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Photonastic materials present a directional and repeatable deformation of the material shape due to transduction from light energy to mechanical energy. Among these materials, light-responsive polymers, composed of photochromic molecules embedded in a polymer matrix, are of high interest. The present work aims at laying the foundation stone of the modeling of the photomechanical behavior of such systems by proposing a computational strategy that is able to investigate (i) the impact of the polymer matrix on the photochromic properties of a dithienylethene (DTE) switch and (ii) the impact of the photochromic reaction on the polymer environment. Contrary to previous approaches, the present model is able to propose a realistic arrangement of the photochrome embedded in the polymer film, thanks to the adaptation of the so-called "controlled-like polymerization algorithm" [Lemarchand, C. A.; J. Chem. Phys. 2019, 50, 224902]. Our strategy relies on molecular dynamics (MD) simulations and time-dependent density functional theory (DFT) calculations. Careful analysis of MD trajectories and comparison with simulations in solution have shown the rigidification of the DTE molecule due to the presence of the polymer chains, which hindered the interconversion between the DTE open-form isomers and can probably modify the photocyclization quantum yield. Besides, the UV-vis absorption properties of the DTE open-form isomers are more impacted by the polymer embedding than its closed-form counterpart. Concerning the impact of the photoreaction on the polymer matrix, the time evolution of the pressure tensor and of the atomic displacements in the matrix have shown that (i) the cyclization reaction has a negligible impact; (ii) the cycloreversion reaction induces a locally large and anisotropic pressure increase and leads to a collective displacement of the polymer matrix away from the reactive center; and (iii) the characteristic time scale associated with these coupled processes is below 1 ps. Therefore, the two processes involved in photonastic motions, namely, the photoreaction and the relaxation of the polymer matrix after the photoswitch structural change, cannot be decoupled.

A dithienylethene (DTE) photochromic compound functionalized by ureidopyrimidinone (UPy) quadruple hydrogen bonding blocks was synthesized by Takeshita and coworkers [Takeshita et al., Chem. Commun., 2005, 761] in order to form a light-responsive supramolecular self-assembling system. In solution, the formation of supramolecular assemblies was only observed for one DTE isomer, namely the closed-form isomer. To rationalize this experimental finding, with the help of Molecular Dynamics (MD) and (time-dependent) DFT calculations, the behaviour of open-form and closed-form monomers, dimers, hexamers and π-stacked dimers in solution is investigated. Our simulations show that, for the open-form oligomers, the progression of the supramolecular assembly is hindered due to (i) the possible formation of a very stable cyclic dimer for the open-form parallel isomer, (ii) the relative flexibility of the open-form oligomers compared to their closed-form counterparts, and (iii) the possible existence of π-stacked dimers that constitute bottlenecks blocking the progression of the supramolecular self-assembly.

International audience; The structural evolution as a function of temperature of a supramolecular copolymer assembly based on multiple hydrogen bonds has been investigated. Ureidopyrimidinone functionalized semi-crystalline poly(ethylene-co-butylene) (PEB-(L-UPy)2) has been characterized by means of in situ WAXS and SAXS experiments. The crystallinity and the nature of crystal forms are associated to the ethylene sequences. Special attention has been paid to the impact of the hydrogen bonding network on the structural arrangements, by comparison to the unfunctionalized PEB. The presence of crystalline domains of in-plane arrangement between neighboring UPy dimers is detected by WAXS up to 80 °C. The SAXS data reflect the presence of L-UPy aggregates stable on the whole temperature range. The correlation of these UPy-based aggregates is effective until the PEB crystal melting temperature (Tm). Beyond Tm, the increase of the macromolecular mobility reduce the number of domains in interaction and, in consequence, implies a decorrelation of the aggregates.

