Unraveling the mechanisms of charge-separation in a dibenzo[b,d]thiophene sulfone polymer photocatalyst using time-resolved electronic absorption spectroscopy.

Organic polymer photocatalysts have gained much interest in recent years, largely because of their photocatalytic activity toward sacrificial hydrogen production from water. Time-resolved electronic absorption spectroscopy is commonly employed to understand the photophysical processes occurring following photon absorption, which in turn is used to rationalize photocatalytic activities. The homopolymer of dibenzo[b,d]thiophene sulfone (P10) is a well-studied and high performing photocatalyst for sacrificial hydrogen evolution from water. While sacrificial reagents are well documented as a prerequisite for this reaction, their roles in the picosecond-nanosecond photodynamics have yet to be determined using transient electronic signatures. By employing lifetime density analysis of time-resolved electronic absorption spectra of P10 in a variety of solvent mixtures, we show that the electron polaron (the required charge for hydrogen evolution) is produced on the 0.5-100 and 50-800 ps timescales via excitonic quenching by triethylamine and methanol, respectively, two common sacrificial electron donors. We conclude that there is significant pre-association of triethylamine with the P10 polymer, resulting in efficient excitonic quenching. This mechanism competes effectively with radiative excitonic relaxation, which occurs on similar timescales, reducing exciton losses and improving polaron yields.