Andrzej Maciejewski

An ultrafast excited state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA) and its “double” analogue salicylaldehyde azine (SAA). A controversial case

Two simple, structurally related photochromic Schiff bases, salicylideneaniline (SA) and salicylaldehyde azine (SAA) were studied in femto– and picosecond time domains. In both systems an ultrafast excited state intramolecular proton transfer (ESIPT) reaction was stated with the characteristic time below 50 fs. For SA this result is in contrast to the recent data published by Mitra and Tamai (S. Mitra and N. Tamai, Chem. Phys. Lett., 1998, 282, 391; S. Mitra and N. Tamai, Chem. Phys., 1999, 246, 463; S. Mitra and N. Tamai, Phys. Chem. Chem. Phys., 2003, 5, 4647), reporting on the corresponding time as long as 200–300 fs. The kinetics of decay of keto-tautomers in S1 states was followed by the transient absorption (410 nm and 470 nm for SA and SAA, respectively) and stimulated emission bands. About 10–30% of excited molecules give birth to the long-lived ground states of photochromic forms.

The measurements of picosecond fluorescence lifetimes with high accuracy and subpicosecond precision

Abstract Systematic studies of the fluorescence picosecond lifetimes determination by laser-excited time-correlated single-photon-counting (TCSPC) have been undertaken. The results have been used to develop methods for determining lifetimes with much smaller error and much greater reproducibility than any hitherto reported. The error in the determination of the lifetimes (±three standard deviations) can be as low as that in simulations and amounts to 0.01 FWHM of the IRF. The lifetimes determined for the second excited singlet state of xanthione in toluene (5.1±0.3 ps ) and in benzene (8.1±0.3 ps ) can be treated as reliable standards of picosecond lifetimes.

The application of perfluoroalkanes as solvents in spectral, photophysical and photochemical studies

Abstract Perfluoroalkanes (PFs) have been shown to be unique solvents in spectral, photophysical and photochemical studies in fluid solutions. Their physical, chemical and spectral properties are discussed. Substantial differences between PFs and alkanes and similarities between PFs and the rare gases argon and krypton are illustrated. Examples are presented which illustrate the advantages of the use of PF solvents in the study of the contribution of non-specific and specific solute—solvent interactions to the experimentally observed spectral shift, in the determination of the role of intermolecular and intramolecular interactions for molecules of rigid and flexible structure, and in measurements of the energies and lifetimes of electronically excited states. The application of PF solvents in photophysical and photochemical studies, especially in the quantitative determination of the contribution of intramolecular and intermolecular processes to the decay paths of electronically excited molecules, is discussed. The results presented demonstrate that PFs considerably extend the polarity scale available for weakly interacting solvents. Therefore they are recommended as solvents in the investigation of the intramolecular properties of reactive molecules in electronically excited states. Some comments on the practical application of PFs are given and some studies using PFs are proposed.

Spectral and photophysical properties of 9,10-diphenylanthracene in perfluoro-n-hexane: the influence of solute-solvent interactions

Abstract The photophysical properties of 9,10-diphenylanthracene (DPA) are found to be unusual in perfluoro-n-hexane (PFH) compared with other solvents. Low values of the fluorescence quantum yield, φF = 0.58, and S1 lifetime, τS1 = 6.7 ns, and relatively large shifts in the absorption and emission spectra in PFH are attributable to changes in the electronic energies of DPA which permit an enhancement of the rate of S1 ⇝ T2 relaxation in this solvent. A large value of the intersystem crossing quantum yield in PFH, φISC = 0.35, supports this conclusion.

Excited state proton transfer and photochromism of an aromatic Schiff base. Pico- and femtosecond kinetics of the N, N'-bis(salicylidene)-p-phenylenediamine (BSP)

Abstract The results of time-resolved pico- and femtosecond absorption and emission study performed for the title photochromic Schiff base, (BSP) are presented. Transient absorption spectra of intermediates, appearing in the excited state intramolecular proton transfer (ESIPT) were identified. A full scheme of deactivation of the excited BSP molecule, including the enol-, keto- and photochromic tautomers, was proposed. In particular, the characteristic time of the ESIPT process was determined as k K − PC =2.8×10 10 s −1 .

Measurements of picosecond lifetimes by time correlated single photon counting method: The effect of the refraction index of the solvent on the instrument response function

The article concerns the possibility of precise picosecond lifetime measurements by the method of the time correlated single photon counting (TCSPC). The lifetimes (τ) of S2 fluorescence of xanthione measured in different solvents have shown that it is possible to obtain τ as short as a few picoseconds. The values of τ varying from ∼5 to ∼38 ps in solvents whose refraction indices range from 1.34 to 1.50 have been proposed as picosecond standards in lifetime measurements. However, in order to obtain reliable results some conditions must be met: the effect of the absorbing and emitting impurities must be eliminated, the instrument response function (IRF) must be very stable and correctly determined, and because of a much lower time resolution of the TCSPC method relative to that of the upconversion method, the procedure of measurements and numerical analysis of results proposed in our work should be applied. In the study reported the effect of impurities was eliminated by applying high performance liquid c...

The phosphorescence of 4h-pyran-4-thione: large quantum yields from room-temperature fluid solutions

Abstract The spectra, phosphorescence quantum yields and triplet lifetimes of 4H-pyran-4-thione (PT) in fluid solution at room temperature have been measured. In inert perfluoroalkane solvents at 293 K, the phosphorescence quantum yield of PT is 0.33 on excitation to S 2 and 0.47 on direct excitation to T 1 . The reasons for these extremely large radiative yields are discussed.

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.

Mechanism and deactivation kinetics of S2-xanthione in acetonitrile, a quenching solvent, and of S2-exciplex measured by pico- and femtosecond laser spectroscopy

Abstract The Letter discusses the mechanism and dynamics of intra- and intermolecular electronic relaxation of the S 2 state of a xanthione molecule in acetonitrile, a quenching solvent, as well as the most important properties of the S 2 -exciplex formed upon the quenching process. Despite very efficient quenching of S 2 -XT and other thiones it has not been possible so far to observe the S 2 -exciplex. Therefore transient absorption system with 120 fs resolution was successfully applied for this purpose: the S 2 -exciplex was identified for the first time, its lifetime (∼30 ps) and transient absorption spectrum were determined as well as the mechanism of its formation and decay. Moreover, the authors observed the presence of a very fast intramolecular electronic relaxation process (S 2 v >0 →S 0 ) during S 2 -state deactivation which competes with intramolecular vibrational redistribution in the S 2 -state.

Determination of ferrocene triplet lifetime by measuring T1→T1 energy transfer to phenylosazone-D-glucose

Abstract The lifetime of ferrocene in the triplet state T1, τT1 = 0.6 ns, was determined in dimethyl sulfoxide at room temperature by ferrocene-sensitized photoisomerization of phenylosazone-D-glucose. Using laser flash photolysis it was found that for ferrocene within the spectral range 300–700 nm, the value of the molar absorption coefficient for T1→Tn transitions is lower than 500 M−1 cm−1.