Knowledge Bank

Photoselected emission studies of rigid glasses indicate that the lowest excited singlet state of all-trans retinal and all-trans retinoic acid is the $\pi -\pi \ast$ state. This is in contrast to the situation in unsaturated aldehydes and ketones containing only one or two carbon-carbon double bonds where intensity and solvent shift data indicate a lowest n-$\pi \ast$ singlet state. Absorption studies of crystals of retinoic acid reveal a low intensity shoulder at about 4650 \AA, an inflection at about 4350 {\AA} and a maximum absorption at about 3640 \AA. Since the lowest state is the $\pi -\pi \ast$, this shoulder must be part of the $\pi -\pi \ast$ transition, probably the strongly Franck-Condon fobidden 0--0 transition. This conclusion is supported by polarized absorption, polarized excitation and crystal fluorescence studies of the triclinic form of retinoic acid whose crystal and molecular structure are known. A reinvestigation of the spectrum of diphenyl-octatetraene reveals a clearly resolved, low intensity peak at 4320 \AA, a position corresponding closely to the vibrational spacing of the previously reported intense bands. The ratio of the extinction coefficients of the weak (0--0) band to the first intense band (0--1) is about 1/30. These experiments demonstrate that the lowest excited state of these polyenes is a $\pi -\pi \ast$ state with a strongly Franck-Condon forbidden 0--0 transition suggesting significant rearrangement in the excited state. The ground state and excited state vibrational frequencies will be compared and the origin of the diffuse nature of the transition in the retinyl series will be discussed.