Relation between conformational structure, vibrational spectra and intramolecular excimer formation in 2,4-dimethyl-2,4-diphenylpentane

Article abstract of DOI:10.1016/0022-2860(81)85040-5

Conformational energy calculations on 2,4-dimethyl-2,4-diphenylpentane have been performed with explicit allowance for methylene bond-angle flexibility. The value of this angle which minimizes the energy is close to 126°, whatever the conformation. The energy contours are given versus main chain bond rotations. An intramolecular mobility study reveals that two kinds of motions are expected: the first includes motions with low activation energies while motions of the second group require a much higher activation energy and involve a large rotation around C-C skeleton bonds. In all cases the phenyl ring rotations are rather strongly coupled with the chain rotations. IR analysis and intramolecular excimer formation indicate that 2,4-dimethyl-2,4-diphenylpentane exists in one of the most stable conformations in the crystalline state. In the liquid state higher-energy conformers are present. Fluorescence emission studies show that 2,4-dimethyl-2,4-diphenylpentane exhibits a higher efficiency of excimer formation than 2,4-diphenylpentane, because of the rather small energy barrier between the excimer and the ground-state conformations.