The single molecule fluorescence spectroscopy of various isolated single-chromophoric dye molecules and multiple-chromophoric conjugated polymer molecules has been investigated. For each system the transient fluorescence “intensity”, I CW(t) (i.e., detected photons/dwell time), has been recorded with continuous wave (CW) irradiation. I CW(t) has been analyzed to yield an occurrence histogram for the different “intensities”, H(I), and an intensity time-autocorrelation function C I (t). The histograms H(I) for the various examples show highly diverse behavior with one, two, or even three peaks as well as “flat regions”. The different features in the histograms are shown to arise from distinct photophysical processes. From the study of model systems, characteristic features in the intensity histograms and autocorrelation functions are shown to result from photon shot noise, “blinking” due to triplet bottlenecks, spectral diffusion due to environmental fluctuations, and interchromophoric energy transfer. Classification of the relevant photophysical processes is aided by single molecule spectroscopic data on these systems, including wavelength-resolved emission spectroscopy and “two-color excitation spectroscopy”, as well as stochastic simulations. The results indicate that a combined analysis of H(I) and C I (t) is a valuable approach in sorting out single molecule behavior involving multiple photophysical processes in complex systems. For single molecule systems that exhibit “on−off blinking” involving the formation of dark states, the paper also explores the practical advantages of studying the duration histograms (H(t on) and H(t off)) versus the intensity autocorrelation function C I (t), for quantifying the underlying photophysical dynamics.

The amphiphilic cyanine dye 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3) self-assembles in aqueous solution to form double-walled, tubular J-aggregates with ∼13 nm diameters and lengths up to several hundred nanometers. The redox and light absorption properties of immobilized J-aggregates on transparent, conductive indium tin oxide (ITO) electrodes have been studied directly using cyclic voltammetry (CV) in conjunction with UV-vis spectroscopy to elucidate unique mechanistic features of J-aggregate oxidation. Morphological properties were examined using in situ atomic force microscopy (AFM). Irreversible J-aggregate oxidation appears to occur primarily along the outer wall of the tubular structure as evidenced by the potential-induced irreversible bleaching of J-band absorption. Voltammetric studies as a function of scan rate and pH indicate that J-aggregate oxidation involves both electrochemical and chemical steps in which dimerization and subsequent dehydrogenation of the J-aggregate leads to the formation of a new dehydrogenated dimer oxidation product. This dehydrogenated dimer exhibits an absorbance band near 560 nm along with a reversible reduction peak characteristic of a surface-confined, redox-active species. Excellent correlation of J-aggregate redox potentials with spectroelectrochemical data is obtained that allows us to understand energetic thresholds for electron transfer in C8S3 tubular J-aggregates.

The formation of flexible molecular fibers via the solution-phase self-assembly of the pseudo-isocyanine dye (PIC) 1,1′-diethyl-2,2′-cyanine, and poly(vinyl sulfate) (PVS) is reported. The physical and electronic properties of these fibers spin-coated into thin films on fused-quartz substrates are studied by fluorescence and topographic imaging with near-field scanning optical microscopy (NSOM) and also by atomic force microscopy (AFM). The scanned-probe images demonstrate that fibers with lengths in the hundred micrometer range, widths of hundreds of nanometers, and thicknesses of a few tens of nanometers are readily formed in aqueous mixtures of PVS and PIC. Unprecedented flexibility in these fibers is exemplified by the formation of numerous curved and looped structures in the spin-coated thin films. A sandwich-like composite structure of alternating anionic PVS and cationic PIC layers is proposed as a model for the assembly of the dye and polymer in these fibers. The alternating layers in this model are held tightly together via the cooperative "cross-linking" of the PVS and PIC layers by electrostatic dye/ polymer interactions, and by hydrophobic van der Waals interactions between the PIC molecules. The intermolecular interactions in the PIC layer result in the formation of a liquid-crystalline-like, well-ordered layer of the PIC which exhibits the spectral characteristics of J-aggregates. The proposed layered structure apparently possesses "reactive" surfaces which link individual fibers into a yarnlike assembly. This cross-linking effect is supported by the presence of continuous circular fibers and by the gel-forming ability of the solutions from which these fibers are grown.

We have examined the effects of dissolved molecular oxygen on multiphoton-excited (MPE) photochemical derivatization of serotonin (5HT) and related cellular metabolites in various buffer systems and find that oxygen has a profound effect on the formation efficiency of visible-emitting photoproducts. Previously, end-column MPE photoderivatization provided low mass detection limits for capillary electrophoretic analysis of hydroxyindoles, but relied on the use of Good's buffers to generate high-sensitivity visible signal. In the present studies, visible emission from 5HT photoderivatized in different buffers varied by 20-fold under ambient oxygen levels but less than 2-fold in the absence of oxygen; oxygen did not significantly alter the photoproduct excited-state lifetime (approximately 0.8 ns). These results support a model in which oxygen interferes with formation of visible-emitting photoproducts by quenching a reaction intermediate, an effect that can be suppressed by buffer molecules. Deoxygenation of capillary electrophoresis separation buffers improves mass detection limits for 5-hydroxyindoles fractionated in 600-nm channels by approximately 2-fold to < or =30000 molecules and provides new flexibility in identifying separation conditions for resolving 5HT from molecules with similar electrophoretic mobilities, such as the catecholamine neurotransmitters.