Knowledge Bank

Vibrationally resolved fluorescence and one photon fluorescence excitation spectra are measured for cis,trans-octatetraene in n-alkane matrices (hexane and octane) at 4.2K. These spectra are surprisingly similar to those measured for trans,trans-octatetraene. There is a weak $S0$ to $S1$ transition (0-0 at $28645cm-1$ in n-hexane) approximately $3400cm-1$ below the strong $S0$ to $S1$ transition at $32134cm-1$. The vibrational development of the $S0$ to $S1$ absorption and fluorescence is that of a symmetry allowed transition unlike the results for the trans,trans isomer. The weak nature of the lowest energy absorption testifies to the importance of multiply excited configurations in the description of $S1$. Vibrational modes in $S1$ are weakly coupled with $S2$ due to the small energy gap; this coupling is energy dependent. Franck-Condon factors for the $S0$ to $S2$ transition are also estimated. Together with the vibrational development of the $S0$ to $S1$ transition, these show that neither of the two lowest excited singlet states are significantly distorted with respect to the ground state.