3RD-ORDER NONLINEAR-OPTICAL PROPERTIES OF POLYAZINE DERIVATIVES

Experimental and theoretical results on third-order nonlinear optical susceptibility chi(3) and molecular hyperpolarizability (gamma) of oli gomeric and polymeric azines are reported for the first time. Molecula r hyperpolarizabilities (gamma) of well-defined conjugation length pol yazines containing terminal electron-donating amino groups were calcul ated by employing the ab initio Coupled Perturbed Hartee-Fock with the 6-31G basis set. An azine monomer showed a molecular hyperpolarizabil ity of the order of 14.7x10(-36) esu, which is the largest gamma value ever reported for such a short pi-electron conjugation length. Molecu lar hyperpolarizability increases significantly as the length of the p i-electron delocalization increases. The gamma values for the dimer, t rimer, tetramer, and pentamer azine derivatives were 79.0x10(-36), 228 .3x10(-36), 575.9x10(-36), and 903.0x10(-36) esu, respectively. Third- order nonlinear optical susceptibilities chi(3) of polyazine derivativ es were determined by third-harmonic generation (THG) measurements. Th e chi(3)(-3omega; omega, omega, omega) values of an azine monomer and a pentamer were 1.39x10(-13) and 15.3x10(-13) esu, respectively. On th e other hand, the chi(3) of a polyazine possessing statistically alter nating methyl and propyl substituents on the carbon atoms along the ch ain was found to be 8.0x10(-12) esu at 1.5 mum. The interesting third- order nonlinear properties of polyazines presumably originate from the multiple quantum-well structure of the main pi-electron conjugated ba ckbone.