Intramolecular charge transfer (ICT) is an important photochemical process. In contrast to those in singlet manifold, triplet ICT states were less studied. In this paper, the lowest excited triplet state (T-1) of 4-amino-4'-nitrobiphenyl (NH2-Bp-NO2) was recorded with nanosecond transient absorption spectroscopy in acidic acetonitrile and alcoholic solutions. By employing the Kamlet-Taft model to analyze the correlation between absorption maxima and alcohol solvent properties including polarity/polarizability, abilities of hydrogen bond donating and hydrogen bond accepting, hydrogen bond configuration in the ground state (S-0) and T-1 was resolved. The results suggest that the hydrogen bond between amino H and alcohol is dominant in S-0, while in T-1, hydrogen bonds between amino Hand alcohol, between nitro O and alcohol have comparable contribution. By examination of the 1-naphthol quench effect on T-1, the hydrogen bond between nitro O and alcohol was confirmed present. Theoretical calculation results on the model of NH2-Bp-NO2-(MeOH)(3) also indicate that hydrogen bonds between amino H and alcohol, between nitro O and alcohol are both much stronger in T-1 than in S-0. In acidic acetonitrile solution, in S-0 of NH2-Bp-NO2 the amino group is protonated with pK(a) of 4.5, meanwhile in T-1 the nitro group is much easier to be protonated than in S-0. Its conjugated acid was measured to have a pK(a) of 3.1. (C) 2019 Elsevier B.V. All rights reserved.
