Circular dichroism in excited triplet states. 2. Chiral dimerlike molecules in the binaphthyl and spirobifluorene series

Chiral binaphthyls and spirobifluorenes have been investigated for the existence of circular dichroism (CD) in their first excited triplet state (triplet-triplet (TT) absorption), by using the technique described in the preceding paper of this issue. A strong triplet circular dichroism has been found for all rigidly 1,1′-bridged binaphthyls. The triplet anisotropy g factor correlates with the specific rotation measured in the ground state and decreases with the interplanar angle of the naphthalene moieties. In contrast, spirobifluorenes were found to be optically inactive in their triplet state, although an achiral triplet conformation can be ruled out from the consideration of their molecular structure. The spirobifluorenes are representative of weakly interacting chromophores. Electron delocalization is kept to a minimum due to the connecting, sp³-hybridized carbon atom. The classical molecular exciton model predicts that such molecules should be optically inactive in their triplet state, due to the constant degeneracy of the triplet exciton levels in first and second order. Consideration of interchromophoric electron exchange releases the constraints of the strict exciton model and allows for optical activity in the triplet state, depending on the amount of electron delocalization. In the binaphthyls, an appreciable orbital overlap at the 1-1′ bond is likely to take place. The real charge displacements accompanying optical excitation are able to sense the chirality of the molecule as a whole and triplet circular dichroism is displayed.