Valery V. Volchkov

Induced Intersystem Crossing at the Fluorescence Quenching of Laser Dye 7-Amino-1,3-Naphthalenedisulfonic Acid by Paramagnetic Metal Ions

The fluorescence and triplet state quenching of 7-amino-1,3-naphthalenesulfonic acid by paramagnetic metal ions have been investigated in an aqueous medium. The basic mechanism of the fluorescence quenching involves the static and dynamic electron transfer to the paramagnetic cation. The induced S1→T1 intersystem crossing at fluorescence quenching of the fluorophore by Cu2+ cation has been found. There is a correlation between triplet state quenching rate constants and values of the efficient paramagnetic susceptibility and spin of the cations. The rate constants for the quenching pathways have been calculated.

Complexation of Donor-Acceptor Substituted Aza-Crowns with Alkali and Alkaline Earth Metal Cations. Charge Transfer and Recoordination in Excited State

Complexation between two aza-15-crown-5 ethers bearing electron donor and acceptor fragments and alkali and alkaline earth perchlorates has been studied using absorption, steady-state fluorescence and femtosecond transient absorption spectroscopy. The spectral-luminescent parameters, the stability and dissociation constants of the complexes were calculated. The intramolecular charge transfer reaction takes place both in the excited state of the crowns and their complexes 1:1; the latter is subjected to photorecoordination resulting in a weakening or a complete disruption of coordination bond between nitrogen atom and metal cation, disposed within a cavity of the crown. The compounds investigated can be viewed as novel optical molecular sensors for alkali and alkaline-earth metal cations. The photoejection of a metal cation into the bulk was not observed.

A fluorescence study of the excited-state dynamics of boron dipyrrin molecular rotors

Noticeable viscosity dependence has been revealed for fluorescence spectra of three phenyl derivatives of boron dipyrrin in aqueous glycerol solutions. This dependence is less pronounced for the fluorescence lifetimes. Such behavior is characteristic of molecular rotors used as media microviscosity sensors. A significant growth of the radiative deactivation constant is observed in the range 298-150 K. Quantumchemical calculation of the model fluorophore support the assumption on barrier-free relaxation of the excited state from the pretwisted to the flattened conformation. The spectral-kinetic parameters of fluorophores have been determined, and viscosity graduation curves are presented.

The Dynamics of Intramolecular Excited State Relaxation of N-Anthryl Substituted Pyridinium Cations

N-(1-Anthryl)-2,4,6-trimethyl-pyridinium (I), N-(2-anthryl)-2,4,6-trimethyl-pyridinium (II) and 10-(1-anthryl)-1,2,3,4,5,6,7,8-octahydro-acridinium cations (III) with anomalously high fluorescence Stokes’ shift have been investigated. Fluorescence kinetics analysis at various temperatures showed that in the range 293–77 K, the radiative deactivation rate constants (kf) increase by 5.5 to 30 times. The low-temperature time-resolved emission spectra of I–III were found to be consistent with the model: A→ A*⇚ B* where A* is the local excited twisted form and B* is the relaxed more planar, bent conformer of the molecule. The rate constants of the excited relaxed state formation (k1) and back reaction (k−1) of compounds studied were estimated.

The dynamics of excited state structural relaxation of 4-dimethylaminobenzonitrile (DMABN) and related compounds

The excited state structural relaxation of 4-dimethylaminobenzenes with various para-acceptor substituents having double-band emission, local excited (LE) and charge transfer (CT), has been investigated. Fluorescence measurements at different temperatures and in different solvents have confirmed the existence of viscosity-dependent, temperature, and polarity-activated relaxation. The kinetics analysis has shown that the radiative deactivation rate constants of the individual LE and CT states differ by 7–112-fold. The dipole moment changes at the excitation for CT states are significantly larger than those for LE states. The spectral-kinetics behavior of compounds studied agrees with the models A→A∗→B∗ or A→A∗↔B∗, where A∗ is the local excited planar and B∗ is the relaxed twisted state of the molecule. The rate constants of the twisted state formation have been calculated in the temperature range 293–77 K. The activation energies of forward process for compounds studied have been estimated.

The Effect of the Complexation of p-N,N-Dimethylaminobenzoic Acid and p-N,N-Dimethylaminobenzonitrile with LaCl3 on Spectral-luminescent Parameters of Fluorophores

The LE band fluorescence enhancement of p-N,N-dimethylaminobenzoic acid (DMABA) and p-N,N-dimethylaminobenzonitrile (DMABN) was found in aprotic acetonitrile and butyronitrile at the addition of LaCl3. The corresponding ICT fluorescence band remains unchanged. This enhancement is explained by the decrease of the internal conversion rate constant in a coordination complex with LaCl3. The formation of the coordination complex between DMABA and LaCl3 in ethanol is accompanied by the efficient fluorescence quenching in LE and ICT bands, in parallel with the enhancement of ICT/LE emission ratio. The experimental data are well described by the proposed kinetic schemes.

Fluorescence Study of the Relaxation Process in Excited Hetarylthiazole Cations

The absorption and fluorescence spectra of five cations protonated at the quinolyl nitrogen atom (IH+−VH+) and one ethylated (IEt+) cation were investigated. For these compounds (except VH+) both an anomalously large fluorescence Stokes shift (up to 238 nm) and a large short-wavelength fluorescence shift (up to 145 nm) at decreasing temperatures (down to 77 K) were observed. This is not the case for unprotonated molecules. The ground-state conjugation between quinolyl and another molecular fragment was found for II, IH+, IIH+, and IEt+. The relaxation process of excited cations is medium viscosity and temperature dependent. The experimental results are explained in terms of excited-state structural relaxation.

Molecular rotors based on styryl dyes. Viscosity dependence of rotation of molecular fragments

Three donor-acceptor styryl dyes R-Het+-CH=CH-C6H4-NR′R″ClO4− (Het is pyridyl) were synthesized. Their spectral-luminescence behavior allows their assignment to a class of molecular rotors. The influence of the viscosity, polarity, and temperature of the medium on their absorption and fluorescence properties was studied. A pronounced enhancement of dye fluorescence accompanied by a short-wavelength shift of the fluorescence maximum is observed with an increase in the viscosity of the medium and temperature decrease. The measurements of the fluorescence spectra of the dyes in the poly(methyl methacrylate) films at 293 and 77 K confirmed the effect of the medium viscosity on the rotation ability of molecular fragments upon photoexcitation. According to the quantum chemical calculations, in excited molecules of the styryl dyes in nonpolar solvents, molecular fragments rotate mainly about the central ethylene bond

Peculiarities of interaction of dual-band fluorophores with manganese halides

(HC\underline \cdots CH)

Protonation of 2-quinolylthiazoles in the ground and excited states in ethanol

. In polar solvents, the rotation barriers around the ordinary bonds of the ethylene fragment decrease.