Photoactive Thiophene-Enriched Tetrathienonaphthalene-Based Covalent Organic Frameworks

Abstract

The optoelectronic properties of covalent organic frameworks (COFs) can be controlled by the design of their molecular building blocks and assembly. Here, a facile and efficient synthetic route is reported for the novel thiophene-enriched tetrathienonaphthalene (TTN)-based node 4,4',4″,4'″-(naphtho[1,2-b:4,3-b':5,6-b″:8,7-b″']tetrathiophene-2,5,8,11-tetrayl)tetraaniline (TTNTA) for constructing imine-linked COFs. Utilizing TTNTA, highly crystalline, thiophene-enriched donor-donor (D-D) and donor-acceptor (D-A) COFs, denoted as TT COF and BDT(BT)₂ COF, are synthesized using two distinct aldehyde-functionalized linear linkers: [2,2'-bithiophene]-5,5'-dicarbaldehyde (TT) and 7,7'-(4,8-diethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(benzo[c][1,2,5]thiadiazole-4-carbaldehyde) (BDT(BT)₂), respectively. Highly crystalline and oriented TTNTA COF films on various substrates via a solvothermal method enabled further comprehensive optical and electronic characterizations. Optical-pump terahertz-probe spectroscopy revealed effective charge-carrier mobility values φμ = 0.34 ± 0.04 and 0.18 ± 0.02 cm²V^-1s^-1 for TT and BDT(BT)₂ COF films, respectively. These results reveal distinct charge-transport characteristics and provide mechanistic insights into their ultrafast charge-carrier dynamics. The COFs are demonstrated to be photoactive, showing promising potential as photocathodes without co-catalysts in photoelectrochemical water splitting, with notable photocurrent densities of 10 and 15.3 µA cm^-2 after 1 h illumination, respectively. This work highlights the potential of TTNTA-based COFs in optoelectronic applications and provides insights into the design of thiophene-enriched COFs with high crystallinity and photoactive behavior.

Keywords: covalent organic frameworks; optical‐pump terahertz‐probe spectroscopy; photoactive; tetrathienonaphthalene.

Scheme 1

a) Key steps for the synthesis of TTNTA via a Scholl reaction to induce ring closure, followed by a Suzuki coupling to functionalize the TTN core with an aniline moiety, to enable the subsequent COF synthesis. b) DFT calculated crystal structure of TTNTA, showing face‐on (left) and edge (right) views. c) Synthetic procedures for the formation of TT and BDT(BT)₂ COFs and structural view of the resulting 2D COFs with rhombic pore architectures.

Figure 1

a,b) PXRD patterns of the highly crystalline TT and BDT(BT)₂ COFs, with simulations based on Pawley refinements. The insets represent Pawley‐refined structure models of TT and BDT(BT)₂ COFs viewed perpendicular to the crystallographic a‐b plane. c,d) Nitrogen sorption isotherms with their corresponding pore size distribution of TT and BDT(BT)₂ COFs. e,f) HRTEM images of TT and BDT(BT)₂ COFs. The insets show magnified regions highlighting the clearly resolved lattice fringes of the corresponding frameworks.

Figure 2

a) GIWAXS 2D pattern of a BDT(BT)₂ COF thin‐film grown on an ITO‐coated glass substrate. b) HRTEM image and electron diffraction pattern of the BDT(BT)₂ COF film, obtained by removing it from an ITO‐coated substrate.

Figure 3

a) UV–vis absorbance and b) PL spectra with 375 nm excitation at room‐temperature of TT COF (red line) and BDT(BT)₂ COF (purple line) films. The insets represent photographs of both COF films in daylight.

Figure 4

a) Fluence‐dependent OPTP transients in TT COF film, measured following a 3.1 eV pulsed photoexcitation with a pump fluence of 80, 115, and 140 µJ cm⁻². Solid lines represent fits to a bi‐exponential model. b) Frequency‐resolved complex photoconductivity of TT COF film after photoexcitation (real (red dots) and imaginary (blue dots) components).

Figure 5

a) Energy levels of the TT and BDT(BT)₂ COFs versus vacuum. The energy level of H₂/H⁺ was adapted at pH 6.8 given the Nernstian behavior under standard conditions. Linear sweep voltammetry (LSV) of b) TT and c) BDT(BT)₂ COF photocathode films on ITO, performed in the dark (green line) and under AM 1.5 G illumination through the substrate (orange line). The red and purple dashed lines inserted in each plot represent the chronoamperometric data of the COF film at different potentials versus RHE. d) Chronoamperometric data of TT and BDT(BT)₂ COF films at 0.3 V versus RHE.