Investigations of the Influence of Molecular Geometry on the Spectroscopic and Photochemical Properties of α-Oxoparacyclophanes (Cyclophanobenzophenones)

Article abstract of DOI:10.1021/ja00198a059

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(n?^*) state (4,4'-dimethylbenzophenone, DM-BZ), photochemical and photophysical evidence is presented that the n?^* 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 1.6E8, 0.93E8, 0.30E8, 0.06E8, and 0.04E8 M^-1s^-1 for triplet 12-8, respectively, a result consistent with decreasing n?^* 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 n?^* character as the cyclophane ring size decreases.