Third-order optical nonlinearity of conjugated poly(4,4-disubstituted-1,6-heptadiyne)s

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Abstract
Large third‐order optical nonlinearity was found with the soluble conjugated polymers (substituted polyacetylenes) synthesized by the ring‐forming metathesis polymerization of 4,4‐disubstituted‐1,6‐heptadiynes. These polymers were of quite high molecular weight (∼105) and were highly soluble in common organic solvents. The linear optical property and the third‐order optical nonlinearity of these polymers were investigated in terms of the chemical structure of the 4,4‐disubstituents using the polymer films of around 100‐nm thickness fabricated by the spin coating of tetrahydrofuran solutions. The π‐π* transition of the conjugated π‐electron main chain was characterized as the strong absorption peak at around 540 nm of the electronic spectra, where the optical density of the transition was found to be largely dependent on the molar volume of the substituents. The near‐resonant values of the third‐order nonlinear optical coefficient, χ(3)(−3ω;ω,ω,ω) were found to be 2.6 to 6.5×10−11 esu, which also were controlled by the molar volume of the substituent group.