Yuang Piao

Qian Jiang

Hao Li

Jinsheng Liang

Hiroaki Matsumoto

Wei Liu

Cuong Pham-Huu

Yuefeng Liu

Fei Wang

Identify Zr Promotion Effects at Atomic Scale for Co-based Catalysts in Fischer-Tropsch Synthesis

Keywords: Fischer-Tropsch synthesis, cobalt nanoparticle, ZrO 2, single-site promoter, atomic resolution STEM TOC

Fischer-Tropsch synthesis (FTS) on cobalt-based catalysts are a vital route for converting syngas into high-value added long-chain hydrocarbons. Introducing promoters into the active phase is an efficient pathway for improving the catalytic performance and has been extensively studied.

However, in the previous works devoted to the study of promoter effects the precise localization of the promoted active sites is seldomly investigated. The present work, for the first time, directly identify the chemical structure as well as the atomic location of promoter additive (e.g. ZrO 2 ) at atomic scale in the cobalt-based FTS catalysts under the wide molar ratio range of Zr/Co (from 0.12 to 1.5). Combining in-situ/ex-situ atomic resolution HAADF-STEM imaging and the related EDS elemental mapping and EELS analysis as well as physicochemical analyses (in-situ XRD, CO chemisorption, CO temperature programmed surface reaction, etc.) and FTS catalytic performance, it has been demonstrated that ZrO 2 promoter is presented as single-site dispersion on the surface of the Co nanoparticles (NPs) and the supports at low content, and further increase the Co NPs roughness which contributes to higher metallic surface area. Such isolated ZrO 2 species plays real active role to promote CO dissociation at the Co-ZrO 2 interface. While at high ZrO 2 content with Zr/Co molar ratio of 1.0 (ZrCo-1.0 catalyst), Zr is more inclined to nucleate on the surface of the support to form amorphous coating structure, whereas the rest still maintain mono-disperse on the surface of the Co nanoparticles. With further increasing the ZrO 2 content (the Zr/Co molar ratio up to 1.5), part of cobalt is encapsulated by ZrO 2 coating thus decreased the FTS activity. The in-situ EELS analysis and density functional theory calculations confirm that Zr atom tends to bind the surface of Co NPs and the charge is transferred from Zr species to Co nanoparticle. The stronger interaction between Zr and cobalt in single site Zr-promoted Co catalyst could enhance the adsorbate binding with the H 2 molecule and the intermediate of the products as well as benefit CO dissociation. It thus displays the enhanced catalytic activity and long-chain hydrocarbon selectivity during the FTS reaction.

Introduction

The metal-based catalysts in heterogeneous catalysis generally require the promoters, such as noble metals, transition metal oxides or rare earth metal oxides, to attain the optimized catalytic performance. [1][2] The intimate contact between active metal and promoter is essential for the catalysis, resulting in the modulation of metal-promoter interaction as an efficient strategy to enhance the catalytic performance. [3][4][5] To date, extensive researches have been devoted to developing the promoters for cobalt-based FTS catalyst [6][7][8] , which is considered as an ideal choice for the synthesis of long-chain products (C 5+ ) like gasoline and diesel via syngas. 9 The addition of noble metal promoters (e.g. Pt) generally promotes the degree of reduction of Co and may further enhance the dispersion of Co or even improve the C 5+ selectivity by increasing the probability of α-olefins re-adsorption. 8,[10][11][12] While considering the cost control for industrial FTS process and limited reserves for the noble metals, transition metal oxides promoters, such as MnO x and ZrO 2 as typical, have been widely studied for enhancing the catalytic activity, C 5+ selectivity and suppress the CH 4 production through CO methanation reaction. [13][14][15] For example, Morales et al. 14 reported that Mn promoted Co/TiO 2 display a rise in C 5+ selectivity and a decrease in CH 4 selectivity. They proposed that the interaction between Co-Mn and solid-solution spinel compound formed in the calcined catalyst that leads to the shift of products selectivity.

It is reasonable that ZrO 2 could play as structure promoter to inhibit the strong metal-support interaction between Co and industrial active oxide supports (e.g. Al 2 O 3 ) and the formation of CoAl 2 O 4 mixture oxide. [16][17][18][19][20] When the interaction between Co and inert oxide support (e.g. SiO 2 ) has been replaced by that of Co and ZrO 2 , the Co dispersion is increased and thus the activity and the product selectivity could be promoted. [21][22] The main conclusions in aforementioned works suggest the interaction between Co and support could be moderated by the introduction of ZrO 2 , resulting better reducibility of Co species, the highly exposed metallic active sites and thus the improved FTS activity. 23 Moreover, the interface of ZrO 2 and Co could also enhance the CO dissociation according to work from Jr. Goodwin. 20 A.T. Bell and co-workers reported the intrinsic promotion role of ZrO 2 on Co-based FTS catalyst for the improved catalytic activity and C 5+ selectivity. 24 Combing the in-situ X-ray absorption spectroscopy with kinetics experiments, it has been clarified that the interface between ZrO 2 and Co nanoparticles increases since the Co nanoparticles are partly covered by ZrO 2 and then the fraction of active sites adjacent to the ZrO 2 species are enhanced. Although the developments concerning the ZrO 2 promoter on Co-based FTS catalysts have been well achieved by using numerous spectroscopic techniques/methodologies, 17,23 the direct evidences of structural origin at atomic-scale of the metal oxide promoters (e.g. ZrO 2 ) on the Co-based FTS catalyst are still missing. In particular, the atomic-scale structural and compositional information of ZrO 2 under the low or high doping concentration is not well identified and thus the structure-activity relationship of the catalysts with the influence of the promoters is still lacking.

With the advanced development of electron microscopy in the last few decades, especially with the aid of the aberration-corrected (AC)-STEM, the atomic arrangement of complex catalyst system could be investigated directly to get the depth insight in the relationship of structure and catalytic activity. [25][26][27][28] Herein, the interactions between the added promoter and the metal or the support are investigated systematically by using the in-situ and ex-situ AC-STEM combined with energy disperse spectroscopy (EDS) and electron energy loss spectroscopy (EELS) as well as chemisorption experiments. Various promoter concentrations of ZrO 2 promoted Co catalysts were prepared with the atomic ratio of Zr/Co varying from 0.12 to 1.5. It is worth noting that the chemical properties of the ZrO 2 promoted Co catalytic system are well identified previously 24 and thus render us the effective observation of the atomic nature of the metal-promoter interaction by using AC-STEM and atomic scale electron spectroscopies (EDS and EELS).

Furthermore, the chemical inert and thermal conductive porous β-SiC was chosen as support for facilitating the heat dispersion since the strong exothermic property of FTS reaction. [29][30][31][32][33][34] Experimental Methods

Catalyst preparation

Cobalt based catalysts were prepared by incipient co-impregnation of porous -SiC (provided by SICAT Corporation) using aqueous solutions of Co(NO 3 ) 2 •6H 2 O (Alfa Aesar, 98%-102% purity) and ZrO(NO 3 ) 2 (Sigma-Aldrich, 99.999% purity). The solution concentration was adjusted to achieve the targeted Zr/Co atomic ratio at a Co loading of 10 wt%. After impregnation, the catalyst was aged at room temperature for 4 hours and then dried at 110 °C for 12 hours followed by the calcination in a Muffle furnace at 350 °C for 3 hours with a heating rate of 2 °C•min -1 unpromoted catalyst is denoted as Co/β-SiC. Zr promoted cobalt based catalysts are named according to the Zr/Co atomic ratio (0.12, 0.5, 1.0, and 1.5), which are abbreviated as ZrCo-x, x = 0.12, 0.5, 1.0, and 1.5. ZrO 2 supported catalyst is also prepared as the reference sample, which is named as Co/ZrO 2 . The other two catalysts for comparison were prepared by step impregnation method with the constant Zr/Co atomic ratio of 1.0. First step was to impregnate the aqueous solution of the ZrO(NO 3 ) 2 into porous -SiC. After aged at room temperature for 4 hours and dried at 110 °C for 12 hours, the precursor was calcinated in a Muffle furnace at 350 °C and 650 o C, respectively, for 3 hours with a heating rate of 2 °C•min -1 .

The samples were then impregnated with aqueous solutions of Co(NO 3 ) 2 •6H 2 O through the same procedure, and the calcination temperature was fixed at 350 °C. The catalysts are named according to the calcination temperature of Zr-containing precursor (350 °C and 650 °C), which are abbreviated as Co@Zr 350 -1.0 and Co@Zr 650 -1.0.

Catalyst evaluation

Fischer-Tropsch reaction was carried out on a home-made three-channel fixed bed setup with the temperature controlled by one oven. The catalysts were loaded into a quartz tube inside the stainless-steel reactor to avoid the influence of metal impurity. The reaction temperature was monitored by thermocouples inserted into the catalyst bed. The gas flow through the fixed bed reactor was regulated by mass flow controller and the pressure was regulated by back pressure regulator. Two traps were designed to collect liquid and wax products where the hot trap was maintained at 100 °C and the cold trap at 0 °C. The gas phase products were analyzed by an online Agilent 7890B gas chromatograph equipped with TCD and FID detector. The non-gasous products were collected after reaction and analyzed by an Agilent 7820A gas chromatograph offline.

The catalyst with the particle size of 160-400 μm diluted with 2 g SiC (particle size ca. 350 μm) was loaded in the middle of the reactor in the isothermal zone to avoid problems linked with inhomogeneous temperature gradient. Prior to reaction, the catalyst was reduced in 60 mL•min -1 H 2 at 350 °C for 8 hours with a heating rate of 2 °C•min -1 at atmospheric pressure. Then it was cooled down to 210 °C and flashed with Ar for 1 hour to remove the H 2 in the system. Afterward, the pressure was raised to 20 bar. When the temperature and pressure is stable, syngas (H 2 /CO=2, N 2 as internal standard) was introduced into the reactor. Considering the strong exothermic character of the Fischer-Tropsch synthesis, a very low heating rate, i.e. 1°C•h -1 was utilized during increasing to the target reaction temperature of 220 °C. The reaction was maintained at 220 °C and 20 bar for 4 days. Notably particularly, the syngas was introduced into the reactor at the same reaction condition for 25 hours before the data was acquired to stabilize the reaction.

