Toward an Enhancement of the Photoactivity of Multiphotochromic

Letter pubs.acs.org/JPCL

Toward an Enhancement of the Photoactivity of Multiphotochromic Dimers Using Plasmon Resonance: A Theoretical Study Arnaud Fihey,*,† Boris Le Guennic,‡ and Denis Jacquemin*,†,§ †

Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS no. 6230, BP 92208, Université de Nantes, 2, Rue de la Houssinière, 44322 Nantes, Cedex 3, France ‡ Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France § Institut Universitaire de France, 103, Boulevard Saint-Michel, F-75005 Paris Cedex 05, France S Supporting Information *

ABSTRACT: Building dimers of organic photochromic compounds paves the way to multifunctional switches, but such architectures often undergo partial photoreactivity only. Combining photochromism of molecules and plasmon resonance of gold nanoparticles (NPs) is known to affect the photochromism of monomers, yet the impact on multimers remains unknown. Here we propose a theoretical study of dimers of dithienylethenes by the mean of a hybrid calculation scheme (discrete-interaction model/quantum mechanics). We aim to assess how the optical properties of multiphotochromes are tuned by the influence of the plasmon resonances. We show that, for a typical chemisorption orientation on the NP, the absorption bands responsible for the photochromism are significantly enhanced for both the doubly open and mixed closed-open isomers of the dyad, hinting that plasmon resonance could be used to boost the generally poor photoactivity of dithienylethene dyads.

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ultiphotochromic organic compounds are a promising way of reaching more intelligent systems where, in theory, light stimuli could grant access to 2n isomers (n being the number of photochromic monomers).1 Among the photochromic families, dithienylethenes (DTEs)2 that undergo an electrocyclization when switching from a colorless open form to a colored closed form constitute the family that has been the most widely applied to obtain multiswitching functionalities.3,4 In practice, however, experimental works have crossed several difficulties in the course of conceiving DTE multimers: in particular, architectures with strongly coupled switches having potential emerging properties are typically limited by partial photochromism, that is, only one of all swicthes remains active in the multimer,1 making such compounds unappealing. Another step into multifunctionality is the grafting of molecular switches onto optically non-innocent metallic aggregates. An example of this strategy is the use of noble metal nanoparticles (NPs) to take advantage of their unique and versatile optical responses.5 They are indeed characterized by an intense absorption band in the visible, i.e., the localized surface plasmon resonance (LSPR), where the electrons of the conduction band collectively oscillate under light stimuli. The LSPR is highly tunable by the intrinsic features of the NP (shape, size) and its environment (dyes or other NPs in the vicinity).6,7 In the past decade, the so-called “molecular plasmonic” field where the photoactivity of a dye is combined to the properties brought by the LSPR has indeed been booming.8 For instance, Nishi et al. recently designed such a hybrid system where the state of the photochrome impacts the

position of the LSPR of a gold NP and allows an indirect reading of the state of the molecule.9 Reciprocally, the photoreactivity of DTE derivatives is known to be influenced by the presence of the NP.10 On the one hand, for the theoretical description of the LSPR, several classical electrodynamic-based models have been successfully applied during the last decades, e.g., the Mie theory,11 the discrete dipole approximation (DDA),12 or the finite-difference time-domain (FDTD) theory.13 On the other hand, quantum mechanics-based methods such as densityfunctional theory (DFT) and its time-dependent extension (TD-DFT) have been used to describe the optical properties of photochromic compounds, allowing a qualitative, if not quantitative, agreement with experiment on a wide range of photochromes.14,15 Bridging those two theoretical worlds, a few hybrid calculation schemes have emerged recently, where TD-DFT is combined to more simple models for the description of the response of the metal.16−20 For instance, to study the evolution of the optical properties of dyes on Ag NPs, Jensen and co-workers developed a hybrid calculation scheme called discrete interaction model/quantum mechanics (DIM/QM),21 where the LSPR of the metallic NP is described at a classical electrodynamic level (similarly to the DDA theory), and TD-DFT is used to access the absorption Received: June 23, 2015 Accepted: July 18, 2015

