Diacetylene polymerization in self-assembled monolayers: influence of the odd/even nature of the methylene spacer

In this study, the interdependence between molecular architecture and photo -polymerization is evaluated for self-assembled monolayers containing diace tylenes. While such assemblies provide added robustness and versatility in molecular design, direct surface-attachment significantly limits the degree s of freedom within the monolayer structure. As a result, the polymer conju gation length as evidenced by the chromatic properties cannot be directly p redicted from the extensive literature on Langmuir-Blodgett (LB) films. Ind eed, a clear odd/even effect is observed in the polymerization properties o f these surface-confined monolayers that is not evident in LB systems. Odd- numbered methylene spacers between the surface and the polymer backbone lea d to the longer conjugation length blue form, whereas even-numbered spacers yield the shorter conjugation length red phase. This observation is consis tent with simple modeling that indicates a release in localized strain upon polymerization for the odd-numbered spacer architectures. While spacers of both 5 and 9 methylene unit yield similar conversion efficiency to the blu e-phase polymer, the kinetic profiles with UV exposure are quite different. The longer conversion times for the shorter spacer are attributed to lower freedom of movement within the monolayer to achieve the spatial alignment required for the topochemical polymerization. (C) 2000 Elsevier Science Ltd . All rights reserved.