International audience; Time-resolved absorption spectroscopy measurements were applied to study the dynamics of photochromic 1,2-Bis(2,4dimethylthiophene-3-yl)perfluoro-cyclopentene (DMTPF) in chloroform including antiparallel conformer ring-closure reaction and parallel conformer photophysics. All characteristic times are given, discussed and compared to a previous publication concerning the close molecule substituted with phenyl rings. (Hamdi et al, pccp, 2016). Apart from the expected photocyclization process, a ring condensed by-product formation is observed and hypothesis concerning the origin of this by-product are presented.

International audience; A new donor–acceptor dyad composed of a BODIPY (4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C60 acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C60 dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C60 dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C60 dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor–acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C60 moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C60 dyad in order to assign the transient species in pump–probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

We report a QM (TD-DFT) and QM/QM' (ONIOM) study of the modulation of emission in a series of thiazolo[5,4,b]thieno[3,2-e]pyridine (TTP) derivatives [Huang et al., J. Mater. Chem. C, 2017, 14, 3456]. By computing the excitation energy transfer couplings and the Huang-Rhys (HR) factors, we rationalize the aggregation-caused quenching (ACQ) observed for the parent molecule and the crystallization-induced emission (CIE) observed for the derivatives presenting intra-molecular H-bonding. We also show that the CIE strategy relying on the rigidification of the arch-bridge-like stator should be considered with caution since it can promote the energy dissipation through vibrational motions.

International audience; Combining different molecular switching functions in a single molecule is a simple strategy to develop commutable molecules featuring more than two commutation states. The present study reports on two molecular systems consisting of two indolino-oxazolidine (Box) moieties connected to an aromatic bridge (phenyl or bithiophene) by ethylenic junctions. Such systems, referenced as BiBox, are expected to show up multiaddressable and multiresponsive behaviors. On one hand, the oxazolidine ring opening/closure of Box moieties can be addressed by chemical stimuli, and on the other hand, the trans-to-cis isomerization of the ethylenic junctions is induced by visible light irradiation (with a thermal back conversion). NMR and UV−visible spectroscopies allowed to characterize up to nine out of the ten theoretically expected commutation states as well as to measure the kinetics of the interconversions. Also, steady state fluorescence spectroscopy measurements highlighted the strong influence of the open/closed states of the Box moieties on their emission properties.

Un materiau photochrome possede la capacite de changer de couleur suite a une absorption de lumiere, et ce de maniere reversible. Cet article traite des molecules et materiaux photochromes depuis les aspects fondamentaux jusqu’aux applications industrielles. Apres avoir defini les concepts generaux, les grandes familles de photochromes, la quantification du photochromisme (determination des rendements quantiques), la comprehension et le controle des mecanismes elementaires sont abordes sous le prisme de la spectroscopie transitoire, des calculs de chimie quantique et de l’optimisation synthetique. Enfin diverses applications dans les domaines de l'industrie et de la recherche sont detaillees.

International audience; Excited state dynamics of constrained photochromic benzodithienylethenes were adressed by considering the bridging with polyether chains (from x=4 to 6 units) at the ortho and meta positions of the aryl group, noticed DTE-ox and DTE-mx, through time-resolved absorption spectroscopy supported with (TD)-DFT calculations. The photochromic parameters and geometrical structures of these series are discussed. A Novel photocyclization pathway via triplet state evidenced recently (Hamdi et al., Phys. Chem. Chem. Phys. 2016, 18, 28091-28100.) is largely dependent on the length and the position of the polyether chain. For the first time, by comparing the two series, we revealed, for the DTE-ox series, an interconversion not only in the ground state but also between the triplet states of the anti-parallel and parallel open form conformers.

