Vertical Positioning of Internal Molecular Scaffolding within a Single Molecular Layer

Abstract

Three-dimensional design within a two-dimensional molecular layer is demonstrated by varying the vertical positioning of internal molecular scaffolding. Monolayer structures are successfully fabricated using photopolymerization of adjacent diacetylene-containing molecules with controlled position of the resultant polymer backbone. Regardless of diacetylene position within the monolayer, polymerization is shown for all molecular architectures. The effective conjugation length of the resulting monolayer polymer, however, appears to be dependent on the molecular architecture. Although no significant expansion or contraction is expected upon polymerization, changes in hybridization must be accommodated within the monolayer structure. While the spatial constraints for polymerization are considerable, the longest conjugation lengths are exhibited by molecular architectures with some disordered component within the alkyl side chains. The excellent long-range order in these monolayers, as demonstrated by heterogeneous electron-transfer measurements, indicates little disordering within interstitial regions at grain boundaries. As a result, it is likely that strain created by the hybridization shift is accommodated by degrees of freedom within the side chains. When the architecture prevents such strain release, the conjugation length of the polymer backbone is compromised. However, this strain is not sufficient to disrupt the long-range monolayer order, and no correlation exists between polymer conjugation length and inhibition of electron transfer. Indeed, a remarkable degree of long-range order is observed for all molecular architectures, demonstrating the viability of fabricating robust monolayer structures with varying positions of internal molecular scaffolding.