N. Kh. Petrov

Solvent effect on magnetic field modulation of excbplex fluorescence in polar solutions

Abstract The magnetic field effect on pyrene/N,N-diethylaniline steady-state exciplex fluorescence was found to depend on solvent polarity. It is explained in terms of a model based on macroscopic properties of the system. Potential barriers between radical ion pair state and fluorescent exciplex, E a = 0.18–0.23 eV at room temperature, have been obtained from comparison of experimental results with theoretical predictions.

A fluorescence spectroscopy study of preferential solvation in binary solvents

Published data are reviewed on the dynamics of nonspecific preferential solvation in binary solvents, as obtained by fluorescence spectroscopy (in particular, by magnetic spin fluorescence method) using probe molecules in which strong charge redistribution yielding dipoles and ion pairs takes place upon excitation. The dynamics of preferential solvation of the excited probe in a binary solvent is described by a sequence of consecutive equilibrium reactions, in which each step is the absorption of a molecule of the polar component by the solvation shell of the excited probe, where transport from the bulk solution occurs via translational diffusion. The characteristic effect of the time saturation of solvation is attributed to the energetically unfavorable process of interface formation between the solvation shell, which a polar nanosized cluster, and the homogeneous binary mixture. Such polar clusters may affect the course of photochemical reactions involving charged species or particles with a large dipole moment. Simple model concepts, such as the Frenkel theory of heterophase fluctuations, the Onsager model, and the Hildebrand theory of solubility used for the description of the physical picture of preferential solvation, are considered.

Supramolecular assembler based on cucurbit[8]uril: Photodimerization of a styryl dye in water

Photolysis of aqueous solutions of styryl dye 1 in the presence of cucurbit[8]uril (CB[8]) has been studied by optical spectroscopic methods for the molar ratios n = cCB[8]/c1 in the range of 0 ≤ n ≤ 6. It has been found that the inclusion complexes (1)2@CB[8] dominate in the solution at n ≤ 0.5, whereas the complexes 1@CB[8] dominate at n ≥ 1. The stability constants have been determined for the 1: 1 (log K1 = 6.2 (L mol−1)) and 2: 1 (log β = 11.9 (L2 mol−2)) complexes. The fluorescence decay kinetics of dye 1 in the presence of CB[8] is two-exponential, with the average lifetime increasing substantially at n ≥ 1. It has been shown that the system can operate in the cyclic mode as an assembler (or supramolecular catalyst) in the photodimerization reaction of dye 1 to form cyclobutane derivative 2. The stability constant of the complex 2@CB[8] (log K3 = 5.9 (L mol−1)) and the quantum yield of cycloaddition (⃜ ≈ 0.07 at n ≈ 0.5) have been determined.

Photophysical properties of aqueous solutions of a styryl dye in the presence of cucurbit[n]uril (n = 5, 6, 8)

Photophysical properties of aqueous solutions of the styryl dye 4-[(E)-2-(3,4-dimethoxyphenyl)-1-ethylpyridinium] perchlorate (1) in the presence of cucurbit[n]urils (CB[n]; n = 5, 6, 8) have been studied by fluorescent spectroscopy methods. The fluorescence intensity of a 10–6 mol L–1 solution of 1 increases by a factor of 12.6 upon the formation of 1 : 1 inclusion complexes with CB[6] or 1.3 in complexes with CB[8]. Upon the formation of inclusion complexes, the average lifetime of the electronically excited state of 1 increases to about 1 ns for both CB[6] and CB[8]. On the basis of fluorescence anisotropy measurements, the rotational relaxation times were estimated to be 408, 314, and 183 ps for the complexes with CB[6], CB[8], and for unbound 1, respectively. Using the fluorescence titration method developed for the case of poorly soluble cavitands, the binding constant of 1 with CB[6] was determined to be 1.1 × 105 L mol–1. The addition of CB[5] does not lead to changes in the photophysical properties of a solution of 1, indicating the absence of complexes between CB[5] and 1. It has been found on the basis of the experimental data that the fluorescence rate constant of 1 decreases about twice in the complex with CB[8], but doubles in the complex with CB[6].

The specific feature of photochemical processes in molecules of 3,3′-dialkylthiacarbocyanines in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photochemical processes of 3,3′-diethylthiacarbocyanine (DETCC) iodide and 3,3′-dimethylthiacarbocyanine (DMTCC) chloride was studied. In the mixtures with the DMSO content more than 20 vol %, DETCC iodide and DMTCC chloride molecules are characterized by a high efficiency of both trans → cis photoisomerization and fluorescence, in contrast to intersystem crossing to the triplet state. With decreasing the DMSO content to 3 vol %, the relative quantum yield of DETCC iodide intersystem crossing increases by two orders of magnitude, which is accompanied by a decrease in the lifetime of DETCC iodide triplet molecules from 1.1 × 10−4 to 2.4 × 10−7 s and a decrease in the yield of the cis-isomers. To explain the results, the concepts of “external heavy atom (iodide) effect” and hyperfine coupling (HFC mechanism) in radical pairs that could be formed via electron transfer between the iodide and an excited dye molecule were used.

Nonradiative relaxation of thiacarbocyanine dyes in binary mixtures.

