Toward Tailored Xerogel Composites: Local Dipolarity and Nanosecond Dynamics within Binary Composites Derived from Tetraethylorthosilane and ORMOSILs,Oligomers or Surfactants

We explore the potential of xerogel composites to tailor the behavior of active dopants that are sequestered within the xerogel. Toward this end, we report on the local dipolarity and dynamics of two fluorescent probes (pyrene and rhodamine 6G, R6G) each co-doped at low concentration directly into a series of binary xerogel composites. The composites that we have investigated are composed of tetraethylorthosilicate (Si(OCH₂CH₃)₄,TEOS) plus one of several organically-modified silanes (ORMOSILs), organic oligomers, or a common surfactant. For convenience we divide these xerogel composites into two classes: (1) xerogels wherein the organic character arises from the addition of an ORMOSIL co-monomer, possessing a non-hydrolyzable organic functional group, that becomes covalently incorporated with in the xerogel and (2) xerogels wherein the organic content is adjusted by adding organic oligomers or a surfactant. Six organically-modified silylalkoxides of the form R _n Si(OR)_4–n were investigated as ORMOSILs. Poly(ethylene glycol), Nafion, and Ionene 6,2 were tested as oligomers. Triton X-100 was used as the surfactant. To estimate the local dipolarity within these composites we used the static fluorescence from pyrene molecules that were sequestered within the composites. These experiments showed that the local dipolarity surrounding the average pyrene molecule can be tuned significantly, but this depends on the actual organic species that one uses to prepare the xerogel composite. Time-resolved fluorescence anisotropy measurements were used to quantify the R6G mobility within the same composites. These results demonstrate that certain organic additive scan be used to adjust the R6G mobility within the xerogel composite.     

Toward Sol-Gel-Processed Chemical Sensing Platforms: Effects of Dopant Addition Time on Sensor Performance

The development of new chemical and biochemical sensing schemes has been a topic of growing interest. Simplicity of preparation and mild processing conditions have made sol-gel-derived composites attractive for many chemical sensing schemes. A portion of our research centers on using sol-gel-processed materials for the development of selective sensors. Over the years we have aimed to characterize the analytical performance of these types of sol-gel-based sensing platforms. In the course of this work we recently discovered that the time (prior to casting) when the sensing chemistry is actually doped into the sol-gel processing solution plays a critical role in a given sensor's analytical performance. In this paper we report on the effects of doping time on the behavior of a model organic dopant (pyrene) sequestered within sol-gel-derived microfiber tips and films. We use O₂ as the analyte and determine the sensor sensitivity and temporal response as a function of doping time. We also quantify the local dipolarity of the immediate environment surrounding the average pyrene molecule as a function of doping time.     

Quantifying surface coverage of colloidal silica by a cationic peptide using a combined centrifugation/time-resolved fluorescence anisotropy approach

Recent experimental studies have shown that time-resolved fluorescence anisotropy (TRFA) is a promising methodology for in situ characterization of the surface modification of aqueous silica nanocolloids. Here we provide a more fundamental insight into the principle of this approach and discuss how the adsorption parameters for a cationic peptide, Lys-Trp-Lys (denoted using the standard shortform KWK), onto Ludox nanoparticles (NPs) are linked to the rotational dynamics of rhodamine 6G (R6G) dispersed in the KWK/Ludox mixture. First, the adsorption isotherm of KWK on hydrophilic controlled pore glass (CPG-3000) was obtained using the traditional centrifugation method, which provides the total molar amount of KWK per unit surface area of the silica. Assuming that both CPG and Ludox particles possess identical surface properties when suspended in the same aqueous buffer, both materials should also have identical adsorption properties. Thus, the adsorbed amount of KWK per unit area at a given total KWK concentration, as determined by the centrifugation method, can be plotted against the fractions of R6G anisotropy decay components at the same KWK concentration to relate the anisotropy components to the absolute surface coverage. Using this approach, it was determined that the concentration of KWK at which the CPG surface was saturated corresponded to the condition g = 0 in the R6G decay, where g is the fraction of the nondecaying anisotropy component. This condition means that there is no R6G bound to the fraction of Ludox NPs with a radius R > 2.5 nm at maximum KWK coverage, consistent with the adsorbed peptide forming a continuous layer on the Ludox surface. Hence, the g value obtained from TRFA analysis can be used to assess the absolute surface coverage of monolayer coatings on colloidal nanoparticles.     

