The photoswitching and competitive processes of two photochromic dithienylethenes (DTEs) functionalized at both sides with 2-ureido-41H-pyrimidone (UPy) quadruple hydrogen-bonding recognition ...patterns have been investigated with NMR experiments, ultrafast spectroscopy, and density functional theory (DFT) calculations. The originality of these molecules is their ability to form large supramolecular assemblies induced by light for the closed form (CF) species while the open form (OF) species exist as small oligomers. Photochromic parameters have been determined and photochemical pathways have been rationalized with clear distinction between the antiparallel (OF-AP) and parallel (OF-P) species. A new photocyclization pathway via triplet manifold has been evidenced. The effect of the supramolecular assembly on the photochemical response is discussed. Unlike the photoreversion process, which is unaffected by supramolecular assembly, rate constants of the photocyclization reaction and intersystem crossing process are sensitive to the presence of small OF oligomers.
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 S
relaxed state, an expected ISC occurred (55 ps) to produce a triplet state,
P, 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,
AP. 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.
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 Ca(2+) 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 log K = 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.
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.
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-dicyano2.nmetacyclophan-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 (ϕ ≈ 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 (ϕ = 0.49, τ = 2.5 ps in nonpolar solvent; ϕ = 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.
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 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.
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.
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 Ca
2+
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 log
K
= 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.
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 or dimethyl-amino groups.
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