PRERESONANCE RAMAN EXCITATION PROFILES OF $CAROTENOIDS^{*}$

Author Institution: Department of Chemistry, Michigan State UniversityIt is now well $established^{1}$ that the lowest excited singlet electronic state in linear polyenes of at least moderate Length has the same parity as the ground state, and is thus forbidden in normal one-photon absorption. The (allowed) $1^{1}B_{u} \leftarrow 1^{1} A_{g}$ transition energy reaches an asymptotic value for long polyene chains; however; the corresponding behaviour for the $2^{1} A_{g} \leftarrow 1^{1}A_{g}$ excitation has not been determined. In order to locate this ""hidden"" state in polyenes possessing $9-15 C = C$ bonds, we are studying the preresonant Raman scattering from a series of carotenoids. The relative intensity of the C = C stretch of the solute to a neighbouring cyclohexane solvent made is measured as the exciting line is tuned through the region of interest. The presence of an underlying state is manifest by interference features on the Raman excitation $profile,^{2}$ and we have interpreted the profile of $\beta$-carotene (n=9) $accordingly,^{3}$ We report here corresponding measurements for lycopene (n = 11) and decapreno-$\beta-carotene (n = 13)$. Estimates of the $2^{1}A_{g}$ state origin based on these observations will be presented, and trends in the energy gaps with variation in chain length will be discussed. $^{*}$Research supported in part by the U. S. National Science Foundation. $^{1}$B. S. Hudson, B. E. Kohler and K. Schulten, in Excited States, Vol, C. E. C. Liz., ed. (Academic Press, New York, 1982). $^{2}$J. Friedman and R. M. Hochstrasser, Chem. Phys. Letters 32, 414 (1975). $^{3}$R. J. Thrash, H. K. L. -B. Fang and G. E. Leroi, J. Chem. Phys. 67, 5939 (1977)