Using a combination of advanced DFT/TDDFT calculations together with ultrafast and stationary spectroscopies we have investigated the photochemistry and cationic complexation ability of 1-pyridinio-benzimidazolate (PyB) and analogs substituted by 15-aza-5-crown (PyB-Aza) or dimethyl-amino groups (PyB-DiMe). Focusing on PyB-Aza, the first aim was to assess the competitive complexation of the imidazole bridge vs. the macrocycle. In acetonitrile, it was found by absorption and emission that the imidazole moiety binds efficiently through lateral electrostatic interaction of high charge density cations and especially Ca2+ to form a 1:1, metal:ligand (M:L) complex. Modulation of the complexation toward para substitution of the phenyl ring with a donor group is reported with values ranging from logK = 3.4 to 6.8. Complexation values are properly predicted by DFT calculations. From a photochemical point of view, for the same series, the trend is parallel to the rate of the photo-release process, found to be less than 200 femtosecond (fs), the fastest photorelease characteristic time reported so far. Unlike photoinduced charge transfer molecules linked with an aza-crown group, the mechanism appears simpler with no participation of loose complexes due to the macrocavity effect. Relaxation mechanisms after cation ejection are discussed as well. Finally, even if any photoinduced translocation of cation is reported for the PyB-Aza molecule between two complexation sites, a discussion about the use of betaine pyridinium as a molecular tool for the smart manipulation of cation systems is initiated.

Photodynamics and excited-state intramolecular proton transfer (ESIPT) of N-(3,5-ditert-butylsalicylidene)-4-aminopyridine (4P) and N-(3,5-ditert-butylsalicylidene)-2-aminopyridine (2P) are studied by steady-state and transient optical spectroscopy in solution and solid state (microcrystalline powder). These two compounds are representative of anils Classes A and B, respectively. The results confirm that in solution both compounds have a similar dynamic behavior: upon excitation at 390 nm, ESIPT leads to different cis-keto* fluorescent species which relax to the photochromic trans-keto photoproduct within a few tens of picoseconds. Furthermore, for the first time, picosecond data are recorded for the two classes of anils in the solid state: in 2P, two cis-keto* fluorescent excited state species (23 and 250 ps lifetime) are relaxing exclusively to the cis-keto ground state, whereas in 4P, a single cis-keto* fluorescent species is observed which leads to the trans-keto photoproduct in 250 ps via an intermediate metastable species.

The photoswitching and competitive processes of the referent photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) and a novel bridged analog DTE-m5 have been investigated by state-of-the-art TD-DFT calculations and ultrafast spectroscopy supported by advanced chemometric data treatments. Focusing on DTE, the overall deactivation pathway of both antiparallel (AP) and parallel (P) conformers of the open form (OF) (1:1 in solution) has been resolved and rationalized starting from the Franck-Condon (FC) region to the ground state recovery. For the photo-excited P conformer, after ultrafast relaxation (∼200 fs) towards the S1 relaxed state, an expected ISC occurred (55 ps) to produce a triplet state, 3P, the latter relaxing within 2.5 μs. Concerning the AP conformer, the photocyclization reaction is reported to proceed immediately (100 fs) starting from the FC region while the relaxed singlet state is populated in parallel. For the first time, we discovered that the latter state evolves through an unexpected ISC process (1 ps) giving rise to a second triplet state,3AP. For DTE-m5, by slightly constraining the molecule with the bridge, this triplet becomes reactive and participates in the formation of 10% of closed form (CF) probably through an adiabatic mechanism. Concerning the photoreversion, in accordance with the literature, we report on a two-step process, a 190 fs vibrational relaxation followed by a 6 ps ring-opening reaction. For the overall species at the singlet or triplet manifold, the use of advanced MCR-ALS allows us to obtain specific spectral signatures. This study is therefore a new step within the comprehension of DTE photochemistry.

Multivariate curve resolution-alternating least squares (MCR-ALS) is a widely used method to resolve multicomponent evolving chemical systems such as processes and reactions into pure contributions. This chapter presents the use of MCR-ALS to analyze time-resolved spectroscopy (TRS) data of molecular processes induced by light occurring in the second to femtosecond time domains. After a short introduction of the photochemical and photophysical processes and a presentation of the basics of TRS measurements, the chapter focuses on the application of MCR-ALS for the resolution of TRS data in order to obtain kinetic profiles and spectra of the species formed after light irradiation. Practical aspects covering data preprocessing, multiset analysis, or the implementation of kinetic constraints are presented. Three illustrative examples are discussed investigating the spectrodynamic of the photoinduced processes occurring at different timescales.