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The Journal of Physical Chemistry Letters Scheme 1. (a) Photochromic Compounds and (b) Gold NP under Studya

a

Experimentally, the multimer does not reach the closed−closed form in solution.25

exist where the DTE is covalently linked to the NP through a Au−S bond.26−28 Thus, for dimers, in addition to a first physisorption-like orientation where the molecule sits perpendicularly to the surface, two other conformations have been studied to gain insights into the optical behavior of chemisorption-like confomations. To this end, we relaxed the geometry of a “cluster-like” model (Au13-2) considering the different isomers of 2, where the dimer is linked to the NP through a −CH2−CH2−S− linker.29 The optimized Au13-2-oc structure is represented in Figure 2. The binding site of the sulfur atom on the gold layer is near a bridge-type position, consistently with previous DFT studies30 (see the SI). The relaxed systems are then kept in a nonbonding limit for the subsequent DIM/QM calculation by replacing the −CH2− CH2−S− linker by a capping hydrogen atom,29 so that the “chemisorbed-like” and “physisorbed-like” molecules in fact only differ by their respective orientations with respect to the NP. From the relaxed orientation and binding site of the DTE onto the Au13 cluster, we built two possible NP-dimer conformations, orientations 2 and 3 in Figure 2, where the molecule is only translated from one binding site to another equivalent one, but further from the center of the gold layer, which tunes the NP-DTE interactions. It has indeed been highlighted that the anchoring position (central or close to a discontinuity of the NP) is another important aspect guiding plasmon-excitation interactions.23,31 The representations of those three geometries in the case of 2-oo and 2-cc systems are given in the SI. Finally, for the consideration of the mixed open-closed isomer covalently grafted on the NP, the choice of the side undergoing the first closure is made following observations on gold-photochrome hybrid systems: the DTE closer to the gold surface is likely to be hindered in the closing process due to excessive electronic perturbation,32,33 and is therefore considered left open in the chemisorbed NP-2-oc form (see Figure 2). The theoretical spectrum of the two isomers of 1, isolated and anchored on the NP, is shown in Figure 1. Let us first focus

(polarizability) of the molecule (see the Supporting Information (SI) for more details). In addition to those static studies, there are also computational works describing dynamic processes like electron injection between a chromophore and a metallic support.22 In a previous work using the DIM/QM framework, one of us showed that the optical properties of a model closed DTE was drastically impacted once physisorbed on gold NPs.23 Several parameters were used to rationalize the plasmon-excitation interaction, but the orientation of the molecule with respect to the NP stood out as the most important.23 In the present Letter we (i) describe the theoretical optical properties of both open and closed forms of a typical DTE monomer (sketched in Scheme 1a) synthesized by Irie et al.,24 once physisorbed on a 5 nm gold NP (sketched in Scheme 1b); and (ii) consider a multiphotochromic DTE dimer in which the two units are coupled through a highly conjugated bridge, rendering the second ring closure impossible in solution (see Scheme 1a).25 For both the monomer and the dimer, we describe the absorption properties of all possible isomers, and we consider several possible orientations for the dimers grafted on the metal. Our goal here is to determine whether LSPR could possibly help making multiphotochromism more efficient, a concept never presented nor tested to date. The orientation of the molecule on the surface of the NP has been highlighted in a former study as the key parameter for increasing (or decreasing) the molecular absorption, through constructive (destructive) interactions.23 It is thus critical to consider plausible orientations in both the physisorbed and chemisorbed cases. A typical physisorption orientation is a conformation where the sulfur atoms interact weakly with the gold surface, leading to a system where the DTE lies perpendicularly on the center of the first layer (see Figure 1). This orientation is chosen for both the open and the closed monomers, 1, on the NP, and is also retained for physisorbed dimers. Furthermore, experimentally, several hybrid systems 3068

DOI: 10.1021/acs.jpclett.5b01333 J. Phys. Chem. Lett. 2015, 6, 3067−3073

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The Journal of Physical Chemistry Letters

Figure 1. Theoretical DIM/QM spectra for (a) 1-c and (b) 1-o isolated and on top of the gold NP, transition dipole moments corresponding to the main transition, and molecular orbitals involved.

