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.

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The effective conjugation length of the delocalized polymer backbone is one of the key factors in designing monolayer polymers for sensor or nonlinear optical applications. In this manuscript, the photopolymerization behavior of self-assembled diacetylene-containing disulfide monolayers is assessed on gold surfaces. Formation of the long conjugation length, so-called blue form, of the polydiacetylene backbone structure is exclusively monitored using resonance Raman spectroscopy as a function of UV exposure time. In these studies, initial formation of the blue polymer form is followed by an irreversible loss with prolonged exposure. This behavior mirrors the chromatic phase transition to shorter conjugation lengths exhibited for multilayer Langmuir−Blodgett films upon extended UV exposure. Although the exact nature of this phase transition remains elusive, most theories focus on factors affecting the alignment of the polymer backbone and influencing the effective conjugation length. Three such factors are examined here:  the Au−S bond with the surface, the crystallinity of the alkyl side chains, and the strain induced by hybridization changes. When the reductive desorption technique is used, the Au−S bond is shown to not be correlated with the polymerization process. In addition, no change in chain crystallinity is observed upon polymerization, but the twist of the methylene chain exhibits significant changes with prolonged UV exposure. This result is consistent with the hybridization-induced strain being translated into the polymer backbone as well as the methylene chains, resulting in a decrease in the effective conjugation length.

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