International audience; The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band ([similar]1300 cm−1) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee′ which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

International audience; State-of-the-art experimental and theoretical tools were used to investigate the gas-phase relaxation dynamics of various photoexcited photochromic dithienylethene molecules in situations where several relaxation channels are simultaneously at play. Unconstrained and constrained dynamics were addressed by considering unbridged and bridged molecules with a polyether bridge of various sizes (from 2 to 4 units). Time-resolved ultrafast ionization spectroscopy techniques were used to probe the dynamics. This revealed the existence of several relaxation pathways from the first excited state to the ground-state. Characteristic times were determined for each process. These channels compete at an early stage of the dynamics only when the initial wavepacket splits into two parts. A striking excited state wavepacket oscillation is observed in bridged molecules. A general reaction mechanism is proposed which rationalizes the carbon-carbon distance rule which is widely used as an empirical tool to predict the photoactivity of photochromic molecules in crystals.

The photophysical properties of a series of dithienylethenes, free or blocked in an ideal photoactive conformation by an alkyl bridge, have been investigated by stationary, ultrafast spectroscopy and state-of-the-art time-dependent density functional theory calculations. Thanks to the clear ultrafast transient signatures corroborating NMR results, we bring strong evidence that the unreactive parallel open form conformer has been efficiently removed by the chain. For the first time, the photophysics of this species, namely an internal conversion of 120 ps is highlighted. In contradiction to the main ideas in the literature, the photocyclization mechanism is rationalized by a direct photocyclization mechanism from the Franck–Condon region passing directly through a conical intersection within ≈100 fs (not few picoseconds) while a competitive mechanism occurs through the relaxed S1 state. Relaxation processes (fluorescence and internal conversion) originating from this relaxed state are sensitive to the length of the blocking chain. Both concomitant pathways are necessary to rationalize: (i) the inverse relationship between emission and cyclization quantum yields and (ii) the non-unity value of the latter for bridged compounds.

This work deals with the photophysics of novel pyridinium betaine based on 2-pyridin-1-yl-1H-benzimidazole (SBPa) substituted symmetrically by mono- (Th2SBPa) and bi-thiophene fragments (Th4SBPa). The study is based on a combination of steady-state, femtosecond transient absorption spectroscopic measurements supported by PCM-(TD)DFT calculations. It is found that the two step ICT process (S0 → S2 excitation followed by S2(CT) → S1(CT) internal conversion) occurring for the parent molecule remains unaffected for Th2SBPa while the situation is less clear for Th4SBPa. Actually, for both molecules, a new decay route involving the π-electron system localized in thiophenic groups is responsible for the production of triplet states. Involvement of this new route in the parallel production of S1(CT) is strongly suspected.

International audience; The nonlinear properties and the photophysical behavior of two π-conjugated chromophores that incorporate an electron-deficient pyrimidine core (A) and γ-methylenepyrans as terminal donor (D) groups have been thoroughly investigated. Both dipolar and quadrupolar branching strategies are explored and rationalized on the basis of the Frenkel exciton model. Even though a cooperative effect is clearly observed if the dimensionality is increased, the nonlinear optical (NLO) response of this series is moderate if one considers the nature of the D/A couple and the size of the chromophores (as measured by the number of π electrons). This effect was attributed to a disruption in the electronic conjugation within the dyes' scaffold for which the geometry deviates from planarity owing to a noticeable twisting of the pyranylidene end-groups. This latter structural parameter also has a strong influence on the excited-state dynamics, which leads to a very efficient fluorescence quenching.

The understanding of the intimate electronic processes in photochromes is essential to optimize the properties of light-controllable devices. For one of the most studied classes of molecular switches, namely, dithienylethenes, the relative efficiencies of the normal and inverse structures remained puzzling. Indeed, despite a larger ratio of the active antiparallel conformers for the latter, the quantum yields of cyclization of inverse dithienylethenes do not exceed those of its normal counterpart. In the present contribution, we provide the first explanation of this experimental outcome using multireference ab initio quantum chemistry. We demonstrate the existence of a fluorescent intermediate on the S1 state of the inverse system that generates a photochemically unreactive conformation in the ground state. This study paves the way toward a rational development of efficient molecular photochromes presenting a photon-quantitative response.

