Third-order nonlinear optical properties of porphyrazine, phthalocyanine and naphthalocyanine germanium derivatives: Demonstrating the effect of pi-conjugation length on third-order optical nonlinearity of two-dimensional molecules

Optical and third-order nonlinear optical properties of germanium porphyraz ine, phthalocyanine and naphthalocyanine derivatives have been measured in order to evaluate the effect of extended pi-electron conjugation on third-o rder optical nonlinearity. Third-order nonlinear optical susceptibilities c hi((3)) Of thin films of trans-dihydroxy- 1,6,11,16-tetra(tert.butyl)porphy razine germanium [t-Bu(4)PazGe(OH)(2)], trans-dihydroxy-tetra-(tert.butyl)p hthalocyaninato germanium [(t-Bu4PcGe(OH)(2)] and naphthalocyanine germaniu m (GeNc) derivative measured by the third-harmonic generation technique are reported. A chi((3))(-3 omega; omega, omega, omega) value as high as 1.3 x 10(-11) esu at 1.65 mu m was observed for [t-Bu(4)PazGe(OH)(2)] thin films from a dominant three-photon resonance. A chi((3))(-3 omega; omega, omega, omega) value of 2.5 x 10(-11) esu and a chi((3))/alpha value of 1.65 x 10( -15) esu cm as figure of merit was observed for [t-Bu4PcGe(OH)(2)] thin fil ms at 1.8 mu m. The chi((3)) values of germanium porphyrazine derivative ha ve been compared with those of structurally related germanium phthalocyanin e and naphthalocyanine analogues to evaluate the effect of extended pi-conj ugation introduced through benzo substitution. A progressive red-shift in o ptical absorption peaks in the Q-band region and an increase in the third-o rder nonlinear optical susceptibility was noticed with sequential increase of benzo substitution from the porphyrazine to naphthalocyanine ring. The e ffect of benzo substitution as well as axial substitution on optical and th ird-order nonlinear optical properties has been discussed. Attempts have al so been made to establish a correlation between third-order optical nonline arity and photoconductivity of metallophthalocyanines in view of the presen t results. (C) 1999 Elsevier Science B.V. All rights reserved.