Fabrication of Extended Conjugation Length Polymers within Diacetylene Monolayers on Au Surfaces:  Influence of UV Exposure Time

Abstract

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.

Monitoring Based on Narrow‐Band Resonance Raman for “Phase‐Shifting” π‐Conjugated Polydiacetylene Vesicles upon Host–Guest Interaction and Thermal Stimuli

“…The Raman spectra in Figure b at 25 °C, taken with light source of 633 nm matching nearly with the maximum absorption of the blue phase in Figure c, exhibits three major features. These are the CCC deformation mode at 694 cm −1 , the alkene stretching mode ν(CC) at 1452 cm −1 , and the alkyne stretching mode ν(CC) at 2085 cm −1 , as reported earlier . The Raman intensity of the blue phase PDA is at its highest, while it abruptly falls off as the phase transition occurs at 80 °C.…”

Use of radiochromic film as a high-spatial resolution dosimeter by Raman spectroscopy

“…[24][25][26][27][28][29][30] On the basis of this knowledge, Callens et al allocated bands in the Raman spectrum of EBT3 film, which included δ CCC deformation mode at 696 cm −1 , ν C-C , ν C= =C , ν C≡ ≡C stretching modes at 1086, 1445, 2058 cm −1 , respectively, and CH 2 wagging and CH 2 twisting modes of all trans-alkyl chain at series of peaks between 1186 and 1333 cm −1 . 17,[24][25][26][27][28][29][30] The characteristic deformation band, δ COH , of diacetylene monomer overlaps with the stretching band, ν C= =C , of diacetylene polymer. 29,30 Moreover, the stretching bands ν C≡ ≡C of both diacetylene monomers and polymers are very close to each other, and the monomer ν C≡ ≡C is absent in partly polymerized diacetylene.…”

Reactions and Reactivity in Self-Assembled Monolayers

“…[69,70] This is a remarkable observation, as polymerization of diacetylenes is only observed in well-ordered systems such as crystals or micelles. The fact that monolayers of diacetylenes can polymerize to give a so-called blue form, characterized by a prolonged conjugation length, [71] indicates a high degree of lateral order in these monolayers. Multilayers of diacetylenes, covalently linked by thioesters, are also sufficiently well-ordered to polymerize.…”