We investigate an inverse (I) dithienylethene, the bis(3,5-dimethyl-2-thienyl) perfluorocyclopentene, using absorption, emission and NMR spectroscopies as well as state-of-art first-principles (TDDFT) calculations. First, we find in addition to the expected antiparallel AP1 and parallel P2 conformers, a new stable antiparallel conformer AP3, but its energy is too high to be significantly populated at working temperature. More importantly, we demonstrate that, instead of an equal proportion of an AP and a P conformer as in normal (N) diarylethenes, the AP conformer is present in large excess. This result is confirmed by both DFT thermodynamical analysis and temperature-dependent NMR experiments modelized with an AP1 ↔ P2 fast interconversion model. With the latter, the relative populations are estimated to be ca. 3/1 for AP1/P2. Furthermore, the 0-0 energies simulated with a model that accounts for both vibrational and state-specific media effects of the ground and the excited states indicate that AP1 and P2 have very similar absorption signatures while only the P2 conformer should give rise to emission. Eventually, within excited state manifold, important topological points along the ring-closure reaction coordinate, and more specifically the unprecedented S1(opt) of the closed isomer, have been identified.

Using Time-Dependent Density Functional Theory and taking into account bulk solvent effects, we investigate the absorption and emission spectra of a betaine pyridinium molecule, the 2-(1-pyridinio) benzimidazolate (SBPa). This molecule exhibits strong photoinduced intramolecular charge transfer (ICT). We have identified two different electronic states involved, respectively, in the strong bathochromic ICT absorption band (S2) and in the moderate emission band (S1). The ICT process is analyzed in terms of charge distribution and dipole moment evolutions upon photoexcitation. These results are compared with steady-state spectroscopic measurements.

The photochromic reaction dynamics of spiroindolinenaphthoxazine and its 6′CN and 5′CHO substituted compounds is investigated in different solvents by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form (ring-opened trans form, OF), another short-lived intermediate species is produced upon photoexcitation, which is not a precursor to the OF product but which is formed in parallel to it via a competing relaxation process. This species is ascribable to either a relaxed s-cis ring-opened isomer on the ground state potential energy surface or to a metastable minimum of the excited S 1 state potential energy surface of the ring-closed form. The observed kinetics suggest that the production of OF (photocoloraton reaction) is controlled by the efficiency of the competing process rather than by an s-cis – trans isomerisation energy barrier.

Photodynamics of 2-hydroxybenzylideneaniline (photochromic salicylidene aniline SAOH) and N-(2-methoxybenzylidene)aniline (SAOMe) are studied by steady state and transient optical spectroscopy in solution and gas phase at different excitation wavelengths (266, 355 and 390 nm). Two competitive processes are observed from the enol* excited state: on one hand a rotation to get a twisted-enol, and on the other hand an excited state intramolecular proton transfer (ESIPT) followed by a cis-trans isomerisation to get the trans-keto photochromic product. For the first time both processes are characterized at an ultrashort time scale for salicylidene aniline. Resolution of the spectrokinetic data is achieved by multivariate curve resolution and attribution of the intermediate species recovered is performed in comparison with the results obtained for SAOMe, which can only undergo enol rotational isomerisation. It shows that ESIPT and rotation to the twisted-enol for SAOH occur within 100 fs, as predicted by recent quantum dynamical simulations, with an efficiency ratio dependent on the excitation wavelength. Therefore a general photoinduced mechanism for salicylidene aniline is drawn.