The cobalt time yield (CoTY) was calculated based on molar CO consumption per gram cobalt per second. F CO, in and F CO, out represent flow rate of CO in the feed gas and CO in the effluent in the unit of mol•S -1 . m Co represents the mass of Co in the catalyst. C 5+ selectivity was calculated as following,

The carbon balance was calculated by following equation, , and represent the amount of the gas phase, oil phase and wax phase. represents the reaction time, which is calculated when the furnace temperature reached 220 °C. The carbon balance of all tested catalysts is in the range of 95±5%.

Catalyst Characterization

The powder X-ray diffraction (XRD) patterns were recorded on an X'pert-Pro diffractometer (PAN Analytical, Netherlands) using Cu Kα radiation at 40 kV and 40 mA in the range of 10°-80° using the step size of 0.2 degree. The catalysts after the reaction were collected from diluting agent by screening and were subject to XRD characterization immediately. The in situ XRD is also recorded on an X'pert-Pro diffractometer in the range of 30°-50° using the step size of 0.12 degree. The flow rate of H 2 is 30 mL•min -1 , the heating rate is 5 °C•min -1 , and from 160 o C the temperature is kept 20 min every 20 o C for measuring the patterns. When the temperature was raised to 350 o C, the reduction was continued for 2 hours.

The specific surface area, pore volume and the average pore diameter of the samples were measured by N 2 physisorption on Micromeritics ASAP 2020 apparatus working at -196 °C.

Before measurement, the samples were degassed under vacuum at 180 °C for 6 hours. The specific surface area was calculated according to the multipoint Brunauer-Emmett-Teller (BET) method. The pore volume and average pore diameter were calculated using the Barrett-Joyner-Halenda (BJH) method with desorption branch of the isotherms.

The temperature-programmed reduction (TPR) was conducted in a fixed bed reactor with a U shape quartz tube (inner diameter 6 mm) at atmospheric pressure monitored by a Pfeiffer Omni star mass spectroscopy. Firstly 50 mg sample was heated to 100 °C under the flow of 99.99% He (40 mL•min -1 ) with the ramp of 10 The mass signal of m/z = 15 was used to record the NH 3 desorbed.

To understand the behaviors for CO adsorption and activation for different catalysts, the CO temperature programmed desorption experiments were performed with the signals recorded by mass spectroscopy. 50 mg catalyst was reduced at 350 °C under pure H 2 (40 mL•min -1 ) for 4 hours and then the gas was switched to He to purge the adsorbed H 2 for 30 min followed by cooling down to 50 °C. Afterwards, pure CO (40mL•min -1 ) was introduced for adsorption at 50 °C for 30 min in the reactor. The reactor was purged with He for 1 hour to remove the physically adsorbed CO until baseline stable. The sample was heated from 50 °C to 700 °C at a ramp of 10 °C•min -1 using the mass signal of m/z of 28 was continuously recorded for CO evolving. In the same device, the reactivity of surface metallic cobalt was evaluated by CO temperature programmed surface reaction (CO-TPSR). The reduction and CO adsorption procedures are the same as those used for CO-TPD mentioned above. The temperature programmed reaction process was conducted with 30 mL gas flow of 5% H 2 /He heated from 50

to 800 °C at a rate of 10 °C•min -1 . The mass signal of m/z = 15 was monitored for the CH 4 formed.

Transmission electron microscopy analysis

Aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on a JEOL JEM-ARM200F microscope. The elemental composition as well as distribution were analyzed on the energy dispersive X-ray analyzer (EDS, EX-230 100 m 2 detector) equipped on the microscope. The sample was ultrasonicated in ethanol solution and a drop was deposited on a copper grid covered with a holey carbon membrane. The catalyst was reduced under 5% H 2 /He for 8 hours followed by a passivation in 0.5% O 2 /He to protect Co particles from excessive oxidation before the microscopy characterization.

In-situ STEM analysis and EELS spectrum was performed on a newly developed HF5000 microscope from the Hitachi Company with a 200 kV cold-field emission gun and a Cs-corrector for the electron probe. The catalysts Co/β-SiC and ZrCo-1.0 was reduced in ex-situ condition at 350 °C for 8h under H 2 flow (40 mL•min -1 ) followed by passivation in 0.5% O 2 /He to protect cobalt particles from excessive oxidation in laboratory. The in-situ experiment was initiated via a MEMS-based heating method (using a MEMS compatible holder designed by Hitachi Company).

Afterwards, the samples were deposited on holder and reduced in microscopy under 500 °C in H 2 atmosphere (P=2 Pa). Maintaining the temperature and atmosphere when collected STEM images and EELS spectra.

Computational and modeling methodology

Spin-polarized density functional theory (DFT) calculations were performed using the VASP code. The core electrons were described by the projector augmented-wave method. 35 The electronic exchange and correlation were described by a generalized gradient approximation method using the Perdew-Burke-Ernzerhof functional. 36 The valence electrons were described by the Kohn-Sham wave functions being expanded in a plane-wave basis with the energy cutoff of 400 eV. 37 Brillouin zone was sampled with (3×3×1) Monkhorst-Pack k-point mesh. 38 For convergence, all the configurations were defined as optimized when the forces of each atom fell below 0.05 eV/Å. The Co surface was modeled as a 4-layer, 4×4, (111) surface, with the lattice constant of 3.53 Å. 39 The bottom two layers were fixed in bulk position, while the rest of the layers were allowed to relax. To model different atomistic environments of Zr, Zr-in-Co(111) and Zr-on-Co(111) were modeled, which respectively represent the Zr inside and outside of the Co lattice. The Zr-in-Co(111) was modelled by replacing a surface Co with a Zr, while the Zr-on-Co(111) was modelled by placing a Zr atom on a Co(111) surface. The H, C, and CO binding energies were calculated respectively using the total energies of H, C, and CO (in vacuum) as the references.

Results

FTS activities

The FTS catalytic performances of the prepared samples with different Zr/Co molar ratios (Zr/Co molar ratios of 0, 0.12, 0.5, 1.0 and 1.5) are evaluated. The results (Figure 1a) show that the catalysts present superior stability and the CoTY value (Cobalt time yield, molar CO conversion per gram cobalt per second) does not significantly decrease along with time on stream.

Surprisingly, the ZrCo-0.12 sample with ultra low ZrO 2 concentration (2.1 wt%) presents more than 2-fold CoTY value (8.410 -5 mol CO g -1 Co s -1 ) compared to that of Co/-SiC (3.310 -5 mol CO g -1 Co s -1 ), demonstrating the positive contribution of ZrO 2 additions. With further increase of the Zr/Co molar ratio (Figure 1b), the CoTY gradually increases to the 12.810 -5 mol CO g -1 Co s -1 over Zr/Co-1.0 sample and subsequently decreases to 7.910 -5 mol CO g -1 Co s -1 over Zr/Co-1.5 and 3.910 -5 mol CO g -1 Co s -1 over pure ZrO 2 supported cobalt catalyst (Co/ZrO 2 ). The highest FTS activity on ZrCo-1.0 catalyst is 7-fold higher than that of unpromoted catalyst (Co/SiC), where the ZrO 2 concentration is 20 wt%. It is worth noting that the increment degree of FTS activity is not linear correlation with the addition amount of ZrO 2 , where 10% and 50% increment degree for CoTY when increment degree of zirconium atoms content is 0.5 and 1.0, respectively, for ZrCo-0.12 to ZrCo-0.5 and to ZrCo-1.0. The results reveal that the role of Zr promotion effects is rather different along with the different ZrO 2 loading. GHSV (STP) = 12,000 mL h -1 g cat -1 for ZrCo-0.12 and ZrCo-0.5; GHSV (STP) = 18,000 mL h -1 g cat -1 for ZrCo-1.0

The ZrO 2 introduction also influences the C 5+ selectivity (Figure 1b). The ZrCo-0.12 catalyst with small amount of ZrO 2 introduction shows higher C 5+ selectivity (85%) compared to that of Co/SiC catalysts (72%), which due to the presence of ZrO 2 near the Co nanoparticle benefits C-C coupling. 24 While further increase the Zr content as to catalyst ZrCo-1. 1d). The trend is consistent with the early reports of Feller et al. 16 , whereas different from the reports from Johnson et al. 24 and Moradi et al. 17 that the C 5+ selectivity increases along with ZrO 2 addition and reach the selectivity plateau at higher ZrO 2 loading. Moreover, these thermally conductive porous -SiC supported ZrCo-x catalyst presents the FTS catalytic performance superior to the most of reported zirconium promoted cobalt catalysts (Table S1).

Physicochemical characteristics of supported CoZr-x catalysts

The XRD patterns for calcined -SiC supported catalysts are presented in Figure 2a HAADF-STEM images (scale bar 100 nm) and the corresponding particle size distribution for passivated ZrCo-0.12 (c, f), ZrCo-0.5 (d, g) and ZrCo-1.0 (e, h)

The XRD patterns of spent catalysts show some apparent new diffraction peaks located at about 21° and 23°, attributed to (C 2 H 4 ) n for the products in oil and wax phase (Figure 2a). The diffraction peaks at 44.4° and 75.8° attribute to the (111) and (220) planes of metallic Co in fcc (face-centered cubic) phase. There still no obvious change of the cobalt crystallite size with various ZrO 2 concentrations, which suggests that Zr promoter slightly influences the cobalt crystallite size and dispersion during the reaction. The HAADF-STEM images and particle size distribution of passivated ZrCo-x catalysts (Zr/Co = 0.12, 0.5 and 1.0) are shown in Figure 2 The cobalt particle size of ZrCo-0.12 is 14 nm, which is slightly smaller than that of ZrCo-0.5 (17 nm) and ZrCo-1.0 (17 nm). However, the crystallite size of cobalt for all ZrCo-x catalysts become larger in a certain extent after FTS reaction (Table 1), which is mainly due to the sintering occurs in the test due to the relative weak metal-support interaction by using chemical inert SiC support. 31,34 In-situ XRD experiment of ZrCo-1.0 catalyst was conducted during the reduction process to were directly compared (Figure S4). The results also clearly show no peak shifting for metallic Co diffraction peak over ZrCo-1.0 catalyst compared to that of unpromoted Co/ SiC catalyst. The particle size of cobalt maintains similar value at the wide range of ZrO 2 addition, however, the ZrO 2 could influence the cobalt metallic surface area (MSA) due to the partial coverage on the cobalt surface. 16 Thus, it is interesting to investigate the relationship between the metallic cobalt surface area and the FTS catalytic activity. The cobalt surface MSA is recognized as the In view of that ZrO 2 is amphoteric oxide that possess the acid property and basic property at the same time. 44 Previous results indicated that Lewis acid sites were beneficial for CO dissociation. 45 It might be ascribed to the stronger interaction between the O atom of adsorbed CO and the promoter that results in a weaker C-O bond that can be cleaved more readily. Similar mechanisms have been proposed for FTS over Co catalysts promoted with metal oxides 13 .

