OBSERVATION OF THE LOW-ENERGY $^{1} A_{g}$ STATES OF DIPHENYLHEXATRIENE AND ALL-TRANS RETINOL BY TWO-PHOTON EXCITATION SPECTROSCOPY

$^{1}$ Howard L. B. Fang, Robert J. Thrash, and George E. Leroi, J, Chem. Phys. 67, 3389 (1977).""Author Institution: Department of Chemistry, Michigan State UniversityThe origin of the lowest-energy $^{1} A_{g}$ excited state of diphenyl-hexatriene (DPH) in 3 rigid EPA matrix at $77^{\circ}\, K$ has been located at $25 050\, cm^{-1}$ by two-photon excitation (TPE) spectroscopy. It lies $990\, cm^{-1}$ below the origin of the lowest one-photon allowed $^{1} B_{u} \leftarrow ^{1} A_{g}$ transition, thus supporting the reordering of polyene excited electronic states suggested by recent theoretical and experimental considerations. As determined also for $diphenyl-octatetraene,^{1}$ one-photon fluorescence originates not from the $^{1}B_{u}$ state, but from the lowest excited $^{1}A_{g}$ state. The excited state $^{1}A_{g} - ^{1}B_{u}$ origin difference is being systematically studied by extension of these experiments to diphenyldecapentaene. TPE measurements of all-trans retinal in the region $11 100-13 900\, cm^{-1}$ will also be reported