Highly Crystalline Semiconducting Tetrathienonaphthalene-Based Covalent Organic Frameworks with Oblique Lattice

Abstract

Highly crystalline semiconducting covalent organic frameworks (COFs) were synthesized from two isomeric tetrathienonaphthalene (TTN) nodes via transimination polymerization. The use of functionalized Schiff-base monomers proved critical for overcoming the low solubility of TTN-tetracarbaldehydes, enabling the solvothermal synthesis of four imine COFs with phenyl or biphenyl linkers. Systematic optimization of amine precursors yielded COFs with very high crystallinity (fwhm1002θ ≈ 0.26°) and porosity (BET surface area up to ∼1660 m2/g). Powder X-ray diffraction combined with DFT modeling revealed oblique lattice stacking and allowed experimental distinction between s-Z and s-E conformations of the imine linkage. Thin films grown on ITO substrates were used to fabricate bilayer photovoltaic diodes with a nonfullerene acceptor IT-4F. Although luminescence quenching (∼50%) suggests appreciable charge separation in these heterojunctions, the power conversion efficiency (0.1%–0.2%) was limited by the low charge mobility of the COF film.