S. I. Druzhinin

Planar and twisted intramolecular charge transfer states of the excited proton transfer products of aminocoumarins

Abstract The excited state proton transfer (ESPT) reaction of aminocoumarins containing primary ( Ia, If ), secondary ( Ib, Ic ) and tertiary ( Id, Ie and IIa, IIb ) 7-amino groups was studied. The limiting value of the fluorescence quantum yield (φ′ ∞ ) of the ESPT product of the 7- aminocoumarins Id and Ie , containing an unfastened tertiary amino group, is unusually low in polar solvents (ethanol (EtOH), dimethylsulphoxide (DMSO)) in contrast with 7-aminocoumarins Ia-Ic and IIb . A decrease in solvent polarity results in a strong (more than 40-fold in cyclohexane) increase in φ′ ∞ , probably due to a decrease in the rate of non-fluorescent, cationic, twisted intramolecular charge transfer TICT) state formation.

Intermolecular excited state proton transfer reactions of diaryloxazoles

Abstract The acid-base interaction of diaryloxazoles (DAO) in the ground and excited states with proton donors in protic and aprotic solvents have been studied. The scheme of DAO-carboxylic acids interaction, including the formation of H-bond complex 1:1 and ionic pairs 1:2 and 1:3 have been proposed. The equilibrium constants of the complex formation and luminescence characteristics of DAO conjugated forms have been determined. It has been shown that the basicity of nitrogen atom of DAO increases at the excitation. In the presence of proton donor the excited state proton transfer reaction (ESPT) takes place. The reaction efficiency which characterizes the probability of the excited product formation in the reaction act is close to unity. This result agrees with the estimation of DAO ESPT efficiency, made on the basis of the least photochemical motion principle. ESPT in the system 2,5-di(biphenyl)-oxazole-trifluoroacetic acid in protic solvents (ethanol, acetic acid) takes place mainly by static mechanism at the excitation of H-bond complex 1:1. The effect of the base and acid structures and the solvent nature on the proton transfer reaction of DAO in the ground and excited states has been discussed.

RADIATIONLESS DEACTIVATION OF EXCITED MOLECULES OF 4-AMINONAPHTHALIMIDES

The effects of alkylation of the 4-amino group, polarity and viscosity of a solvent, and temperature on the fluorescence ofN-(2,4-dimethylphenyl)imides of 4-amino-1,8-naphthalenedicarboxylic acid were studied. A decrease in the temperature and polarity of 4-dimethylaminonaphthalimide to decrease and have almost no effect on the rate of emissive deactivation of excited state. The abnormally low efficiency of fluorescence of 4-dimethylaminonaphthalimide in polar low-viscosity media at room temperature is associated with the formation of a nonfluorescent charge-transfer state. The increase in the rate of radiationless deactivation of 4-aminonaphthalimide in the alkylation of its amino group is associated with the increase in its electron-donor ability.

CROWN-CONTAINING STYRYL DYES. XIV: SYNTHESIS, LUMINESCENCE, AND COMPLEXATION OF THE TRANS-ISOMERS OF CHROMOGENIC 15-CROWN-5-ETHERS

Novel crown ether styryl dyes containing heterocyclic residues with various substituents were synthesized. The fluorescence and absorption spectra of thetrans-isomers of crown ether styryl dyes and their complexes with metal cations were studied. Based on spectral data, the influence of the nature of a heterocyclic residue, the crown ether fragment, and the temperature on the fluorescence of dyes and their complexes with metal cations was elucidated.

The mechanism of fluorescence quenching of aromatic compounds by acids: ZDO calculations

Abstract The fluorescence quenching of cyano, hydroxy, methoxy, and amino derivatives of naphthalene, anthracene and pyrene by acids in polar solvents has been studied by quantum-chemistry methods in semi-empirical approximation. Quenching mechanisms, including protonation of the aromatic nucleus and electron transfer have been considered. It is shown that the mechanism of radiationless deactivation in encounter complexes of reagents consists in the specific interaction of the solvated proton with certain carbon atoms of the aromatic nucleus, not in the electron transfer. It is found that the rate constant of radiationless deactivation correlates with the charge at the corresponding carbon atom of the excited molecule. It is shown that the fluorescence quenching is determined mainly by the nature of the fluorescent state and electron donor-acceptor properties of the aromatic nucleus and the substituent. The present model makes it possible to predict the fluorescent properties of some aromatic compounds in the presence of acids.

Excited state proton transfer in 2-heteroaryloxazoles. The principle of least photochemical motion

Abstract The excited state proton transfer (ESPT) of 2-heteroaryloxazoles with acetic acid in ethanol has been studied by means of fluorescence spectroscopy. The reactions take place in the excited state by static and dynamic mechanisms. Various 4-derivatives of pyridine have similar acid-base properties in the ground and excited state. This agrees with quantum-chemical calculations by CNDO/S method. The quantum yields, equilibrium constants and rate constants of these reactions have been determined. The efficiency of ESPT of 2-heteroaryloxazoles is close to unity. It is 2.4 times greater than that of azaderivatives of aromatic hydrocarbons. The ESPT and induced deactivation have the same reaction center — the nitrogen atom of the heterocycle. The principle of least photochemical motion has been proposed. According to this principle the quantum efficiency of formation of excited photoreaction products depends inversely on the extent of change of the wavefunction during the reaction. The experimental data agree qualitatively with this principle.

