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

J. RODRÍGUEZ, D. A. SCHERLIS, D. A. ESTRIN, P. F. ARAMENDÍA, R. M. NEGRI

JOURNAL OF PHYSICAL CHEMISTRY A

Año: 1997 vol. 101 p. 6998 - 7006

1089-5639

Ground (S0) and first excited singlet state (S1) potential energy surfaces were calculated for a series of sixsymmetric carbocyanines as a function of the twisting angle (õ), around a carbon-carbon bond of thepolymethine 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 (E0). For that rotation, thecomputed values of E0 are in good agreement with values extrapolated from experiments in solutions ofn-primary alcohols. The same holds for the computed transition energies between both surfaces for thethermodynamically stable N isomer (õ ) 0°) and the P photoisomer (õ ) 180°). The effects of chain lengthand pattern substitution of the indoline moiety on E0 were also analyzed for both surfaces. The shape of thepotential surfaces referred as the Rullie`re’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 ofõ. The calculations show that the dipole moment remains almost constant except in the vicinity of õ ) 90°,where a sudden increase with a sharp peak was obtained in both surfaces. This gives a simple explanationfor the well-known experimental observation that the activation energy on the excited state surface isindependent of solvent polarity, as the angle of the transition state is smaller than 90°. On the other hand,the transition state is at õ ) 90° on the ground state, and a polarity influence is predicted. An improvementin the description of the experimental isomerization rate constants in S0 is obtained for the two smallestcarbocyanines considered when polarity contributions are included.