Photochemical switching through protonation in merocyanines

The ground- and excited-state energies for the planar and various 90-degree twisted structures of the merocyanine Me2N-CH=CH-CH=CH-CH=O and its protonated form +Me2N=CH-CH=CH-CH=CH-OH have been calculated by the AMI method involving complete geometry optimization. On successively twisting bonds in the polymethine chain, the energies of the resulting twisted forms display a pronounced alternation which is opposite in the ground and the excited state. This trend in computed energy values as well as the reversed alternation pattern for the two types of polymethine compounds can be anticipated from simple qualitative MO considerations and in the topological long-chain approximation. (C) 1999 Published by Elsevier Science S.A. All rights reserved.