Electron spin resonance of the lowest excited triplet states .alpha.-Oxo[1.n]paracyclophanes [cyclophanobenzophenones]. Effect of molecular geometry on the electronic character of the triplet state

We report a low-temperature electron spin resonance (ESR) investigation of the first triplet excited states of a series of cyclophanobenzophenones, the a-oxo[ 1 .n]paracyclophanes (n = 8-12). The zero-field D splitting parameters for the series range from about -0.12 cm-1 for n = 12 to about -0.05 cm-l for n = 8. Relative triplet sublevel populating rates range from about 90% Pz and 0% Px for n = 12 to about 50% Pz and 40% Px for n = 8. The results for n = 12 are comparable to those for benzophenone and 4,4'-dimethylbenzophenone. Molecular mechanics show that as the methylene tether that connects the two phenyl rings is shortened, the phenyl rings are forced to rotate away from conjugation with the carbonyl ?r system. We propose that as this conformational change occurs, unpaired electron density in the n orbital of oxygen is transferred to the phenyl rings. This proposal is supported by semiempirical quantum mechanical calculations. Interpretation of our spectroscopic results show that the 15-20% of nominal ?rx* character found in the lowest triplet of the n = 12 cyclophane increases to around 90% for then = 8 cyclophane. The extremeconformational sensitivity of the phosphorescence lifetimes and T1 ESR spectra of benzophenone, dimethylbenzophenone, and cyclophanes n = 9-12 suggests that TI changes from n?r* to ?ra* character between phenyl ring twist angles of between 40° and 55°