Excited-state properties of a blue luminescent covalent organic framework by the time-dependent density-functional tight-binding method

Excited-state properties of a blue luminescent two-dimensional (2D) layered covalent organic framework (COF) have been studied by the time-dependent density-functional tight-binding (TD-DFTB) method and compared with available experimental data. For single layer fragment, signatures in absorption and emission spectra are revealed, which show the typical π–π∗ transitions and intramolecular charge transfer (ICT) character. The influence of interlayer stacking has also been considered. Our calculations on the interlayer spacing of the COF suggest that adjacent layers are considerably more stable if their stacking arrangement is horizontally offset by ≈1.4 Å compared with the eclipsed AA stacking structure. Excite-states calculations of high-symmetry eclipsed AA stacking and low-symmetry offset stacking are also considered. The low-symmetry offset stacking shows higher luminescence intensity but lower carrier transport ability. These calculations will be helpful for understanding interlayer excitation and carrier mobility, and the balance of the two processes is essential to the designing of the COF-based optoelectronic devices.