Influence of Extended π-Conjugation on the Nonlinear Optical Properties of Triphenylamine-Based Covalent Organic Frameworks

Covalent organic frameworks (COFs) are considered optimal candidates for third-order nonlinear optical (NLO) materials due to their extended conjugation networks and structural tunability. Nevertheless, the comprehension of the NLO response mechanism in donor–acceptor (D–A) type COFs, particularly the augmentation of the conjugation degree of the donor or acceptor unit, remains inadequate. Accordingly, in the present study, a D–A type COF, BT-COF1, was constructed utilizing triphenylamine as an electron donor; BT-COF2 and BT-COF3 were synthesized by increasing the π-conjugation degree of the donor and acceptor units in BT-COF1. The NLO properties of these materials were investigated using Z-scan techniques. Results reveal that all three BT-COFs exhibit saturable absorption and self-focusing effects, with β values of −9.31 × 10^–7 m/W for BT-COF1, −8.19 × 10^–7 m/W for BT-COF2, and −4.3 × 10^–6 m/W for BT-COF3. In comparison to BT-COF1, the augmented donor conjugation in BT-COF2 results in a widening of the band gap and a decline in the NLO performance. Conversely, the augmented acceptor conjugation in BT-COF3 results in a narrower band gap and an enhanced NLO performance. This may be attributed to the fact that the enhanced π-conjugation of the acceptor unit is more conducive to intramolecular charge transfer in BT-COFs than that of the donor unit. This study presents a theoretical framework for elucidating the relationship between the structural design of COF materials and their NLO properties, as well as providing guidance for the design of COFs with optimal NLO performance.