ORGANOMETALLIC NONLINEAR-OPTICAL POLYMERS .4. ORGANOMETALLIC MAIN-CHAIN, SIDE-CHAIN, AND GUEST-HOST POLYMERS - A STUDY OF THEIR ORIENTATIONAND RELAXATION USING 2ND-HARMONIC GENERATION

In this paper several new organometallic polymers have been prepared. Structure-property relationships for local polymer mobility and the ne t orientation of the organometallic nonlinear optical (NLO)-phores hav e been investigated using second harmonic generation (SHG). The methac rylate organometallic derivatives {eta5-C5H4CH2O2CC(CH3)=CH2}Fe{eta5-C 5H4CH=C(CN)X} [4a, X = p-C6H5Br; 4b, X = 4-pyridyl; 4c, X = CN; 4d, X = CO2Et] and 5-C5H4CH=C(CN)CO2(CH2)2O2CC(CH3)=CH2}Fe{eta5-C5H5} (7) we re prepared and polymerized with methyl methacrylate (5/95, mol/mol) t o afford copolymers 8a-d and 9, respectively. Comonomer {eta5-C5H4CH=C (CN)CO2(CH2)2OH}Fe{eta5-C5H4CH2OH) (5) was synthesized and reacted wit h 1,6-diisocyanatohexane to yield a main-chain NLO organometallic poly urethane 10 (M(n) = 7600, T(m) = 176-degrees-C). In addition, a poly(m ethyl methacrylate) guest-host film of NLO-phore 5 was prepared. Coron a poling and SHG measurements were made under a variety of carefully c ontrolled experimental conditions. In the case of the covalently bound ferrocenyl NLO-phore, temporal stability was greatest for the smaller acceptor group CH=C(CN)2 (i.e., copolymer 8c). The rate of relaxation for copolymer gd was faster than that for 9, indicating that the poin t of attachment on the ferrocenyl NLO-phore to the polymer backbone wa s an important consideration. It was found that physically aging the o rganometallic polymers prior to poling produced samples which displaye d a smaller initial SHG signal; however, the signal was significantly more stable for a longer time. The guest-host system using NLO-phore 5 , with its multiple hydrogen-bonding sites, was observed to have very good long-term temporal stability. The guest-host polymer was poled ne gatively and showed enhanced temporal stability in comparison to a pos itively poled sample. The organometallic main-chain copolymer 10 respo nded well to poling but had concomitant decomposition, leading to an u nderestimation of orientational stability,