Photophysics of Single Rhodamine Dye Molecules on TiO2 Substrates

Author

Natalie Wong, College of William and MaryFollow

Date Thesis Awarded

5-2014

Access Type

Honors Thesis -- Access Restricted On-Campus Only

Degree Name

Bachelors of Science (BS)

Department

Chemistry

Advisor

Kristin Wustholz

Committee Members

Christopher Abelt

William McNamara

Abstract

Dye-sensitized solar cells (DSSCs) comprised of sensitizers within a TiO₂ semiconductor matrix are promising photovoltaics that use electron transfer (ET) to convert photons into current. Improvements in DSSC efficiency require fundamental understanding of the distribution of forward and back ET dynamics at the dye-semiconductor interface, which ensemble-averaged experiments are incapable of observing. In this study, the distributions of ET dynamics in dye-sensitized TiO₂ systems are probed using single-molecule fluorescence (SMF) microscopy. The time-dependent emission (i.e., blinking dynamics) of rhodamine 6G (R6G), rhodamine B (RB), and 5-carboxy-x-rhodamine (5-ROX) sensitized colloidal anatase TiO₂ films are quantified by constructing histograms of emissive (“on”) and non-emissive (“off”) events. Robust statistical analysis reveals that the off-time distributions for molecules on TiO₂ are not consistent with the power-law hypothesis and are instead well represented by log-normal distributions. These ET data are consistent with the Albery model of a Gaussian distribution of activation energies, where the power-law and log-normal distribution fit parameters are sensitive to experimental time resolution and cannot be analyzed as absolute values. Relative comparison of the fit parameters of R6G, RB, and 5-ROX ET distributions on colloidal TiO₂ provides insight into the effect of sensitizer structure on the photophysical system. Notably, log-normally distributed off times produce a larger scaling parameter μ for 5-ROX than for R6G or RB, suggesting the dye binds more strongly to TiO₂ than R6G or RB and produces slower back electron transfer (BET) rates due to increased electron delocalization. The on-time distributions for rhodamine dyes on TiO₂ are fit by power laws, but they are only operative for long emissive durations comprising less than 15% of the photophysical data. Changes in fluorescent spot density on false-colored images of R6G on bare glass, colloidal anatase TiO₂, and single-crystal rutile TiO₂ illustrate the impact of substrate on ET dynamics, but single-molecule studies of rhodamine molecules on single-crystal rutile could not be completed due to extremely efficient ET. Future investigations will apply single-molecule methodologies with picosecond resolution to dye-semiconductor systems.

Recommended Citation

Wong, Natalie, "Photophysics of Single Rhodamine Dye Molecules on TiO2 Substrates" (2014). Undergraduate Honors Theses. William & Mary. Paper 44.
https://scholarworks.wm.edu/honorstheses/44

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