Study of diacetylene monomer and polymer monolayers using second- and third-harmonic generation

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Abstract
Monolayers of several diacetylene monomers and polymers spread at the air–water interface have been studied using optical second-harmonic generation (SHG) and third-harmonic generation (THG). Owing to the centrosymmetry of the diacetylene core, SHG from these molecules arises mainly from their side groups. THG from a single monolayer of polydiacetylene is reported, to our knowledge, for the first time—the THG signal (in reflection) arising from the polydiacetylene monolayer is several times larger than that from the water subphase. THG was observed both when a polymer monolayer was spread directly and when a monomer monolayer was UV polymerized on the water surface. Values of the third-order nonlinearity of polydiacetylenes derived from these measurements are in agreement with earlier studies.