Knowledge Bank

For centrosymmetric chromophores, such as linear polyencs, approximately half of the electronic excited state manifold is inaccessible from the ground state by conventional absorption spectroscopy because of parity selection rules. Such ``hidden'' states are typically studied by two-photon induced fluorescence spectroscopy, or by high resolution absorption and fluorescence excitation spectroscopy of samples placed an a Shpolskii matrix environment. In cases where these techniques are not applicable, (e.g. non-fluorescent molecules), preresonance Raman spectroscopy may be a suitable alternative. The preresonance Raman excitation profile is determined point-by-point from the ratio of the scattered intensity of a sample vibrational band to that of a nearby solvent mode as a function of the excitation energy, and the hidden states manifest themselves as interference $features.1$ However, the interpretation of the interference features, and their relationship to the hidden state origin, which is not active in the excitation profile, rely on self-consistency arguments. In an effort to establish more firmly the utility of preresonance Raman profiles, we have chosen diphenyldecapentaene (DPDP) as a model compound. The low-lying, hidden $21Ag$ state origin of DPDP has been located at low temperatures by both two-photon $excitation2$ and matrix absorption/$emission3$ spectroscopy. We present here the preresonance Raman excitation profile of DPDP in the region just below the allowed $\mathrm{<mrow\; data-mjx-texclass="ORD">\ast \ast \ast </mrow>}Bu$ electronic state, in the $20000-22700cm-1$ energy range. The results are in excellent $accord1$ with those of the more direct methods, and establish preresonance Raman excitation as a viable technique for locating parity-forbidden electronic states.