M. Szymanski

Subpicosecond pump–probe measurements of the electronic relaxation rates of the S1 states of azulene and related compounds in polar and nonpolar solvents

The lifetimes of the S1 states of azulene, azulene‐d8, 4,6,8‐trimethylazulene, and guaiazulene (1,4‐dimethyl‐7‐isopropylazulene) have been measured in three nonviscous solvents of different polarity and structure using a two‐photon, two‐color, pump–probe method with subpicosecond time resolution. A significant solvent effect is measured. The rate constants for S1■S0 internal conversion in all four compounds in all three solvents exhibit one common S1–S0 energy gap law correlation, indicating that variations in the electronic relaxation rates are governed exclusively by changes in the Franck–Condon factors for the transition. No effect is observed when the exciting wavelength is changed, indicating that vibrational relaxation is occurring on a time scale which is faster than that of electronic relaxation in these systems. No significant deuterium isotope effect is measured in azulene, indicating that high frequency C–H(D) stretching vibrations do not act as significant accepting modes in the radiationless ...

T1–T2 inversion in aromatic thiones

Abstract The emission spectra and phosphorescence lifetimes and quantum yields, of xanthione (XT), 4H-pyran-4-thione (PT) and 4H-1-benzopyran-4-thione (BPT) have been measured in five solvents of varying polarities at room temperature. A dramatic change in the T 1 →S 0 phosphorescence spectrum is observed, and this is interpreted in terms of inversion of the 3 (n, π*) and 3(π,π*) states with increasing solvent polarity. Only modest variations in the radiative and non-radiative decay constants for the triplet are observed, and no evidence of bi-exponential decay is found. Strong coupling between T 1 -T 2 and between the singlet and triplet manifolds is indicated. Comparisons are drawn with ketones of similar structure.

Concerning apparent intersystem crossing efficiencies in molecules with small S1T1 energy gaps

Abstract Accurate values of the apparent quantum yields φ ST of S 1 ⇝ T 1 intersystem crossing were determined for several aromatic thiones in fluid solutions at room temperature. All values of φ ST are significantly less than unity at 295 K; this can be compared with a value of unity based on limited data previously obtained at 77 K, which has been assumed to hold at room temperature. The importance of triplet decay via back intersystem crossing to S 1 followed by radiative and non-radiative decay to S 0 is assessed. The possible involvement of T 2 in the subpicosecond decay of vibrationally excited S 1 is discussed. The potential importance of back intersystem crossing in other molecules with small S 1 T 1 gaps is briefly addressed.

Deactivation of the S2 and Two Lowest Triplet States of 4H-1-Benzopyran-4-thione and Its Alkyl Derivatives in Methanol Solution

Spectral and photochemical properties of three aromatic thioketones, 4H-1-benzopyran-4-thione and its two alkyl derivatives, in their second excited singlet and two lowest triplet states have been a subject of stationary and time-resolved studies. Quantum yields of emissions from the above-mentioned states as well as lifetimes of these states have been measured. The results indicate an important role of intermolecular solvent-induced processes in deactivation of the excited states of the thioketones studied. The possible mechanism of these processes involves hydrogen atom abstraction and/or formation of exciplex. For relatively high thione concentrations, the contribution of self-quenching in deactivation of the triplet states becomes significant. The effect of the alkyl chain in a thione molecule on deactivation parameters is discussed.

Photochemistry of aromatic thiones in their S2 and T1 states. Role of triplet self-quenching and excimer formation

The quantum yields of photodecomposition of xanthione (XT), 4H- 1-benzopyran-4-thione (BPT) and 4H-pyran-4-thione (PT) subsequent to excitation to their S2 and S1 states have been measured as a function of thione concentration in inert perfluoroalkane solvents. The quantum yields of intramolecular photodecomposition of these thiones in S2 are immeasurably small. Intermolecular processes dominate the photochemical removal of the triplet states even at very low ( < 10–5 mol dm–3) thione concentrations. In inert perfluoroalkane solvents a photochemical channel in the diffusion-controlled triplet self-quenching process is responsible for the removal of the thione. A mechanism involving the intermediacy of triplet excimers is proposed.

