E. N. Khodot

Effect of the nature of inert solvent on the decay kinetics of the carbocation generated in the photolysis of 1,2-dihydroquinolines in methanol

The dependence of the quantum yield and the decay rate constant for the carbocation generated in the photolysis of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline (6-EtO-DHQ) in methanol on the solvent composition was studied in the mixtures of methanol with isopropanol, acetonitrile, and pentane by pulse photolysis. The addition of these solvents decreases the yield of the carbocation and differently affects the kinetic parameters of its decay. The carbocation decay in the mixtures MeOH-i-PrOH and MeOH-C5H12 is described by the pseudo-first order equation (k1), with the dependence of k1 having a maximum at 50 vol % of MeOH in the MeOH-i-PrOH mixtures, and k1 increasing with a decrease in the MeOH concentration in the MeOH-C5H12 mixtures. In the MeOH-MeCN mixtures, the value of k1 decreases with a decrease in the MeOH concentration, and, at the concentration of MeOH lower than 50 vol %, the contribution of the second-order reaction (k2) is observed. The activation energies and preexponential factors were determined in the MeOH-C5H12 mixtures of different compositions, and it was shown that Eact practically did not depend on the solvent composition and were close to Eact for other carbocations obtained in MeOH. The increase in k1 with a decrease in the MeOH concentration is caused by an increase in the preexponential factor. The results were discussed on the basis of the reaction mechanism involving the competing reactions of the carbocation combination with two nucleophilic particles, the MeOH molecule and the MeO− anion. The composition of the mixture and the nature of the inert solvent affect strongly the course of these reactions.

Acid-base properties of N-methyl-substituted 1,2-dihydroquinolines in the ground and excited states

Changes in the absorption and fluorescence spectra of 1,2,2,3-tetramethyl-(1), 1,2,2,4-tetramethyl-(2), 6-ethoxy-1,2,2,4-tetramethyl-(3), and 1,2,6-trimethyl-1,2-dihydroquinolines (4) were studied in aqueous solution over a wide pH range from 1.0 to 12.0. The quantum yields of fluorescence and the values of pKa of dihydroquinolines (DHQs) under study in the ground and excited states were determined, pKa = 4.5, 3.8, 4.5, and 4.2 for the ground state of compounds 1–4, respectively, and pKa ∼ 1.7 for the S1* state for all DHQs.

Spectral and time-resolved properties of novel hetarylazo dyes containing hydrogenated quinolines and triazole moieties

The spectral characteristics of novel hetarylazo dyes containing triazole and hydrogenated quinoline moieties and the spectral and time-resolved parameters of photochemical processes occurring upon their photoexcitation were studied by stationary spectrophotometry and pulse photolysis. All compounds have an intense absorption maximum in the visible spectral range at 470–500 nm with ɛ = 17000–27000 L mol−1 cm−1. Upon excitation with the visible light, reversible trans-cis photoisomerization occurs, which is followed by thermal cis-trans isomerization. The temporal parameters of the isomerization depend on the azo dye structure. The introduction of bulky substituents in the triazole and hydroquinoline moieties results in the increase in characteristic times of the thermal transformation of the generated transient species from milliseconds to seconds and in the partial irreversibility of the process.

Spectral and luminescence properties and photochemical transformations of 7,7,9-trimethyl-6,7-dihydrofuro[3,2-f]quinoline

Spectral, luminescent, and photochemical properties of 7,7,9-trimethyl-6,7-dihydrofuro[3,2-f]quinoline (FDHQ) have been studied in hexane, alcohols, and water. Unlike 1,2-dihydroquinolines without the furo substituent, the fluorescence quantum yields for FDHQ in water and methanol are high (0.4 and 0.2, respectively), thereby indicating a difference in routes of excitation energy degradation in these compounds. The regularities in the FDHQ phototransformations have been studied by steady-state photolysis. The composition and the yield of final products depend on the presence or absence of oxygen in the system to a higher extent than on the solvent nature. This indicates the participation of the triplet excited state in the photo-chemical reactions in oxygen-free systems.

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λpump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λmax = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.

