Detection of parity-forbidden excited states by preresonance Raman excitation: Diphenyldecapentaene

Abstract

The presence of dipole-forbidden (‘‘hidden’’) excited electronic states in centrosymmetric chromophores can in principle be inferred from preresonance Raman excitation profiles (REPs). As the excitation radiation is tuned through the appropriate energy range, vibronic coupling between the state of interest and nearby ‘‘allowed’’ electronic states will produce interference effects in the ground state scattering intensity. We have used the Kramers–Heisenberg dispersion relation to establish a one-to-one correspondence between the vibrational modes of the 2 1Ag excited state and the interference features in the experimental preresonance REP of all-trans diphenyldecapentaene (DPDP). The parameters utilized to predict the REP of DPDP were obtained without adjustment from absorption and fluorescence excitation spectra. The satisfactory representation and interpretation of the structure in the experimental spectrum establishes preresonance Raman excitation as a viable technique for characterizing such hidden states in non-fluorescing molecules.