The photophysics of thioflavone in solution

Abstract The absorption, emission and emission excitation spectra, S 2 → S 0 fluorescence and T 1 → S 0 phosphorescence quantum yields and S 2 and T 1 excited state lifetimes of thioflavone (TF) were measured in perfluoro-1,3-dimethylcyclohexane (PFDMCH) and 3-methylpentane (3MP) at room temperature. The results were analysed to provide a quantitative description of the decay processes of excited TF, with emphasis on the effect of the phenyl rotor on the radiationless decay rates. In the inert perfluoroalkane solvent, both S 2 and T 1 relax intramolecularly via an S 2 → S 1 → T 1 → S 0 path. S 2 → S 0 fluorescence and T 1 → S 0 phosphorescence also account for a small fraction of the excited state decay events, but no thermally activated delayed fluorescence, S 1 → S 0 + hv df , is observed. In 3-MP, the decay mechanism is dominated by intermolecular interactions between excited TF and the solvent. The rates of intramolecular radiationless decay are larger in TF than in rigid thiones having the same electronic energy gaps owing to the influence of the torsional motion of the phenyl group. By comparison of the observed rate constants for intramolecular radiationless decay of excited TF with those of a hypothetical rigid thione having the same electronic energy spacings, contributions to the radiationless decay rates of S 2 and T 1 due to torsional motion of the phenyl group were quantified. The second-order rate constants for quenching of triplet thione by molecular oxygen and by ground state thione were also measured. Excitation of TF to singlet excited states higher in energy than S 2 results in decay to S 1 and T 1 which partially bypasses S 2 .

A method of determination of quantum yields of S3⇝S2,S3⇝S1, and S3⇝S0 intramolecular radiationless transitions

An effective method for determination of the quantum yields of the S3⇝S2, S3⇝S1 and S3⇝S0 intramolecular radiationless transitions, based on steady-state measurements of absorption, S2 fluorescence and T1 phosphorescence or S1 fluorescence under selective excitation to the S2 and S3 states has been described and used for three aromatic thioketones. The results show that in the compounds studied the deactivation of the S3 state is not strictly sequential and the S3⇝S1 as well as, unexpectedly, S3⇝S0 transitions occur with relatively high yields. Measurement of the quantum yields of S3⇝S2, S3⇝S1, and S3⇝S0 intramolecular radiationless transitions is essential for the determination of the intramolecular properties of molecules in the S3 state as well as pathways of their decay.

Photochemistry and photophysics of osazones. Part 3.—Properties of D-arabino-hexulose phenylosazone in excited singlet and triplet states

Steady-state measurements of the sensitization and quenching of N—PH → O—PH photoisomerization and the fluorescence of the N—PH isomer, together with dynamic measurements of the decay of fluorescence and of T1→ Tn absorption, have been performed to determine the singlet and triplet energy levels of the planar and twisted configurations for D-arabino-hexulose phenylosazone (PH). The emission and excitation spectra have been measured and the fluorescence quantum yields have been obtained at 293 and 77 K. With a nanosecond laser photolysis system triplet–triplet absorption spectra have been measured and the molar absorption coefficient of the T1→ Tn transition has been determined for N—PH.

Photophysics of UV-excited Cl2CS in a perfluoroalkane solvent

The quantum yields of fluorescence from UV-excited Cl2CS in inert perfluoroalkane solutions have been measured as a function of the analytical concentration of the solute in the range 5 × 10–5⩽C/mol dm–3⩽ 1, and as a function of excitation energy in the range 245 ⩽λex/nm ⩽ 330. The excited-state decay rates have also been measured at 272 ⩽λex/nm ⩽ 300 over the same concentration range. A quantitative description of the photophysics of the system has been achieved, which accounts for significant self-aggregation of the ground state solute at C > 10–4 mol dm–3, absorption by a nearby dark excited state of the monomer in the 285 ⩽λex/nm ⩽ 330 range, different lifetimes and rate constants of concentration quenching of the excited monomer and the excited dimer, and the excitation wavelength dependence of fluorescence-weakening non-radiative relaxation processes in each excited species.

Application of picosecond laser spectroscopy in investigation of initial stages of fluorescence quenching

The initial stage of fluorescence quenching (the transient quenching) has been studied for the S2-xantanione/3,3-diethylpentane/perfluoro-1,3-dimethylcyclohexane (as a donor/quencher/solvent, respectively) system. A picosecond time-correlated single photon counting system with a proximity type microchannel plate photomultiplier tube was used in dynamic studies of a strong time dependence of the transient quenching of S2-xanthione fluorescence. A satisfactory description of the data within the Smoluchowski-Collins-Kimball model has been obtained, permitting the determination of realistic values of the encounter radius, the sum of diffusion coefficients, and the intrinsic rate coefficient.