We suggest for linear conjugated polyenes a twin state model which represen
ts the ground state (S-0) and first excited state (S-1) as a superposition
of mainly two mesomeric structures, the fully spin-paired one and a diradic
al. This model rationalizes why the bond-length alternation, which is prono
unced in S-0, more or less disappears in S-1 and why the bond-alternation v
ibration (highest frequency C=C stretch) is raised in S-1 and depressed in
S-0. The similarity to the Peierls effect and Kohn anomaly in one-dimension
al metals is emphasized. Moreover, the conical intersection between S-2 and
S-1 is qualitatively predicted, and invoking in addition, some spectroscop
ic and other observations and the phase-change rule, that between S-1 and S
-0 can also be predicted. Compared with the consideration of densities of s
tates and matrix elements, these intersections more satisfactorily explain
the S-2/S-1 and S-1/S-0 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 in
ternal-conversion rates, which is derived from a proposed dependence of the
energy at the intersection on the S-1-S-0 energy gap. (C) 2000 Elsevier Sc
ience B.V. All rights reserved.