Surface-Enhanced Photopolymerization of a Diacetylene Derivative in Langmuir−Blodgett Films on a Silver Island Film

Surface-enhanced resonance Raman scattering (SERRS) spectroscopy was applied to monitor the photopolymerization process of one monolayer ( L = 1) of a Langmuir−Blodgett (LB) film of the cadmium salt of a diacetylene monocarboxylic acid derivative, 10,12-pentacosadiynoic acid (DA), prepared on an evaporated silver island film with a mass thickness of 60 Å. The result indicated that the polymerization proceeds in a way that is similar to that of the multilayer LB films ( L ≥ 3) of DA prepared on a smooth silver substrate with a mass thickness of 1000 Å, exhibiting a transition from a blue phase (λ max ≈ 635 nm) to a red phase (λ max ≈ 540 nm). In addition, the photopolymerization on the SERS-active substrate with d = 60 Å was found to proceed much faster than that on the smooth (SERS-inactive) substrate ( d = 1000 Å). The rate of the photopolymerization of the monolayer on the island film was compared with the rates of a series of LB films of DA prepared on evaporated silver films with d = 60−1000 Å, including the DA Langmuir−Blodgett monolayer sample, into which the spacer layer of LB films of arachidic acid was inserted to control the separation distance between the reacting monolayer and the silver substrate ( d = 60 Å). On the basis of these results, we proposed that the enhanced photopolymerization is due to an accelerated propagation reaction (an addition reaction of DA to reactive polydiacetylene oligomers) resulting from an enhanced band-to-band transition of the oligomers caused by a strong coupling of the transition to the localized plasmon resonance of the substrates.