Multiprobe spectroscopic evidence for "hyperpolarity" within 1-butyl-3-methylimidazolium hexafluorophosphate mixtures with tetraethylene glycol

A hybrid, potentially green solvent system composed of tetraethylene glycol (TEG) and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) was investigated across all mole fractions with regard to the solvent properties of the mixture. For this purpose, a suite of absorbance- and fluorescence-based solvatochromic probes were utilized to explore solute-solvent and solvent-solvent interactions existing within the [bmim][PF(6)] + TEG system. These studies revealed an interesting and unusual synergistic solvent effect. In particular, a remarkable "hyperpolarity" was observed in which the E(T) value, comprising dipolarity/polarizability and hydrogen bond donor (HBD) acidity contributions, at intermediate mole fractions of the binary mixture well exceeded that of the most polar pure component (i.e., [bmim][PF(6)]). Independently determined dipolarity/polarizability (pi*) and HBD acidity (alpha) Kamlet-Taft values for the [bmim][PF(6)] + TEG mixtures were also observed to be anomalously high at intermediate mole fractions, whereas hydrogen bond acceptor (HBA) basicities (beta values) were much more in line with the ideal arithmetic values predicted on a mole fraction basis. Two well-established fluorescent polarity probes (pyrene and pyrene-1-carboxaldehyde) further illustrated notable hyperpolarity within [bmim][PF(6)] + TEG mixtures. Moreover, the steady-state fluorescence anisotropy of the molecular rotor rhodamine 6G and the excimer-to-monomer fluorescence ratio exhibited by the fluidity probe 1,3-bis-(1-pyrenyl)propane demonstrated that solute rotation and microfluidity within the [bmim][PF(6)] + TEG mixture were significantly reduced compared with expectations based on simple solvent mixing. A solvent ordering via formation of HBD/HBA complexes involving the C-2 proton of the [bmim(+)] cation and oxygen atoms of TEG, as well as interactions between [PF(6)(-)] and the terminal hydroxyl groups of TEG, is proposed to account for the observed behavior. Further spectroscopic evidence of strong intersolvent interactions occurring within the [bmim][PF(6)] + TEG mixture was provided, inter alia, by substantial frequency shifts in the [PF(6)(-)] asymmetric stretching mode observed in the infrared spectra as TEG was incrementally added to [bmim][PF(6)]. Overall, our observations contribute to a growing literature advocating the notion that ionic liquids and certain organic solvents form ordered, nanostructured, or microsegregated phases upon mixing.     

Synthesis,Characterization, and Hydrolytic Degradation of Polylactide/Poly(ethylene glycol)/Nano-silica Composite Films

Poly(lactic acid) (PLA)/PEG/nano-silica composite degradable films have been prepared by solvent casting method. IR measurements showed that vibration of C–O–C group was confined by silica network. SEM results showed that nano-silica particles were dispersed uniformly in the PLA/PEG matrix. TGA results indicated that the thermal decomposition temperature rose with the increase of nano-silica content. The tensile strength of composite film increased by the addition of nano-silica particles into PLA/PEG matrix. The degradation rate of PLA/PEG/nano-silica composites increased with the acidic medium of degradation. On the other hand, the slower degradation was obtained in the neutral buffer solution. PLA/PEG/nano-silica composites were found to exhibit almost similar degradation behavior as that of PLA/PEG films.     

