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

Optical and third-order nonlinear optical properties of germanium porphyrazine, phthalocyanine and naphthalocyanine derivatives have been measured in order to evaluate the effect of extended pi-electron conjugation on third-order optical nonlinearity. Third-order nonlinear optical susceptibilities chi((3)) Of thin films of trans-dihydroxy- 1,6,11,16-tetra(tert.butyl)porphyrazine germanium [t-Bu(4)PazGe(OH)(2)], trans-dihydroxy-tetra-(tert.butyl)phthalocyaninato germanium [(t-Bu(4)PcGe(OH)(2)] and naphthalocyanine germanium (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-Bu(4)PcGe(OH)(2)] thin films at 1.8 mu m. The chi((3)) values of germanium porphyrazine derivative have been compared with those of structurally related germanium phthalocyanine and naphthalocyanine analogues to evaluate the effect of extended pi-conjugation introduced through benzo substitution. A progressive red-shift in optical absorption peaks in the Q-band region and an increase in the third-order nonlinear optical susceptibility was noticed with sequential increase of benzo substitution from the porphyrazine to naphthalocyanine ring. The effect of benzo substitution as well as axial substitution on optical and third-order nonlinear optical properties has been discussed. Attempts have also been made to establish a correlation between third-order optical nonlinearity and photoconductivity of metallophthalocyanines in view of the present results. (C) 1999 Elsevier Science B.V. All rights reserved.