Barriers to conformational transformations in symmetrical linear conjugated systems

A quantum-chemical study has been carried out on conformational transformations in the ground and first excited states of symmetrical polymethine dyes and related α, ω-disubstituted polyenes. The magnitude of the trans-cis isomerization barriers depends on the length of the conjugation chain, position of the rotated bond, electron-donor capacity of the terminal groups, and occupancy of the electron shell. The rotation of molecular fragments in the excited state may lead to a change in the nature of the first electronic transition.