Twin states and conical intersections in linear polyenes Original Research Article

We suggest for linear conjugated polyenes a twin state model which represents the ground state (S0) and first excited state (S1) as a superposition of mainly two mesomeric structures, the fully spin-paired one and a diradical. This model rationalizes why the bond-length alternation, which is pronounced in S0, more or less disappears in S1 and why the bond-alternation vibration (highest frequency CC stretch) is raised in S1 and depressed in S0. The similarity to the Peierls effect and Kohn anomaly in one-dimensional metals is emphasized. Moreover, the conical intersection between S2 and S1 is qualitatively predicted, and invoking in addition, some spectroscopic and other observations and the phase-change rule, that between S1 and S0 can also be predicted. Compared with the consideration of densities of states and matrix elements, these intersections more satisfactorily explain the S2/S1 and S1/S0 internal conversions and their dependence on chain length, substituents, solvent and temperature and is furthermore consistent with photochemistry. This also includes an exponential gap rule for the internal-conversion rates, which is derived from a proposed dependence of the energy at the intersection on the S1–S0 energy gap.