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

Abstract

In this study, the interdependence between molecular architecture and photo-polymerization is evaluated for self-assembled monolayers containing diacetylenes. While such assemblies provide added robustness and versatility in molecular design, direct surface-attachment significantly limits the degrees of freedom within the monolayer structure. As a result, the polymer conjugation length as evidenced by the chromatic properties cannot be directly predicted from the extensive literature on Langmuir–Blodgett (LB) films. Indeed, a clear odd/even effect is observed in the polymerization properties of these surface-confined monolayers that is not evident in LB systems. Odd-numbered methylene spacers between the surface and the polymer backbone lead to the longer conjugation length blue form, whereas even-numbered spacers yield the shorter conjugation length red phase. This observation is consistent 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 blue-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.