Therefore, the acid property of the samples with different ZrO 2 content is also considered.

NH 3 -TPD measurements are carried out for the catalysts after reduction to evaluate the acidity of the catalysts. The ZrO 2 normally present acid sites of weak strength, 46 thus the desorbed NH 3 below 400°C is attributed to the weak acid sites on ZrO 2 and the NH 3 released above 400 °C is ascribed to the strong interaction between NH 3 molecular and metallic cobalt surface. 19,47 NH 3 -TPD profiles (Figure S3) for Co/SiC and supported ZrCo-x catalysts reveal that the amount of acid sites increases with the Zr content in all catalysts except the ZrCo-1.0, which shows more acid sites than other samples. The amount of the acid site shows a positive effect on the CoTY (Figure 4b). The results imply that the presence of zirconia in ZrCo-x catalyst causes the enhancement in acidity and consequently influences the catalytic performance which is in agreement with current literature. 22,48 It is known that under the reaction condition the CO chemisorption on the catalyst surface is strong, and the coverage ratio is approximately unity, while the H 2 chemisorption is weak and its coverage is very low. 49 To further confirm the influence of Zr addition on CO adsorption and dissociation on the metallic cobalt surface, 50 the temperature dependent evolution of methane on behalf of the CO adsorption was monitored by CO-TPSR experiments. 51 To avoid interference from the fragments of H 2 O molecule, the m/z signal of 15 was monitered for methane. From the TPSR (Figure 4c), it shows that both the onset and the peaking temperature of methane for Zr promoted catalyst (ZrCo-1.0) are lower than the none-promoted one (Co/β-SiC). The fact that the Zr reduces the adsorption of CO, yielding the enhanced FTS performance is probably due to the Zr promotion of the CO dissociation. This agrees well with the reported literatures. [23][24] S5.

Atomic resolution investigations of supported CoZr-x catalysts

Aberration-corrected HAADF-STEM was employed to explore the structure details of the ZrCo-x catalysts including the particle size, lattice spacing of grains and the status of Zr species. -SiC areas. To identify the distribution status of Zr species, the Zr/Co atomic ratios on the cobalt particle and the support are measured by EDS mapping (Figure S7). The results (Table S2) show a similar value of Zr/Co ratio for either an individual cobalt particle or the whole area containing both SiC support and cobalt particles. This suggests in the catalyst with low Zr content, Zr species are in homogeneous mono-dispersion on both the cobalt particle and support. The schematic illustration based on above structural analysis is given in Figure 5c. When promoting the Zr/Co molar ratio to be 0.5 via increasing the ZrO 2 concentration, microscopy results (Figure 5d) reveal that the surface of the -SiC support is encapsulated with a bright layer that is recognized to be ZrO 2 species according to EDS elemental mapping evidence (Figure S8). Considering that there are no diffraction peaks in XRD patterns belonging to Zr species (Figure 2a), the ZrO 2 layer should exist in amorphous form, which could also be confirmed by the enlarged STEM image of Figure 4e. Cobalt particle presents a Co 3 O 4 oxidized layer with the thickness around 2 nm covering the metallic core of fcc cobalt. Distinguished from the situation on SiC support, Zr species on Co particle are highly dispersed without aggregation or forming coating layers. The EDS statistics (Table S3) as well as their corresponding STEM images (Figure S11) from three randomly selected regions disclose that Zr species preferentially concentrate on the surface of the SiC support leaving less Zr on Co particles. Such trend agrees well with microscopy observation (Figure 5d). Since Zr species accumulate on the support forming a layer outward, an interface between cobalt particle and Zr species shell appear as demonstrated by the EDS analysis (Figure S8). The above analysis concludes a structural illustration for ZrCo-0.5 as shown in Figure 5f, where the amorphous ZrO 2 covers on the support surface yielding the ZrO 2 -Co interface, meanwhile those Zr species locating on Co particle distribute as single site promoters similar to the behavior in sample ZrCo-0.12.

Further increase the Zr/Co ratio to 1.0, partial Zr species remain oxide layer encapsulating on SiC supports (Figure S12), while the mono-dispersed Zr on cobalt particles becomes more abundant (Figure 5g, 5h) compared to the previous two catalysts ZrCo-0.12 and ZrCo-0.5. It is worth to be noticed that even the atomic ratio of Zr/Co is as high as 1.0, there is still no sign of ZrO 2 aggregations on the Co particles. The EDS and STEM evidences from multiple sample areas (Table S4 and Figure S12) reveal that Zr/Co atomic ratios for the support areas retain higher than those specific Co particles, which is analogous to the findings for catalyst ZrCo-0.5 as illustrated by Figure 5i. Further quantitative data analysis will be discussed in section 4.2.

When the Zr content is consecutively increased reaching to the Zr/Co atomic ratio of 1.5 (Figure S13), it is intriguingly to find a distinct amorphous ZrO 2 layer outside the Co nanoparticle. Combined with aforesaid analysis from samples of all Zr/Co atomic ratios, we can draw brief conclusion that Zr species preferentially concentrate on the -SiC support, while when the Zr additive exceeds a certain amount, the distribution on the Co surface would evolve from the isolated mono-dispersion into an continuous amorphous layer, which explains why the reaction activity of ZrCo-1.5

decreased compared with ZrCo-1.0. From the above results, a model can be established that the encapsulated amorphous ZrO 2 locates not only on the surface of the support but also on the Co nanoparticles, which is the cause for the large drop of the CoTY of the catalyst.

To investigate the chemical interaction of Co and Zr in atomic scale, the representative Co/SiC and ZrCo-1.0 catalysts were characterized by an environmental aberration corrected STEM (Hitachi HF5000) combined with the EELS analysis (Figure 6). The Zr M 4,5 -edge EELS spectra of bulk ZrO 2 and CoZr alloy were also performed for better comparison. After in-situ reduction in the microscope of passivated catalyst, the cobalt oxide layer could be totally removed according to the EELS spectra of O K-edge (530 eV in Figure 6c) over Co/ SiC and ZrCo-1.0 catalysts.

Consistent with the results of ex-situ STEM-HAADF analyses (Figure 5), the Zr species distribute into two different ways on the ZrCo-1.0 catalyst, one is coated on SiC support (Figure 6a) and another is interacted with cobalt nanoparticle (Figure 6b).

It is clearly shown that the bulk ZrO 2 presents the energy loss peak of 210 eV and 330

eV attributed to M 4 and M 3 edge for Zr and 530 eV owing to O K-edge. As shown in Figure 6c, the EELS spectra of Zr species on SiC and cobalt nanoparticles present the similar shape as bulk ZrO 2 , revealing the Zr atoms not embedded into the cobalt lattice to form the alloy compound. The Co L-edge spectra of cobalt nanoparticles on Co/SiC and interacted with Zr on ZrCo-1.0 are selected to identify how the chemical interaction between Zr and Co (Figure 6d). It is known that the intensity ratio between the L 2 and L 3 edges is sensitive to the chemical valence for the cobalt species 52 . The L 3 /L 2 ratio of ZrCo-1.0 catalyst is 1.47, which is much lower compared to that of Co/SiC catalyst (L 3 /L 2 ratio of 1.88, Figure 6d). Such L 3 /L 2 ratio decrement over The cobalt particle size of the reduced ZrCo-x catalysts is about 10 nm according to the XRD and STEM characterizations (Figure 2 and Figure 5). The slightly larger appearance for particle size in sample ZrCo-1.5 is due to contribution of contrast overlay between the ZrO 2 layer and the Co core. In general, given the similar particle size, the surface area of Co NPs is expected to be also similar. However, the exposed MSA over the ZrCo-x catalysts shows remarkable variation illustrating a volcanic curve with maximum of 14.2 m 2 /g for ZrCo-0.5 (Figure 4). Such MAS increment at lower ZrO 2 concentration mainly attributes to the rough surface of cobalt ascribed to introduction of isolated Zr species (Figure 4a). When the Zr/Co atomic ratio gets higher than 0.5 (ZrO 2 loading of 10 wt%), the amorphous ZrO 2 phase starts to accumulate over the cobalt surface that reduces the MSA meanwhile generates the times from Co/SiC (3.710 -5 mol CO g -1 Co s -1 ) to ZrCo-0.12 (8.510 -5 mol CO g -1 Co s -1 ), but it only increases by 10% from ZrCo-0.12 to ZrCo-0.5 (CoTY of 9.210 -5 mol CO g -1 Co s -1 ). Such disproportion behavior indicates that not the amorphous ZrO 2 species but the isolated ZrO 2 sites on cobalt surface possess a dominant contribution to the CO dissociation.

To further understand the interaction between Zr species and cobalt nanoparticle, two more catalysts by the ZrO 2 pre-deposition over SiC support with the same Zr/Co molar ratio as ZrCo-1.0. As confirmed by XRD patterns in Figure S14, ZrO 2 presents as the amorphous state for Co@Zr 350 -1.0, whereas displays well crystalline for Co@Zr 650 -1.0. The interaction degree between cobalt and ZrO 2 could be labeled in the following order, ZrCo-1.0 > Co@Zr 350 -1.0 > Co@Zr 650 -1.0. It should be noted that, even the ZrO 2 was introduced on the support before cobalt, the single site Zr species could be not eliminated during the nucleation of Co NPs in calcination process as well as the reduction of metallic cobalt from the cobalt oxide. Such dramatic reconstruction of the bi-metallic nanoparticles during the reduction and reaction process was detected in our previous work 53 . However, lower interaction degree between Co and Zr could results in lower probability of single site of Zr species. It is clearly shown in Figure S15 that the FTS catalytic activity decrease along with the decrement of the interaction efficiencies of between ZrO 2 and cobalt, which further confirm efficient active role of single-site Zr species.

