SINGLE-MOLECULE FLUORESCENCE SPECTROSCOPY OF PHASE SEPARATED 10, 12-PENTACOSADYNOIC ACID AND PERFLUOROTETRADECANOIC ACID TO INVESTIGATE ENERGY TRANSFER DYNAMICS.

Abstract

Polydiacetylenes (PDAs) display several intriguing structural and optical properties, including a strong, structure-dependent fluorescence. Phase-separated Langmuir Blodgett monolayer films prepared from mixtures of photopolymerizable 10, 12-pentacosadiynoic acid (PCDA) and perfluorotetradecanoic acid (PF) form fluorescent polydiacetylene fibers and aggregates upon laser or UV-illumination. In this thesis, the single-molecule fluorescence behavior of the photopolymer in 7PF: 1PCDA mixed monolayer films has been probed using single-molecule spectroscopy imaging supported by ensemble fluorescence spectroscopy and atomic force microscopy. The fluorescence experiments revealed significant fiber-to-fiber heterogeneity, but in all cases, single-molecule emission from the fibers was observed. Analysis of the fluorescence intermittency (blinking) from the individual emitters was carried out. Autocorrelation and cross correlation analysis showed that the blinking of individual spots on the fiber is random, with no evidence for intermolecular energy transfer in the system. Also, single-molecule normalized intensity time correlation functions showed rapid decay of fluorescence intensity time traces at higher illumination intensities. Crystallographic measurements of mixed Langmuir monolayers of PCDA and PF were also carried out at the air/water interface, before and after polymerization, using grazing incidence X-ray diffraction (GIXD). Mixed films before and after 1 minute polymerization proceeded gave diffraction signals exclusively from the PF component. After increasing polymerization time to 30 minutes, diffraction peaks corresponding to the red phase PDA and that of the PF was observed. The combination of measurements used in this thesis have been used to provide a thorough understanding of the structure and composition of the mixed films.