High second-order NLO responses of dehydrogenated hydrogen cyanide borane(1) oligomers

Second-order non-linear optical (NLO) properties of dehydrogenated hydrogen cyanide borane(1) oligomers (up to dodecamers) are investigated by the Hartree-Fock, hybrid DFT (including long-range corrected functionals) and MP2 approaches. Due to one-dimensional extension of the systems the longitudinal component of the properties is addressed with focus on the electronic contributions. Direct and indirect electron correlation effects are evaluated at the MP2 level paying particular attention to the basis set effects. Unit-cell asymmetry and delocalization effects in the NLO responses are discussed in terms of the evolution of the bond-length alternation parameter, dipole moment and polarizability (reduced per unit cell) with the increasing chain length. It was found that suitable combination of the unit-cell asymmetry and electron delocalization leads to extremely large NLO response (MP2/6-311++G(d,p) value for the dodecamer is 43.7 × 103 a.u. per unit cell) what makes the conjugated BCN oligomers (if one takes into account their thermodynamic stability) a promising material for NLO applications.