OPTICAL 2ND-HARMONIC GENERATION FROM LANGMUIR-BLODGETT-FILMS OF AN ASYMMETRICALLY SUBSTITUTED PHTHALOCYANINE

Phthalocyanine, which is a pi-conjugated macrocyclic molecule, usually shows third-harmonic generation. After chemical modification by attaching donor-acceptor substituents to its peripheral ring, intramolecular charge transfer through the pi-conjugated system is created. This makes phthalocyanine a second-harmonic-generation (SHG) active molecule. The Langmuir-Blodgett (L-B) film technique offers a way to create noncentrosymmetric structure, which in principle is a requirement for SHG materials. Here we report the L-B film fabrication of an asymmetrically substituted phthalocyanine consisting of three tert-butyl groups as donor substituents and one nitro group as an acceptor substituent, namely, nitrotri-tert-butylphthalocyanine both in pristine form and in alternating form alternating with either tetra-tert-butylphthalocyanine or arachidic acid. SHG from these L-B films was investigated, We find that the tilt angle of such phthalocyanine molecules on the substrate is almost the same as that at the air-water interface, indicating that the configuration of these molecules was maintained during the deposition process. We demonstrate an alternative strategy for improving the SHG behavior in L-B films. A nearly quadratic dependence of second-harmonic intensity on the number of bilayers is observed up to nine bilayers.