Knowledge Bank

The origin of the lowest-energy $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ excited state of diphenyl-hexatriene (DPH) in 3 rigid EPA matrix at $77\circ K$ has been located at $25050cm-1$ by two-photon excitation (TPE) spectroscopy. It lies $990cm-1$ below the origin of the lowest one-photon allowed $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bu\leftarrow 1Ag$ 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bu$ state, but from the lowest excited $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ state. The excited state $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag-1Bu$ origin difference is being systematically studied by extension of these experiments to diphenyldecapentaene. TPE measurements of all-trans retinal in the region $11100-13900cm-1$ will also be reported.