Active sites at the atomic interface of Zr/Co.

To put insights into the interaction details of Zr and Co species, the atomic resolution STEM as well as the EDS characterization are employed to identify and quantify the characteristics of ZrO 2 promoter. To study the distribution variation between SiC support and Co NPs, two types of Zr content parameters have been focused (Figure 7a), the contents of the element averaged from a whole area (denoted as "A" for images), the contents of the element specialized from randomly selected Co NPs (denoted as "N" for images). All the data issues are from the statics over three randomly-selected areas/NPs and normalized using the mass percentage as shown in the Table S2 (10% ZrCo-0.12), Table S3 (10% ZrCo-0.5) and Table S4 (10% ZrCo-1.0). Their microscopy images correspond to Figures S10-S12, respectively. As shown in Figure 7b, the Zr/Co molar ratio on the Co NPs is 0.023 slightly lower than the whole area for the ZrCo-0.12 sample. Whereas the significant increment of Zr/Co molar ratio on the whole area compared to that on the individual Co NPs, indicating a clear tendency of preferential aggregation of ZrO 2 species on the support surface. The CoTY value for sample ZrCo-0.5 slightly increases compared to that of ZrCo-0.12, which is fitted well the same trend as the Zr/Co molar ratio on the individual cobalt NPs. For the catalyst ZrCo-1.0, the Zr/Co atomic ratio on the individual cobalt NPs is 5-7 times higher than that for the ZrCo-0.5 facilitating significantly promoted CoTY.

Based on the data collected from the FTS performance, HAADF-STEM and EDS (Figure 8). Interestingly, it clearly demonstrates that Zr-in-Co(111) leads to very strong (i.e., negative) binding energies for all the three adsorbates, indicative of the formation of hydride-or carbide-like materials. In contrast, the Zr-on-Co(111) surface has the H binding energy closer to that of Co(111). 55 The CO bindings on Zr-on-Co(111) is similar to that of Co(111), with the reason that CO tends to bind at the atop-site where the atomic ensemble effect is less predominant to tune the CO bindings. 56 This is also in excellent agreement with the XRD results that there was no significant shift of the cobalt oxides diffraction peaks induced by Zr doping (Figures 2 and3). These results indicate that a Zr-on-Co(111) structure has the active site for C-C coupling and binding site of cobalt atoms to increase the FTS activity and selectivity in consistent with the experimental observations. Therefore, we suggest that Zr is not embedded into the lattice of Co; instead, it is expected to bind to the surface of Co NPs for the ZrCo-x catalysts. This is also in excellent agreement with the in-situ XRD results that there was no significant shift of the cobalt oxides diffraction peaks induced by Zr doping (Figure 3). The Zr promotion effect probably came from the combination of electronic and strain effects, 56 since the adsorption site (the surface ensemble) remained the same (Figure 8). Based on this comment, we carried out additional calculations to analyze the electron charge transfer and the bond length change at the Co sites after the doping of Zr (Table S5 and Figure S16). Interestingly, it can be clearly seen that doping of Zr on the Co (111) surface leads to the electron charge gain at surface Co (Figure S17), while it meanwhile leads to the enlarged Co-Co bond lengths.

According to the d-band center theory 57 , the charge gain of Co leads to the increase of Co d-band filling, which in turn down-shifts the d-band and weakens adsorbate bindings. But meanwhile, the enlarged average bond length tunes the adsorbate binding stronger due to the strain effect. From the enhanced binding strength of H and C at the Co sites (Figure 8), it is clear that the strain effect is predominant in determining the adsorbate binding after doping Zr onto Co (111). Such stronger bindings of H and C on Zr-on-Co(111) support that the Zr species act as the promoter that can strengthen the adsorbate bindings, which in turn promote the adsorption abilities of H 2 and intermediates of the products. 54 The results presented in this work provide a new insight into the ZrO is not covered by ZrO 2 . According to the previous reports that the increase of Zr content facilitates enhanced CO dissociation thus promoting CO conversion. [58][59] Therefore, under the promotion effect of Zr, it would be expected that the stronger binding of CO would decrease the ratio of H/CO on the catalyst surface, thereby enhancing the formation of C 5+ products. S1-S5) (PDF)

Figure 1 .

 1 Figure 1. (a) FTS catalytic performance as a function of time on stream and (b) the average value

  . The -SiC support presents three main diffraction peaks at 35.5°, 59.9° and 72.2° corresponding to (111), (220) and (311) planes, respectively. The additional diffraction peaks emerge at 36.8°, 31.3°and 65.2° after Co introduction, which are attributed to (311), (220) and (440) planes of Co 3 O 4 . As shown in Table 1 and Figure2a, the Zr introduction brings no significant influence on the crystal phase composition of the catalyst in aspects of either Co 3 O 4 dispersion or the new phase formation even when the Zr content is as high as 20 wt%, which indicates that the Zr species are well dispersed in sub-nano sizes or in amorphous phase without contributing periodic diffraction contrast.

Figure 2 .

 2 Figure 2. XRD patterns for calcined catalysts (a) and catalysts after 80 hours FTS reaction (b).

  investigate the effects of Zr species on the microstructure of cobalt NPs. It can be clearly seen that the as-prepared ZrCo-1.0 catalyst had significant Co 3 O 4 diffraction peaks, but not the zirconium-related diffraction peak, indicating the ZrO 2 only presented in the amorphous state. As shown in in-situ XRD patterns in Figure 3b, the diffraction peak of Co 3 O 4 start to disappear and a CoO (200) peak (42.4°) emerges when the reduction temperature increases from room temperature to 250 °C. When the temperature reaches 350 °C, the diffraction peak of CoO gradually disappears, replaced by that of metallic Co (111) at diffraction peaks of 44.4°. The in-situ reduction was then maintained at 350 °C for 2 hours. It is found that the CoO diffraction peak completely disappeared, and the FWHM of the Co diffraction peak gradually increased, indicating that Co particles gradually formed during the reduction process. The in-situ XRD results are consistent with the two-step reduction characteristics of Co 3 O 4 in the previous literature 40 . Although in presence of ZrO 2 on the ZrCo-1.0 catalyst, the diffractions (lattice spacings) of Co phase do not exhibit a visible shift, and no diffraction peak of Zr species is detected during the reduction process, which further confirmed that Zr species are in either mono-atom dispersion or the amorphous form in the Zr-Co catalyst without contributing any periodic diffraction. Furthermore, the ex-situ XRD patterns for passivated Co/SiC and ZrCo-1.0

Figure 3 .

 3 Figure 3. In-situ XRD experiments of ZrCo-1.0 catalyst. (a) Reduced ZrCo-1.0 catalyst, (b) XRD

Figure 4 .

 4 Figure 4. Metallic cobalt surface area (columns) determined by CO chemisorption and CoTY

Figure 5

 5 presents the atomic structure and elemental distribution of ZrCo-x catalysts via characterizations of atomic resolution HAADF-STEM imaging and EDS elemental mapping. It shows for the ZrCo-0.12 (Figure 5a-5c), the cobalt particles are in a core-shell configuration with a passivated oxide layer on the surface and Zr species are highly dispersed over the whole sample. The lattice fringes in spacings of 2.02 Å and 2.51 Å can be indexed to the (002) plane of fcc metallic cobalt phase and the (111) plane of -SiC, respectively. Relatively bright contrast from strong scattering of Zr atoms in a HAADF imaging illustrates that Zr species exist in form of isolated atoms (labeled by purple circle) locating in both Co and

Figure 5 .

 5 Figure 5. HAADF-STEM images for ZrCo-0.12 (a, b), ZrCo-0.5 (d, e), ZrCo-1.0 (g,

ZrCo- 1 .

 1 0 catalyst probably attributed to the charge transfer from Zr species to Co nanoparticle, resulting in the more negative chemical valence for Co NPs (i.e. Co δ- species in the surface of cobalt NPs).

Figure 6 .Discussion 4 . 1

 641 Figure 6. In-situ STEM experiments of Co/SiC and ZrCo-1.0 catalysts. Atomic

ZrO 2 -

 2 Co interface (Figures5d-5i). The relative dense coverage of ZrO 2 amorphous layer in Zr/Co-1.5 catalyst leads to the worse FTS activity although it contains large amount of the Lewis acid sites (Figure4b). According to the previous microscopy results, two forms of Zr species have been identified on the Co NPs. The atomically isolated ZrO 2 at low ZrO 2 concentration and the amorphous ZrO 2 layer over Co surface at high ZrO 2 concentration. It can be found that the CoTY increases by 2.2

Figure 8 .

 8 Figure 8. DFT-optimized configurations of H, C, and CO bindings on Co(111),

  2 promotion effects on the cobalt FTS catalyst. The single-site Zr species is revealed on the surface of cobalt nanoparticle, as illustrated in Figure 9a. The single-site Zr species anchored on the surface of Co NPs and the charge is transferred from Zr species to Co nanoparticle. Thanks to charge transfer between Zr and Co atoms, the adsorbate binding with the H 2 molecule and the intermediate of the products are enhanced as well as beneficial for the CO dissociation during the FTS reaction. It thus enhances FTS catalytic activity and C 5+ selectivity (Figure 9b) when the surface of cobalt NPs

Figure 9 .

 9 Figure 9. (a) Diagram illustrate the promotion effect of single site Zr anchored on the

  5, the C 5+ selectivity decreases to 76%, with the similar level as pure ZrO 2 supported catalyst and Co/SiC. To shed light on the influence of ZrO 2 promotion for the product selectivity, the GHSV was adjusted to maintain the CO conversion at around 25% for all investigated ZrCo-x catalysts. The products selectivity of CH 4 , C 2 -C 4 and C 5+ , as well as the C 2 -C 4 olefin/paraffin ratio (C

o /C p ) are presented in Figures 1c, 1d, S1 and S2. The C 5+ selectivity of ZrCo-x catalyst increases with the increment of Zr/Co ratio and reach to 84.2 % at maximum value for ZrCo-1.0 catalyst and subsequently decrease to the 77.8% for ZrCo-1.5 catalyst. The increase in the selectivity of the C 5+ product could attribute from the improvement of hydrogenation ability by adding the Zr species, which also result in the lower C 2 -C 4 olefin/paraffin ratio (Figure

Table 1 .

