Organic Conjugated Trimers with Donor-Acceptor-Donor Structures for Photocatalytic Hydrogen Generation Application

Organic donor-acceptor-donor (D-A-D) polymers or small molecules are widely investigated in organic solar cells due to their broad light absorption, narrow band gap, excellent charge mobility, and exciton seperation at the interface. However, studies of conjugated small molecules with D-A-D molecule structures as photocatalytically active materials are still rare. In this study, an unprecedented demonstration that photocatalytic activity can in fact be affected by tuning the D and A is given. Especially, the EBE trimer, comprising 3,4-ethylenedioxythiophene (E) and benzothiadiazole (B) units, exhibits the best photophysical, chemical, and photocatalytic properties compared to other D-A-D combinations of D and A. Detailed kinetic studies show that all trimers in organic solution present relatively long-lived and highly emissive photogenerated singlet excitons (tau = 4-13 ns; phi(em) = 0.5-0.9) as judged by photoluminescence and transient absorption measurements, while in specific cases formation of long-lived triplet states can be identified. Organic microparticles of the trimers are efficiently formed in aqueous solution by nanoprecipitation, and rapid photoinduced electron release/injection to the solvent is evidenced spectroscopically. The results indicate that organic small molecule structures with D-A-D structures pave a new pathway for photocatalytic solar-to-chemical energy conversion of novel small organic molecules.