corresponding to this excitation are sketched in Figure 1a. This dipole moment is found in the plane of the molecule, along the length of the system. The first intense absorption band of the open form 1-o is found at 313 nm (see Figure 1b): this S0 → S7 corresponds to a combination of HOMO−1 → LUMO+1 and HOMO → LUMO+2 contributions. As in 1-c, the corresponding transition dipole moment is in the plane of the molecule. In the open form case, it is possible to estimate the efficiency of the ring-closure by considering the topology of the virtual molecular orbitals populated by the electronic transition of interest. More precisely, the presence in this orbital of a bonding interaction between the two carbon atoms involved in the bond formation is required to attain ring-closure.34 For 1-o, both LUMO and LUMO+2 possess such a bonding interaction between the reactive carbon atoms, and are denoted as “photochromic” orbitals. The intense transition populating the LUMO+2 is then considered as the photochromic transition, in line with previous TD-DFT study on similar DTEs.34 As a technical note, we underline that even if pure DFT is used here, which leads to red-shifted transition energies compared to more accurate approaches,35,36 the main character of the key transitions composing the theoretical spectrum of both the open form (with a weak transition involving the LUMO and a more intense involving the LUMO+2) and the closed form (with an intense HOMO → LUMO in the visible) is accurately obtained (see comparisons with hybrid functionals in the SI). The isolated 2-oo presents an intense absorption band at 421 nm, and a less intense and less energetic one at 468 nm.

Figure 2. Geometries of NP-2-oc hybrid systems under study. For the sake of visibility, only the top of the gold NP is sketched.

on the isolated case. The optical features of the highly conjugated 1-c, taken alone, are rather simple as the calculated spectrum is dominated by an intense S0 → S1 transition in the visible, at 701 nm, that corresponds to a HOMO → LUMO excitation. This LUMO and the transition dipole moment 3069


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Figure 3. Theoretical DIM/QM spectra for (a) NP-2-oo, (b) NP-2-oc and (c) NP-2-cc systems for the three orientations, compared to their isolated counterpart. Transition dipole moments of the transition of interest are also sketched along with virtual molecular orbitals involved.

The latter corresponds to S0 → S3 and involves HOMO → LUMO+2, HOMO−1 → LUMO and HOMO−2 → LUMO contributions, whereas the former is a S0 → S7 transition, mixing HOMO → LUMO (mainly), HOMO−2 → LUMO and HOMO−2 → LUMO+2 excitations. The involved virtual molecular orbitals and the corresponding transition dipole moments are sketched in Figure 3a. As in the monomer, both LUMO and LUMO+2 are “photochromic orbitals” and both S0 → S3 and S0 → S7 transitions could induce the photoreaction of the first DTE. The spectrum of 2-oc is more difficult to rationalize as it combines “closed” and “open” features. One can find an intense transition in the visible that can be attributed to the closed unit, and transitions involving the open unit are to be found around 400 nm in a more dense spectroscopic region encompassing

several rather weak transitions. In this 350−450 nm energy range, one can find a set of transitions involving the LUMO+1 photochromic orbital, sketched in Figure 3b. Out of these transitions the most “efficient” ones are the S0 → S11 transition, populating the LUMO+1 in the greatest amount (73%) and peaking at 440 nm, and the S0 → S17 transition at 417 nm significantly involving the photochromic orbital (24%). However, both present tiny transition probabilities (f = 0.02 and f = 0.04, respectively): clearly, irradiating in this 350− 450 nm region is not sufficient to effectively trigger photochromism. This is well in line of the absence of a second ring closure that is observed experimentally.25 Finally, 2-cc absorbs at longer wavelengths, thanks to an intense HOMO → LUMO transition fully delocalized on the long dimer conjugation path (see Figure 3c). 3070


