECs based on either complexes Cu-NIR1 or Cu-NIR2 doped with complex YIr are summarized in Table 3. The LECs based on complex YIr have been shown to have superior device carrier balance and higher EQE. [23] Incorporating a material with better carrier balance in the emissive layer can help to balance the number of carriers. [24] However, some EL properties are altered simultaneously upon doping. The EL spectra of the LECs based on complexes Cu-NIR1 and Cu-NIR2 doped with various complex YIr concentrations are shown in Figure 10. When doped with complex YIr, the EL emission of both complexes showed hypsochromic shift due to reduced intermolecular interactions, [25] especially for higher doping concentrations of YIr complex. To maintain the EL emission peak wavelength longer than 700 nm, the concentration of complex YIr is kept below 20 wt.%.

In addition to EL spectra, carrier transport properties in the LECs can be modified by incorporating complex YIr. [23] The device current density, light output power, and EQE of the LECs based on complex Cu-NIR1 doped with various concentrations of complex YIr are shown in Figure 11. The bias voltage for each LEC with different complex YIr concentration was chosen to obtain the optimized device efficiency. When doped with complex YIr, comparable device current and light output power can be obtained under a lower bias voltage. It reveals improved carrier transport properties of the mixed emissive layer. Furthermore, more balanced electrons and holes in the device enhanced the EQEs of the NIR LECs based on complex Cu-NIR1 by 34%. The peak EQE for NIR EL ( max = 707 nm, 16 wt.% complex YIr) was 0.43% while the peak EQE can be further increased to 0.7% for deep-red EL ( max = 692 nm, 20 wt.% complex YIr). Similarly, doping of complex YIr also improved the carrier transport properties and carrier balance of the LECs based on complex Cu-NIR2 (see Figure S52). The peak EQE reached 0.26%, which is enhanced by 53% ( max = 714 nm, 16 wt.% complex YIr). As such, the proposed doping strategy is effective in further enhancing the device performance of these NIR LECs.

Inspired by the fact that the EL emissions of the proposed complexes show hypsochromic shift when the intermolecular interactions are reduced, red EL emission can be harvested by doping lowconcentration NIR complex in a higher-gap host. Based on this concept, white LECs (188 nm) based on the blue-green complex B [26] doped with 0.6 wt.% complex Cu-NIR1 were fabricated and their EL characteristics were measured (see supplementary discussion #4 for further details). With the aid of efficient host complex B, the peak EQE of the white LEC reached 1.78%, which is higher than that of the LEC employing complex Cu-NIR1. The Commission Internationale de l'Éclairage (CIE) 1931 coordinate of the stabilized EL spectra approached the equal-energy point (0.33, 0.33). These results reveal that the proposed copper complex is not only suitable for NIR LECs, but also useful in white LECs.

Conclusion

A series of binuclear cationic Cu I complexes is disclosed that displays photoluminescence into the NIR region in solution with maximum centered at 746 nm. Despite the hypscochromic shift observed going from solution to condensed samples, for the herein presented binuclear complexes NIR emission is still preserved in the solid state. This emission was achieved thanks to a dinuclearisation strategy and establishment two π-π stacking interligand interactions that provide a doubly locked architecture, disfavoring large excited state geometrical distortion as confirmed by single-crystal X-ray. No suchstacking is present in the parental mononuclear counterparts. The radiative process is assigned to a longlived excited state with 3 MLCT character as supported by in depth photophysical and computational

Figure 1 .

 1 Figure 1. Chemical structure of the dinuclear complexes Cu-NIR1 and Cu-NIR2 along with their mononuclear M1 and M2 counterparts. All the compounds were prepared as PF -salt.

Figure 2 .

 2 Figure 2. ORTEP diagram of compound Cu-NIR2 with thermal ellipsoids shown at 50% probability level obtained by single-crystal X-ray diffractometric analysis along two different views (box a and b). Right view highlights the interligand -stacking interactions involving phosphines and the TzTz ligand. Hydrogen atoms, PF -anions and solvent molecules are omitted for clarity. Selected bond lengths (Å): Cu-N(1) = 2.097(5), Cu-N(2) = 2.072(4), Cu-P(1) = 2.2748(18), Cu-P(2) =2.259(19). Selected bond angles (°): N(1)-Ir-N(2) = 79.52(18), P(1)-Ir-P(2) =124.53(6).

Figure 3 .

 3 Figure 3. CV recorded for copper complexes Cu-NIR1 (blue curve) and Cu-NIR2 (red curve) at concentration of 1 mM and carried out in CH2Cl2/0.1 M TBPF6. Scan rate 0.2 V s -1 .

Figure 4 .

 4 Figure 4. Electronic absorption (solid traces) and emission spectra (dashed traces) of dinuclear complex Cu-NIR1 (red) and Cu-NIR2 (black) and mononuclear M1 (blue) and M2 (orange) Cu I complexes recorded for CH2Cl2 samples at concentration of 3x10 -5 M at room temperature upon excitation at λexc = 480 nm for Cu-NIR1 (red) and Cu-NIR2 and 425 nm for M1 and M2.

