The effects of dye structure, driving force of photoinduced electron transfer, and adsorption affinity on the dispersive electron-transfer dynamics of rhodamines on TiO2 films are investigated using single-molecule microscopy. The time-dependent emission (i.e., blinking dynamics) of rhodamine-sensitized TiO2 films are quantified using maximum likelihood estimation (MLE) and quantitative goodness-of-fit tests based on the Kolmogorov–Smirnov (KS) statistic to determine the best fit to the photophysical data. Although the observation of significant p-values (i.e., ranging from 0.16 to 0.53) seems to support the power-law model for the on-time distributions, only a minor subset of the data are actually represented by power laws (i.e., ∼15–20% of events). Instead, the MLE/KS analysis reveals that log-normal distributions, not power laws, most closely represent the entire on-time and off-time distributions for RB, R6G, R123, and 5-ROX on TiO2. Monte Carlo simulations based on the Albery model for dispersive electron transfer (i.e., activation barriers to electron transfer are Gaussian distributed) demonstrate that the log-normal fit parameters (i.e., μon/off and σon/off) are dependent on the average rate constants for injection and recombination as well as the extent of energetic dispersion about the mean activation barrier. The single-molecule results for RB, R6G, R123, and 5-ROX on TiO2 are interpreted in the context of ensemble-averaged spectroscopic and electrochemical characterization and suggest that heterogeneity in electronic coupling and reorganization (i.e., static and dynamic disorder) play a decisive role in the observed dispersive kinetics.

The blinking and photobleaching dynamics of alizarin (1,2dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) are investigated using single-molecule spectroscopy. The time-dependent emission of alizarin and purpurin on glass under N 2 is analyzed using the change point detection (CPD) method to compile on-and off-event distributions. The number of distinct emissive events per molecule is about four times higher for alizarin relative to purpurin, consistent with an excited-state intramolecular proton transfer (ESIPT) process to populate an emissive tautomer state. To elucidate the mechanism for blinking (i.e., switching between on and off events), maximum likelihood estimation (MLE), goodness-of-fit tests based on the Kolmogorov−Smirnov (KS) statistic, and the loglikelihood ratio (LLR) tests are used to establish the best fits to the on-and offinterval probability distributions. For both alizarin and purpurin the on intervals are log-normally distributed, and off intervals are Weibull distributed, consistent with a dispersive electron-transfer (ET) kinetics model for blinking (i.e., involving Gaussian-like distributions of activation barriers to ET). Further analysis of the blinking dynamics reveals that ET to a long-lived dark state most often precedes molecular photobleaching, where extended residency in the dark state increases the probability of photobleaching. Based on these findings, mechanisms for the blinking and photobleaching of alizarin and purpurin are proposed. The ability of alizarin to undergo ESIPT enables fast excited-state decay and decreases the probability of ET. In contrast, purpurin exhibits faster injection and slower back ET relative to alizarin, leading to increased photobleaching via a dark radical cation state.

We demonstrate the first biosensing strategy that relies on quantum dot (QD) fluorescence blinking to report the presence of a target molecule. Unlike other biosensors that utilize QDs, our method does not require the analyte to induce any fluorescence intensity or color changes, making it readily applicable to a wide range of target species. Instead, our approach relies on the understanding that blinking, a single particle phenomenon, is obscured when several QDs lie within the detection volume of a confocal microscope. If QDs are engineered to aggregate when they encounter a particular target molecule, the observation of quasi-continuous emission should indicate its presence. As proof of concept, we programmed DNAs to drive rapid isothermal assembly of QDs in the presence of a target strand (oncogene K-ras). The assemblies, confirmed by various gel techniques, contained multiple QDs and were readily distinguished from free QDs by the absence of blinking.

The aggregation and photodegradation of rhodamine derivatives adsorbed to TiO2 are investigated using diffuse reflectance spectroscopy, steady-state fluorescence, and time-correlated single-photon-counting (TCSPC) measurements. Rhodamine dyes containing substituted amines (i.e., 5-ROX, R101, RB) exhibit an ∼50 nm hypsochromic shift in λmax upon adsorption to TiO2 relative to solution. By examining a rhodamine derivative with primary amines (i.e., R560) as well as control experiments on insulating ZrO2 substrates, we demonstrate that photocatalyzed N-dealkylation is largely responsible for the spectral changes observed upon surface adsorption to TiO2. For R560, which does not undergo N-dealkylation, diffuse reflectance spectra show that mainly monomers and J-aggregates are present on TiO2. Comparative lifetime measurements for R560 on TiO2 and ZrO2 show that the injection yield for R560/TiO2 is increased with dye-loading concentration (i.e., from 0.63 for monomers to ∼0.80 for heavily doped films), indicating that the presence of aggregates enhances electron injection. The residual fluorescence of R560/TiO2 is attributed to subpopulations of monomers and weakly fluorescent J-aggregates of R560 that do not undergo efficient electron injection to TiO2. The fluorescence intensity, energy, and lifetime of R560 on TiO2 and insulating ZrO2 films are dependent on dye concentration, consistent with a resonance energy transfer quenching process. This study shows that contributions due to molecular photodegradation and energy transfer interactions must be considered when pursuing the development of a controlled aggregation strategy for solar energy conversion materials and devices.

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