In-situ formatting donor-acceptor polymer with giant dipole moment and ultrafast exciton separation

Abstract

Donor-acceptor semiconducting polymers present countless opportunities for application in photocatalysis. Previous studies have showcased their advantages through direct bottom-up methods. Unfortunately, these approaches often involve harsh reaction conditions, overlooking the impact of uncontrolled polymerization degrees on photocatalysis. Besides, the mechanism behind the separation of electron-hole pairs (excitons) in donor-acceptor polymers remains elusive. This study presents a post-synthetic method involving the light-induced transformation of the building blocks of hyper-cross-linked polymers from donor-carbon-donor to donor-carbon-acceptor states, resulting in a polymer with a substantial intramolecular dipole moment. Thus, excitons are efficiently separated in the transformed polymer. The utility of this strategy is exemplified by the enhanced photocatalytic hydrogen peroxide synthesis. Encouragingly, our observations reveal the formation of intramolecular charge transfer states using time-resolved techniques, confirming transient exciton behavior involving separation and relaxation. This light-induced method not only guides the development of highly efficient donor-acceptor polymer photocatalysts but also applies to various fields, including organic solar cells, light-emitting diodes, and sensors.

Fig. 1. Synthesis and morphology of KDBT polymer.

a FeCl₃-catalyzed Friedel-Crafts polycondensation route for KDBT. b, c Transmission electron microscopy (TEM) images of KDBT. HAADF image (d) and the corresponding elemental carbon (e) and sulfur (f) maps of the KDBT nanotube.

Fig. 2. Photocatalytic performance, driving force, and transformation of the polymers.

a Time course of H₂O₂ production over the polymer. The base point was set when the system reached absorption-desorption equilibrium in the dark. The error bars (mean ± standard deviation) were obtained from three independent photocatalytic experiments. b 8 long-time cycle tests. c The possible structure transformation from KDBT to KDBT-A. d Band structure diagrams of KDBT and KDBT-A. e Light-irradiated EPR spectra of DMPO-•O₂⁻ adducts. Experimental conditions: 2 mg of samples, 10 mL of anhydrous CH₃CN, irradiated by LED (λ = 365 nm) for 5 min. High-resolution X-ray photoelectron spectra (XPS) of C 1 s (f) and S 2p (g) in KDBT and KDBT-A.

Fig. 3. In situ transformation process of KDBT to KDBT-A and the corresponding mechanism.

a Evolution of S 2p XPS spectrum of the polymer during photocatalysis. A new and weaker signal gradually emerges on the spectrum, indicating that the polymer is progressively oxidized. b Blue background: Photocatalytic performance for KDBT and KDBT-A in AgNO₃ (0.01 M), methanol (10 v.%), isopropanol (10 v.%), or benzoquinone (0.01 M) aqueous solution under air condition. Yellow background: Photocatalytic performance for KDBT and KDBT-A in pure water or AgNO₃ (0.01 M) aqueous solution under Ar condition. c S 2p XPS spectra of KDBT before and after irradiation in Ar atmosphere for 6 h. d Oxidation pathway of DBT in photocatalysis. e, f Theoretical calculations performed to investigate the electrostatic potential distribution and dipole moments of KDBT and KDBT-A at the B3LYP/631 G(d,p) level.

Fig. 4. Macroscopic insights into the photophysical processes.

a, b Steady-state fluorescence spectra of KDBT and KDBT-A under air and Ar conditions, where F1 and F2 denote the observed fluorescence peaks. c, d Schematic representation of photocatalytic processes (excitation, electron transfer) in KDBT and KDBT-A polymers.

Fig. 5. Visualization of charge separation.

a, b Atomic force microscopic images of KDBT and KDBT-A. c–f Corresponding surface potential distributions of KDBT and KDBT-A. Line-scan surface potential across KDBT (g) and KDBT-A (h) nanotube. Surface photovoltage (SPV) could be obtained as follows: $SPV={CPD}_{\mathrm{light}}-{CPD}_{\mathrm{dark}}$.

Fig. 6. Validation of ultrafast excitonic behavior.

a–c Two-dimensional (2D) TA spectra mapping of KDBT with or without scavengers. d–f 2D TA spectra mapping of KDBT-A with or without scavengers. All data were acquired under excitation of 400 nm and optical power of 260 μW cm⁻². g, h Extracted decay kinetics of KDBT and KDBT-A with or without TEOA at λ = 1150 nm. i Schematic illustration of the ESA signal formation process.

Fig. 7. Microscopic insights into the photophysical processes.

Normalized decay kinetic plots for KDBT (a) and KDBT-A (b) at 1150 nm. c Schematic illustration for the decay pathways of photogenerated electrons.