This work has examined the photoreactivity of benzophenone (3), 2-benzoylthiophene (4), 4-methoxybenzophenone (5), 4,4 '-dimethoxybenzophenone (6), and 4-carboxybenzophenone (7) with 2-aminobenzimidazole (1). Laser flash photolysis (LFP) revealed quenching of the aromatic ketone triplets by 1, leading to formation of ketyl radicals plus aminyl radical 1-H center dot. The quenching rate constants obtained for 3 (n pi* triplet) and 4 (pi pi* triplet) were 6.2 x 10(9) and 3.9 x 10(9) M-1 s(-1,) respectively. The similarity between the two values suggests that the process is not a pure hydrogen abstraction but rather a charge transfer followed by proton transfer. This is in agreement with thermodynamic calculations, using the Rehm-Weller equation. In the case of 5 and 6, the transient absorption spectra showed distinct bands corresponding to both types of triplets (n pi* and pi pi*); their ratio was found to depend on solvent polarity. In the presence of 1, spectral changes were also consistent with formation of the aminyl/ketyl radical pairs. The rate constants for quenching of both types of triplets were very high, in the range 10(9)-10(10) M-1 s(-1.) When an electron acceptor substituent was attached to the aromatic ring, as in 7 (n pi* triplet), the quenching rate constant was higher (8 x 10(9) M-1 s(-1)), close to diffusion control. The reaction mechanism for hydrogen abstraction from 1 by triplet excited 3 or 4 was theoretically studied using density functional theory (DFT) methods. The results suggest formation of ground state molecular complexes, where one electron is transferred from the 2-aminobenzimidazole to the benzophenone or benzoylthiophene moieties upon excitation, giving radical ion pairs; subsequent proton transfer from the amino group to the carbonyl oxygen atoms leads to the neutral biradicals. A comparison between the relative energies and geometries of the species involved in the photochemical reactions indicates that all ketones follow a similar mechanism.