  Cu-NIR1 and Cu-NIR2 at their electronic ground state (GS) was carried out by means of density functional theory (DFT). The structure of L1 possess Ci symmetry and the fused thiazolo[5,4-d]thiazole core adopts an almost planar geometry, with a (N1-C2-C3-N2) value of -13.0°. Expectantly, the two peripheral phenyl rings are twisted and show a (C4-C5-C6-C7) = 44.1°. See Scheme S3 for atom labelling.Coordination of [Cu(P^P)] + fragment onto ligand L1 induces large distortions due to multiplestacking interactions between the phenyl rings of the diphosphine ligands and the π -extended aromatic scaffold of L1 (see TableS9-S10 for geometrical parameters). As far as complex Cu-NIR1 at its lowestenergy GS geometry is concerned, four -stacking interactions are present involving two phenyl rings of each of the two DPEPhos ligands and the thiazolo[5,4-d]thiazole core of L1 and two with the pyridine rings (FigureS37and Supplementary Discussion #2). The maximization of these π -stackings leads to a distorted conformation of ligand L1 characterized by a twist of the N1-C2-C3-N2 and C4-C5-C6-C7 dihedral angles by -23.3° (23.8°), an elongation of the Cu-N1 and Cu-N1' distances by ca. 0.1 Å and an increase of the (N1-C2-C3-N2) by ca. 20° compared to the values observed in the experimental structure. In complex Cu-NIR2, only two interligand π -stacking interactions are present involving one phenyl ring of each phosphine and the pyridine ligand L1, owing to the more rigid nature of the diphosphine. The computed structure for Cu-NIR2 agrees well with the experimental one. Overall, bent geometry of the complex is also observed with M-C2-C9 and M-C2'-C9' angles of 156.2° and 160.5°, respectively. Additionally, N1-C2-C3-N2 (N1'-C2'-C3'-N2') dihedral angle of 20.9° (-10.1°) are computed, yielding a more distorted structure than that computed for Cu-NIR1, and characterized by rupture of the Ci symmetry. Optimization with Ci constrain yield a less distorted structure that lies 0.250 eV higher in energy with (N1-C2-C3-N2) = 8.7°. Overall, these results show that both dinuclear complexes are quite flexible and probably exhibit several

Figure 7 .

 7 Figure 7. Nature and energies of the frontier molecular orbitals of Cu-NIR1 complex. Hydrogens are omitted for clarity.

  πPh-π*TzTz nature involving and electron density displacement from the terminal phenyl rings to the TzTz core of ligand L1. Conformation of ligand L1 and rotation about the C5-C6 and C5'-C6' bonds is further investigated in terms of energetics and optical properties highlighting the crucial role played by TzTz TzTz geometrical distortion on the optical properties in such flexible structure (see Supplementary Discussion#3).

Figure 8 .

 8 Figure 8. EDDMs computed between ground and excited state for S0 S1 (a) and S0 S3 (b) transition for complexes Cu-NIR1. Electronically enriched and depleted areas are colored in green and red, respectively. Hydrogens are omitted for clarity.

Figure 9 .

 9 Figure 9. EDDMs computed for T1a ( 3 MLCT, box a) and T1b ( 3 IL, box b) state for complex Cu-NIR1 determined by electron density difference between ground and excited state. Electronically enriched and depleted areas are colored in green and red, respectively. Hydrogens are omitted for clarity

Figure 11 .

 11 Figure 11. Time-dependent (a) current density, (b) light output power, and (c) EQE of the LECs based on complex Cu-NIR1 doped with x wt.% complex YIr. The bias voltages for the LECs doped with 0, 8, 12, 16, and 20 wt.% complex YIr are 5, 4, 3, 2.5 and 2.2 V, respectively.

  analysis. Despite the PLQY values are still modest, they are amongst the highest for Cu I derivatives in this spectral region, which prompted their successful use as EL materials in LECs devices. NIR electroluminescence was demonstrated in LEC fabricated comprising an EML with either neat binuclear Cu I complexes or by doping a wider bandgap host with the NIR emitting Cu I as the guest. The so-fabricated LECs achieved EL maximum wavelength up to 756 nm and peak EQE of 0.43%. Owing to the possibility to tune the EL spectral feature via EML thickness white-emitting LEC was demonstrated as well using a bluered two component approach. To the best of our knowledge, this work demonstrated for the first time the possibility to achieve challenging NIR EL by using earth-abundant metal complexes

Acknowledgements M.M. gratefully acknowledges the Université de Strasbourg and CNRS for financial support. The French Agence Nationale de Recherche (ANR) is kindly acknowledged for the grants ANR-21-CE29-0015 "ChirON" and ANR-22-CE07-0049-02 "BoostOLED" This work was supported by the Higher Education Sprout Project of the National Yang Ming Chiao Tung University and Ministry of Education (MOE), Taiwan.

Supporting Information

The authors have cited additional references within the Supporting Information. Cartesian coordinates of the optimized structures are provided as .xyz file.