Charged substituents and the effect of their electric field on the electronic spectra of some aminocoumarins

Abstract The influence of a charged substituent which is not conjugated with a chromophor system on the maximum and intensity of the absorption and fluorescence spectra of organic compounds (intramolecular electrochromism) has been found. The maximum shift of the absorption and fluorescence maxima for 7-aminocoumarins is 1790 cm −1 . This is in accordance with the theoretical estimation for an electric field ≈ 3 × 10 7 V/cm. The results have been discussed at the assumption of the field nature of the inductive effect. The shift value correlates linearly with the σ*-Taft inductive constant of substituent and decreases as the refraction index of the solvent increases.

Proton-transfer-induced radiationless deactivation in 2,4,6-triarylpyridines

Abstract Excited state proton transfer (ESPT) of 2,4,6-triarylpyridines (TAPs) with carboxylic acid in dioxane, tetrahydrofuran, ethylacetate, acetonitrile and ethanol has been studied by absorption and fluorescence spectroscopy and kinetic fluorometry. The electronic structures of the TAPs and their cations have been calculated by the CNDO/S method. The ESPT proceeds via a dynamic mechanism. The fluorescence-quenching rate constant is lower than the limiting diffusion rate constant of bimolecular reaction by more than an order of magnitude. This proves the existence of a potential barrier for the ESPT reaction of the TAPs. The excited product quantum yield in the ESPT reaction (ϕ r ) is less than unity and depends on the concentration of acetic acid (AcOH). This dependence can be explained neither by a static ESPT reaction nor by different reaction efficiencies for acid solvates of different structure. The limiting ([AcOH] = 0) value of ϕ r increases as the TAP basicity and acid strength increase. This value does not depend on the medium polarity but increases as the “concentration” of hydroxyl groups in the TAP solvate shell grows. The efficiency of ESPT correlates with the electron-accepting ability of the excited pyridinium ion.

Fast dynamic excited state proton transfer (ESPT) reaction of weak oxoaromatic bases. Aminocoumarins with fixed amino group

Abstract The excited state protonation of the carbonyl group of 7-aminocoumarins, cotaining a 7-amino group fixed in the yulolidine fragment, is an irreversible diffusion-limited process in protic solvents and proceeds by a dynamic mechanism. The effect of the non-stationary diffusion of reagents contributes significantly to fluorescence quenching (25% in the presence of 0.1 M H 2 SO 4 for IIb in EtOH). The limiting value (acid concentration →∞) of the fluorescence quantum yield of the product of the excited state proton transfer (ESPT) reaction ( φ ′∞) reaches 0.48. The quenching of 7-aminocoumarins in protic solvents is effected by the solvated proton. The dependence of k q on the solvent is determined by the limiting proton conductance in the solvent. The activation energy of the decay of the encounter complex between the solvated base and proton is determined by two factors: (1) the energy of HOR bond breakage in this complex; (2) the difference between the stabilization energies of the encounter complex and the transition state in the polar solvent.

Acid-base interaction of 2,3-dimethyl-6-phenylimidazo-[1,2-b]-1,2,4-triazine in the ground and excited states

Abstract The acid-base interaction of 2,3-dimethyl-6-phenylimidazo-[1,2-b]-1,2,4-triazine (DMPIT) in the ground and excited states was studied. The electronic structures of 6-phenylimidazo-[1,2-b]-1,2,4-triazine and its cations were calculated by the CNDO/S and MNDO methods. The ground state protonation of the DMPIT molecule proceeds at the N5 atom of the imidazole ring. In hydrocarbon solvents, a hydrogen-bonded 1 : 1 complex and 1 : 2 ionic pair of DMPIT with trifluoroacetic acid are formed. In polar and basic solvents, a 1 : 1 ionic pair is formed (pKa=2.76). On excitation of DMPIT, the basicity of the N5 atom decreases ( p K a ∗ =−0.88 ) and that of the N4 atom increases. In the excited state, the dissociation of N5-protonated DMPIT takes place. The rate of dissociation increases as the solvent basicity increases. This reaction induces a radiationless deactivation (quantum yield of adiabatic reaction fr=0.46 in water). In dioxan, ethanol and water, excited DMPIT is protonated by an acid at the N4 atom by a dynamic mechanism. In hydrocarbon solvents, the phototautomerization of the N5 cation to the N4 cation takes place in the hydrogen-bonded 1 : 2 ionic pair. This photoreaction is similar to that in the 7-azaindole-solvent complex.