High-rate Solar Hydrogen Evolution Using Small Molecule Donor/Acceptor Organic Semiconductors

#organic-semiconductors #alberta #resilient

Organic semiconductors, comprised of π-conjugated aromatic systems, have attracted considerable research interests owing to the earth abundance of their constituent elements, including carbon, hydrogen, nitrogen, oxygen, and so on. In contrast to conjugated polymeric semiconductors, small molecule organic semiconductors are particularly appealing due to simpler synthetic routes, facile purification, well-defined molecular structures, improved processability, and high reproducibility. Organic semiconductor possesses a small Frenkel exciton radius (~5 Å) and a high exciton binding energy (0.3-1.0 eV), which limit the electron-hole (e⁻-h⁺) pairs separation within the same molecule. However, the dissociation of e⁻-h⁺ pairs into free charge carriers can be facilitated through the incorporation of the donor-acceptor (D-A) architecture. In donor-acceptor systems, intramolecular charge transfer (ICT) occurs through electron donation from the electron-rich donor unit to the electron-deficient acceptor unit. Moreover, the energies of the frontier molecular orbitals (HOMO and LUMO) in donor-acceptor-donor (D-A-D) systems can be tuned by varying the donor units, thereby modulating their optical and electrochemical properties. Herein, we employed a palladium-catalyzed cross-coupling reaction, namely Suzuki-Miyaura coupling, to synthesize different small molecules having D-A-D structure with a common benzoselenadiazole acceptor unit. We tested these D-A-D systems to generate solar hydrogen by methanol photoreforming. The D-A-D systems exhibited high-rate solar hydrogen evolution due to improved charge separation, favorable redox potentials, and reduced Gibbs free energy for methanol photoreforming compared to water splitting.