Primary Photophysical and Photochemical Processes in the Photolysis of 1,2-Dihydroquinolines in Different Solvents

Abstract

1 Recently, it has been shown the photochemical transformations of alkyl and alkoxy substituted dihydroquinolines are very sensitive to the solvent nature, with the critical change of the reaction direction occurring on passing from aprotic solvents to water and methanol [1][2][3]. In aprotic solvents and alcohols with C > 1, the homolytic cleavage of the N-H bond to give corresponding aminyl radicals was observed in the case of sec -DHQ [4]. The quantum yields of the homolytic photodissociation of the N-H bond in aprotic solvents upon excitation of the long-wavelength absorption band are low, and the fluorescence quantum yields are high [5,6]. This is accounted for by the fact that the reaction occurs from the first excited singlet rather than triplet state, and this process, as was shown earlier [5,7,8], has low efficiency. N -Alkylated DHQ ( tert -DHQ) are photochemically stable in hydrocarbon solvents. In water and methanol, the photoinduced addition of solvent molecules to the double bond of the heterocycle occurs under irradiation without formation of intermediate radicals in the case of both sec-and tert -DHQ [1]. The reaction occurs from the excited singlet state. The process is accompanied by a drastic increase in the quantum yield of the reaction with a decrease in the quantum yield of fluorescence [3,6]. A carbocation formed in the reaction of proton transfer from the solvent molecule to the substrate in the excited singlet state to the C(3) atom of the heterocycle is the primary transient species observed in the flash photolysis of tert -DHQ. In the case of the sec -DHQ photolysis in MeOH, cyclic o -quinomethane imine is the primary transient species, which is formed as a result of the dou-1 e-mail: nekip@sky.chph.ras.ru ble proton transfer: from the solvent molecule to the substrate on the C(3) position and from the substrate N-H bond to the solvent molecule (Scheme 1).A study of the spectral characteristics of DHQ in different protic and aprotic solvents and their mixtures has shown that changes in the mechanism of photochemical reaction correlate with changes in absorption and fluorescence spectra [6]. Hydrogen bonding with a proton-donating solvent molecule plays the key role in this process. Since the reaction proceeds from the excited singlet state and the reaction and fluorescence quantum yields strongly depend on the solvent, the results of measurements of DHQ fluorescence lifetimes in different solvents and of the wavelength dependence of the fluorescence quantum yield presented in this work are very important for understanding the mechanism of the photophysical and photochemical processes occurring upon interaction of DHQ with light. EXPERIMENTAL 2,2,4,6-Tetramethyl-1,2-dihydroquinoline ( 1 ) (mp 42° C, Reakhim, Russia), 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline ( 2 ) (bp 150-152° C , 12 mmHg, synthesized according to a procedure described in [9] by reacting 1 with methyl iodide), 2,2,4-trimethyl-1,2-dihydroquinoline ( 3 ) (mp 18° C, synthesized in the condensation reaction of acetone with ...

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

“…The results obtained support the assumption made in ref. 16 from the analysis of the wavelength dependence of the DHQ fluorescence quantum yield that the PT reaction between the excited singlet state of DHQ and MeOH bound in the H-bonded complex occurs in the unequilibrated S 1 state and give a reasonably clear picture of the excited-state dynamics of DHQ in MeOH (Scheme 2) and Pr i OH. On excitation of DHQ in MeOH two concurrent processes occur on the fs time scale: (i) PT between DHQ and the solvent (∼200 fs) and (ii) vibrational relaxation to the thermally equilibrated S 1 state (∼500 fs).…”

“…Recently, the lifetimes of fluorescence were measured for various DHQ in different solvents and these are s fl = 5.6 and 7.2 ns in Pr i OH and 1.1 and 0.96 ns in MeOH for compounds 1 and 2, respectively, 16 i.e., the lifetime of the excited singlet state S 1 differs in the two solvents almost by an order of magnitude. Simple calculations show that these values of the fluorescence lifetime correspond to the decay during 350 ps of ca.…”

“…It has been found 16 that the quantum yield of fluorescence for DHQ depends on the excitation wavelength within the longwavelength absorption band in water and methanol, in which the photoinduced PT reaction occurs, and is independent of the excitation wavelength in hexane and isopropanol. This allowed us to assume that the photoinduced PT occurs from the unequilibrated excited singlet state of DHQ.…”