Femtosecond excited state dynamics of stilbene-viologen complexes with a weakly pronounced charge transfer.

The femtosecond dynamics of photoinduced electron transfers in supramolecular donor-acceptor complexes between (E)-bis(18-crown-6)stilbene (D) and tetraperchlorates of 2,7-di(2-ammonioethyl)(2,7-diazapyrenium) (A1), 3,3'-(E)-ethene-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A2) and 4,4'-ethane-1,2-diylbis[1-(3-ammoniopropyl)pyridinium] (A3) was studied. The acceptors A2 and A3 are weak electron acceptors whose first reduction potentials are equal to -1.0 and -1.2 V (Ag), respectively, while A1 is a strong acceptor with a reduction potential of -0.42 V. It was shown that the back electron transfer time in CT-states of the complexes D·A2 and D·A3 is 30-40 ps, which is approximately 50 times greater than the analogous time for the charge transfer complexes studied earlier. The complex D·A1 is characterized by ultrafast back electron transfer (770 fs). The relaxation pathway of excited states of D·A1 depends on the wavelength of the excitation light. When excited at 356 nm, the accumulation of a transient locally excited (LE) state with a 250 fs lifetime was observed. But when excited at 425 nm, the formation of the LE-state was not observed.

Femtosecond excited state dynamics of a stilbene-viologen charge transfer complex assembled via host-guest interaction.

The dynamics of the excited states of a supramolecular complex with a charge transfer between (E)-bis(18-crown-6)stilbene and 4,4'-(E)-ethene-1,2-diylbis[1-(2-ammonioethyl)pyridinium]tetraperchlorate was studied by means of femtosecond transient spectroscopy. It is found that the characteristic time of the conversion of the locally excited (LE) state into the charge transfer (CT) state is equal to 300 fs, whereas the characteristic time of the conversion of the CT state into the ground state is equal to 400 fs. Due to host-guest interaction involving hydrogen bonds, the complex possesses high thermodynamic stability. As a result of ultrafast photoinduced processes of the direct and back electron transfer, the complex does not fluoresce. Upon the interaction of the complex with alkaline-earth metal cations, "switch-on" of its fluorescence occurs.

Dynamics of excited-state intramolecular proton-transfer in 2-amino-3-(2'-benzazolyl)quinoline cations.

It was found that cations formed by the protonation of 2-amino-3-(2'-benzoxazolyl)-quinoline (ABO) and 2-amino-3-(2'-benzothiazolyl)-quinoline (ABT) at the nitrogen atom of the quinoline ring exhibit excited-state intramolecular proton transfer (ESIPT). The two-band fluorescence of these cations is due to the emission from two species: the initial tautomer (short-wavelength band) and the ESIPT product (long-wavelength band). The relative intensity of the long-wavelength band depends on the basicity of the proton-accepting moiety and temperature. Quantum-chemical calculations demonstrated that ESIPT in cations involves overcoming a significant potential barrier, which increases with the decreasing basicity of the proton-accepting benzazole moiety. Using femtosecond absorption spectroscopy and nanosecond fluorescence spectroscopy, the effective ESIPT time in the studied cations was determined, which increased with decreasing temperature.

Relaxation Photoprocesses in a Crowned Styryl Dye and its Metal Complex.

The effects of solvent and crown-ether moiety on spectral properties of pyridinium styryl dye were studied by steady-state absorption and fluorescent spectroscopy. Analysis of viscosity and polarity effects on fluorescence quantum yield and Stokes shift permitted us to suggest that there is a two stage process of excited state relaxation. The macrocyclic moiety has a little influence on the first stage of relaxation, which manifests itself in a magnitude of Stokes shift, but suppresses considerably the second stage, which manifests itself in a magnitude of fluorescence quantum yield. The metal complex shows an additional stage of excited state relaxation, namely, photorecoordination of metal cation within the macrocyclic cavity.

Competition and interplay of various intermolecular interactions in ultrafast excited-state proton and electron transfer reactions.

The main features of the photoinduced kinetics of both ultrafast excited-state proton and electron transfer reactions that occur in the picosecond (ps) and femtosecond (fs) time domains are compared. Proton transfer (PT) reaction kinetics can be described in terms of several discrete values of rate coefficients in the form of polyexponential functions where each value of the rate coefficient can be attributed to a definite physical behavior of the reaction mechanism. In contrast, electron transfer (ET) reaction kinetics requires a consideration of a continuous distribution of rate coefficients. This difference can be related to structure of the ground-state reactant pairs for each reaction. Excited-state ET can occur at various configurations of reactant molecules and its rate reflects the fluctuations of the distances and orientations of these molecules. In contrast, excited-state PT requires preliminary formation of a ground-state H-bonded complex with definite structure where the reaction occurs after photoexcitation.

Photodegradation of 2-mercaptobenzothiazole and 1,2,3-benzotriazole corrosion inhibitors in aqueous solutions and organic solvents.

The photochemical degradation of 2-mercaptobenzothiazole (MBT) and 1,2,3-benzotriazole (BTA) inhibitors was studied in the present work in aqueous and in organic solutions. The extent of photodegradation was assessed by UV-Vis spectroscopy and the main reaction products were identified by tandem electrospray ionization mass spectrometry (ESI-MS/MS). The analysis of degradation products upon UV irradiation revealed the predominant formation of dimeric compounds from MBT and oligomeric structures from BTA, which were further converted into aniline. The increase of the quantum yield of MBT and BTA photodegradation reactions under aerobic conditions both in aqueous and organic solvents was explained by an increase of the spin-orbit conversion of the singlet radical pairs into the triplet radical pairs in the presence of oxygen. These triplet pairs further dissociate into free radicals, or convert to the parent compounds. At the early stage of UV irradiation, free radical coupling leads essentially to dimer formation in the case of MBT and to the formation of oligomers in the case of BTA irradiation.

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 LaCl(3). 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 LaCl(3). The formation of the coordination complex between DMABA and LaCl(3) 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.

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 S(1)-->T(1) 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.

Magnetic isotope effect in the photolysis of organotin compounds.

Photolysis of organotin molecules RSnMe3 is shown to be a spin selective radical reaction accompanied by fractionation of magnetic, (117,119)Sn, and nonmagnetic, (118,120)Sn, isotopes between starting reagents and products. A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope effect.