Single-Molecule Fluorescence Spectroscopy of Phase-Separated 10,12-Pentacosadynoic Acid Films

Phase-separated monolayers of 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid can be photopolymerized to produce micrometer-sized, fluorescent polydiacetylene fibers at the air-solid interface. The photopolymer fibers were not uniformly fluorescent but rather showed a series of fluorescent spots along their lengths. The spots exhibited the classic properties of single-molecule fluorescence emission, including diffraction-limited size and fluorescence intermittency ("on-off blinking"). We have analyzed the fluorescence blinking dynamics of these spots using a variety of single-molecule analysis approaches, including fluorescence intensity histograms, autocorrelation analysis, as well as cross-correlation analysis as a function of distance between individual transition dipole moments, and propose a simple physical model for the fiber structure based on the observed blinking dynamics, in which the polymer fibers contain numerous structural defects. The model was supported by grazing incidence X-ray diffraction measurements of the mixed monolayer films at the air-water interface, in which it was observed that the presence of perfluorocarbon in the mixed monolayers significantly inhibited the ability of the 10,12-pentacosadiynoic acid to polymerize.