Enhancement of the second-order NLO responses of boron-nitrogen oligomers by copolymerization with polyyne

The enhancement of the static electronic second-order NLO properties of the linear boron-nitrogen chains by copolymerization with polyyne that is related mainly to the changes in the bond length alternation (BLA) parameter has been investigated at the HF and MP2 levels using the 6-311G(d,p) and 6-311+G(d,p) basis sets. The non-zero BLA parameter combined with ununiform electron distribution along the chain yields a unit cell asymmetry necessary for large second-order NLO responses. Alternance of the single and triple bonds brings an efficient electron delocalization resulting in large polymeric electronic dipole polarizability (186 ± 1 a.u. per unit cell). The unit cell asymmetry and large delocalization lead to sizeable longitudinal first hyperpolarizability (5850 ± 100 a.u. per unit cell).