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S1 and S2 singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S1 vertical excitation induces direct efficient ( ø ≈ 0.9−1), and ultrafast ( ∼ 120 fs) photocylization from S1 open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S1 state, then evolves toward the cyclized S1 state and relaxes by ultrafast S1−S0 internal conversion. Alternatively, the possibility for a second conical intersection near hot S1 state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (ø = 0.49, τ = 2.5 ps in nonpolar solvent; ø = 0.7, τ = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

In this work, we discuss the reliability of soft-modelling multivariate curve resolution of transient absorption spectra of benzophenone which is chosen as a case study. The main issue concerns the recovery of short-time intermediate excited state that is probed in the UV–Visible on pico-second time scale. The corresponding spectrum and kinetic profile are provided. As such information has never been reported elsewhere, we propose to evaluate the pertinence and the robustness of the resolution through different data analysis strategies. It is shown how overfit situation can be detected analyzing the 2-way matrix of residuals. Another approach concerns the analysis of augmented data matrix. It enables to provide global solutions that are thus less affected by rotational ambiguities. The application of hard-modelling constraint on the kinetic profiles in MCR-ALS is also evaluated as a way to validate the results.

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.

The photophysical and photochemical properties of four 3,3-diphenyl-3H-naphtho[2,1-b]pyrans substituted, via an acetylenic junction, to (thiophene)n oligomers (n = 0−3 units) were investigated by transient absorption in the femtosecond to microsecond time domain and by stationary absorption and fluorescence. The decay of the initially produced excited S1(ππ*) state is found to occur via three competing processes: fluorescence, intersystem crossing, and a ring-opening reaction leading to a colored merocyanine product, with relative yields varying drastically with n. Whereas ultrafast (sub-picosecond) reaction dynamics and high product quantum yield are observed for n = 0 and 1, the reaction is considerably slowed down on going to the n = 2 (105 ps) compound and does not occur for n = 3. A reaction scheme that accounts for this behavior is proposed and the effect of the oligothiophenic chain length on the photoinduced properties is discussed. It is suggested that increasing the chain length from 1 to 3 thiophene units stabilizes the S1(ππ*) state by π conjugation and induces an excited-state potential barrier along the reaction pathway.

For the two photochromic molecules, 3-benzoyl-2-benzyl-1-methyl-1H-quinolin-4-one (QC1) and 3-benzoyl-1,2-dibenzyl-1H-1,8-naphthyridin-4-one (QC18a) as well as the nonphotochromic 3-benzoyl-1-benzyl-2-methyl-1H-1,8-naphthyridin-4-one (QC18b), the full photochemical mechanism, which is based on the photoenolization process, has been elucidated using stationary and time-resolved spectroscopy techniques. After photoexcitation, the S1(n,pi*)-T1(n,pi*) ISC process involving the exocyclic carbonyl chromophore is demonstrated to occur. Subsequently, gamma-hydrogen transfer proceeds very rapidly to give rise to the triplet photoenol with a probable 1,4-biradical structure. For all three molecules, the biradical is clearly detected and proved quantitatively to be the direct precursor of the colored form (photochromic compounds) or ground state starting material (nonphotochromic compound). Solvent effects for the three molecules studied may suggest the existence of intramolecular hydrogen bonding in both biradical and colored form species. Structural effects on the gamma-hydrogen transfer rate and biradical decay are related to the photochromic performances.

The photophysical and photochemical behavior of a 3H-naphtho[2,1-b]pyran compound substituted at position 8 by a p-nitroaniline group has been investigated by transient absorption spectroscopy in the femto/picosecond and nano/microsecond time domains. Measurements were undertaken at two different pump excitation wavelengths, adjusted in resonance with the lowest energy transition S0–S1 (abs λmax 392 nm) and with a higher energy transition S0–Sn (abs λmax 265 nm), respectively. In both cases, the results show the contribution of three transient species to the photoinduced processes: the excited S1 and T1 states of the initial ring-closed molecule and a long-lived colored species ascribed to a ring-opened photoproduct, attesting the presence of some photochromic activity. The S1 and T1 states are mainly localized on the p-nitroaniline substituent and do not contribute highly to the photochromic efficiency. The ring-opening photochromic process occurs essentially in ≤0.4 ps following excitation of the Sn state, in competition with the relaxation to the S1 state via internal conversion.

