SINGLE ORIENTATION IN POLY(DIACETYLENE) FILMS FOR NONLINEAR OPTICS - MOLECULAR EPITAXY OF 1,6-BIS(9-CARBAZOLYL)-2,4-HEXADIYNE ON ORGANIC-CRYSTALS

Oriented thin films of a diacetylene derivative with carbazolyl substituents (DCH) have been prepared under vacuum by epitaxial nucleation on an organic single crystal. Polymerization performed by thermal heating preserves the anisotropic orientation of the diacetylene crystals. A good alignment of the poly(diacetylene) chains (pDCH) has been obtained on large-area single crystals of a series of acid phthalate salts used as substrates. In optical studies, the dichroism observed by UV-visible spectroscopy confirms a long-range organization established mainly by electron microscopy. The uniaxially oriented film is constituted of an arrangement of microcrystals oriented along privileged direction of the potassium acid phthalate single crystal (KAP). The pDCH chain axis, b(pDCH), is parallel to c(KAP) and we propose a new type of epitaxy, "molecular epitaxy", based on the insertion of the DCH molecule on the KAP surface. Third-order harmonic generation has been measured on these highly oriented poly(diacetylene) films. Experimental values of CHI(3) measured at two wavelength, 1.064 and 1.907-mu-m, confirm the three-photon resonance to the exciton in poly(diacetylene) at 650 nm (1.91 eV). A high value of the cubic susceptibility CHI(3) = 6 x 10(-10) esu is observed at 1.907-mu-m (0.65 eV).