T. D. Nekipelova

Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.

Ultrafast excited state proton transfer dynamics of 1,2-dihydroquinolines in methanol solution

Femtosecond and picosecond dynamics of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) were studied in MeOH, MeOD, and Pr(i)OH to probe the early events of the photoinduced proton transfer (PT) between 1,2-dihydroquinolines (DHQ) and a solvent. From studies in the two solvents MeOH and Pr(i)OH and by examining the effect of deuterium replacement of proton, it has been established that PT takes 150-200 fs in MeOH, but does not occur in Pr(i)OH. The formation of PT products in the ground state proceeds concurrently to the relaxation of the higher vibrational excited singlet state to the thermally equilibrated state S(1) of DHQ. The absorption spectrum of the S(1) state was registered, and the time constant of its decay in MeOH (ca. 1 ns) agrees well with the lifetime of fluorescence measured recently by single photon counting.

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.

Mechanism of Photoinduced Addition of Water and Methanol to the Double Bond of 1,2-Dihydroquinolines

The kinetics and mechanism of photoinduced addition of water and methanol to the double bond of 2,2,4-trimethyl-1,2-dihydroquinoline (DHQ) bearing different substituents in the 6- and 8-positions and N-methylated DHQ were studied by flash photolysis. The reaction affords corresponding Markovnikov adducts, 4-hydroxy- and 4-methoxy-1,2,3,4-tetrahydroquinolines. On the basis of time-resolved experiments and the measurement of quantum yields of the reaction in media of different acidities or basicities, a general scheme of the reaction is suggested, which involves the formation of two intermediate species. The principal rate constants, activation parameters, and their dependence on substituents were determined.

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.

Studies of photoinduced addition of water and alcohols to substituted dihydroquinolines

Steady-state photolysis products of 6- and 8-substituted 2,2,4-trimethyl-1,2-dihydroquinolines in water and lower alcohols were identified by 1H NMR. In the case of electron-donor substituents, the solvent molecule is added to the double bond of the heterocycle affording the corresponding 4-hydroxy- or 4-alkoxytetrahydroquinolines. Nitro-substituted dihydroquinolines are photostable. The addition of EtOH and PrnOH occurs only in the presence of water to give a mixture of alkoxy- and hydroxy-adducts. A reaction scheme is suggested.

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the spectral and luminescent properties of 1,2-dihydroquinolines

The dependence of absorption and fluorescence spectra, quantum yields, and lifetimes of fluorescence on the solvent composition in the MeOH-C5H12 and MeOH-MeCN mixtures was studied for 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMDHQ). The variations in the parameters of deconvolution of the absorption and fluorescence spectra by the Gaussian functions in the MeOH-C5H12 mixtures of various compositions indicate the specificity of methanol clustering in saturated hydrocarbons and hydrogen bonding between TMDHQ and the methanol clusters of different compositions. At low MeOH concentrations (∼0.2 vol %), TMDHQ molecules are practically completely bound with the MeOH molecules by hydrogen bonds. In the MeOH-MeCN mixtures, the changes in the absorption and fluorescence spectra are observed at a substantially higher MeOH concentration (≥10 vol %) and monotonically change at the further increase in the MeOH concentration that is caused by the peculiarities of MeOH clustering in acetonitrile and the distribution of the TMDHQ molecules between the solvent components. At 50–95 vol % of MeOH in the mixture with MeCN, the fluorescence decay kinetics is described by the biexponential curve with the lifetime of the major component (τ1) decreasing from 7.5 to 1.1 ns in pure MeCN and MeOH, respectively, and the lifetime of the minor component τ2 ≈ 4 ns corresponding to the fluorescence lifetime in the solution containing 50 vol % MeOH. This indicates the existence of the free TMDHQ molecules, which are not bound with MeOH molecules or their clusters.

Synthesis of advanced fluorescent probes — water-soluble symmetrical tricarbocyanines with phosphonate groups

A method for the synthesis of a series of water-soluble heptamethine indocyanine dyes containing a phosphonate group in the substituent bonded to the quaternary nitrogen atom of the indolenine moiety has been developed.

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.