On the Origin of the Heterogeneous Emission from Pyrene Sequestered Within Tetramethylorthosilicate-Based Xerogels: A Decay-Associated Spectra and O2 Quenching Study

Steady-state and time-resolved fluorescence spectroscopy are used to determine the local microheterogeneity surrounding pyrene molecules sequestered within tetramethylorthosilicate-derived xerogels. After compensation for the intrinsic background emission from the xerogel, we find that the pyrene intensity decay kinetics are best described by a two-term rate law. This is consistent with the pyrene molecules distributing primarily into two microenvironments. Under ambient conditions, the individual pyrene microenvironments exhibit excited-state fluorescence lifetimes that differ by 100 ns. However, the pyrene I₁ to I₃ band ratios that are associated with each microenvironment are statistically equivalent to one another. These results show that the local dipolarity surrounding these pyrene microenvironments are similar, but the decay rates associated with each microenvironment are very different. The longer-lived pyrene species (Environment #1) constitutes 1/2 of the total fluorescence and it exhibits an O₂ quenching sensitivity (K_sv1) of (5.19 ± 0.52 × 10^–3 %O₂ ^–1 and a bimolecular quenching constant (k_q1) of (2.30 ± 0.23) × 10⁴ %O₂ ^–1 s^–1. Environment #2, associated with the shorter-lived pyrene species, exhibits an O₂ quenching sensitivity (K_sv2) of (2.31 ± 0.16) × 10^–2 %O₂ ^–1 and a bimolecular quenching constant (k_q2) of (2.11 ± 0.23) × 10⁵ %O₂ ^–1 s^–1. These results are interpreted as follows: Environment #1 consists of pyrene molecules sequestered within a relatively rigid siloxane network wherein non-radiative decay pathways are lessened, but these pyrene molecules are not quenched readily by O₂. Environment #2 consists of pyrene molecules adsorbed onto surface silanols within the xerogel. These pyrene molecules are quenched by the silanols and they are simultaneously more accessible to O₂ compared to Environment #1.     

Characterization of rhodamine 6G aggregates intercalated in solid thin films of laponite clay. 2 Fluorescence spectroscopy

The photoluminescence response of Rhodamine 6G (R6G) laser dye intercalated into solid thin films of Laponite (Lap) clay is studied as a function of dye loading. Fluorescence spectroscopy (steady-state and time-resolved techniques) was used to characterize the R6G species adsorbed into the solid films. For very diluted R6G loadings (40% CEC), with a reminiscent fluorescence band at around 600 nm.     

Micelles with highly branched nanoporous interior: Solution properties and binding capabilities of amphiphilic copolymers with linear dendritic architecture

This article describes the solution behavior of model amphiphilic linear‐dendritic ABA block copolymers that self‐assemble in aqueous media and form micelles with highly branched nanoporous cores. The materials investigated are constructed of poly(ethylene glycol), PEG, with molecular weight 5,000 or 11,000 as the water‐soluble B block and poly(benzyl ether) monodendrons [G] of second and third generation as the hydrophobic A fragments. The process of self‐assembly in aqueous media and the character of the micellar core are investigated by fluorescence spectroscopy using pyrene as the molecular probe. The data obtained by different methods indicate that the critical micelle concentrations (cmc) for these systems are between 1.1 × 10⁻⁵ and 2.0 × 10⁻⁵ mol/L for [G‐2]‐PEG5000‐[G‐2] and between 7.08 × 10⁻⁶ and 7.94 × 10⁻⁶ mol/L for [G‐3]‐PEG11000‐[G‐3]. It is found that the ratio of the first and third vibronic bands (I₁/I₃ ) in the fluorescence spectrum of the encapsulated pyrene changes from 1.77 to 1.32 when the concentration of [G‐2]‐PEG5000‐[G‐2] increases from 1.1 × 10⁻⁶ mol/L to 1.1 × 10⁻⁴ mol/L. For [G‐3]‐PEG11000‐[G‐3] these changes are between 1.77 and 1.17 in the same concentration range. The hybrid copolymers form host‐guest complexes with several polyaromatic compounds (phenanthrene, pyrene, perylene and fullerene, C₆₀) that are stable over extended periods of time (more than 12 months). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2711–2727, 2000     

Coulombic interactions on the deposition and rotational mobility distributions of dyes in polyelectrolyte multilayer thin films

