Dual-wavelength resonance Raman spectroscopy of polydiacetylene monolayers on Au surfaces

In this paper, resonance Raman spectroscopy with dual-wavelength laser excitation is applied to study the chromatic phase properties of self-assembled diacetylene monolayers on atomically flat Au surfaces. Laser excitation at 632.8 nm is utilized to exclusively monitor the longer conjugation length blue-phase polydiacetylene (PDA) backbone structure, while laser excitation at 514.5 nm is applied to probe the shorter conjugation length red phase. This experimental design allows in situ measurement of the photophysics and chemistry of the polymerization process within a single molecular layer. These studies demonstrate the presence of multiple chromatic species of PDAs within the same monolayer structure. Similar to observations for analogous thin film and multilayer assemblies, systematic conversion from the higher conjugation length blue form of the polymer to the shorter conjugation length red form is observed upon extended UV irradiation. Moreover, a very large Raman dispersion of 130 cm^-1/eV is measured, demonstrating the highly conjugated nature of these monolayer polymers. Finally, backbone planarity and strain during the polymerization process are assessed by using the correlation of double- and triple-bond stretching frequencies.