Date Thesis Awarded

5-2016

Access Type

Honors Thesis -- Access Restricted On-Campus Only

Degree Name

Bachelors of Science (BS)

Department

Chemistry

Advisor

Kristin Wustholz

Committee Members

William McNamara

Elizabeth Harbron

Drew LaMar

Abstract

Dye-sensitized solar cells (DSSCs) are promising solar devices to provide for future energy needs. Improvements in DSSC efficiency require an understanding of the complete distributions of electron injection and charge recombination. In this thesis, a series of rhodamine dyes is investigated to probe the impact of structure, driving force for photoinduced electron transfer, and adsorption affinities to TiO₂ on electron transfer (ET) dynamics. A combination of ensemble-averaged techniques, single-molecule spectroscopy, and modeling approaches are used to interpret the dispersive ET kinetics.

Ensemble-averaged measurements provided insight into aggregation effect on binding affinity to and differences in driving forces for electron injection and recombination. Absorbance and fluorescence measurements revealed that adsorption affinity to TiO₂ increases as follows: rhodamine 6G (R6G) < rhodamine 123 (R123) < rhodamine B (RB) < 5-carboxy-X-rhodamine (5-ROX). 5-ROX contains a para-substituted carboxyl group that is less sterically hindered for TiO₂ binding relative to the ortho-substituted carboxyl group in RB. Electrochemical measurements evaluated the range of driving forces for electron injection and recombination in all of the dyes, where RB had the smallest driving force for electron injection and the largest driving force for recombination. Overall, these ensemble-averaged characteristics demonstrated the diversity of driving forces and adsorption geometries exhibited by these series of rhodamine dyes.

Single-molecule blinking measurements were compiled from ~100 molecules of RB, R6G, R123, and 5-ROX. Blinking traces were separated into on-time and off-time distributions, which were fitted to various heavy-tailed exponential forms (i.e., power-law, log-normal, and Weibull) to establish the best fit with robust statistical tests. Our analysis reveals that although power law seems to be statistically significant for on-time distributions for all of the rhodamine dyes, the onset time of the power-law fit only describes a small portion of the data (i.e., less than 25%). Instead, we observe that lognormal distributions better capture the entire on-time distribution. Furthermore, the lognormal function also characterizes off-time distributions, which supports the Albery model of dispersive ET kinetics (i.e., ET rates will be log-normally distributed). The physical interpretation of the associated log-normal fit parameters was found with Monte Carlo (MC) simulations based on the Albery model. Changes to rates of injection and recombination resulted in changes to μ_on/off, respectively, while the extent of energetic dispersion around the mean activation barrier was associated with σ_on/off. The single-molecule results for RB, R6G, R123, and 5-ROX, on TiO₂ are interpreted in the context of ensemble-averaged data and suggest that heterogeneity in electronic coupling and reorganization play a significant role in the observed dispersive kinetics.

Time-correlated single photon counting (TCSPC) measurements are implemented to study fluorescence in combination with single-molecule studies. Control experiments are taken to establish precision and accuracy in the TCSPC setup. The instrument response function (IRF) of the system is minimized to an appropriate full width at half maximum for lifetime measurements. Further work is needed on obtaining literature appropriate values for lifetime standards of common fluorophores. Once a complete series of lifetime standards are measured, fluorescence decay measurements from rhodamine dyes will enrich our model of the excited state in a dye-TiO₂ system.

Recommended Citation

Tan, Jenna Adrienne Uy, "Probing Single-Molecule Photophysics of Rhodamines on TiO2" (2016). Undergraduate Honors Theses. William & Mary. Paper 898.
https://scholarworks.wm.edu/honorstheses/898