Knowledge Bank

Conjugated polyenes have been the focus of much attention in recent years. Interest in these compounds is due to the important roles they play in biological processes (i.e., visual transduction and photosynthesis). The polyene essential to the visual process is 11-cis-retinal. $\beta$-carotene, a carotenoid, is the precursor of retinal. It also acts as a light harvesting component in photosynthetic systems as well as protecting such systems from photooxidation reactions. Short polyenes and their phenyl-substituted derivatives are often used as model compounds through which the functions of retinal, $\beta$-carotene, and related compounds can be described. These model compounds include trans-stilbene (tS), diphenylbutadiene (DPB), diphenylhexatriene (DPH), and diphenyloctatraene (DPO). The $S1$, spectra of tS, DPH, and DPO have been studied by many groups and been well $characterized.1,2,3$ DPB is not unlike the other model compounds mentioned above. However, there exists some ambiguity regarding its lowest excited singlet state. According to supersonic jet studies, the $S1$ state of DPB is of $Ag$ symmetry while it is believed to have $Bu$ symmetry in solution. This observation has been explained by a solvent-enhanced inversion of energy levels in the excited state. The $S1$ transient Raman spectrum of DPB has been previously $reported.4$ We have significantly improved the S/N compared to these previous results, and have observed changes in the shape and position of several bands as the solvent and delay time are varied. These changes and their origins will be addressed.