Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2 '-aminoacetophenone and its derivatives in solution

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X - C6H4 - COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2 (AAP), NHCH3 (MAAP), N(CH3)(2) (DMAAP), NHCOCH3 (AAAP), NHCOCF3 (TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n, pi* - pi, pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic) = 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S-1(pi,pi*) and S-2( n, pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.