Knowledge Bank

In centrosymmetric chromophores, such as linear polyenes, approximately half of the electronic excited state manifold is inaccessible from the ground state by conventional absorption spectroscopy because of parity selection rules. However, the presence of such hidden'' excited states can sometimes be inferred from preresonance Raman excitation profiles. 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 this method in conjunction with two-photon excitation data 1 for a diphenyldecapentaene (DPDP) to locate the lowest-lying excited gerade singlet state $(1Ag)$ of DPDP, Excitation was supplied by an Ar pumped tunable dye laser in the wavelength region covered by coumarin 480 and stilbene 420. The excitation spectrum was determined point-by-point from the patio of the scattered intensity of the DPDP C-C pymmetric stretch $(1565cm-1)$ to that of a cyclohexane solvent made at $1445cm-1$. The technique has been applied to other polyenes, and results for all systems investigated to date will be described.