Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots

Artificial photosynthetic systems consisted of nanocrystal light absorbers, molecular redox mediators, and catalysts are one of the most promising and flexible approaches for solar fuel generation because their constituents can be independently tuned. In this work, we investigate the photoreduction of three viologen derivatives, one of the most widely investigated molecular redox mediators, of different redox potentials, 7,8-dihydro-6H-dipyrido[1,2-a:2',1'-c][1,4]diazepinediium (PDQ(2+)), methyl viologen (MV2+), and benzyl viologen (BV2+), using CdS quantum dots (QDs) as the light absorber and mercaptopropionic acid as a sacrificial electron donor in aqueous (pH = 7) solution. Under continuous 405 nm light-emitting diode illumination, the steady-state radical generation quantum yield (QY) follows the order of PDQ(center dot+) (15.99%) > MV center dot+ (12.61%) > BV center dot+ (6.56%). Transient absorption spectroscopy studies show that while the rates of initial electron transfer (ET) from the excited QD conduction band to the mediators, following the order of BV2+ > MV2+ > PDQ(2+), decrease for mediators with more negative redox potentials (and lower ET driving force), the initial transient charge separation QYs are unity in all samples because these ET rates are much faster than the intrinsic exciton decay within the QD. The steady-state QYs are much smaller than unity because of charge recombination (CR), whose rates, following the order of BV2+ > MV2+ > PDQ(2+), decrease for mediators with more negative redox potentials (and higher ET driving force in the Marcus' inverted regime). In these systems, there exists a long-lived component in the radial decay kinetics, whose amplitudes determine the steady-state radical generation QYs. We speculate that the desorption of the radical from the QD surface is essential for the suppression of CR and is responsible for the steady-state generation of radicals. This work provides new insight for rational design and improvement of efficient QD/redox mediator-based photoreduction systems.