Supramolecular assemblies for electron transfer

Ternary I-III-VI2 semiconductors, such as CuInS2 and AgInS2 (compliant with RoHS, restriction of hazardous substances), are useful as light-harvesting materials. However, the presence of sub-bandgap states (donor-acceptor pair or DAP) introduces complexity during their activation through photoexcitation. When photoirradiated, the photogenerated charge carriers in AgInS2 quantum dots undergo rapid relaxation to populate intrinsic DAP states while competing with charge carrier recombination. Interestingly, these defect-related DAP states can be activated through sub-bandgap excitation and, thus, extend the absorption range to the near-infrared region. We have now employed time-resolved absorption and emission techniques to glean mechanistic insights into the photophysical properties of intragap states of AgInS2 quantum dots (QDs) and their participation in interfacial electron transfer. When the AgInS2 QDs are excited with above bandgap excitation (400 nm), we observe a prompt formation (<1 ps) of the bleach at wavelengths closer to the bandgap, indicating the formation of a charge-separated pair. This transient bleach shifts to lower energies with time (similar to 5 ps), indicating population of sub-bandgap states via relaxation of electrons and holes from the conduction and valence bands, respectively. These sub-bandgap states which can also be populated via direct excitation using low energy (lambda < E-g) excitation exhibit prompt bleach (<1 ps) in contrast to bandgap excitation. The excited DAP states are long-lived (similar to 1 mu s) and can participate in the electron transfer process. We have elucidated the electron transfer dynamics from these midgap states of AgInS2 by employing ethyl viologen (EV2+) as a probe molecule. The role of surface-anchored viologen as an electron shuttle was further exploited by using free-floating benzoquinone (BQ) as a secondary electron acceptor. The sub-bandgap response of AgInS2 to promote electron transfer paves the way to extend the photoresponse of ternary I-III-VI2 semiconductor-based photocatalytic systems.