Investigations of the Influence of Molecular Geometry on the Spectroscopic and Photochemical Properties of α-Oxo[1.n]paracyclophanes (Cyclophanobenzophenones)

The spectroscopic and photochemical properties of a family of α-oxo[1.n]Paracyclophanes (8-12) for n = 8–12, respectively) have been investigated. Compared to a model structure possessing a nominal T₁(nπ*) state (4,4'-dimethylbenzophenone, DM-BZ), photochemical and photophysical evidence is presented that the nπ* character of T₁ 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 10⁸, 0.93 X 10⁸, 0.30 X 10⁸, 0.06 X 10⁸, and 0.04 X 10⁸M^-1s^-1 for triplet 12–8, respectively, a result consistent with decreasing nπ* 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 nπ* character as the cyclophane ring size decreases.