M. Cai et al., Fabrication of extended conjugation length polymers within diacetylene monolayers on Au surfaces: Influence of UV exposure time, LANGMUIR, 15(4), 1999, pp. 1215-1222
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 us
ing resonance Raman spectroscopy as a function of UV exposure time. In thes
e studies, initial formation of the blue polymer form is followed by an irr
eversible loss with prolonged exposure. This behavior mirrors the chromatic
phase transition to shorter conjugation lengths exhibited for multilayer L
angmuir-Blodgett films upon extended UV exposure. Although the exact nature
of this phase transition remains elusive, most theories focus on factors a
ffecting the alignment of the polymer backbone and influencing the effectiv
e conjugation length. Three such factors are examined here: the Au-S bond w
ith 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 polymerizatio
n process. In addition, no change in chain crystallinity is observed upon p
olymerization, but the twist of the methylene chain exhibits significant ch
anges with prolonged UV exposure. This result is consistent with the hybrid
ization-induced strain being translated into the polymer backbone as well a
s the methylene chains, resulting in a decrease in the effective conjugatio
n length.