We employed negatively charged fluorescein (FL), positively charged rhodamine 6G (R6G), and neutral Nile Red (NR) as molecular probes to investigate the influence of Coulombic interaction on their deposition into and rotational mobility inside polyelectrolyte multilayer (PEM) films. The entrapment efficiency of the dyes reveals that while Coulombic repulsion has little effect on dye deposition, Coulombic attraction can dramatically enhance the loading efficiency of dyes into a PEM film. By monitoring the emission polarization of single dye molecules in polyethylenimine (PEI) films, the percentages of mobile R6G, NR, and FL were determined to be 87 +/- 4%, 76 +/- 5%, and 68 +/- 3%, respectively. These mobility distributions suggest that cationic R6G enjoys the highest degree of rotational freedom, whereas anionic FL shows the least mobility because of Coulombic attraction toward cationic PEI. Regardless of charges, this high percentage of mobile molecules is in stark contrast to the 5-40% probe mobility reported from spun-cast polymer films, indicating that our PEI films contain more free volume and display richer polymer dynamics. These observations demonstrate the potential of using isolated fluorescent probes to interrogate the internal structure of a PEM film at a microscopic level.     

Two-site ionic labeling with pyranine: implications for structural dynamics studies of polymers and polypeptides by time-resolved fluorescence anisotropy

Time-resolved fluorescence anisotropy (TRFA) is widely used to study dynamic motions of biomolecules in a variety of environments. However, depolarization due to rapid side chain motions often complicates the interpretation of anisotropy decay data and interferes with the accurate observation of segmental motions. Here, we demonstrate a new method for two-point ionic labeling of polymers and biomolecules that have appropriately spaced amino groups using the fluorescent probe 8-hydroxyl-1,3,6-trisulfonated pyrene (pyranine). TRFA analysis shows that such labeling provides a more rigid attachment of the fluorophore to the macromolecule than the covalent or single-point ionic labeling of amino groups, leading to time-resolved anisotropy decays that better reflect the backbone motion of the labeled polymer segment. Optimal coupling of pyranine to biomolecule dynamics is shown to be obtained for appropriately spaced Arg groups, and in such cases the ionic binding is stable up to 150 mM ionic strength. TRFA was used to monitor the behavior of pyranine-labeled poly(allylamine) (PAM) and poly-d-lysine (PL) in sodium silicate derived sol-gel materials and revealed significant restriction of backbone motion upon entrapment for both polymers, an observation that was not readily apparent in a previous study with entrapped fluorescein-labeled PAM and PL. The implications of these findings for fluorescence studies of polymer and biomolecule dynamics are discussed.     

Direct and indirect monitoring of peptide-silica interactions using time-resolved fluorescence anisotropy

The present work extends the application of time-resolved fluorescence anisotropy (TRFA) of a cationic probe rhodamine 6G (R6G) in aqueous Ludox to in situ monitoring of peptide adsorption onto the silica particles. Steady-state anisotropy and TRFA of R6G in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles in the presence of varying levels of tripeptides of varying charge, including Lys-Trp-Lys (KWK), N-acetylated Lys-Trp-Lys (Ac-KWK), Glu-Trp-Glu (EWE), and N-acetylated Glu-Trp-Glu (Ac-EWE). The results were compared to those obtained by direct observation of peptide adsorption using the steady-state anisotropy of the intrinsic tryptophan residue. Ionic binding of the peptides to Ludox particles produced an increase in the steady-state Trp anisotropy that was dependent on the number of cationic groups present, but the limiting anisotropy values were relatively low, indicating significant rotational freedom of the indole residue in the adsorbed peptides. On the other hand, R6G showed significant decreases in anisotropy in the presence of cationic peptides, consistent with the cationic peptides blocking the adsorption of the dye to the silica surface. Thus, R6G is able to indirectly report on the binding of peptides to Ludox particles. It was noteworthy that, while there were similar trends in the data obtained from steady-state anisotropy and TRFA studies of R6G, the use of steady-state anisotropy to assess binding of peptides overestimated the degree of peptide adsorption relative to the value obtained by TRFA. The study shows that the competitive binding method can be used to assess the binding of various biologically relevant compounds onto silica surfaces and demonstrates the potential of TRFA for probing peptide-silica and protein-silica interactions.     

