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. 2023 May 29;14(1):3083.
doi: 10.1038/s41467-023-38884-w.

Computation-based regulation of excitonic effects in donor-acceptor covalent organic frameworks for enhanced photocatalysis

Yunyang Qian #  1 , Yulan Han #  2 , Xiyuan Zhang  1 , Ge Yang  1 , Guozhen Zhang  2 , Hai-Long Jiang  3
Affiliations

Computation-based regulation of excitonic effects in donor-acceptor covalent organic frameworks for enhanced photocatalysis

Yunyang Qian et al. Nat Commun. .

Abstract

The strong excitonic effects widely exist in polymer-semiconductors and the large exciton binding energy (Eb) seriously limits their photocatalysis. Herein, density functional theory (DFT) calculations are conducted to assess band alignment and charge transfer feature of potential donor-acceptor (D-A) covalent organic frameworks (COFs), using 1,3,5-tris(4-aminophenyl)triazine (TAPT) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) as acceptors and tereph-thaldehydes functionalized diverse groups as donors. Given the discernable D-A interaction strengths in the D-A pairs, their Eb can be systematically regulated with minimum Eb in TAPT-OMe. Guided by these results, the corresponding D-A COFs are synthesized, where TAPT-OMe-COF possesses the best activity in photocatalytic H2 production and the activity trend of other COFs is associated with that of calculated Eb for the D-A pairs. In addition, further alkyne cycloaddition for the imine linkage in the COFs greatly improves the stability and the resulting TAPT-OMe-alkyne-COF with a substantially smaller Eb exhibits ~20 times higher activity than the parent COF.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Schematic illustration.
Construction of D-A COFs composed by amine monomers and functionalized tereph-thalaldehydes for photocatalytic H2 production. The TAPB, TAPT and TEA are abbreviations for 1,3,5-tris(4-aminophenyl)benzene, 1,3,5-tris(4-aminophenyl)triazine and triethylamine, respectively.
Fig. 2
Fig. 2. HOMO and LUMO energy level diagram for the two types of building blocks for constructing D-A pairs.
The TAPTc and TAPBc represent the core part (without the amine group) of the amino-based monomers (TAPT and TAPB), and the Clc, Hc, OCCHc, OHc and OMec represent the core part (without the aldehyde group) of the terephthalaldehyde-based monomers.
Fig. 3
Fig. 3. Computed charge density difference between the ground and the excited states of the D-A pairs at an isovalue of 0.0005 a.u.
(For each COF: Δρ is shown at the left side; centroids of charges are shown at the right side). The green and red represent increase and decrease in electron density, respectively. Quantitative charge-transfer analysis is based on the atomic dipole corrected Hirshfeld (ADCH) atomic charges. DCT represents the distance between the bary-center of the density increment and depletion regions upon electronic excitation.
Fig. 4
Fig. 4. Calculated exciton binding energy and band alignment of D-A pairs.
a Extrapolation results of fundamental gap (Efund, red line) and optical gap (Eopt, green line) energies of D-A pairs. Inset: double arrows represent the exciton binding energy (Eb = Efund − Eopt). b The calculated bandgap and band position of TAPB-OMe-COF (black), TAPT-Cl-COF (green), TAPT-H-COF (blue), TAPT-OCCH-COF (khaki), TAPT-OH-COF (purple) and TAPT-OMe-COF (red) relative to the vacuum level. The dashed lines are the redox potential of water at pH = 0.
Fig. 5
Fig. 5. Characterization and activity performance of D-A COFs.
a Powder XRD patterns. b Photocurrent responses. c EIS Nyquist plots. d Photocatalytic H2 production rate. TAPB-OMe-COF, TAPT-Cl-COF, TAPT-H-COF and TAPT-OMe-COF are represented by black, green, blue and red colours, respectively.
Fig. 6
Fig. 6. Activity performance and fluorescence spectra of D-A COFs after modification.
a Photocatalytic H2 production rate. b Powder XRD patterns after the photocatalytic H2 production. c Recycling performance in photocatalytic H2 production. d Steady-state fluorescence emission under excitation at 380 nm. The TAPB-OMe-COF, TAPT-OMe-COF, TAPB-OMe-alkyne-COF and TAPT-OMe-alkyne-COF are represented by black, green, blue and red colours, respectively.
Fig. 7
Fig. 7. DFT calculations to clarify the reasons for boosting activity after modification.
a Computed charge density difference between the ground and the excited states for TAPT-OMe-alkyne and TAPB-OMe-alkyne at an isovalue of 0.0005 a.u. (For each COF: Δρ is shown at the left side; centroids of charges are shown at the right side). The green and red represent increase and decrease in electron density, respectively. b Extrapolation results of fundamental gap (Efund, red line) and optical gap (Eopt, green line) energies of TAPB-OMe and TAPT-OMe before and after the post-synthetic modification. Inset: double arrows represent the exciton binding energy (Eb = Efund − Eopt).
See this image and copyright information in PMC

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