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to the gold layer and is partially aligned with the plasmon dipole, explaining both the increase in the intensity and the redshift. In Figure 3b, the absorption spectra of isolated and grafted 2-oc are compared, considering the three orientations. As for the 2-oo isomer, the resulting DIM/QM spectra are quite different depending on the orientation of the dyad. For this mixed open-closed dyad, let us focus on the evolution of transitions involving the open DTE unit, potentially allowing the second ring closure. As described above, the so-called photochromic transitions of the isolated 2-oc are located in the 350−450 nm region, where several weak transitions, with an oscillator strength not greater than 0.05, populates the LUMO +1. Given the proximity of these different transitions in this region, their low intensities, and their different dipole moment orientations, simple chemical intuition cannot be used to identify their corresponding bands in the DIM/QM spectra that are rather broad. However, two general observations can be made. On the one hand, in the physisorption orientation 1, the intensities of the absorption bands globally decrease, in the whole 350−650 nm range. On the other hand, in both orientations 2 and 3, two regions of the computed spectrum of 2-oc present clearly enhanced absorption intensities once under the influence of the NP. The first zone is found at ca. 475 nm and the second below 400 nm. The former corresponds to transitions populating mainly the closed DTE-localized LUMO, represented in Figure 3b, while the latter includes the transitions involving the photochromic LUMO+1. Irradiating in this second area would most probably increase the population of “photochromic states” which could, in turn, help reaching the second ring-closure that remains out-of-reach in solution. The intense low-energy S0 → S2 transition of 2-cc is globally decreased when the dyad is placed on top of the NP, and its behavior is very similar to the 1-c case discussed above, with a transition dipole moment perpendicular to the NP induced dipole moment. The strength of the destructive interaction between the NP and the dipoles of the molecule has an intensity depending of the orientation of the latter. The original band intensity is decreased by a factor of 5 for orientations 1 and 3, but a factor of 2 for orientation 2, as a consequence of the changes in the relative orientation of the molecule and NP dipole moments. The redshift of the hallmark band is more important in orientations 2 and 3 (140 and 160 nm, respectively) than in orientation 1 (25 nm). Concerning the shift of the key transitions, the results obtained for 2-oo and 2cc are similar: the perfectly perpendicular orientation 1 leads almost no modification of the position of the band, whereas for the two other orientations, the less perpendicular situation yields a redshift, very similar for both orientations as the tilt of the molecule on the surface is equivalent. The blueshift observed previously for the band of 1-o is not recovered, likely because the dimer is a bit further than the monomer from the surface, due to intrinsic geometical constraints. Overall, orientation 2, where the photochrome presents an orientation of a chemisorbed system, placed at the center of the gold first layer, is clearly the most advantageous, as it leads to optical properties always superior to the isolated case. Importantly, the absorption bands of interest for photochromism are more intense in both 2-oo and 2-oc after grafting. In this Letter we presented a theoretical description of the optical properties of a DTE dyad under the influence of the plasmon resonance of a 5 nm gold NP, using a hybrid calculation scheme (DIM/QM) that considers the molecule at

Let us now turn toward the impact of the grafting onto the NP. For the interested reader, a short description of the formalism describing the optical properties of the molecule in the DIM/QM scheme is given in the SI. To put it in a nutshell, and as explained in previous works,21,23 the interactions between the LSPR of the NP and the electronic excitation of the organic molecule are well-understood by considering the orientation of the molecular transition dipole moment with respect to the global induced dipole generated by the plasmon of the NP, that was observed in SERS and enhanced fluorescence.37,38 In our case, the dipole of the plasmon band always points to the exterior of the NP, perpendicularly to its surface.23 As an empirical general rule,21 a parallel orientation between the dipole moments of the plasmon and the transition moment of the dye leads to an increase in the absorption intensity of the compound while a perpendicular orientation yields the reverse effect. The shift in the wavelength of the transition follows the same kind of rules and is highly dependent on the orientation: the more the DIM and QM dipoles are aligned, the more the electronic transition is redshifted, a perfectly perpendicular situation ultimately nullifying the shift, or even inducing a slight blue-shift if the molecule is very close to the surface.39 Of course those qualitative rules should be viewed as simplifications of the physical behavior of the system, and they cannot explain all the differences in absorption profiles. Figure 1 compares the optical properties of both 1-o and 1-c physisorbed onto the NP and isolated.40 The optical properties of 1-c onto the NP are in line with previous results obtained for a model DTE:23 the intense S0 → S1, while peaking at the same wavelength with and without the NP, undergoes a destructive interaction and its intensity is dramatically reduced. As shown in Figure 1a, this destructive interaction is consistent with the orientation of the transition dipole moment of the S0 → S1 transition, which is parallel to the gold surface and perpendicular to the NP induced dipole moment. Although the transition dipole moments of the intense transitions in 1-o and 1-c possess similar orientations, a clear difference arises between the evolution of the open and closed DTE optical properties in the presence of the NP. If the absorption of 1-c is drastically impacted, the spectrum of 1-o retains its global shape. The decrease in the intensity of the S0 → S7 transition is limited, and accompanied by a small blueshift of 8 nm. These different evolutions can be rationalized by the weaker overlap between the molecular absorption and the LSPR band in 1-o than in 1-c. Indeed this LSPR peaks at 2.47 eV and is closer from the 1-c maximum (1.77 eV) than from its 1-o counterpart (3.97 eV). Figure 3a presents the absorption properties of the 2-oo dimer anchored on the NP. The intense band corresponding to a photochromic transition in the isolated dyad case is impacted distinctively depending on the orientation of the multiphotochrome. For orientation 1, the S0 → S7 transition is decreased by a factor of 4 compared to the isolated scenario, and roughly remains at the same wavelength. The transition dipole moment of this S0 → S7, represented in Figure 3a, is indeed parallel to the NP surface and perpendicular to the NP dipole moment. For both orientations 2 and 3, this transition undergoes a consequent redshift of 29 nm (from 421 to 450 nm). The band is still slightly decreased for orientation 3 but, in contrast, orientation 2 leads to a superior intensity than in the isolated 2-oo case. In fact, in those tilted conformations the S0 → S7 dipole moment is no more parallel 3071