 1 Physicochemical properties of various porous -SiC supported ZrCo-x catalysts (Co loading=10 wt%). The degree of reduction is obtained by integrating the consumption peaks for H 2 which is shown in Figure

		Textural property a		Crystallite size b		Particle size c CO-chemisorption	Reducibility d (%)
		S BET	Pore	Pore	After	Used	Reduced	Metallic	D (%)
	Catalyst	(m 2 g -1 )	volume (cm 3 g -1 )	diameter (nm)	calcination (Co 3 O 4 ) (nm)	(nm) (Co 0 )	(nm) (Co 0 )	area (m²/g Co )
	Co/SiC	28	0.13	16.5	14.3	19.2	-	8.1	1.2	93.3
	ZrCo-0.12	26	0.17	24.3	11.6	18.1	14	10.0	1.5	84.7
	ZrCo-0.5	20	0.18	34.9	12.6	18.3	17	14.1	2.0	90.0
	ZrCo-1.0	44	0.15	13.8	11.9	18.9	17	11.3	1.6	89.6
	ZrCo-1.5	44	0.12	10.3	12.1	20.5	13	8.4	1.2	67.8
	Co/ZrO 2	6	0.03	20.2	25.3	-	-	5.3	0.7
	a. BET profiles are shown in Figure S4.
	b. Calculated from XRD
	c. Determined from TEM
	d.

ACKNOWLEDGMENT This work was financially supported from the National Natural Science Foundation of China (Nos. 51874115, 91645117, 21972140 and 21872144), Excellent Young Scientist Foundation of Hebei province, China (No. E2018202241), Enterprise Science and Technology Commissioner Project of Tianjin City, China (No. 19JCTPJC56100), CAS Youth Innovation Promotion Association (Nos. 2019190, 2018220), Liaoning Revitalization Talents Program (No.XLYC1907053), China Postdoctoral Science Foundation (No.2018M641726) and Talents Innovation Project of Dalian city (Nos. 2016RD04 and 2017RQ032). The authors acknowledge the SICAT Co. (www.sicatcatalyst.com) for providing porous β-SiC materials. Prof. Yunjie Ding and Dr. Ziang Zhao are acknowledged for the assistance of in-situ XRD experiment. We sincerely cherish our memory to Prof. Dang Sheng Su for his indispensable supports and fruitful discussions.

place for CO and H 2 dissociation and also C-C coupling, which is an important factor to evaluate the FTS activity. [41][START_REF] Cheng | In Advances in catalysis[END_REF] In this work, the cobalt MSA is determined by CO chemisorption (Figure 4a and Table 1) and its influence on the FTS activity has been analyzed. The MSA increases with the Zr/Co molar ratio from 0 to 0.5, reaching the maximum of 14 m 2 •g Co -1 for ZrCo-0.5. When the ZrO 2 content is further increased, the MSA is sharply decreased. It is known that higher MSA means more Co active sites for FTS reaction [START_REF] Spadaro | [END_REF] thus leading to superior catalytic activity.

Surprisingly, the ZrCo-0.5 catalyst shows the highest MSA, whereas the highest CoTY is achieved by ZrCo-1.0 rather than the ZrCo-0.5, suggesting that MSA is not the sole influence factor for the catalytic performance.

analysis, it reaches a conclusion that the ZrO 2 species disperse on cobalt NPs are the real promoted sites that contribute to the improved FTS performance.

Theoretical investigations.

To understand the activity of FTS on different ZrCo-x catalytic surfaces, a previous volcano-like model was derived with the functions of adsorbate binding energies based on DFT calculations. 54 Notably, Co binds to adsorbates slightly weaker than the optimal activity as indicated from the volcano peak. In our study, DFT calculations were performed to evaluate: i) the promotion effect of Zr in catalysis and ii) the possible position of Zr when mixed with Co-catalysts (denoted as 'Zr-in-Co' for Zr atoms embedded in cobalt lattice; 'Zr-on-Co' for Zr atoms supported on cobalt surface). Therefore, the adsorption binding energies of H, C, and CO species were calculated on the surface of Co(111), Zr-in-Co(111), and Zr-on-Co(111), respectively

AUTHOR INFORMATION

Corresponding Author

*E-mail: weiliu@dicp.ac.cn (W. Liu), yuefeng.liu@dicp.ac.cn (Y. Liu), wangfei@hebut.edu.cn (F. Wang)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡Y.P. and Q.J. contributed equally.

Notes

The authors declare no competing financial interest.


Temitope T Abiola

Natércia D N Rodrigues

Ho Casey

Daniel J L Coxon

Michael D Horbury

Josene M Toldo

Mariana T Do Casal

Benjamin Rioux

Cédric Peyrot

Matthieu M Mention

Patrick Balaguer

Mario Barbatti
email: mario.barbatti@univ-amu.fr

Florent Allais
email: florent.allais@agroparistech.fr

Vasilios G Stavros
email: v.stavros@warwick.ac.uk

New Generation UV-A Filters: Understanding their Photodynamics on a Human Skin Mimic

Keywords: Ultrafast spectroscopy, sunscreens, nature-inspired, photoprotection, photochemistry, photophysics

The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge towards developing effective broadband sunscreens with minimal human-and eco-toxicity. To combat this, we have designed a new class of Meldrum-based phenolic UV-A filters. We explore the ultrafast photodynamics of coumaryl Meldrum, CMe, and sinapyl Meldrum, SMe, both in an industry standard emollient and on a synthetic skin mimic, using femtosecond transient electronic and vibrational absorption spectroscopies, and computational simulations. Upon photoexcitation to the lowest excited singlet state (S1), these Meldrum-based phenolics undergo fast and efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast twisted intramolecular charge transfer mechanism as these systems evolve out of the Franck-Condon region towards an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. Importantly, we correlate these findings to their long-term photostability upon irradiation with a solar simulator and conclude that these molecules surpass the basic requirements of an industry standard UV filter.

INTRODUCTION

Biological systems rely on solar radiation from the Sun to provide light, warmth and energy to sustain life. [START_REF] Ahmed | Worldwide regulation of UV filters: current status and future trends[END_REF] However, excessive exposure to ultraviolet (UV) radiation from the Sun at the Earth's surface (UV-B: 280 -315 nm, UV-A: 315 -400 nm) causes adverse effects such as tanning (which occurs as a response to photodamage to the skin), sunburn, DNA mutations and photoaging in humans. [START_REF] Baker | Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents[END_REF][START_REF] Rodrigues | Photophysics of the sunscreen ingredient menthyl anthranilate and its precursor methyl anthranilate: A bottom-up approach to photoprotection[END_REF][START_REF] Dahle | Induction of delayed mutations and chromosomal instability in fibroblasts after UVA-, UVB-, and X-radiation[END_REF] The naturally occurring UV-absorbing pigment in human skin, melanin, often provides insufficient protection against high levels of UV radiation and sunscreens are commonly used for additional photoprotection. [START_REF] Baker | Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents[END_REF] While the sunscreen industry has focused on the more energetic UV-B radiation, UV-A is very abundant at the Earth's surface and penetrates deeper into the skin than UV-B. [START_REF] Battie | New insights in photoaging, UVA induced damage and skin types[END_REF][START_REF] Brenner | The protective role of melanin against UV damage in human skin[END_REF] Moreover, the adverse effects of overexposure of human skin to UV-A are widely reported to include DNA mutation, pigmentation, suppression of acquired immunity and production of harmful reactive oxygen species in the skin; all of which facilitate carcinogenesis. [START_REF] Battie | New insights in photoaging, UVA induced damage and skin types[END_REF][START_REF] Matsumura | Molecular mechanisms of photocarcinogenesis[END_REF] Therefore, photoprotection against UV-A radiation is of crucial importance, but it is hindered by the sparsity of approved commercial UV-A filters, as well as their lack of photostability (e.g. avobenzone). [START_REF] Afonso | Photodegradation of avobenzone: stabilization effect of antioxidants[END_REF][START_REF] Mturi | Photostability of the sunscreening agent 4-tert-butyl-4′methoxydibenzoylmethane (avobenzone) in solvents of different polarity and proticity[END_REF] In addition to being photostable, and blending into a non-toxic and aesthetically pleasing sunscreen formulation, an effective UV-filter within a sunscreen formulation should be able to dissipate the excess energy absorbed safely (i.e. without generating photoproducts) and quickly, both to prevent harmful side reactions and to ensure the UV-filter is 'recycled' (i.e. that it returns from the electronic excited state to the electronic ground state without detriment to its molecular integrity), to maintain photoprotection. [START_REF] Baker | Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents[END_REF][START_REF] Rodrigues | Photophysics of sunscreen molecules in the gas phase: a stepwise approach towards understanding and developing next-generation sunscreens[END_REF] Recently, the photodynamics of sinapoyl malate (see Figure 1), a natural UV-absorbing compound in Arabidopsis thaliana, [START_REF] Fraser | The phenylpropanoid pathway in Arabidopsis[END_REF][START_REF] Dean | Plant sunscreens in the UV-B: ultraviolet spectroscopy of jet-cooled sinapoyl malate, sinapic acid, and sinapate ester derivatives[END_REF] have been reported and found to enable efficient dissipation of absorbed energy in the form of heat, through a proposed trans-cis isomerization across the allylic bond. [START_REF] Baker | Ultrafast photoprotecting sunscreens in natural plants[END_REF][START_REF] Luo | Ultrafast barrierless photoisomerization and strong ultraviolet absorption of photoproducts in plant sunscreens[END_REF] Furthermore, in methyl and ethyl sinapate (where R = CH3 and C2H5 in Figure 1a, respectively), photoexcitation to the lowest excited singlet state (S1) was found to lead to ultrafast trans-cis isomerization mediated via a conical intersection (CI) between the S1 state and the electronic ground state (S0), denoted S1/S0 CI. [START_REF] Baker | Ultrafast photoprotecting sunscreens in natural plants[END_REF][START_REF] Baker | Gas-Solution Phase Transient Absorption Study of the Plant Sunscreen Derivative Methyl Sinapate[END_REF][START_REF] Horbury | Investigating isomer specific photoprotection in a model plant sunscreen[END_REF][START_REF] Horbury | Elucidating nuclear motions in a plant sunscreen during photoisomerization through solvent viscosity effects[END_REF][START_REF] Zhao | Substitution Dependent Ultrafast Ultraviolet Energy Dissipation Mechanisms of Plant Sunscreens[END_REF] The same CI that allows a portion of the population to isomerize, leading to the cis-isomer, also facilitates repopulation of the original ground state of the trans-isomer. [START_REF] Baker | Gas-Solution Phase Transient Absorption Study of the Plant Sunscreen Derivative Methyl Sinapate[END_REF][START_REF] Horbury | Investigating isomer specific photoprotection in a model plant sunscreen[END_REF] The cis-isomer of related cinnamates has been reported to possess a lower UV absorption (i.e. lower extinction coefficient) and, importantly, significantly higher genotoxicity. [START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF][START_REF] Sharma | Different DNA damage response of cis and trans isomers of commonly used UV filter after the exposure on adult human liver stem cells and human lymphoblastoid cells[END_REF] One way to circumvent concerns related to the genotoxicity of potential photoexcitation by-products, such as geometric isomers, is to design UV-filters with identical functional groups around the allylic bond, so that isomerization results in no significant alteration of molecular structure and properties.

