ABSTRACT:

Most energy of sunlight is concentrated in the near-infrared (NIR) region, which is a challenge for the practical application of current typical photocatalysts. Organic semiconductors can possess NIR light absorption and tunable energy levels simultaneously via precise molecular engineering, which shows a great potential in solar catalysis. However, an individual organic semiconductor usually has low dielectric constant (ε≈3-4) and generates Frenkel excitons with large binding energy (E_B≈0.5 eV), making it difficult to separate electrons and holes. In this dissertation, we thoroughly investigate the photocatalytic activities in select organic semiconductor catalysts and systematically explore the photophysical mechanisms under NIR photon excitation.

Firstly, we develop molecular-level organic heterojunction to suppress electron-hole recombination, thereby achieving a boosted photocatalytic hydrogen evolution reaction (HER) rate of 25.54 μmol h⁻¹ (12.77 mmol h⁻¹ g⁻¹) under visible–near-infrared (Vis–NIR) light illumination. Intriguingly, heterojunction nanoparticles (NPs) comprising the donor polymer PBDB-T matched with a novel A-D₁-D₂-D₁-A type acceptor BTPT-IC4F exhibit a promising external quantum efficiency of 6.3% at 730 nm. Transient absorption spectroscopy monitors the effective extraction of photogenerated holes from the highest occupied molecular orbital (HOMO) of BTPT-IC4F to the HOMO of PBDB-T, while first-principle calculations confirm the prolonged lifetime of excited BTPT-IC4F due to efficient hole capture by the PBDB-T phase.

Secondly, we have successfully prepared single-component organic photocatalyst based on double-cable polymer as-DCPIC. Consequently, as-DCPIC NPs exhibit significantly enhanced hydrogen evolution performance (11.88 mmol h⁻¹ g⁻¹) compared to pristine PBDB-T or TPDIC NPs. Transient absorption spectroscopy in conjunction with steady-state photoluminescence spectra elucidates the effective electron-hole separation within as-DCPIC NPs, while decay kinetics monitor the long-lived excited species (38.44 ns) in such organic NPs based on double-cable polymers. The theoretical calculation of molecular surface electrostatic potential (ESP) also reveals that ESP difference between donor backbone and acceptor side unit can create an intermolecular electric field between two different as-DCPIC molecular chains, thus promoting exciton dissociation.

Thirdly, we develop organic heterojunction with extended diffusion length via strong exciton-CT state coupling, thus achieving enhanced H₂ generation. Remarkably, the HER rate of heterojunction NPs combining PFBDB-T with BTOR-IC4F reaches up to 18.33 μmol h⁻¹, which is 27 times higher than that of PFBDB-T NPs and 7 times higher than that of BTOR-IC4F NPs. Decay dynamics analysis demonstrates the significantly extended exciton diffusion (17.1 nm) in PFBDB-T:BTOR-IC4F blend compared to that in pristine PFBDB-T NPs or BTOR-IC4F NPs, which could be ascribed to strong exciton-CT state coupling in terms of time-dependent density functional theory (TD-DFT) calculation.