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Zusammenfassung:
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°.
