Energies of low-lying excited states of linear polyenes

Room temperature absorption and emission spectra of the all-trans isomers of decatetraene, dodecapentaene, tetradecahexaene, and hexadecaheptaene have been obtained in a series of nonpolar solvents. The resolved vibronic features in the optical spectra of these model systems allow the accurate determination of S₀ (1¹A_g^-) → S₂ (1¹B_u⁺) and S₁ (2¹A _g^-) → S₀ (1¹A_g ^-) electronic origins as a function of solvent polarizability. These data can be extrapolated to predict the transition energies in the absence of solvent perturbations. The effects of the terminal methyl substituents on the transition energies also can be estimated. Franck-Condon maxima in the absorption and emission spectra were used to estimate differences between S ₀ (1¹A_g^-) → S₁ (2¹A_g^-) and S₀ (1¹A _g^-) → S₂ (1¹B_u ⁺) electronic origins and "vertical" transition energies. Experimental estimates of the vertical transition energies of unsubstituted, all-trans polyenes in vacuum as a function of conjugation length are compared with long-standing multireference configuration interaction (MRCI) treatments and with more recent ab initio calculations of the energies of the 2 ¹A_g^- (S₁) and 1¹B _u⁺ (S₂) states.