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

Abstract

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,π*–π,π* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k_ic = 1.0 × 10¹¹ s⁻¹ for AAP and 3.9 × 10¹⁰ 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 S₁(π,π*) and S₂(n,π*) 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.