An example of such a symmetrically substituted UV-filter is diethyl sinapate (DES), the photodynamics of which were recently investigated. [START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF] The study showed not only that ground state recovery of DES is very fast, but also that the absorption spectrum of DES is spectrally red shifted (further into the UV-A region) relative to its monoester building block, ethyl sinapate. [START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF] We have synthesized a new class of symmetrically substituted sinapate esters [START_REF] Peyrot | Innovative Bio-Based Organic UV-A and Blue Light Filters from Meldrum's Acid[END_REF] in an attempt to further enhance UV-A absorption whilst maintaining the efficacy of ground state recovery.

In this work, we present a multi-pronged experimental and computational study to unravel the photodynamics of a novel class of Meldrum-based phenolic UV-A filters inspired by nature, specifically CMe and SMe, shown in Figure 1b andc, respectively. We utilize transient electronic and vibrational spectroscopies to gain unprecedented insight into the electronically excited and ground state dynamics of these UV-A filters. Importantly, we have adapted our experimental setup to ensure that our experiment models the conditions these UV-A filters are found in commercial formulations (dissolved in an emollient) as closely as possible; we have deposited the bulk solution of these UV-A filters (dissolved in an emollient) on a synthetic skin mimic to model the application environment of the sunscreen. This will enable us to discern if the skin surface perturbs the dynamics of the UV-filters. As detailed further in section A of the Supporting Information (SI), we have supplemented these ultrafast experiments with ultraslow experiments, ascertaining the long-term photostability of these UV-filters when exposed to radiation from a solar simulator, as well assessing their endocrine disruption and antioxidant properties. To complement these experiments and to better understand the dynamics of CMe and SMe, we employ high-level calculations. These reveal a new relaxation mechanism in these plant-derived UV filters involving a twisted intramolecular charge transfer state that allows the photoexcited molecule to return to its electronic ground state with exceptionally high efficiency.

RESULTS

Experimental Findings

Steady-state UV-vis absorption spectra were obtained for separate solutions of CMe and SMe in the industry standard emollient caprylic capric triglyceride (CCT) and in ethanol. As shown in Figure 2 and SI section B, the absorption maxima (λmax) are located at 362 nm for CMe in CCT (denoted CMe/CCT, similarly for SMe, and in all other solutions henceforth) and 375 nm for CMe/ethanol. In the case of SMe/CCT and SMe/ethanol, λmax is located at 396 nm and 407 nm, respectively. The molar extinction coefficient of CMe and SMe at their λmax are 29,877 and 32,105 mol -1 dm 3 cm -1 , respectively, which is comparable to that of the globally most common UV-A filter, avobenzone, which has a reported molar extinction coefficient of 34,140 mol -1 dm 3 cm -1 . [START_REF] Shaath | Ultraviolet filters[END_REF] We investigated the long-term photostability of CMe and SMe in CCT, as shown in Figure 2, by collecting UV-vis spectra at various time-intervals after irradiation with a solar simulator. These measurements revealed that CMe and SMe in CCT experience only a minor reduction in absorbance of less than 10% over an irradiation period of two hours. Equivalent photostability tests could not be performed for CMe/CCT or SMe/CCT deposited on synthetic skin mimic (VITRO-CORNEUM®, VC) due to excessive scattering from the sample precluding any reliable measurements. For the transient electronic absorption spectroscopy (TEAS) presented in the results section, we have modified our experimental setup by increasing the pump-probe incidence angle to minimize this scattering and use off-axis parabolic mirrors to enhance collection of scattered transmitted light (see SI, section A). In addition to photostability tests, the critical wavelengths (CW) of CMe and SMe were determined from their corresponding UV-vis spectra; these values are shown as a vertical dashed lines in Figure 2a,b. The CW of a UV-filter is defined as the wavelength at which the integrated area under the spectral absorbance curve reaches 90% of the total area between 290 and 400 nm. [START_REF] Diffey | A method for broad spectrum classification of sunscreens[END_REF] The concept of CW in UV-A protection has been widely reported in previous literature. [START_REF] Diffey | A method for broad spectrum classification of sunscreens[END_REF][START_REF] Wang | Comparison of ultraviolet A light protection standards in the United States and European Union through in vitro measurements of commercially available sunscreens[END_REF][START_REF] Diffey | The FDA final rule on labeling and effectiveness testing of sunscreens: Too little, too late?[END_REF] To be labelled as broadband spectrum protection in the USA, a sunscreen must have a CW of at least 370 nm. The CW of CMe/CCT and CMe/ethanol were determined to be 384 nm and 395 nm, respectively (SI Figure S2); for SMe/CCT and SMe/ethanol, the CW takes values of 395 nm and 396 nm, respectively (cf. 378 nm for avobenzone in ethanol [START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF] ). Furthermore, endocrine disruption analysis was conducted for SMe and reported in the SI, section D, to supplement those already reported for CMe; [START_REF] Peyrot | Innovative Bio-Based Organic UV-A and Blue Light Filters from Meldrum's Acid[END_REF] both molecules are found to be innocuous in this sense.

To elucidate the origin of the photostability of CMe and SMe, we investigated their ultrafast (femto-to pico-seconds) photodynamics with TEAS. A full description of our experimental setup and procedures are detailed in the SI, section A. TEAS measurements obtained for separate solutions of CMe/CCT, SMe/CCT, CMe/ethanol and SMe/ethanol are presented in the SI, in are, in all environments (CCT, ethanol and VC), dominated by four features: first, there is a negative feature centred at ~362 nm for CMe and ~396 nm for SMe, attributed to a ground state bleach (GSB) through comparison with the steady-state UV-vis absorption spectra (see Figure 2a,b); a second, negative feature centred at ~450 nm and ~475 nm in CMe and SMe, respectively, is attributed to stimulated emission (SE) between the S1 (1 1 ππ*) state and the S0 state, based on our electronic structure calculations (discussed below) and the emission spectra presented in SI section F. We note, for the purpose of the discussion below, that the SE feature observed for CMe appears to decay back to baseline within 1 ps, while that of SMe persists up to 5 ps. Thirdly, there is an intense positive feature attributed to excited state absorption (ESA) of the S1 (1 1 ππ*) state, centred at ~365 nm in both CMe and SMe; finally, close examination of the TEA spectra of both molecules reveals a second ESA centred at ~420 nm, which decays on a similar timescale to the GSB and is more clearly visible in the evolution associated difference spectra (EADS) in Figure 3 and SI Figure S4. We suggest that this ESA feature results from the formation of vibrationally hot electronic ground state following internal conversion from S1 to S0 mediated by the S1/S0 CI. SMe VC/CCT photoexcited at 396 nm. The TEA spectra are presented as false color maps in panel (iii) and (iv), respectively. The same data is presented as a line plots of mOD vs probe wavelength at selected pump-probe time delays in panels (v) and (vi). The top panels (i) and (ii) show transients (raw data as symbol and fits as solid lines) at selected probe wavelengths. The bottom panels (vii) and (viii) show the EADS produced by the fitting procedure (see main text for details), EADS4 is multiplied by three as a visual aid. The masked region of the TEA spectra, line plots and EADS of CMe VC/CCT and SMe VC/CCT corresponds to the wavelengths for which excess scatter from the pump pulse (caused by the uneven surface of VC) interferes with data subtraction and renders the data void.

) EADS 1 ,  FC EADS 2 ,  IC EADS 3 ,  Vib EADS 4 (X3),  Fl (a) CMe VC/CCT (i) (iii) (v) (vii)
To extract kinetic information from the TEA spectra, we carried out global fitting employing a sequential (A

 1 → B  2 → C  3
→ D…) kinetic model using the Glotaran software package. [START_REF] Snellenburg | Glotaran: A Javabased graphical user interface for the R package TIMP[END_REF][START_REF] Mullen | TIMP: an R package for modeling multi-way spectroscopic measurements[END_REF] The extracted time-constants τn (where n denotes the dynamical process associated with each timeconstant) are shown in Table 1. The errors quoted for these fits are those produced by the fitting software; however, the quality of each fit is best evaluated by analysis of the associated residuals shown in SI section G (Figure S7). The solvent only time-zero responses, which correspond to our instrument response, are shown in Figure S8 discussed in SI section H. [a] dynamics simulations of 70 trajectories.

[b] dynamics of 10 trajectories.

