ELECTRON-SPIN-RESONANCE OF THE LOWEST EXCITED TRIPLET-STATES OF 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 alpha-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(-1) for n = 8. Relative triplet sublevel populating rates range from about 90% and and Oro P-z for n = 12 to about 50% P-z and 40% P-x 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 a 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 pi pi* character found in the lowest triplet of the n = 12 cyclophane increases to around 90% for the n = 8 cyclophane. The extreme conformational sensitivity of the phosphorescence lifetimes and T-l ESR spectra of benzophenone, dimethylbenzophenone, and cyclophanes n = 9-12 suggests that T-l changes from n pi* to pi pi* character between phenyl ring twist angles of between 40 degrees and 55 degrees.