Electronic origin for enhanced nonlinear optical response of complexes from tetraalkylammonium halide and carbon tetrabromide: Electrostatic potentials of intermolecular donor-acceptor dyads

Electronic origin for nonresonant enhancement of nonlinear optical response in the complexes formed from tetraalkylammonium halide and carbon tetrabromide is provided in view of electrostatic potentials of intermolecular donor (halide ion)-acceptor (CBr₄). The calculated electrostatic potentials of donor-acceptor range from -4.83 to -7.70 kcal mol^-1 and show a decreasing order of [Et₄Cl^-⋯Br] > [Et ₄Br^-⋯Br] ≅ [Et₄I ^-⋯Br] > [Bu₄Br^-⋯Br]. The calculated second-order susceptibilities of solid complexes are in an increasing order of [NEt₄Cl·CBr₄] < [NEt ₄Br·CBr₄] ≅ [NEt₄I·CBr ₄] < [NBu₄Br·CBr₄·C ₃₆O]. It has been shown that the donor/ac ceptor dyads make the exclusive contribution to nonlinear optical response. A large size of halide or tetraalkylammonium ion results in a small electrostatic potential and large nonlinear optical response in these charge-transfer complexes. It indicates that a small supermolecular interaction will create a large nonlinear optical response, and it gives a clue to design the molecular complexes with large non-linear optical susceptibility.