Joint spectroscopic and theoretical investigation of cationic cyanine dye Astrazon Orange-R: solvent viscosity controlled relaxation of excited states.

In this paper, the first study of cationic cyanine dye Astrazon Orange-R by combined spectroscopic and theoretical investigation is presented. It is shown that molecular modeling of Astrazon Orange-R is in very good agreement with experiment, allowing us to gain insight into its complicated photophysics. A solvent viscosity controlled relaxation of excited states, involving cyanine isomerization, is also outlined.

A two-step ICT process for solvatochromic betaine pyridinium revealed by ultrafast spectroscopy, multivariate curve resolution, and TDDFT calculations.

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 S(0) → S(2) vertical transition and a weakly-solvatochromic emission due to S(1) → S(0) transition. Advanced TDDFT optimizations of the Franck-Condon states S(2)(FC) and S(1)(FC) led to two additional CT levels with planar geometry, S(2)(CT) and S(1)(CT), respectively, allowing prediction of a two-step photoinduced ICT process, i.e., S(0) → S(2)(FC) and S(2)(CT) → S(1)(CT), separated by a S(2)(FC) → S(2)(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 S(2)(CT) state, ultrafast production of the emissive S(1) state (the only channel observable in the gas phase) was observed to occur in competition with a further ICT process toward the S(1)(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 S(1)(CT) state decays essentially through IC in the 3-40 ps time range whereas the emissive S(1) state decays within 130-260 ps via IC, ISC and fluorescence.

The excited state dipole moments of betaine pyridinium investigated by an innovative solvatochromic analysis and TDDFT calculations.

This work reports on the solvatochromic properties of a simple heterocyclic betaine pyridinium, 2-(1-pyridinio)benzimidazolate (SBPa), having promising potentialities in non-linear optics. From advanced PCM-TDDFT calculations, the solvatochromism of SBPa was found to be unusual, involving two different electronic states for absorption (S(0)→ S(2)) and emission (S(1)→S'(0)). To account for this behavior, we developed an innovative physical treatment which consists in a non-linear fit of the solvatochromic data using the Bilot-Kawski theoretical model and visualizing the least-square coefficient χ(2) on a 2D map as a function of the solute polarizability and gas phase absorption energy. In parallel, Kamlet-Taft correlations were undertaken to select a propitious set of electrostatic solvents usable in this treatment. Protic solvents that lead to specific interactions and nonpolar solvents that favor dimerization processes were excluded. From a choice of aprotic solvents with sufficiently high polarity, 4 dipole moments µ(g)(S(0)) = +9.1 D, µ(e)(S(2)) = -1.5 D, µ(e)(S(1)) = 0 D and µ(g)(S'(0)) = +3.31 D were determined, the 3 former values being in close agreement with TDDFT values, although the solute polarizability values seem underestimated. Anyhow, disregarding this discrepancy, we evaluated the static hyperpolarizability to ß(0) = -64 × 10(-30) esu from the solvatochromic data in close agreement with DFT calculations.

Ultrafast excited-state dynamics of a series of zwitterionic pyridinium phenoxides with increasing sterical hindering.

A series of pyridinium phenoxides that differ by the dihedral angle between the pyridinium and the phenoxide rings because of substituents with increasing steric encumbrance has been investigated by ultrafast spectroscopy. Like the related betaine-30, these molecules are characterised by a zwitterionic electronic ground state and a weakly polar S(1) state. Their fluorescence lifetime was found to lie between 200 to 750 fs, decreasing with increasing dihedral angle, and increasing with solvent viscosity. This was assigned to a non-radiative deactivation of the emissive state coupled to a large amplitude motion involving the dihedral angle. The transient absorption spectra suggested that emission occurs from the Franck-Condon S(1) state, which decays to a dark excited state, that itself most probably corresponds to the relaxed S(1) state. Finally, this relaxed state decays to the vibrationally hot ground state through an intramolecular charge separation process with a time constant ranging between 0.4 and 3 ps, increasing with the dihedral angle and with the solvent relaxation time. These variations were discussed in terms of the Jortner-Bixon model of electron transfer, where the charge separation dynamics depends on both electronic coupling and solvent relaxation. The results suggested that charge separation slows down with increasing dihedral angle.

Photoinduced coupled charge and proton transfers in gradually twisted phenol-pyridinium biaryl series.

A detailed photophysical analysis of a phenol-pyridinium biphenyl series with gradual twisted geometry is presented in this paper. The low-energy CT absorption band of the compounds undergoes a decrease of intensity with a progressive blue shift by increasing the twist angle of the central bond (Theta(AD)). These effects are well described and quantified within the framework of the Mulliken-Murrel approach, which allows us to extend such a model to the charge-shift process. The biaryl compounds exhibit broadened fluorescence bands assigned to the radiative deactivation of a charge shift (CSh) species generated by an intramolecular twisting relaxation of the locally excited (LE) state. Parallel to the rotamerism of the central single bond, excited-state proton-transfer (ESPT) processes are occurring from both excited states and lead to the nonemissive phenolate forms. Solvatochromic shifts of the emission bands are correlated by the Kamlet-Taft parameters (pi*, alpha, and beta). The correlation first confirms the pi* dependence of the CSh band shift but also demonstrates a clear beta dependence. The contribution of the latter parameter to the band hypsochromy is markedly increasing with Theta(AD). Such an unusual effect was ascribed to a much higher ESPT rate relative to the highly twisted conformation with respect to that of more planar geometry. Despite the suppression of the geometrical relaxation in ethanol glass at 77 K, the fluorescence of the phenolate species produced by ESPT from LE state is detected. The relative increase of its fluorescence band intensity with Theta(AD) confirms the gradual enhancement of the excited states acidity.

Ultrafast decay of the excited singlet states of thioxanthone by internal conversion and intersystem crossing.

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet pi pi* state is shown to decay through internal conversion and intersystem crossing processes via intermediate n pi* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet-triplet crossing near the singlet pi pi* minimum and the large spin-orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.

Crystal structure, spectroscopic, and theoretical investigations of excited-state proton transfer in the doubly hydrogen-bonded dimer of 2-butylamino-6-methyl-4-nitropyridine N-oxide.

The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.

Singlet exciplexes between a thioxanthone derivative and substituted aromatic quenchers: role of the resonance integral.

Fluorescence quenching of a thioxanthone derivative by methyl- and methoxy-substituted benzenes (MeB and MeOB, respectively) is performed in solvents of different polarity. Emissive exciplexes are observed even in polar solvents and provide kinetic and spectroscopic data over a large scale of solvent polarity. These data were subsequently analyzed by use of a new theoretical model that leads to a thermodynamic relationship between exciplex and electron-transfer driving forces Delta G(exc) and Delta G(et), respectively. The remarkable agreement found between this model and both kinetic and spectroscopic data supports its validity. Moreover, the difference observed between MeB and MeOB compounds in quenching efficiency is analyzed by this model and provides the main parameters governing exciplex features, especially the resonance integral between locally excited and charge-transfer states.