Investigations of the influence of molecular geometry on the spectroscopic and photochemical properties of alpha-oxo[1.n]paracyclophanes (cyclophanobenzophenones)

The spectroscopic and photochemical properties of a family of - [ 1.«] paracyclophanes (8-12) for = 8-12, respectively) have been investigated. Compared to a model structure possessing a nominal )( *) state (4,4'-dimethylbenzophenone, DM-BZ), photochemical and photophysical evidence is presented that the * character of T] decreases as the cyclophane ring size decreases. For example, the rate constants for hydrogen atom abstraction from 1,4-cyclohexadiene in acetonitrile are 1.6 X 10s, 0.93 X 10s, 0.30 X 10s, 0.06 X 108, and 0.04 X 108 M-1 s-1 for triplet 12-8, respectively, a result consistent with decreasing * character and decreasing reactivity toward hydrogen atom abstraction with decreasing value of n. The spectroscopic properties (vibrational structure of the phosphorescence spectrum, phosphorescence lifetime, phosphorescence excitation spectra, triplet-triplet absorption spectra) also vary in a manner consistent with decreasing * character as the cyclophane ring size decreases