Continuous-wave two-photon spectroscopy on a ladder-type conjugated polymer

Employing continuous-wave two-photon fluorescence-excitation spectroscopy on a methyl-substituted ladder-type poly(para-phenylene) we studied the symmetry properties of the vibronic wave functions of the lowest electronically excited states and found that the symmetry selection rules for optical transitions are rather strictly fulfilled in this rigid, rod-like conjugated polymer. Additionally, these data allowed us to show that the energies of the predominant vibrational modes, the inter-ring stretching vibration and the aromatic ring CC stretching mode, are significantly lower in the second excited singlet state with respect to the corresponding energies in the first excited singlet state and in the electronic ground state.