Polyethylene glycol-polyamidoamine dendritic micelle as solubility enhancer and the effect of the length of polyethylene glycol arms on the solubility of pyrene in water

Unimolecular dendritic micelles designed as solubility enhancers were obtained by coupling polyethylene glycol (PEG) to Starburst polyamidoamine (PAMAM) dendrimers. Micelles-750, -2000, and -5000 have a generation 3.0 dendrimer core (32 primary amine end groups) and PEG arms with molecular weights of 750, 2000, and 5000, respectively. The conjugate of dendrimer core and PEG was characterized by MALDI-TOF MS and 1H NMR. 1H NMR was also used to estimate the average number of PEG arms on each dendrimer molecule. A typical hydrophobic compound, pyrene, was sonicated in an excess amount together with micelles at 50 degrees C for 6 h to produce its saturated water solution. The change of the solubility of pyrene was monitored at 334 nm, its maximum adsorption wavelength, by UV-VIS spectra. Concentrated micelles tended to dissolve more pyrene. However, there is no obvious linear relationship between micelle type and the amount of pyrene entrapped within micelles. Micelle-2000 could solubilize more pyrene than micelle-750. It is hypothesized that micelle-5000 did not solubilize more pyrene than micelle-2000 because of the PEG shell disruption by adjacent interpenetration of individual micelles when PEG arm length increased.     

Unusual solvatochromism within 1-butyl-3-methylimidazolium hexafluorophosphate + poly(ethylene glycol) mixtures

Hybrid "green" solvent systems composed of room-temperature ionic liquids (ILs) and poly(ethylene glycols) (PEGs) may have enormous future potential. Solvatochromic absorbance probe behavior is used to assess the physicochemical properties of the mixture composed of IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) and PEG (average molecular weights of 200, 400, 600, and 1500) at ambient conditions. Lowest energy intramolecular charge-transfer absorbance maxima of a betaine dye, i.e., E(T)(N), indicates the dipolarity/polarizability and/or hydrogen-bond donating (HBD) acidity of the [bmim][PF 6] + PEG mixtures to be even higher than that of neat [bmim][PF(6)], the solution component with higher dipolarity/polarizability and/or HBD acidity. Dipolarity/polarizability (pi*) obtained separately from the electronic absorbance response of probe N, N-diethyl-4-nitroaniline shows a trend similar to E(T)(N ) thus confirming the unusually high dipolarity/polarizability of the [bmim][PF(6)] + PEG mixtures. Similar to E(T)(N ) and pi*, the HBD acidity (alpha) of [bmim][PF(6)] + PEG mixtures is also observed to be anomalously high. Contrary to what is observed for E(T)(N ), pi*, and alpha, the hydrogen-bond accepting (HBA) basicity (beta) of the [bmim][PF(6)] + PEG mixtures is observed to be lower than that predicted from ideal additive behavior indicating diminished HBA basicity of the mixture as compared to its neat components. A four-parameter simplified combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) equation is shown to satisfactorily predict the solvatochromic parameters within [bmim][PF(6)] + PEG mixtures. It is demonstrated that [bmim][PF(6)] + PEG mixtures possess physicochemical properties that are superior to those of either the neat IL or the neat PEG.     

Evidence for rigid binding of rhodamine 6G to silica surfaces in aqueous solution based on fluorescence anisotropy decay analysis

Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.     

Preparation, characterization and application of pyrene-loaded methoxy poly(ethylene glycol)–poly(lactic acid) copolymer nanoparticles