This work deals with the photophysics of a pyridinium betaine, 2-pyridin-1-yl-1H-benzimidazole (SBPa), based on a combination of steady-state, femtosecond photoionization (gas phase) and femtosecond transient absorption (solution) spectroscopic measurements, supported by (LR)-PCM-(TD)DFT calculations. Preliminary and new electrochemical results have revealed a strongly negative solvatochromic charge transfer (CT) absorption due to a S0 → S2 vertical transition and a weakly-solvatochromic emission due to S1 → S0 transition. Advanced TDDFT optimizations of the Franck-Condon states S2(FC) and S1(FC) led to two additional CT levels with planar geometry, S2(CT) and S1(CT), respectively, allowing prediction of a two-step photoinduced ICT process, i.e., S0 → S2(FC) and S2(CT) → S1(CT), separated by a S0(FC) → S2(CT) back charge transfer relaxation. While the pyridinium ring is the acceptor group in both steps, two different donor groups, the benzene ring and the imidazole bridge, are involved in the excitation and internal conversion processes, respectively. Femtosecond transient absorption experiments supported by MCR-ALS decomposition confirmed indeed the contribution of two distinct CT states in the photophysics of SBPa: following excitation to the S2(CT) state, ultrafast production of the emissive S1 state (the only channel observable in the gas phase) was observed to occur in competition with a further ICT process toward the S1(CT) state, with a time constant ranging from 300 fs to 20 ps depending on the solvent. While in aprotic media this ICT process was found to be purely solvent controlled (double polarity and viscosity dependency), in protic solvents, the influence of the hydrogen bond network has to be taken into account. Comparison with data obtained for a pre-twisted SBPa analogue led us to exclude the presence of any large-amplitude geometrical change during ICT. Analyzing the solvent dependency using the power law approach, we concluded that the S1(CT) state decays essentially through IC in the 3-40 ps time range whereas the emissive S1 state decays within 130-260 ps via IC, ISC and fluorescence.

This work concerns the photoreactivity of photochromic dithienylethenes A (in solution and thin film) able to form a film exhibiting an opto-mechanical effect under UV or Visible irradiation. The production of this film is based on the reversible supramolecular assembly by quadrupolar hydrogen bonds between A and elastomers B, both being functionalized by UPy groups. The first step of this work focused on the kinetics of supramolecular assemblies by DLS, NMR, AFM, and DFT calculation. Concerning the self-assembly AAAA, a strong sensitivity to initial conditions and a typical kinetics (overshoot) have been interpreted in terms of nucleation and 3D growth of nano-objects. After addition of B, these effects disappear in favor of a mixture which homogenizes within several hours. The second step, conducted by nanosecond / femtosecond absorption spectroscopy and TDDFT calculations, focused on the photocommutation mechanisms of A in different environments (oligomer, large assembly). Considering the S1 states, the characteristic times of photoreversion (≈5 ps) and photocyclization (≈150 fs) processes appear to agree with literature data: Two triplet states have been identified (antiparallel and parallel conformers), one of which is photoactive (Tau≈10 μs). Compared to the photoreversion process which is insensitive to changes in the environment, the photocyclisation process appears to be very sensitive. These, the interface between the open and closed forms within the films (A, B) is probably one of the keys to understand the optomechanical effect.; Ce travail porte sur la photoréactivité de dithiényléthènes A (solution et film mince), molécules photochromiques permettant l’obtention d’un film présentant un effet opto-mécanique sous irradiation UV ou Visible. Son obtention est basée sur l’assemblage supramoléculaire réversible par liaisons hydrogènes quadripolaires entre A et des élastomères B, tous deux fonctionnalisés par les mêmes groupements UPy. La première étape de ce travail s’est concentrée sur la cinétique des assemblages supramoléculaires par DLS, RMN, AFM, et calculs DFT. Concernant l’auto-assemblage AAAA, une forte sensibilité aux conditions initiales et des particularités cinétiques (temps de latence, "overshoot") ont été interprétées en termes de nucléation et croissance 3D de nano-objets. Après ajout de B, ces effets disparaissent au profit d’un mélange qui s’homogénéise sur plusieurs heures. La deuxième étape, conduite par spectroscopie d’absorption nanoseconde/femtoseconde et calculs TDDFT, s’est focalisée sur les mécanismes de photocommutation de A sous différents environnements (oligomères, grands assemblages). Si, concernant les états S1, des temps caractéristiques de photoréversion (≈5 ps) et photocyclisation (≈150 fs) proches de ceux de la littérature ont été observés, deux états triplets ont pu être identifiés (conformère antiparallèle et parallèle) dont l’un est photoactif (Tau≈10 µs). Contrairement au processus de photoréversion qui est insensible aux changements de l’environnement, le processus de photocyclisation y est lui très sensible. Donc, il est très probable que l’interface forme ouverte-forme fermée au sein des films (A,B) soit l'une des clefs pour comprendre l’effet optomécanique.

