Electrostatic effects on the energetics and geometry of a cyanine dye

Computations using B3LYP density functional method with 6-31G (d,p) and 6-311 + G (d,p) basis sets were carried out on a cationic cyanine dye at various locations of the Br⁻ counter ion. It was found that the Br⁻ position and its accompanying electric field have a significant affect on the charge distribution within the cationic dye. Furthermore, the enthalpy of anion/cation interaction was highly sensitive to the Br⁻ location. Bringing the Br⁻ either closer or further than the optimum is substantially destabilizing. The flexible cyanine dye molecule tends to fold to accommodate electrostatic interactions. On the other hand, less activation energy was required for the approach of F⁻ towards the cyanine dye (6 kcal/mol vs. 15 kcal/mol for the Br⁻ approach).