Effect of the substituent position at the heterocycle on proton transfer from alcohols to the excited molecules of 1,2-dihydroquinolines

A novel compound 1,2,2,3-tetramethyl-1,2-dihydroquinoline (1223TMDQ) was synthesized, and the products of its steady-state photolysis in water, MeOH, EtOH, trifluoroethanol (TFE), PrnOH, and BuiOH were analyzed by 1H NMR and mass-spectrometry. The corresponding adducts with water and alcohols were identified. The presence of the adducts for alcohols with a number of carbon atoms n > 1 distinguishes 1223TMDQ from 2,2,4-trimethyl-1,2-dihydroquinolines with methyl-, alkoxy-, and hydroxy-substituents at positions 1, 6, and 8, for which the formation of adducts was observed only in the presence of water and MeOH. The results were interpreted in terms of the effect of steric hindrance caused by substituents in the heterocycle and increasing size of the alkyl group of alcohol on proton transfer from a solvent to the molecule of 1,2-dihydroquinoline, which occurs in the complex between the solvent and the dihydroquinoline molecule in the excited singlet state. It was shown that the main steric hindrance for the photoinduced proton transfer in 2,2,4-substituted 1,2-dihydroquinolines is the substituent at position C(4) of the heterocycle.

Dynamics of trans-cis photoisomerization of hetarylazo dyes

Dynamics of trans-cis photoisomerization of novel hetarylazo dyes containing hydrogenated quinoline and triazole or tetrazole moieties has been studied by femtosecond laser photolysis with spectrophotometric detection. For all the dyes under study, the absorbance dynamics after photoexcitation in the long-wavelength absorption band (λpump = 550 nm) is described by three fast processes with characteristic times of 0.07–0.27, 0.4–1.0, and 3–7 ps. The effect of the solvent and the azo dye structure on the dynamics of transient species has been investigated.

Spectral and kinetic parameters of the triplet state of 7,7,9-trimethyl-6,7-dihydrofuro[3,2-f]quinoline

The spectral and kinetic properties of the transient species generated in the photolysis of 7,7,9-trimethyl-6,7-dihydrofuro[3,2-f]quinoline (FDHQ) in the absence of oxygen have been studied by lamp and laser flash photolysis. The triplet origin of the short-lived transient species that absorbs in the range of 600–750 nm has been established in experiments with a donor and an acceptor of triplet energy. The rate constants of triplet-triplet annihilation and the interaction of the FDHQ triplet with oxygen (1010 L mol−1 s−1) have been estimated.

Role of steric hindrance in photoinduced proton transfer from solvent to excited molecules of 1,2-dihydroquinolines

It was shown that the photolysis of 1,2,6-trimethyl-1,2-dihydroquinoline (126TMDHQ) in water, methanol, ethanol, and isopropanol affords the corresponding adducts of water and the alcohols, unlike the case of 2,2,4-trimethyl-1,2-dihydroquinolines bearing the methyl, alkoxyl, and hydroxyl substituents in the 1-, 6-, and 8-positions, which were previously found to form adducts only in the presence of water and MeOH. The quantum yield of the 126TMDQ photolysis (Φ) in this solvent series changes as ΦMeOH:ΦEtOH:ΦPrOH = 10:3:1. The results were rationalized in terms of the effect of steric hindrance caused by substituents on the heterocycle and increasing size of the alcohol alkyl group on proton transfer from the solvent to the 1,2-dihydroquinoline molecule in the excited singlet state. The existence of two adduct isomers was revealed. The preferential formation of one of the isomers was considered from the standpoint of carbocation accessibility to the solvent by nucleophilic attack.

Novel hetarylazo dyes containing tetrazole and hydroquinoline moieties: spectral characteristics, solvatochromism and photochemistry

Novel hetarylazo dyes containing tetrazole and tetra- or dihydroquinoline moieties were synthesized and their spectral properties in solvents of different polarities and H-bonding abilities were examined. The dyes exhibit solvatochromism dramatically depending on the proton accepting ability of solvents: (DMSO > H2O > MeOH > ACN > CH2Cl2) and the dye concentration. Upon dilution the absorption maximum of the visible band shows a blue shift and the absorption coefficient of the maximum decreases. This was accounted for by complex formation either between the dye molecules or between the dye and solvent molecules. The H-bond with partial proton transfer is formed between the acidic NH group of the tetrazole moiety of a dye molecule and the basic NH group of the hydroquinoline moiety of the other dye molecule or with a solvent with proton accepting ability. Upon dilution the equilibrium shifts to the complex with a solvent. The coexistence of several forms of the dye molecules with different absorption spectra was demonstrated in pulse photolysis upon excitation by light with different wavelengths. Three forms of cis-isomers were registered. The photogenerated cis-isomers decay with lifetimes from 200 μs to 5 ms. The fast cis-trans dark isomerization determines the photostability of the dyes.