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

In this paper several new organometallic polymers have been prepared. Structure-property relationships for local polymer mobility and the net orientation of the organometallic nonlinear optical (NLO)-phores have been investigated using second harmonic generation (SHG). The methacrylate organometallic derivatives {eta5-C5H4CH2O2CC(CH3)=CH2}Fe{eta5-C5H4CH=C(CN)X} [4a, X = p-C6H5Br; 4b, X = 4-pyridyl; 4c, X = CN; 4d, X = CO2Et] and {eta5-C5H4CH=C(CN)CO2(CH2)2O2CC(CH3)=CH2}Fe{eta5-C5H5} (7) were prepared and polymerized with methyl methacrylate (5/95, mol/mol) to afford copolymers 8a-d and 9, respectively. Comonomer {eta5-C5H4CH=C(CN)CO2(CH2)2OH}Fe{eta5-C5H4CH2OH) (5) was synthesized and reacted with 1,6-diisocyanatohexane to yield a main-chain NLO organometallic polyurethane 10 (M(n) = 7600, T(m) = 176-degrees-C). In addition, a poly(methyl methacrylate) guest-host film of NLO-phore 5 was prepared. Corona poling and SHG measurements were made under a variety of carefully controlled 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 point of attachment on the ferrocenyl NLO-phore to the polymer backbone was an important consideration. It was found that physically aging the organometallic polymers prior to poling produced samples which displayed 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 negatively and showed enhanced temporal stability in comparison to a positively poled sample. The organometallic main-chain copolymer 10 responded well to poling but had concomitant decomposition, leading to an underestimation of orientational stability,