Three-component systems, 1a-c and 2a,b, comprising 1,8-naphthalimide and 4-methoxy-1,8-naphthalimide as fluorophore, a dimethylamino moiety as guest binding site and a polymethylene group as spacer, have been synthesized and the fluorescence behavior of these systems has been studied in the absence and in the presence of the salts of several transition metal ions. The systems are found to be very weakly fluorescent compared to their constituent fluorophores (3 and 4) and this observation has been ascribed to photoinduced intramolecular electron transfer (PIET) between the electron rich amino moiety (donor) and relatively electron deficient fluorophore component (acceptor). Spectral and electrochemical data indicate the thermodynamic feasibility of PIET (exergonic free energy changes) in these multicomponent systems and PIET is found to be most efficient in systems where the fluorophore and the amino moiety are separated by two methylene groups. Fluorescence decay behavior of the systems suggest that PIET occurs by a through-space mechanism. In the presence of the transition metal ions, well-known for their fluorescence quenching abilities, the present systems exhibit significant fluorescence enhancement (FE). Moreover, it has been observed that guest-induced FE can even be severalfold higher than that expected from consideration of PIET in the system. It is suggested that a system can exhibit unusually high FE when the guest is capable of inducing FE by more than one means. In the present case, it is shown that preferential solvation of the fluorophore by the water molecules of the hydrated metal salts could be partially responsible for the high FE values.

Single-molecule fluorescence spectroscopy is used to study dye diffusion within organically modified silicate (ORMOSIL) films. ORMOSIL films are prepared from sols containing tetraethoxysilane and isobutyltrimethoxysilane in 2:1 and 1:9 molar ratios. Nile red and a new silanized form of nile red that can be covalently attached to the silicate matrix are used as fluorescent probe molecules. The number and rate of single molecules diffusing through these films increases dramatically with increasing film organic content. Autocorrelation of the fluorescence images yields a quantitative measure of the relative populations of fixed and diffusing species. Surprisingly, both "free" and silicate-bound nile red exhibit relatively facile translational motions. Single-molecule/single-point fluorescence correlation spectroscopy (FCS) is used to measure the dye diffusion coefficients in submicrometer-scale film regions. The most common diffusion coefficients for "free" and silicate-bound nile red molecules in the 1:9 films are 3.9 × 10 -10 and 1.6 × 10 -10 cm 2 /s, respectively. The unexpectedly rapid diffusion of silicate-bound nile red is attributed to the presence of liquidlike silicate oligomers in the films. A lower bound for the molecular weight of the oligomers is estimated at 2900. Bulk solution-phase FCS experiments performed on "free" and silicate-bound nile red species extracted into chloroform solutions provide valuable support for these conclusions. Comparison of the results derived from experimental and simulated time transients indicates film heterogeneity occurs on sub-100-nm-length scales and likely results from the presence of inorganic-and organic-rich domains.

The photophysical properties of 11-(4-N,N-dimethylaminophthalimido)undecanoic acid (DAPL), a surfactant covalently labeled with a fluorophore, in homogeneous and micellar media are reported. The remarkable sensitivity of the fluorescence properties of DAPL to the polarity of the media is attributed to the existence of a low-lying nonfluorescent twisted intramolecular charge transfer (TICT) state. DAPL is found to be an excellent fluorescence sensor for following the micellar aggregation process. Although DAPL shows enhanced binding with the micelles, the quenching experiments and the fluorescence spectral and intensity data clearly indicate that the fluorescing moiety does not penetrate into the nonpolar core region of the micelles even though the fluorophore is covalently attached to the nonpolar end group of the fatty acid. It is unambiguously established from the fluorescence data that both of the terminal groups of DAPL are located in the interfacial region of the micelles by folding of the long polymethylene chain.

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