It was found that preferential solvation of cyanine dyes in binary mixtures can strongly affect both their isomerization and aggregation; the comparison of absorption and fluorescence excitation spectra might be a useful tool for studying these nonradiative processes.

Ultrafast kinetics of fluorescence decay of aqueous solutions of styryl dye derivatives and their complexes with cucurbit[7]uril

The ultrafast relaxations of electronically excited states of styryl dyes, viz., 4-[(E)-2-(3,4-dimethoxyphenyl)- 1-ethylpyridinium] perchlorate (1) and 1-(3-ammoniumpropyl)-4-[(E)-2-(3,4-dimethoxyphenyl) vinyl]pyridinium diperchlorate (2), and their inclusion complexes with cucurbit[7]uril (СВ[7]) have been studied in aqueous solutions as a function of the fluorescence wavelength by femtosecond laser-induced fluorescence spectroscopy.For all dyes and their complexes, the fluorescence decay curves are fitted satisfactorily by the sum of two and three exponentials. The relaxation of electronically excited states in the blue wing of fluorescence band contains a component with a short decay time of about 100–200 fs and the red wing displays a rising component with a characteristic time of the same order of magnitude. In addition, the fluorescence decay includes a component with a characteristic time of about 2 ps, which is independent of complexation and fluorescence wavelength. The addition of СВ[7] to aqueous solutions of dyes 1 and 2 results in an increase in the longer relaxation times from ≈50 ps for the free dyes to 100–150 ps for the bound ones. The extra positive charge of dication 2 has almost no effect on the photophysical properties compared to cation 1, which agrees with quantum chemical calculations of the structures of inclusion complexes. A considerable increase in the fluorescence lifetime can be attributed to the increase in the potential energy barrier between the excited state of dye and the conical region on the potential energy curve, which is assumed to be caused by the displacement of the dye cation from the equilibrium state deep into the cavity within the first several picoseconds after laser pulse excitation.

Primary photoprocesses in molecules of carbocyanine dyes in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photophysical processes of thia-, indo-, and imidacarbocyanine dyes was studied. In the mixtures with the DMSO content of more than 20 vol %, the dyes representing solvated cations are characterized by high efficiency of trans → cis photoisomerization and fluorescence, in contrast to the extremely low efficiency of intersystem crossing to the triplet state. With growing the toluene content in the mixture, ion pairs between the dye cation and Cl−, Br−, I−, or BF4− anion are formed. In the cases when the dye counterions are Br− or I−, a sharp increase in the yield of the triplet molecules and a decrease in their lifetime take place. The results are discussed in terms of “the external heavy atom effect” in ion pairs.

Effect of counterions on photoprocesses of thiacarbocyanine in a binary solvent blend

A model describing the effect of counterion X− (X = Cl, I) on the deactivation kinetics of the S1 state of thiacarbocyanine Cy+X− is presented. According to the model, the ion pair Cy+X− in a binary solution is characterized by a distribution function f(r) over interatomic distances r, which depends on the composition of the mixture. The assumption of kinetically independent local states of the ion pair, which decay with the rate constants ki(r)(i = 1–4 is the index of the decay channel), is made. The statistic analysis of the experimental data in terms of the model permitted us to find the functions f(r) and to estimate the parameters of the constants ki(r).

Time-resolved fluorescence anisotropy of styryl dye–cucurbituril complexes

It is known that photophysical properties of a dye molecule change significantly in supramolecular host–guest complexes of organic luminophores with cucurbit[n]urils (CBn) [1]. For example, the f luorescence intensity of a styryl dye–CB6 complex increases more than 12-fold in comparison with the free dye [2]. This phenomenon is relatively poorly studied and is very interesting property from both fundamental and applied points of view. In this communication, we present new experimental results on ultrafast f luorescence depolarization of a styryl dye, 4-[4-(dimethylamino)styryl-1-methylpyridinium] iodide (DSM), in the host-guest complexes with CB6, in ethylene glycol and water solutions. Ethylene glycol (Khimmed, analytical grade), water (Millipor Simplicity), cucurbit [6]uril (Sigma Aldrich), and DSM (Sigma Aldrich) were used without additional purification. The measurements were carried out at ambient temperature, with the DSM concentration being 5 × 10–6 mol/L in steady-state experiments or 2 × 10–4 mol/L in the study on picosecond timescale. The concentration of CB6 aqueous solution was 2.3 × 10–4 mol/L. Steady-state f luorescence spectra and fluorescence anisotropy were measured on a Fluorolog 3 Tau spectrofluorimeter. Fluorescence decay kinetics on nanosecond timescale was studied by correlated single-photon counting on a PicoQuant spectrofluorimeter. Femtosecond fluorescence depolarization was studied by the upconversion (frequency summation) method using a chromium-forsterite laser (pulse duration of 90 fs, first harmonics of 1280 nm). The detailed description of the experimental setup is given elsewhere [3]. The upconversion nonlinear BBO crystal is physically a polarizer (analyzer). The transmission direction of the crystal is determined by the direction of oscillations of the electric vector of a linearly polarized gate pulse. The time-resolved f luorescence anisotropy as an important characteristic of the f luorophore was determined experimentally as