Photoactive Thiophene-Enriched Tetrathienonaphthalene-Based Covalent Organic Frameworks

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.