Multipathway photodimerization of 2-methylthianaphthene 1,1-dioxide. An example of heavy-atom effect on intersystem crossing in a 1,4 diradical

The photodimerization of 2-methylthianaphthene 1,1-dioxide (MeTND) at 313 nm produces two photodimers whose structures are shown to be anti head to head (HH) and anti heat to tail (HT) about the cyclobutane ring. The major isomer produced is always the HH dimer in oxygen-free solutions. Fluorescence quenching of MeTND by 1,4-diazabicyclo[2.2.2]-octane produces a linear Stern--Volmer relation that yields an estimated singlet lifetime for MeTND of 2 x 10-9S. The kinetics of the photodimerization of MeTND were studied as a function of concentration of MeTND, solvent, triplet quenchers, and triplet sensitizers. The data suggest that the HH and HT dimers arise from different excited-state precursors. A monomeric excited triplet is the proposed progenitor of the HH dimer while an excimer is proposed to be the precursor of the HT dimer. Heavy-atom perturbation is found to occur on the S1 → T1 transition of MeTND but is not effective for the radiationless T1 → S0 transition. The triplet-sensitized dimerization of MeTND is found to be sensitive to heavy-atom perturbation and this is interpreted as a spin-orbital perturbation in intersystem crossing in the 1,4 diradical. 6 figures, 6 tables