This work concerns the photoreactivity of photochromic dithienylethenes A (in solution and thin film) able to form a film exhibiting an opto-mechanical effect under UV or Visible irradiation. The production of this film is based on the reversible supramolecular assembly by quadrupolar hydrogen bonds between A and elastomers B, both being functionalized by UPy groups. The first step of this work focused on the kinetics of supramolecular assemblies by DLS, NMR, AFM, and DFT calculation. Concerning the self-assembly AAAA, a strong sensitivity to initial conditions and a typical kinetics (overshoot) have been interpreted in terms of nucleation and 3D growth of nano-objects. After addition of B, these effects disappear in favor of a mixture which homogenizes within several hours. The second step, conducted by nanosecond / femtosecond absorption spectroscopy and TDDFT calculations, focused on the photocommutation mechanisms of A in different environments (oligomer, large assembly). Considering the S1 states, the characteristic times of photoreversion (≈5 ps) and photocyclization (≈150 fs) processes appear to agree with literature data: Two triplet states have been identified (antiparallel and parallel conformers), one of which is photoactive (Tau≈10 μs). Compared to the photoreversion process which is insensitive to changes in the environment, the photocyclisation process appears to be very sensitive. These, the interface between the open and closed forms within the films (A, B) is probably one of the keys to understand the optomechanical effect.; Ce travail porte sur la photoréactivité de dithiényléthènes A (solution et film mince), molécules photochromiques permettant l’obtention d’un film présentant un effet opto-mécanique sous irradiation UV ou Visible. Son obtention est basée sur l’assemblage supramoléculaire réversible par liaisons hydrogènes quadripolaires entre A et des élastomères B, tous deux fonctionnalisés par les mêmes groupements UPy. La première étape de ce travail s’est concentrée sur la cinétique des assemblages supramoléculaires par DLS, RMN, AFM, et calculs DFT. Concernant l’auto-assemblage AAAA, une forte sensibilité aux conditions initiales et des particularités cinétiques (temps de latence, "overshoot") ont été interprétées en termes de nucléation et croissance 3D de nano-objets. Après ajout de B, ces effets disparaissent au profit d’un mélange qui s’homogénéise sur plusieurs heures. La deuxième étape, conduite par spectroscopie d’absorption nanoseconde/femtoseconde et calculs TDDFT, s’est focalisée sur les mécanismes de photocommutation de A sous différents environnements (oligomères, grands assemblages). Si, concernant les états S1, des temps caractéristiques de photoréversion (≈5 ps) et photocyclisation (≈150 fs) proches de ceux de la littérature ont été observés, deux états triplets ont pu être identifiés (conformère antiparallèle et parallèle) dont l’un est photoactif (Tau≈10 µs). Contrairement au processus de photoréversion qui est insensible aux changements de l’environnement, le processus de photocyclisation y est lui très sensible. Donc, il est très probable que l’interface forme ouverte-forme fermée au sein des films (A,B) soit l'une des clefs pour comprendre l’effet optomécanique.