ELECTRONIC ENERGY LEVELS IN LONG POLYENES: $S_{2}\to S_{0}$ EMISSION IN ALL-TRANS-1,3,5,7,9,11,13-TETRADECAHEPTAENE

Author Institution: Department of Chemistry, Rowdoin CollegeAbsorption, fluorescence, and fluorescence excitation spectra of all-trans-1,3,5,7,9,11,13-tetradecaheptaene have been obtained in room temperature solutions and 77K glasses. The heptaene, unlike shorter fluorescent polyenes, does not exhibit the characteristic gap between the origins of absorption $(1^{1}A_{g}^{-} + 1^{1} B_{u}{+})$ and emission. Excitation spectra and solvent shift studies lead to the assignment of two distinct emissions, $S_{2} \to S_{0}\ (1^{1}B^{+}_{u} + 1^{1} A{^{-}}_{g})$, and $S_{1} \to S_{0} (2^{1} A{^{-}}_{g} + 1^{1} A{^{-}}_{g})$, with the ratio of $S_{2}$ to $S_{1}$ emission increasing with the $S_{2} - S_{1}$ energy gap. Extrapolation of room temperature solution data gives a gas phase $S_{2} - S_{1}$ difference of $8400 cm^{-1}$. Tetradecaheptaene’s ``anomalous” $S_{2} \to S_{O}$ emission, the first observed for a linearly conjugated system, is compared to similar violations of Kasha’s Rule by azulene and other molecules. Solvent and substituent effects on the $S_{2}$ and $S_{1}$ emissions also will be discussed