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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 chromatography to choose the excitation and emission wavelength for which the emission of impurities is negligible for fluorescence decay measurements of the sample (S) and reference (R). It has been shown that the IRF in TCSPC depends on the refraction index of the solvent used. In order to avoid the systematic errors related to different IRF for S and R, the calculations of fluorescence lifetimes should be performed for S and R in solvents of the possibly most similar refraction indices. The effect of the solvents difference in the refraction indices on the shape of IRF has been shown and then on the fluorescence lifetimes below 20 ps. The proposed procedure of determination of single picosecond lifetimes with the use of TCSPC is particularly suitable when it is necessary to use a very low concentration (<=10-5 M) of the compound studied in order to avoid dimers and oligomers formation (in the upconversion method concentration >10-4 M is generally used). A very good statistics of the TCSPC method—much better than in the other time-resolved methods—together with the use of the procedure of measurements—the procedure of numerical analysis and procedure of elimination of emission of impurities proposed in our article—permit elimination of the systematic errors (usually ±3 ps) in the lifetimes determined for monoexponential decays and obtaining a highly reliable description of the fluorescence decay kinetics also when the fluorescence decay is nonexponential. [ABSTRACT FROM AUTHOR]

Presented herein are the luminescence properties of several 1,4,5,8- polinahthalisoimides. The luminescence decay curves after deconvolution exhibit three decay times: τ 1 , τ 2 , and τ 3 .The luminescence lifetimes change along with the growth of excitation energy, as well as the contribution of each luminescence decay mechanism ( A 1 , A 2 , and A 3 ). The manner of change is distinct for one sample, namely, the pure E-isomer, in comparison to the others. The radiative deexcitation coupled with radiationless vibrational energy transfer to other luminescence centers, mainly other isomer units, is discussed as a possible deexcitation mechanism of fluorescence. The Kasha–Vavilov empirical rule is not fulfilled for the studied samples, as the luminescence quantum efficiency ( Φ F ) distinctly depends on the excitation wavelength ( λ exc ). The quantum yield of one sample (about 63% of Z-isomer) is quite high, at 1%. [ABSTRACT FROM AUTHOR]

Abstract: Interactions of tyrosine and phenylalanine analogues with β-cyclodextrin have been examined in terms of structural features of the ligand such as the separation of the charged amino group and aromatic ring, the presence of additional functional group attached to the amino or phenyl ring, and the presence of a charge on amino or carboxyl group, and steric effects using steady-state and time-resolved fluorescence spectroscopy and microcalorimetry. The studied aromatic amino acids possess low binding constant to β-cyclodextrin, diversified with respect to the presence or absence of a substituent in para position of the phenyl ring. However, calculated, based on the global analysis of the fluorescence intensity decays, binding constants do not allow to estimate unequivocally the influence of the distance between the charged groups and phenol/phenyl ring on the inclusion complex stability because of their low diversification. [Copyright &y& Elsevier]

Abstract: A series of N-(tert-butoxycarbonyl)-3-[2-(phenyl)benzoxazol-5-yl]alanine methyl ester derivatives possessing electron-donor and electron-acceptor substitutents in the phenyl ring were synthesized and their photophysical properties in methanol, acetonitrile and methylcyclohexane were studied by means of absorption and fluorescence spectroscopies as well as theoretical calculations. The structures of compounds studied in the ground and excited state were calculated using the MOPAC 2002 package with COSMO model of the solvent whereas the positions of vertical absorption and emission transitions were calculated using a INDO/S or CNDO/Param method within a SCRF approximation for the solvent but they are not in good agreement with the experimental values. High molar absorption coefficient values and fluorescence quantum yields are characteristic for all compounds studied. Moreover, in the case of derivatives possessing electron-donor substituents in position 2 of the phenyl ring the fluorescence quantum yield increases with the solvent polarity whereas the opposite effect is observed for derivatives substituted by electron-acceptor groups. Also, the position of the absorption and emission band depends on the solvent polarity and is shifted to longer wavelengths in more polar solvents as a result of a charge transfer from a substituent to the benzoxazole moiety. In the case of derivatives substituted by 2-hydroxyphenyl, the intramolecular proton transfer process is observed. However, the character of the group in position 4 of the phenyl ring influences on it. An electron-acceptor substituent facilitates the proton transfer whereas the opposite is true for an electron-donor group. [Copyright &y& Elsevier]

The properties of the lowest excited triplet state (T1) of 2′,3′,5′-tri-O-acetyl-4-thiouridine (TU) and its 5-halogeno substituted derivatives (XTU, X = F, Cl, Br and I) in CCl4 solutions are presented. Steady-state and pulsed techniques were used to characterize the T1 state of TU and XTU in terms of the phosphorescence quantum yields (φp0), intrinsic lifetimes (τT10) and the self-quenching rate constants (ksq). The quantum yields of photochemical decay (ϕpch) of TU and XTU were also determined. From the values of quantum yields and under assumption that the formation of the T1 state can be approximated by ϕT1=1, the rate constants of radiative (kp) and non-radiative (knr) decay processes from the T1 state were calculated. The results of the studies show that the T1 states of TU and XTU have a ππ* nature. Comparison of the absorption and phosphorescence spectral properties of TU and XTU molecules with those of 1,3-dimethyl-4-thiouracil (DMTU) confirms that TU and XTU exist in CCl4 solution as (C-2)-keto-(C-4)-thione tautomers, exclusively, and that the phosphorescence arises from the excited T1 state of these single tautomeric forms. The effect of the halogen substituent on the T1 state properties of XTU molecules is discussed. [Copyright &y& Elsevier]