ELSEVIER November 1997 Materials Letters 33 (1997) 23-26 Third-order nonlinear optical properties of pyridine- and ferrocene-containing polyazines Hari Singh Nalwa zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Hitachi Research Laboratory, Hitachi Ltd., 7-l-l Ohmika-cho, Hitachi City, lbaraki 319-12, Japan Received 14 January 1997; revised 25 February 1997; accepted 26 February 1997 Abstract Third-order nonlinear optical susceptibilities x (3) of pyridine and ferrocene-containing polyazines have been measured by the third harmonic generation in the wavelength region 1.0 pm to 1.95 pm. The xC3)( - 3~; o, w, w) values of 7.75 X lo-” esu at 1.5 pm for pyridine-containing polyazine and 2.23 X lo- ” esu at 1.8 pm for ferrocene-containing polyazine were observed. Third order optical nonlinearities of polyazines with and without aromatic units in the n-conjugated backbone are compared to develop a structure property relationship. 0 1997 Elsevier Science B.V. Keywords: Nonlinear optics: Polyazines: Third harmonic generation Organic r-conjugated polymers have been of great interest both as electronic and photonic materials over the past decade [l-4]. From the nonlinear op- tics (NLO) viewpoint, a number of highly conjugated polymers such as polyacetylenes, polydiacetylenes, polythiophenes, poly(thieny1 vinylene)s, poly( p- phenylene vinylene)a, polyquinolines, poly(aniline)s, etc., have been discovered to show large third-order optical nonlinearity [5,6]. We have focused our stud- ies on polyazines which are isoelectronic to poly- acetylene. The polyazine backbone consists of pairs of nitrogen atoms substituted for pairs of carbon atoms in the polyacetylene backbone. The presence of nitrogen atoms in the rr-conjugated backbone yields environmentahy stable materials (against oxy- gen, water vapors)1 unlike polyacetylene. On the other hand, it is well known that third-order optical nonlinearity is associated with bandgap, the smaller the band gap of a 7r-conjugated polymer, the higher the third-order optical nonlinearity. The band gap of polyazine (2.3 eV) is about twice that of polyacety- lene (1.1 eV> [7] which indicates that polyacetylene will be a better semiconductor than polyazine. A similar assumption holds true for third-order optical nonlinearity therefore based on the band gap consid- eration, third-order optical nonlinearity of polyazine could not be achieved as high as for polyacetylene. The nitrogen atoms within the backbone chain are quite basic and give rise to localized dipole mo- ments. The basicity is manifested by the presence of nonbonding electrons on the chain nitrogen atoms. These filled levels are presumably the highest occu- pied orbitals and have an effect on the allowed electronic transitions, the polymer systems can un- dergo. The presence of this dipole moment also should influence the magnitude of the second-order optical nonlinearity. This article describes the prepa- ration and nonlinear optical properties of derivatized azine polymers. Some qualitative comments regard- ing correlation between electron delocalization, band 00367-577X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SO167-577X(97)00063-3