Experimental and theoretical studies on the mechanism of photochemical hydrogen transfer from 2-aminobenzimidazole to nπ* and ππ* aromatic ketones.
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•. The quenching rate constants obtained for 3 (nπ* triplet) and 4 (ππ* triplet) were 6.2 × 10(9) and 3.9 × 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 (nπ* and ππ*); 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 (nπ* triplet), the quenching rate constant was higher (8 × 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.
Jornet
Dolors
D
Departamento de Química/Instituto de Tecnología Química UPV-CSIC, Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, E-46022 Valencia, Spain.
Bartovský
Pavel
P
Domingo
Luis R
LR
Tormos
Rosa
R
Miranda
Miguel A
MA
D001562:Benzimidazoles / Q000737:chemistry*; D001577:Benzophenones / Q000737:chemistry; D004583:Electrons; D006859:Hydrogen / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry*; D007834:Lasers; D008962:Models, Theoretical; D015394:Molecular Structure; D010777:Photochemistry / Q000379:methods*; D010782:Photolysis; D013050:Spectrometry, Fluorescence / Q000379:methods; D013816:Thermodynamics
D001562:Benzimidazoles; D001577:Benzophenones; D007659:Ketones; C047723:benzophenone; D006859:Hydrogen; C027391:2-aminobenzimidazole