Photoinduced intramolecular charge transfer and relaxation dynamics of 4-dimethylaminopyridine in water, alcohols, and aprotic solvents

Abstract

4-Dimethylaminopyridine (1), a charge-transfer (CT) probe and strong acceptor of proton and hydrogen bond, has been studied by stationary and broadband transient absorption (TA) spectroscopy. Upon photoexcitation S0→S1°, intramolecular CT occurs only in polar solvents suggesting the CT is solvent- and solvation-controlled. In n-hexane, there is no CT the state S1° decays with 81 ps directly to ground state S0. In aprotic acetonitrile, charge transfer S1°→S1n develops with 16 ps (n indicates non-protonated 1), much slower than solvation, implying a reaction barrier of 12 kJ/mol. Subsequently S1n decays to S0 on a 10 ns time scale with a large triplet yield. In water, 1 is fully protonated at the pyridine nitrogen, the charge transfer S1°→S1p (p indicates protonation) proceeds nearly barrierless with 0.15 ps, followed by a biexponential evolution with 2.6 (55 %) and 26 ps. The behavior in primary alcohols, from methanol to n-octanol in decreasing acidity, show mixed features characteristic for both acetonitrile and water, suggesting the presence of non-protonated and protonated species in solution. Decomposed emission spectra in n-propanol and n-butanol provide an estimate of the excited-state basicity, pKa ∼20, much higher than pKa = 9.3 in S0. The stationary spectra in alcohols originate both from protonated and non-protonated species. They can be well modelled by the spectra in water/acetonitrile mixture with varying water content, however differently for absorption and emission. In moderately acidic methanol, the emission spectra are very similar to those in water, while in very weakly acidic n-octanol or tert-butanol they resemble more those in acetonitrile. This picture is corroborated by transient absorption spectra and kinetics in the alcohols and water/acetonitrile mixtures. Deviations from the model, such as possible contribution from additional emission bands, are discussed.