[c] first-order perturbation theory. FC corresponds to the time to relax from the Franck-Condon (FC) to the twisted CT minimum, IC is the S1/S0 internal conversion time, Vib is the time for vibrational cooling in the ground state, and Fl is the fluorescence time for a minor fraction of the population that does not undergo IC. TEA spectra timeconstants under other conditions are given in SI, Table S1.

Complementary transient vibrational absorption spectroscopy (TVAS) measurements, i.e. UVpump/infrared(IR)-probe, were obtained for CMe and SMe in order to probe changes in vibrational state on the S0 electronic surface following internal conversion from the S1 state. Due to the strong IR absorption of CCT, TVAS measurements were taken in ethanol, which we have found from TEAS measurements presented in SI section E to be comparable to a CCT environment. The transient vibrational absorption (TVA) spectra for CMe and SMe are reported in Figure 4 (see SI Figure S9 for additional TVA spectra and section I for discussion). The strongest GSB features observed in the TVA spectra of CMe and SMe were fitted with tri-exponential (CMe) and biexponential (SMe) functions, shown in Figure 4, yielding the time-constants, shown in Table 1.

The fit of the GSB feature at 1510 cm -1 for CMe and 1558 cm -1 for SMe, as well as the GSB feature at 1714 cm -1 (reported in SI Figure S9) for both UV-filters are reported in SI Table S2. data as open circles and fit as solid red line) of signals at selected wavenumber. For CMe/ethanol the normalized integration of the 1558 cm -1 GSB signal was fitted with a tri-exponential function, while the normalized integration of the 1505 cm -1 GSB signal was fitted with a bi-exponential function for SMe/ethanol. Attempt to fit CMe/ethanol data with a bi-exponential function returned a poor fit. Frequency calculations (see SI section J) suggest the vibrational feature probed is the allylic C=C stretching at 1558 cm -1 and aromatic C-H bending and C=C stretching at 1505 cm -1 .

Computational Studies of CMe and SMe

In interpreting experimental results, we have made use of electronic structure calculations (details in SI section J) carried out for CMe and SMe using an implicit solvation model for ethanol.

The inclusion of explicit solvation does not have significant effect on the vertical excitations, although it slightly stabilizes the lowest ππ* states and destabilizes the lowest nπ* state (see Tables S3 andS4 in the SI). Optimized geometries are shown in SI Figure S10, along with the initial excitation in Figures S11 andS12. In addition, molecular orbital characterization for the singlet states is shown in Figure S13 and S14. The calculated vertical excitations presented in Table 2 show that photoexcitation at λmax of CMe and SMe corresponds to population of the bright S1 state

(1 1 ππ*). For CMe, this state has a large oscillator strength and lies below the 1 nπ* state (S3 state) by 0.9 eV, according to the TD-ωB97XD and CASPT2 results (see SI Table S3). This energy difference is slightly greater for SMe (1.1 eV) due to stabilization of the ππ* states arising from the electron donor methoxy groups.

Following the vertical excitation, the optimization of the S1 (1 1 ππ*), S2 (2 1 ππ*), and S3 (1 1 nπ*)

states shows that the 1 1 ππ*state is largely stabilized, lying below the nπ* state by ~1.8 eV for both molecules (see the adiabatic energies in Table 2). This large energy difference between the states hinders any possibility of internal conversion to a dark 1 nπ* state, previously reported as a competitive decay channel for other cinnamate and sinapate derivatives. [START_REF] Dean | Plant sunscreens in the UV-B: ultraviolet spectroscopy of jet-cooled sinapoyl malate, sinapic acid, and sinapate ester derivatives[END_REF][START_REF] Tan | Excited state dynamics of Photoactive Yellow Protein chromophores elucidated by high-resolution spectroscopy and ab initio calculations[END_REF][START_REF] Kinoshita | The direct observation of the doorway 1nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens[END_REF] The strong stabilization of the S1 state occurs through twisting around the C2-C3 (see Figure 1) allylic bond towards a twisted intramolecular charge transfer (TICT) minimum, S1-TICT, at ϕ ~ 90° twist (state characterization is given in SI Table S5; ϕ is the C1'-C2-C3-C4 dihedral angle). The excited-state topography of SMe and CMe revealed by the theoretical approach is schematically shown in Figure 5. While CMe shows a barrierless S1 relaxation towards the S1-TICT twisted minimum, in SMe the equivalent relaxation leads to a flat region on the potential energy surface (PES), with a locally excited minimum (S1-LE) and partially twisted geometry (ϕ ~ 40°).

The barrier between the TICT and LE minima is less than 0.1 eV (see SI Figure S15 andS16).

Near the 90° twisted geometry, a S1/S0 minimum energy crossing point was found 0.2 eV (at TD-DFT) above the S1-TICT minimum for both CMe and SMe (SI Figure S16). This up-hill intersection was determined for both molecules using TD-DFT and further confirmed by CASPT2//CASSCF calculations for CMe, which provided a 0.37 eV barrier. (a) (b)

 FC  IC  Vib  FC  IC  Vib
To verify the time evolution of the molecules along this pathway, we performed excited-state dynamics simulations by propagating an ensemble of trajectories using TD-DFT (ωB97XD/cc-pVDZ) in implicit ethanol until S1 reached a crossing with S0 (see SI section J-iii). The time to reach the crossing point for each trajectory was assumed to correspond to the internal conversion (IC) time. For CMe, all trajectories undergo an ultrafast twist around the allylic bond, which drives it towards the intersection with the ground state. Monitoring the dynamics starting from the vertically excited-state, we have observed that the S1/S0 energy gap quickly drops to ~0.5 eV with a mean time of 334 ± 12 fs, when twisted geometries near 90° are reached (SI Figure S19), in good agreement with the experimental FC (see Table 1). After this gap closure, the molecule requires a further 300 fs to reach the S1/S0 intersection which, as we mentioned, is energetically above the S1-TICT minimum. During this time, the S1/S0 energy gap remains small (<0.5 eV) and we can observe oscillations in bond lengths, ring puckering and pyramidalization of the C3 atom (see Figure 1). The IC follows an exponential decay pattern with a mean time of 628 ± 56 fs, in line with the experimentally obtained IC (see Table 1). For SMe, only a few trajectories with exploratory character and low statistical significance were run. For this molecule, the dynamics evolution is similar, although it is elongated to ~600 fs due to the flat region around the S1-LE minimum. Like in CMe, IC takes a further 300 fs, occurring at ~940 fs.

DISCUSSION

We now discuss the implications of our results with regards to photoprotection, drawing on the different aspect of the experimental and computational studies. The presented UV-vis spectra, photostability and CW tests reveal that CMe and SMe are photostable UV-A absorbers. In addition, previous studies have reported not only promising antioxidant properties for this class of molecules, [START_REF] Peyrot | Innovative Bio-Based Organic UV-A and Blue Light Filters from Meldrum's Acid[END_REF] but also lack of endocrine disruption effect for the CMe. [START_REF] Peyrot | Innovative Bio-Based Organic UV-A and Blue Light Filters from Meldrum's Acid[END_REF] Endocrine disruption analysis for SMe highlighted the same innocuousness (see SI section D). Together, these observations make CMe and SMe promising candidates for inclusion in commercial formulations, [START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF] although further analysis of their toxicity is needed. With regards to their ultrafast photodynamics, which should allow for fast and efficient dissipation of excess energy, we draw on our experimental results, calculations, and previous related studies to assign dynamical processes to extracted time-constants and to discuss the implications of our findings.

Previous studies on related systems have shown that, following initial photoexcitation to the 1 1 ππ* state, these molecules tend to undergo rapid geometry and vibrational relaxation out of the Franck-Condon region, usually evidenced by slight changes in the EADS. [START_REF] Baker | Ultrafast photoprotecting sunscreens in natural plants[END_REF][START_REF] Horbury | Investigating isomer specific photoprotection in a model plant sunscreen[END_REF][START_REF] Horbury | Towards symmetry driven and nature inspired UV filter design[END_REF] While we might expect CMe and SMe to undergo a similar dynamical process, the EADS associated with FC (EADS1) is significantly different from EADS2 (see Figure 3). This marked difference suggests either a change of state or excited-state population migration to a drastically different region of the PES. Indeed, our calculations reveal that the relaxation of the initially excited 1 1 ππ* state of both

CMe and SMe follows a large energy stabilization alongside a marked geometrical change. As indicated in Figure 5, the 1 1 ππ* state evolves into a global minimum with a geometry twisted ~90°

around the allylic double bond and large charge-transfer character prior to reaching a CI with the ground state. TICT processes have been previously reported for systems in which electron withdrawing and donating substituents are positioned para to each other (i.e. position 1 and 4 on the benzene ring), lending further support to our hypothesis. [START_REF] Rodrigues | Substituent position effects on sunscreen photodynamics: A closer look at methyl anthranilate[END_REF][START_REF] Weersink | Complexes of Methyl 4-(N, N-Dimethylamino) benzoate: Spectroscopy and Dynamics of the Charge Transfer State[END_REF][START_REF] Howell | Fluorescence of jet-cooled dimethylamino benzonitrile, its aggregates and solvated complexes[END_REF] Hence, we assign FC for CMe and SMe (see Table 1), to an ultrafast relaxation into the TICT minimum (S1-TICT). Once this minimum is reached, we propose that IC takes place from the TICT to the ground state within the timescale of IC. These assignments of FC and IC are confirmed by the results of our excited-state dynamic simulations, which yield times for movement out of the Franck-Condon region that are in excellent agreement with experimental FC value (see Table 1).

In addition, the time constants associated with the recovery of GSB features in the TVA spectra effectively correspond to the rate of vibrational cooling of the vibrationally hot S0, as previously described for other systems. [START_REF] Baker | Probing the ultrafast energy dissipation mechanism of the sunscreen oxybenzone after UVA irradiation[END_REF][START_REF] Roberts | On the participation of photoinduced N-H bond fission in aqueous adenine at 266 and 220 nm: a combined ultrafast transient electronic and vibrational absorption spectroscopy study[END_REF] The first time-constant (~1 ps) extracted from the fit to each GSB feature of the TVA spectra in CMe, as well as the GSB feature of SMe at 1714 cm -1 , encompasses several initial processes, including solvent heating and any overlapping features. Consequently, we refrain from conclusive assignment of this time constant in these cases. However, Vib for CMe and SMe from their TVA spectra match the equivalent Vib values in TEA spectra in ethanol (see Table 1): Vib ~8 ps observed for CMe in TVA spectra and TEA spectra, and Vib ~5 ps in TVA spectra and ~4 ps in TEA spectra for SMe. This agreement confirms our assignment of Vib for CMe and SMe in TEA spectra (in both ethanol and CCT) to vibrational cooling within their respective S0 states.

