Knowledge Bank

Two-photon excitation spectra of several retinyl polyenes (all-trans retinol, all-trans retinal and all-trans anhydrovitamin A), and of diphenyloctatetraene in random glass matrices at $77\circ K$ have been obtained in the energy region 10 000--14 000 $cm-1$. A ruby laser-pumped tunable dye laser was used as the excitation source. The presence of two-photon, allowed, electronic bands in all of these linear polyenes, at energies below the strongly allowed one-photon, absorption to the lowest $1Bu$ state, has been established. A possible assignment for the upper state is $1Ag$, to which one-photon transitions from the ground state would be symmetry forbidden. The location of this low energy transition removes several discrepancies between the expected and observed fluorescence properties of these polyenes.