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 photophysical properties of 6-methoxyquinoline (6MeOQ) and 6-hydroxyquinoline (6HQ) in organic solvents and in neutral, acidic, and alkaline aqueous solutions have been studied by picosecond ...transient absorption spectroscopy. The dynamics of photoinduced tautomerization by excited state proton transfer(s) arising in the case of 6HQ has been analyzed. Hydroxyl deprotonation of the cationic (quinolinium) form in acidic solution (pH 1.5) is 1 order of magnitude faster (τ dp ∼ 2.2 ps) than imine protonation of the anionic (phenolate) form in 1 M KOH alkaline solution (τ p ∼ 30 ps). In neutral solution, our results suggest a two-step process of initial proton release from the hydroxyl group followed by proton capture by the nitrogen atom with characteristic times close to the τ dp and τ p values, respectively.
The decatungstate anion W10O324– is a widely used photocatalyst for promoting hydrogen atom transfer (HAT) reactions. The mechanism implicated in the activation of organic substrates, however, still ...needs to be clarified and has been claimed to involve an unknown relaxed excited state of triplet multiplicity, tagged wO. A subpicosecond investigation allowed us to follow early events leading to the chemically reactive species. A hot singlet excited state (S1 HOT) has been individuated through pump–probe experiments, yielding S1 by ultrafast decay (<1 ps). The reactive species wO arises from S1 in competition with decay to S0 (efficiency ca. 0.5) and has been detected spectroscopically by flash photolysis experiments, with peculiar absorption bands in the near-UV (370 nm) and visible (600–800 nm) regions. TD-DFT calculations demonstrated that excitation to S1 occurs through a ligand to metal charge transfer (LMCT) transition, involving a displacement of electron density from dicoordinated (bridging) oxygen to tungsten atoms. Population of wO ensues and involves a reorganization of the singly occupied orbital centered on oxygen (not tungsten) atoms. As a result, monocoordinated O centers acquire a partial radical character that well explains the known chemistry, essentially hydrogen atom transfer (HAT), and highlights the similarity with nπ* carbonyl triplets. This rationalization may help in devising other photocatalysts able to promote HAT processes from unactivated precursors.
A spirobenzopyran containing 6-chloro and 8-diphenyloxazole substituents has been investigated in acetonitrile solution by nanosecond laser photolysis at room temperature. In degassed solution, a ...short-lived transient (5 μs) has been identified as the triplet state of the closed spiro form. The ratio between the singlet and triplet pathways of ring opening has been determined from oxygen quenching measurements. In the presence of Co(II), UV irradiation is needed to induce the complexation. The fast second order rate constant for the 1:1 complex formation has been determined. It is suggested that such kinetic parameter could be used to characterise the properties of metallochromic spiropyrans.
The photoreduction of 4,4‘-bipyridine (44BPY) by diazabicyclo2.2.2octane and triethylamine (TEA) is investigated by using picosecond transient absorption and time-resolved resonance Raman ...spectroscopy in various acetonitrile−water mixtures. The results are interpreted on the basis of a preferential solvation effect resulting from the presence of a specific interaction between 44BPY and water by hydrogen bonding. Below 10% water, the free 44BPY species is dominant and leads upon photoreduction to a contact ion pair that undergoes efficient intrapair proton transfer if TEA is the amine donor. Above 10% water, most of the 44BPY population is H-bonded and leads upon photoreduction to a hydrated ion pair in which the intrapair proton transfer is inhibited. Instead, the 44BPY-• species is protonated by water through the hydrogen bond with a rate constant that increases by more than 3 orders of magnitude on going from 10% to 100% water. The dependence of this rate constant on the solvent mixture composition suggests that the reaction of intracomplex proton transfer is controlled by the hydration of the residual OH- species by three molecules of water, leading to a trihydrated HO-(H2O)3 species.
Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an ...aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S
2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile.
Raman spectra of the fluorene radical cation (FR
+
) have been recorded for four isotopic derivatives in the 300–1800
cm
−1 spectral range, in resonance with two different electronic transitions at ...370 and 625
nm. Quantum chemical investigations, at the B3LYP/6-31G* level of theory, of the ground state vibrational modes of FR
+
provided a reliable assignment of the resonance Raman active (totally symmetric) modes and emphasized the complexity of the experimental vibrational features with the presence of a great number of harmonic and combination modes sometimes superimposed to fundamental frequencies. The predicted structure of FR
+
was validated on the basis of a good agreement between the observed and calculated vibrational data. Simulating the resonance Raman intensities in the Hartree–Fock formalism (ROHF and CIS) allowed us identifying the resonant states involved in both the investigated electronic transitions.
A series of emitting push-pull triarylamine derivatives, models of their widely used homologues in photonics and organic electronics, was investigated by steady-state and time-resolved spectroscopy. ...Their structural originality stems from the sole change of the electron-withdrawing substituent X (-H: 1, -CN: 2, -NO2: 3, -CHC(CN)2: 4), giving rise to efficient emission tuning from blue to red upon increasing the X electron-withdrawing character. All compounds are highly fluorescent in alkanes. The more polar compounds 2-4 undergo considerable Stokes shift and emission quenching in polar solvents. Femtosecond transient absorption data allowed us to identify the nature of the emissive state which varies as a function of the compound and surrounding polarity. A long-lived ππ* excited state with weak charge transfer character was found for 1. This excited state evolves into a long-lived ICT state with red-shifted emission for 2 in polar solvents. For 3 and 4, the ICT state is directly populated in all solvents. Long-lived and emissive in n-hexane, it relaxes in toluene to a new ICT' conformation with stronger charge transfer character and enhanced Stokes shift. In more polar THF, ethanol, and nitrile solvents, ICT relaxes to a dark excited state ICT'' with viscosity-dependent kinetics (<10 ps). The ICT'' state lifetime drops with increasing solvent polarity (150 ps for 3 in THF, 8.5 ps in butyronitrile, 1.9 ps in acetonitrile), denoting an efficient radiationless deactivation to the ground state (back charge transfer). This result reveals a very small S0-S1 energy gap at the relaxed ICT'' geometry, with a possible close-lying S0-S1 conical intersection, which suggests that the ICT → ICT'' process results from a structural change involving a large-amplitude molecular distortion. This fast structural change can account for the strong fluorescence quenching observed for 3 and 4 in polar solvents. Finally, the magnitude of intersystem crossing between the singlet and triplet excited states largely depends on the electron-deficient X unit and the solvent itself. These observations help one conclude on the prevailing role played by the electron-withdrawing groups and the surrounding polarity in the photophysical performances of triphenylamine derivatives, largely employed in numerous emissive solid-state devices.
The photochromic reaction dynamics of 6
′-cyano-spiroindolinenaphthoxazine (6
′CN-SNO) in solution is followed by femtosecond transient absorption spectroscopy. In addition to the formation of the ...merocyanine coloured form, we demonstrate the presence of a new metastable species with intense absorption bands in the 400–600 and 650–975 nm spectral regions that was not observed for the non-substituted parent molecule. The formation of this species parallels the photochromic reaction. The existence of this concurrent photophysical process can explain the 50% reduction in the photocoloration quantum yield of this molecule relative to that of the parent molecule.
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