AM1 study of the ground and excited state potential energy surfaces of symmetric carbocyanines

Ground (S-0) and first excited singlet state (S-1) potential energy surfaces were calculated for a series of six symmetric carbocyanines as a function of the twisting angle (theta), around a carbon-carbon bond of the polymethine chain. The surfaces were computed using AM1 semiempirical quantum mechanical calculations, Rotations around different bonds were considered in order to determine the relevant rotation for isomerization, that is, the rotation with the lowest activation energy for the isolated molecule (E-0). For that rotation, the computed values of Eo are in good agreement with values extrapolated from experiments in solutions of n-primary alcohols, The same holds for the computed transition energies between both surfaces for the thermodynamically stable N isomer (theta = 0 degrees) and the P photoisomer (theta = 180 degrees), The effects of chain length and pattern substitution of the indoline moiety on Eo were also analyzed for both surfaces. The shape of the potential surfaces referred as the Rulliere's model holds in all cases for at least one rotational coordinate, The electrical dipole moment with respect to the center of electrical charges was calculated as a function of theta. The calculations show that the dipole moment remains almost constant except in the vicinity of theta = 90 degrees, where a sudden increase with a sharp peak was obtained in both surfaces. This gives a simple explanation for the well-known experimental observation that the activation energy on the excited state surface is independent of solvent polarity, as the angle of the transition state is smaller than 90 degrees. On the other hand, the transition state is at theta = 90 degrees on the ground state, and a polarity influence is predicted, An improvement in the description of the experimental isomerization rats constants in So is obtained for the two smallest carbocyanines considered when polarity contributions are included.