Enhanced Electron Transfer Rates Between Surface-Attached Dye Molecules with Large Pendant Moieties and Co3+/2+ Complex Redox Mediators
Publication Name
Journal of Physical Chemistry C
Abstract
Enhancing electron transfer (ET) kinetics between densely packed, electrode surface-attached dye molecules and redox couples in solution is a key challenge toward more efficient solar to electrical energy conversion. In densely packed dye layers required for efficient light harvesting, the access of the redox mediator to the π-conjugated backbone of the dyes is restricted, and therefore, ET is blocked. Attaching large pendant groups to the end of the dyes facing the electrolytes has been proposed as one of the strategies for enhancing ET. The pendant group would be the first interaction area for the redox mediator designed to give higher electronic coupling. ET can also be enhanced by confining the electron density in smaller dyes, providing strong electron coupling sites. Because of these countering effects of exposed surface area of the orbitals versus high electron probability density, it is currently not known to what extent attaching large pendant groups can enhance ET rates or, in other words, how to design molecules with increased electronic coupling. Herein, we measured the ET kinetics of over 20 dye molecules with the same thiophene backbone and with pendant units having varied size, electron-donating ability, and conjugation. The electronic structure of the dyes and their redox potential was calculated using density functional theory following a thermodynamic cycle. Conformational search of the lowest energy structures together with calculating the contribution of solvation free energies to the redox potential were found to be the key to get redox potential values closely matching the values measured by cyclic voltammetry in solution. To determine the enhancement in ET rates due to changes to the molecular structure, the ET rates of dyes with the smallest pendant groups (3 dyes) in combination with two Co-bipyridyl electron donors were measured, and the data points were fitted to nonadiabatic Marcus theory. Then, the enhancement factor (EF) for the dyes with extended pendant groups is defined by the ratio between the measured rate of the dyes with extended pendant groups and the value on the fitted curve above at the same driving force. The ET rates were enhanced by up to six times for dyes with larger pendant moieties, with clear trends between EFs and the size of the molecular surface area. Furthermore, a key correlation between the EF and the spatial delocalization index (SDI) of the spin density of the oxidized dyes has been uncovered, suggesting the SDI is an appropriate index to evaluate dyes for ET. EF of the largest pendant group dye PX99 was not as large as expected, mainly because the spin density did not spread uniformly to the large pendant group. Further dye designs should focus on concentrating the spin density more evenly on extended donor moieties by exploring stronger push-pull dye designs and the effect of end groups of the pendant units.
Open Access Status
This publication is not available as open access
Volume
128
Issue
24
First Page
9847
Last Page
9860
Funding Number
DP190100687
Funding Sponsor
Australian Government