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the TD-DFT level and the NP at the classical electrodynamic level. First, we studied a DTE monomer: we found a dramatic decrease of the absorption of the closed form in the visible, in agreement with previous studies, while the spectrum of the open form is far less perturbed due to the smaller spectral overlap with the LSPR. For the DTE dimer, in which the electronic coupling between the two photochromes is ensured by a highly conjugated linker, we considered orientations corresponding to potential chemisorption situations often used experimentally. Interestingly, for the mixed closed-open isomer, which is often a dead-end for photochromism in DTE multimers, the absorption spectra is more intense in the key regions when interacting with the NP. The previously weak photochromic transitions become more likely, hinting to a potential superior photoreactivity of multimers grafted onto NPs. Of course, this initial study deserves further extensions, e.g., the consideration of other metals, the investigation of the dynamics of the process, the use of more accurate functionals,41 etc. Nevertheless, it provides first insights into the potential interest of using NPs to enhance the activity of DTE multimers.

METHODS All DIM/QM spectra were obtained using the ADF software.42,43 The linear response calculations were conducted with the help of the AORESPONSE module, with a finite lifetime of 0.03037 au,44 and the BP86/DZP functional/basis set combination. Geometry optimizations of Au13-DTE-DTE systems were realized with fixed positions for the gold atoms, in vacuum, using the BP86 functional, with a LANL2DZ pseudopotential and a 6-31G(d) basis set, respectively for gold and light atoms (C, S, H, F). Geometries of the isolated molecules were relaxed with the same DFT method, and followed by frequency calculations. We used the Gaussian 09 package for this step.45 Excited states TD-DFT calculations were conducted in parallel with the DIM/QM calculations, at the same level of theory, to clarify the molecular orbital composition of the different absorption bands obtained with linear response. Additional details about the DIM/QM approach can be found in the SI. ASSOCIATED CONTENT

S Supporting Information *

Computational details for the DIM/QM method, CAM-B3LYP results for 1-o and 1-c, binding site on the Au13 cluster, conformations for 2-oo and 2-cc on the NP. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01333.

