Electron Energies, Dipole Electric Moments, and Distribution of the Laurdan Molecule over the Conformation States of Methyl Groups

Using the methods of molecular simulation and HyperChem v.5.0 programs (PM3 method), we carried out calculations of the principal spectroscopic characteristics and of the structure of the laurdan molecule in the ground and the first excited electronic states. The thermal static distribution of molecules over various possible orientations of the plane of methyl groups relative to the plane of the naphthalene bicycle was taken into account. The energies and dipole moments of these electronic states have been calculated as functions of the torsion angle of methyl groups. The existence of an additional mechanism of electronic spectrum broadening is shown; it is associated with thermal mismatch of the equilibrium orientations of the rotational fragment of the molecule and with the dependence of the electron transition frequency on the degree of deviation of the angle from the equilibrium value. The dependence of dipole moments on this angle has been found and calculated. This dependence is the strongest for the ground state. The maximum values of dipole moments in the ground and excited states are 4.0 and 7.6 D.     

Luminescence of Molecules with Internal Charge Transfer upon Longwave Excitation

We have studied the spectral properties of luminescence of laurdan molecules in glycerin upon excitation at the red edge of the absorption band at different temperatures. The most significant red-wave shift of the spectra (10 nm) for the longwave band of dual fluorescence is observed depending on the excitation wavelength at a low temperature of 260 K when a solvent forms a fairly rigid matrix. At the same time, at increased temperatures of up to 370 K a small bathochromic shift and a change in the shape of the luminescence bands are also recorded reliably. Changes in the excitation spectra were observed when luminescence was recorded in the bands of the LE- and CT states. The difference spectrum responsible for the additional absorption that does not make a contribution to the longwave luminescence component has been isolated. The decay kinetics of both luminescence components have been measured and their expansions in decay constants have been analyzed. The experimental dependences obtained point to the complex mechanism of inhomogeneous broadening of spectra.     

Effects of long-wavelength fluorescence excitation in liquid solutions of N-pyrrolobenzonitrile

The absorption, dual fluorescence, and fluorescence excitation spectra of N-pyrrolobenzonitrile (P5C) in a set of solvents with different polarities are studied upon irradiation of solutions by light with different photon energies. The dual fluorescence from the locally excited Frank-Condon and charge-transfer states are recorded in all cases. The change in the excitation photon energy does not affect the fluorescence band positions, but noticeably changes the intensity ratio between the bands in favor of the long-wavelength band belonging to the charge-transfer state. The effects observed are explained using the data of quantum-mechanical calculations, which demonstrate that the solutions of these systems can very likely contain rotational isomers with different pyrrole orientations with respect to the benzene ring. In the excited state, these isomers have different charge-transfer reaction rates, which leads to different intensity ratios of the recorded fluorescence bands.     

Broadening of electronic absorption bands and the kinetics of luminescence of dapoxyl

Stationary spectra of selectively excided dapoxyl and time characteristics of the decay of its luminescence in polar solvents with the radiation excitation by picosecond pulses were studied. It was found that the absorption and luminescence stationary spectra are characterized by significant inhomogeneity, more pronounced in higher viscous solutions. Using the solvatochromic method, the change in the dipole moment of dapoxyl on the electron transition was estimated to be 18 ± 3 D. It was concluded that time characteristics of the decay of luminescence and the displacement of instantaneous luminescence spectra with time toward the range of long wavelengths are related to the mutual orientational relaxation of molecules in the solution.     

Kinetic spectroscopy of orientational states of solvated dye molecules in polar solutions

Some intermolecular processes, especially those due to a change of orientational structure of the medium surrounding a dye molecule in solution, are studied by means of kinetic spectroscopy. The first part of the paper is devoted to the question of inhomogenous orientational broadening of electronic transitions of dyes in polar solutions, in particular, the dynamic character of such broadening is demonstrated. Then we discuss the importance of inhomogeneous orientational broadening of dye spectra for the mechanism of directed energy transfer in rigid solutions including biological systems. Finally we try to show how the orientational relaxation, i.e. the temporal change of the spatial configuration of the solvate, influences the kinetics of the energy transfer between two different dyes in a mixed liquid solution.     

Tunable,narrow bandwidth, 2 MW dye laser pumped by a KrF1 discharge laser

Characteristics of p-terphenyl and PBD, the most efficient uv dyes, have been investigated under high intensity pumping by a multi-atmosphere KrF¹ discharge laser at 248 nm wavelength. Energy conversion efficiencies, spectral features and temporal behaviour of the dyes were studied parametrically. A maximum powerof 2 MW uv emission with a bandwidth of ～2×10^-2 nm was obtained for PBD. It was found that a mixture of both dyes provided higher (by a factor of 1.3) output in the range 348–366 nm than either of the dyes alone. Significant pump intensity dependent conversion efficiency and pulse shape variations were observed for the most efficient uv dye, p-terphenyl and were related to triplet-triplet adsorption processes in the dye.     

Investigation of excited-state proton transfer in 3-hydroxyflavone molecules

The dual fluorescence spectra of 3-hydroxyflavone molecules excited by electromagnetic radiation in the region of the S ₁ and S ₂ absorption bands in the temperature region of 20–80°C are studied using the dynamic quenching of the excited state. An analysis of the fluorescence parameters shows that heating the solution from room temperature to 60°C increases the proton transfer rate by a factor of 1.24 in the case of standard excitation into the main absorption band and even stronger (by a factor of 6.9) in the case of excitation into the second absorption band. The presence of a quencher reduces the yield of the two emission bands and noticeably increases the proton transfer rate, by a factor of 1.16 at room temperature and by a factor of 1.25 at 80°C. Upon excitation into the second singlet band, the transfer rate increases even more (especially at higher temperatures), by a factors of 1.24 and 3.5 for the same temperatures. The temperature dependences of the transfer rate constant allowed us to estimate the activation energies of the proton transfer reaction under different physical conditions and reach conclusions about the mechanism by which this reaction proceeds. It is found that the proton transfer activation energy decreases from 500 to 360 cm⁻¹ when measured in temperature ranges of 20–40 and 20–60°C. The introduction of a quencher with a concentration of 5 × 10⁻³ M increases the activation barrier to 534 and 471 cm⁻¹ in the same temperature ranges.