Pyrene-loaded biodegradable polymer nanoparticles were prepared by incorporating pyrene into the polymer nanoparticles formulated from amphiphilic diblock copolymer, methoxy poly(ethylene glycol)–poly(lactic acid) (MePEG–PLA). Their morphological structure and physical properties were characterized by nuclear magnetic resonance (NMR), dynamic light scattering, fluorescence spectroscopy, transmission electronic microscopy and zeta potential measurements. Further, MePEG–PLA nanoparticles containing pyrene as fluorescent marker were administered intranasally to rats, and the distribution of nanoparticles in the nasal mucosa and the olfactory bulb were visualized by fluorescence microscopy. NMR results confirmed that MePEG–PLA copolymer can form nanoparticles in water, and hydrophilic PEG chains were located on the surface of the nanoparticles. The particle size, zeta potential and pyrene loading efficiency of MePEG–PLA nanoparticles were dependent on the PLA block content in the copolymer. Following nasal administration, the absorption of nanoparticles across the epithelium was rapid, with fluorescence observed in the olfactory bulb at 5 min, and a higher level of fluorescence persisted in the olfactory mucosa than that in the respiratory mucosa. These results show that pyrene could serve as a useful fluorescence probe for incorporation into polymer nanoparticles to study tissue distribution and MePEG–PLA nanoparticles might have a great potential as carriers of hydrophobic drugs.     

End-group dynamics of fluorescently labeled dendrimers

The molecular dynamics of dendrimer branches were studied by observing the excimer formation of pyrenyl end-groups. By a selective synthesis only one pyrene moiety per dendron was attached, resulting in a dendrimer with 3 fluorophors per molecule. The first 3 generations of the dendrimer were synthesized by the convergent-growth approach. By time resolved measurements of the eximer and monomer fluorescence an increased mobility of the end-groups was observed.     

Strong cellulose nanofibre–nanosilica composites with controllable pore structure

Flexible nanocellulose composites with silica nanoparticle loading from 5 to 77 wt% and tunable pore size were made and characterised. The pore structure of the new composites can be controlled (100–1000 nm to 10–60 nm) by adjusting the silica nanoparticle content. Composites were prepared by first complexing nanoparticles with a cationic dimethylaminoethyl methacrylate polyacrylamide, followed by retaining this complex in a nanocellulose fibre network. High retention of nanoparticles resulted. The structural changes and pore size distribution of the composites were characterised through scanning electron microscopy (SEM) and mercury porosimetry analysis, respectively. The heavily loaded composites formed packed bed structures of nanoparticles. Film thickness was approximately constant for composites with low loading, indicating that nanoparticles filled gaps created by nanocellulose fibres without altering their structure. Film thickness increased drastically for high loading because of the new packed bed structure. Unexpectedly, within the investigated loading range, the level of the tensile index on nanocellulose mass basis remained constant, showing that the silica nanoparticles did not significantly interfere with the bonding between the cellulose nanofibres. This hierarchically engineered material remains flexible at all loadings, and its unique packing enables use in applications requiring nanocellulose composites with controlled pore structure and high surface area.     

Effects of cluster formation on spectra of benzo[a]pyrene and benzo[e]pyrene

Absorption and fluorescence emission spectra of the polycyclic aromatic hydrocarbons benzo[a]pyrene (BaP) and benzo[e]pyrene (BeP) in solution and adsorbed on silica have been obtained and compared to examine the spectroscopic effects of clustering. Molecular mechanics calculations with the UFF potential were done to optimize monomer, dimer and trimer geometries, and energy differences were determined by MP2/6-31G* calculations. Fluorescence emission spectra of adsorbed BeP and BaP display a red shift that progresses with increased loading, and the two differ in their photodegradation kinetics. The experimental and theoretical results are found to be consistent.     

Interfacial electron transfer dynamics in dye-modified graphene oxide nanosheets studied by single-molecule fluorescence spectroscopy

Graphene oxide (GO) nanosheets have received a great deal of attention for a wide range of applications from optoelectronic devices to biological sensors. We now report a mechanistic study of the interfacial electron transfer (ET) processes between organic dye molecule, 9-phenyl-2,3,7-trihydroxy-6-fluorone (PF), and nanometre-sized GO sheets using ensemble-averaged and single-molecule spectroscopies. The ET dynamics was characterized by the direct observation of the PF radical cation during the laser flash photolysis, and its reaction rate was determined to be ~10(11) s(-1). The single-molecule fluorescence spectroscopy was utilized to clarify the heterogeneous nature of the interfacial ET within individual composites. Their fluorescence lifetimes and spectra were found to vary from composite to composite, possibly due to the different local structures and molecular interactions. The autocorrelation analysis of fluorescence intensity trajectories also revealed the temporal fluctuation of the ET reactivity.