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REFERENCES

(1) Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Multiphotochromic Molecular Systems. Chem. Soc. Rev. 2015, 44, 3719−3759. (2) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 12174−12277. (3) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M. Full-Color Photochromism of a Fused Dithienylethene Trimer. J. Am. Chem. Soc. 2005, 127, 8922−8923. (4) Areephong, J.; Logtenberg, H.; Browne, W. R.; Feringa, B. L. Symmetric Six-Fold Arrays of Photo- and Electrochromic Dithienylethene Switches. Org. Lett. 2010, 12, 2132−2135. (5) de Heer, W. A. The Physics of Simple Metal Clusters: Experimental Aspects and Simple Models. Rev. Mod. Phys. 1993, 65, 611−676. (6) Morton, S. M.; Silverstein, D. W.; Jensen, L. Theoretical Studies of Plasmonics using Electronic Structure Methods. Chem. Rev. 2011, 111, 3962−3994. (7) Thomas, R.; Swathi, R. S. Organization of Metal Nanoparticles for Surface-Enhanced Spectroscopy: A Difference in Size Matters. J. Phys. Chem. C 2012, 116, 21982−21991. (8) Long, R.; Prezhdo, O. V. Instantaneous Generation of ChargeSeparated State on TiO2 Surface Sensitized with Plasmonic Nanoparticles. J. Am. Chem. Soc. 2014, 136, 4343−4354. (9) Nishi, H.; Asahi, T.; Kobatake, S. Light-Controllable Surface Plasmon Resonance Absorption of Gold Nanoparticles Covered with Photochromic Diarylethene Polymers. J. Phys. Chem. C 2009, 113, 17359−17366. (10) Nishi, H.; Asahi, T.; Kobatake, S. Plasmonic Enhancement of Gold Nanoparticles on Photocycloreversion Reaction of Diarylethene Derivatives Depending on Particle Size, Distance From the Particle Surface, and Irradiation Wavelength. Phys. Chem. Chem. Phys. 2012, 14, 4898−4905. (11) Mie, G. Beiträge zur Optik Trüber Medien, Speziell Kolloidaler Metallösungen. Ann. Phys. 1908, 330, 377−445. (12) Yang, W.; Schatz, G. C.; Van Duyne, R. P. Discrete Dipole Approximation for Calculating Extinction and Raman Intensities for Small Particles with Arbitrary Shapes. J. Chem. Phys. 1995, 103, 869− 875. (13) Bian, R. X.; Dunn, R. C.; Xie, X. S.; Leung, P. T. Single Molecule Emission Characteristics in Near-Field Microscopy. Phys. Rev. Lett. 1995, 75, 4772−4775. (14) Patel, P. D.; Masunov, A. E. Theoretical Study of Photochromic Compounds: Part 3. Prediction of Thermal Stability. J. Phys. Chem. C 2011, 115, 10292−10297. (15) Perrier, A.; Maurel, F.; Aubard, J. Theoretical Investigation of the Substituent Effect on the Electronic and Optical Properties of Photochromic Dithienylethene Derivatives. J. Photochem. Photobiol., A 2007, 189, 167−176. (16) Corni, S.; Tomasi, J. Enhanced Response Properties of a Chromophore Physisorbed on a Metal Particle. J. Chem. Phys. 2001, 114, 3739−3751. (17) Chen, H.; McMahon, J. M.; Ratner, M. A.; Schatz, G. C. Classical Electrodynamics Coupled to Quantum Mechanics for Calculation of Molecular Optical Properties: a RT-TDDFT/FDTD Approach. J. Phys. Chem. C 2010, 114, 14384−14392. (18) Mullin, J.; Valley, N.; Blaber, M. G.; Schatz, G. C. Combined Quantum Mechanics (TDDFT) and Classical Electrodynamics (Mie Theory) Methods for Calculating Surface Enhanced Raman and Hyper-Raman Spectra. J. Phys. Chem. A 2012, 116, 9574−9581. (19) Andreussi, O.; Biancardi, A.; Corni, S.; Mennucci, B. PlasmonControlled Light-Harvesting: Design Rules for Biohybrid Devices via Multiscale Modeling. Nano Lett. 2013, 13, 4475−4484. (20) Andreussi, O.; Caprasecca, S.; Cupellini, L.; Guarnetti-Prandi, I.; Guido, C. A.; Jurinovich, S.; Viani, L.; Mennucci, B. Plasmon Enhanced Light Harvesting: Multiscale Modeling of the FMO Protein Coupled with Gold Nanoparticles. J. Phys. Chem. A 2015, 119, 5197− 5206.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.F. thanks the European Research Council (ERC, Marches -278845) for supporting his postdoctoral grant. D.J. acknowledges the ERC and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches −278845) and a recrutement sur poste stratégique, respectively. This research used resources of CCIPL (Centre de Calcul Intensif des Pays de Loire) and a local Troy cluster. 3072


Toward an Enhancement of the Photoactivity of Multiphotochromic

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