Photochemistry and photodissociation of benzosultine and naphthosultine: electronic relaxation of sultines and kinetics and theoretical studies of fragment o-quinodimethanes

Fluorescence decays of benzosultine and naphthosultine excited at 263 nm are detected with time-correlated single-photon counting (TCSPC). Both molecules display biexponential decay with a rapid component (time constants 90 and 350 ps for benzosultine and naphthosultine, respectively) assigned to an electronically excited state Sn at high energy and the slow one (constants 7.5 and 9.0 ns for benzosultine and naphthosultine, respectively) to the S1 state. Dissociation of both molecules was investigated with nanosecond laser flash photolysis combined with transient absorption to detect the intermediates and products. With excitation at 266 nm, benzosultine yields a transient with absorption maximum at λmax = 370 nm; this transient has a short-lived component with lifetime around 1 μs and a long-lived component. Both components are insensitive to molecular oxygen. The short-lived component is tentatively assigned to the dissociation intermediate and the long-lived to be singlet o-benzoquinodimethane (o-BQDM). Photolysis of naphthosultine yields two transient species with absorption at λmax = 420 and 520 nm; we assign the former band to triplet–triplet absorption of naphthosultine and the latter to absorption by the product singlet o-naphthoquinodimethane. Optimal geometries, energetics, and vertical transitions of benzosultine, naphthosultine, o-benzoquinodimethane, and o-naphthoquinodimethane are calculated using methods based on density-functional theory (B3LYP/6-311++G**) and time-dependent density-functional theory (TD-DFT). The results of these calculations imply that the ground electronic state of these two o-quinodimethanes (o-QDM) is singlet with a structure of diene form. Their triplet states display a biradical structure. The energy separation between singlet and triplet states of o-benzoquinodimethane is calculated to be ca. 93.6 kJ/mol, but for o-naphthoquinodimethane, it is only ca. 27.8 kJ/mol.