Ab initio studies of electronic first hyperpolarizability of AB systems containing phosphorus and silicon atoms

Longitudinal first hyperpolatizability () calculations were carried out in three phosphorus- and one silicon-containing oligomers among which three are isovalent to polymethineimine (PMI). Extremely large values have been predicted in two of the phosphorus-containing oligomers, namely, polyphosphaacetylene (PPA), -(P=CH)(n)- and polyphosphasilyne (PPS), -(P=SiH)(n)-, i.e. 212777 and 90811 a.u. respectively, at n=16. These values are 5.3 and 2.3 times that of polymethineimine (PMI), which is well known for its higher nonlinear response. In striking contrast, extremely small values are predicted in another phosphorus-containing polymer, namely, polyborophosphene (PBP), -(PH=BH)(n)-, with a /n approaching zero at large n. Furthermore, consistently negative values are predicted for the polysilanitrile (PSA) oligomers, which are opposite to all the phosphorus-containing polymers as well as PMI The causes for the relative values in all these series of oligomers are discussed, in light of the bonding structure, the bond strength, the size of the nuclei, and the degree of delocalization.