Influence of Extendedπ‑Conjugation onthe Nonlinear Optical Properties of Triphenylamine-Based CovalentOrganic 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, <b>BT-COF1</b>, was constructed utilizing triphenylamine as an electron donor; <b>BT-COF2</b> and <b>BT-COF3</b> were synthesized by increasing the π-conjugation degree of the donor and acceptor units in <b>BT-COF</b>1. The NLO properties of these materials were investigated using <i>Z</i>-scan techniques. Results reveal that all three <b>BT-COFs</b> exhibit saturable absorption and self-focusing effects, with <i>β</i> values of −9.31 × 10^–7 m/W for <b>BT-COF1,</b> −8.19 × 10^–7 m/W for <b>BT-COF2,</b> and −4.3 × 10^–6 m/W for <b>BT-COF3</b>. In comparison to <b>BT-COF1</b>, the augmented donor conjugation in <b>BT-COF2</b> results in a widening of the band gap and a decline in the NLO performance. Conversely, the augmented acceptor conjugation in <b>BT-COF3</b> 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 <b>BT-COFs</b> 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.