From seconds to femtoseconds: Solar hydrogen production and transient absorption of chalcogenorhodamine dyes

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO₂ catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore's triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO₂. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.