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Stepwise versus Concerted Mechanism of Photoinduced Proton Transfer in sec-1,2-Dihydroquinolines: Effect of Excitation Wavelength and Solvent Composition

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journal contribution
posted on 12.02.2015 by Tatiana D. Nekipelova, Ivan S. Shelaev, Fedor E. Gostev, Victor A. Nadtochenko, Vladimir A. Kuzmin
Excited-state proton transfer in 2,2,4,6-tetramethyl-1,2-dihydroquinoline was studied in dependence on solvent composition in binary mixtures of MeOH with nonpolar pentane and polar acetonitrile by an ultrafast pump–probe technique with λpump = 350 nm and spectroscopic detection. The concerted mechanism was observed earlier at λpump = 308 nm, whereas the stepwise mechanism is clearly observed at λpump = 350 nm. The “fast” (200 fs) component is attributed to PT from MeOH molecule to the C(3) of the heterocycle to give the carbocationic structure in the highest vibrational ground state. This process is succeeded by the “slow” PT from the NH group to the MeO anion in the cluster of MeOH molecules to give corresponding 2,3-dihydroquinoline. The former process is independent of the solvent composition, whereas the latter depends on the solvent composition in the MeOH–C5H12 mixtures with a characteristic time increasing from 15 ps in bulk MeOH to 150 ps in the mixture containing 2 vol % MeOH. This result is explained in terms of stabilization of the ionic species in the less polar microenvironment.