EXPERIMENTAL CHARACTERIZATION AND MOLECULAR MODELING OF THE INTRAMOLECULAR EXCIMER IN POLY(PARA-TERT-BUTYLSTYRENE)

Steady-state fluorescence spectra have been obtained for isotactic poly(p-tert-butylstyrene) and for the atactic polymer prepared by two different methods. Excimer emission is observed in all samples of poly(p-tert-butylstyrene) in dilute solution in 1,2-dichloroethane. The monomer and excimer bands are more closely spaced in poly(p-tert-butylstyrene) than in polystyrene, and the values of I(D)/I(M) are smaller in the former polymer. Here I(D)/I(M) denotes the ratio of the intensities of excimer and monomer emission. The isotactic sample has a value of I(D)/I(M) that is slightly larger than that of the atactic samples. Molecular modeling identifies the tt conformation of the meso diad and the tg- and g-t conformations of the racemic diad as the conformations most conducive to formation of an excimer by neighboring rings. The angle, psi, between the normals to the best planes of the two rings in these compounds is almost-equal-to 20-degrees, which is substantially larger than the corresponding angles in the molecules without the pendant tert-butyl groups. This increase in psi-undoubtedly accounts for the diminished energetic separation of the monomer and excimer bands.