A MECHANISM OF PHOTODISSOCIATION OF DIPHENYLMETHANE TO A DIPHENYLMETHYL RADICAL IN SOLUTION

The photodissociation of diphenylmethane by excitation to the S-1 stat e at 266 nm in n-heptane solution is studied by nanosecond fluorescenc e and absorption spectroscopy. The formation of the diphenylmethyl rad ical is identified by its fluorescence, which is induced by excitation at 308 nm, and by its absorption. The growth rate of (3.7 +/- 0.4) x 10(7) s(-1) for the radical is equal to the decay rate of (3.8 +/- 0.4 ) x 10(7) s(-1) for the precursor fluorescence. The quantum yield of t he radical is of the order of similar to 10(-3). Neither dissociation to the radical nor intersystem crossing to the T-1 state is thermally activated, whereas activated internal conversion to the S-0 state is o bserved. The formation of the radical depends linearly on the photolys is pulse fluence. The data are consistent with a mechanism that the mo lecule undergoes intersystem crossing from thermally equilibrated leve ls of the S-1 state to vibrationally excited levels of the T-1 state a t which it dissociates in competition with vibrational relaxation. The mechanism is explained in terms of electronic coupling between the pr ecursor and product states. The S-1 state does not correlate adiabatic ally to the ground state of the C-H bond fission products, so intersys tem crossing to the T-1 state precedes dissociation. In the T-1 state, avoided crossing between the pi pi (benzene) configuration and the s igma sigma (C-H) repulsive configuration results in the adiabatic pot ential energy surface which evolves to the ground state of the C-H bon d fission products allowing rapid dissociation. (C) 1998 American Inst itute of Physics.