Finally, the remaining time-constant extracted from the TVA spectra, Fl, is associated with the residual GSB feature observable in the EADS of both CMe and SMe. This feature persists beyond the 2 ns temporal window of our experiments, with < 1% of the feature's intensity remaining (see SI Figure S20). The linear power dependence observed for these measurements (see SI section L, Figure S21) suggests that it does not correspond to radical formation, which has previously been attributed to a two-photon ionization mechanism. [START_REF] Baker | Ultrafast photoprotecting sunscreens in natural plants[END_REF][START_REF] Horbury | Photoisomerization of ethyl ferulate: A solution phase transient absorption study[END_REF] We address this incomplete recovery of the ground state by once again considering the results of our electronic structure calculations: as discussed, the only difference between the potential energy profiles of CMe and SMe is the partially-twisted LE minimum in the S1 state of SMe. Although this minimum has a small energy barrier (<0.1 eV), we show in SI section J-iv that a small fraction of the SMe population could be trapped in the S1-LE minimum long enough to fluoresce at 448 nm emission, within 5.7 ns (in reasonable agreement with the TEAS result in ethanol, 475 nm in > 2 ns). We note, however, that the absence of a SE feature beyond 5 ps and up to 2 ns in the TEA spectra could simply reflect the small amount of trapped population in the S1-LE state, leading to a weak SE that is within the signal-to-noise of the TEA spectra. For CMe, although we do not observe S1-LE minimum on the PES, a small fraction of the population could be trapped within the shallow TICT minimum or some other state not captured by our calculation, which could explain the incomplete recovery of the GSB feature in EADS4, clearly visible in the 2 ns TEA spectrum in the SI (Figure S20). An alternative relaxation pathway for the long-lived species (represented by Fl) could be intersystem crossing (ISC) from the S1 to triplet state. However, we are unable to confirm or rule out either mechanism; the weak fluorescence emission from CMe and SMe has precluded us from ascertaining any reasonable fluorescence lifetime measurements. Independently of the Fl assignment, it is crucial to note that additional TEAS measurements, NMR and HPLC measurements presented in SI Figure S22, S23 and S24 lead us to confidently conclude that the long-lived Fl is not likely to be the result of interaction with the skin mimic, nor is it associated with the formation of any potentially harmful photoproduct in either CMe or SMe.

In summary, combining ultrafast transient absorption spectroscopy, excited-state calculations, and steady-state studies, we have provided unprecedented insight into the excited-state photodynamics of CMe and SMe in closer-to-real environments (i.e. in industrial grade emollient and on skin mimic surface, providing more than just conventional solvent-chromophore interaction). Following photoexcitation of CMe and SMe to their respective S1 states (in either bulk ethanol, CCT, or deposited on a skin mimic), our results reveal fast relaxation involving TICT followed by IC to the S0 state and vibrational cooling on the ground state surface. Finally, we note a mild incomplete ground state recovery within 2 ns, which we assigned to excited-state population trapped at a locally excited minimum. Steady-state studies revealed that these molecules are highly photostable over prolonged irradiation, surpassing the most widely used UV-A filter on the market (avobenzone). These studies also demonstrate that these filters do not produce any observable photoproducts after two hours of simulated solar irradiation (see SI section M). Importantly, our results showed that the dynamics of CMe and SMe are, at best, mildly influenced by environment following our studies in ethanol, emollient and on a skin mimic. Allied to the strong UV-A protection they afford and the favorable antioxidant and endocrine disruption tests, [START_REF] Peyrot | Innovative Bio-Based Organic UV-A and Blue Light Filters from Meldrum's Acid[END_REF] CMe and SMe present as highly promising candidates for inclusion in commercial sunscreen formulations.

Given the importance of structure-dynamics-function relationships in a multitude of fields, including the present studies linked to sunscreen science, our work clearly demonstrates that the dynamics of CMe and SMe UV-filters are only mildly influenced by surrounding environment (solution versus skin mimic). Coupled to the demonstration that symmetrical substitution across the allylic bond circumvents the formation of potentially toxic geometric isomer photoproducts, our nature-inspired UV-filters offer a promising avenue of photon-to-molecule heat generation.

This has ramifications, not just to researchers within the sunscreen industry, but researchers whose work targets, more broadly, photon-to-molecule energy conversion such as light capturing for solar fuels, light driven catalyst for fine chemicals and fluorophores with environment specific emission for medical imaging.

Figure 1 .

 1 Figure 1. Chemical structure of (a) plant UV-filter in Arabidopsis thaliana, sinapoyl malate (SM)

Figure 2 .

 2 Figure 2. Photostability of (a) CMe and (b) SMe in CCT recorded at different intervals during

Figure

  Figure S4 and S5, and discussed in SI section E. Presented in Figure 3 are the TEAS measurements

Figure 3 .

 3 Figure 3. TEA spectra obtained for 20 mM (a) CMe VC/CCT photoexcited at 362 nm, and (b)

Figure 4 .

 4 Figure 4. TVA spectra obtained for 30 mM solution of (a) CMe/ethanol following 375 nm

Figure 5 .

 5 Figure 5. Topography of the PES for CMe (left) and SMe (right). Minima and state intersections

Table 1 .

 1 Summary of the time-constants and associated errors extracted from data collected for CMe and SMe in CCT, VC/CCT and ethanol with TEAS, TVAS, and theory in implicit ethanol.

				TEAS		TVAS	Theory
			CCT	VC/CCT	Ethanol	Ethanol	Implicit
			(1 mM)	(20 mM)	(1 mM)	(30 mM)	ethanol
		FC / fs	220 ± 40	190 ± 40	190 ± 40	1240 ± 20	334 ± 12 [a]
		IC / fs	460 ± 40	450 ± 40	450 ± 40	-	628 ± 56 [a]
	CMe	Vib / ps 10.02 ± 0.22 6.92 ± 0.26 7.78 ± 0.13 7.44 ± 0.61 Fl / ns ->2 ->1.5	--
		FC / fs	300 ± 40	280 ± 40	350 ± 40	-	600 [b]
	SMe	IC / ps Vib / ps	1.15 ± 0.04 8.26 ± 0.04	1.21 ± 0.05 0.79 ± 0.04 8.69 ± 0.43 4.26 ± 0.09	-5.06 ± 0.08	0.94 [b] -
		Fl / ns	>2	>2	>2	>1.5	5.7 [c]

Table 2 .

 2 Vertical absorption energy into Sn (ΔEvert), vertical emission energy (ΔEem), and adiabatic energy (ΔEadiab) for CMe and SMe, calculated with TD-DFT (B97XD/aug-cc-pVDZ//B97XD/cc-pVDZ; PCM (ethanol)). Oscillator strengths are given in the parentheses.

				ΔEvert / eV	ΔEem / eV	ΔEad / eV
		S1	ππ*	3.83 (0.8765)	ππ* 0.79 (0.0000)	ππ* 2.51 (0.0000)
				3.31 [a]
	CMe	S2	ππ*	4.62 (0.0074)	nπ* 3.14 (0.0321)	ππ* 4.25 (0.0511)
		S3	nπ*	4.70 (0.0038)	ππ* 3.64 (0.0244)	nπ* 4.30 (0.0160)
		S1-LE	ππ*	3.57 (0.6662)	ππ* 2.77 (0.5281)	ππ* 3.18 (0.5281)
				3.05 [a]
	SMe	S1-TICT		--	ππ* 0.82 (0.0000)	ππ* 2.49 (0.0000)
		S2	ππ*	3.86 (0.0183)	ππ* 2.76 (0.0168) [b]	NO [c]
		S3	nπ*	4.68 (0.0019)	nπ* 3.14 (0.0968) [b] nπ* 4.29 (0.0000)

[a] Experimental values in ethanol. [b] Relative to S1-TICT minimum. [c] NO: Not obtained; the state optimizes to S1-FC.

University of Warwick for PhD studentship through the Chancellor Scholarship. M.D.H thanks the Leverhulme Trust for postdoctoral funding. M.T do C, J.M.T, and M.B thank the project Equip@Meso (ANR-10-EQPX-29-01) funded by the French Government "Investissements d'Avenir" program for the computational resources. B.R, C.P, M.M.M and F.A thank the Agence Nationale de la Recherche (grant number ANR-17-CE07-0046), as well as the Grand Reims, Conseil Départemental de la Marne, and the Grand Est region for financial support. V.G.S thanks the Royal Society and Leverhulme Trust for a Royal Society Leverhulme Trust Senior Research Fellowship. The authors thank Nikola Chmel, Abbigail Whittock and Michael Staniforth for their help and useful discussion.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge in PDF.

Experimental methods, photostability measurements of CMe and SMe in ethanol, critical wavelength of CMe and SMe in ethanol, endocrine disruption measurements of SMe, additional TEAS measurements of CMe/CCT, SMe/CCT, CMe/ethanol and SMe/ethanol at 1 mM concentration, emission spectra of CMe and SMe in ethanol, residuals between fits returned by sequential fit and raw data for CMe and SMe in CCT and VC/CCT, instrument response, TVAS experimental setup and additional data, pump power dependence study, TEAS measurements of CMe/CCT and SMe/CCT at higher concentration, 1 H NMR of pre-and post-irradiated CMe and SMe, HPLC chromatogram of pre-and post-irradiated CMe and SMe on VC, kinetic fits.

Computational details: further description of methodology, vertical and adiabatic energies at different levels of theory, optimized structures, representative trajectories, analysis of fluorescence in SMe.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

The authors thank the support of the FetOpen grant BoostCrop (Grant agreement 828753). The authors would like to thank the Warwick Centre for Ultrafast Spectroscopy (WCUS) for the use of transient vibrational absorption spectroscopy (TVAS) and Fluorolog 3. T.T.A thanks The