Exploring the effect of squaraine infusion on the two-photon absorption cross-section of a few functionalized...

The current work focuses on elucidating the structural influence on the electronic properties and absorption spectra of a diverse series of functionalized BODIPY systems, inspired by squaraine dyes. In the current work, we considered two fusion frameworks of BODIPY-squaraine (bdp-sq) systems and functionalized them with selected polar groups (-X; -enX where en = ethenyl; X = NMe₂, PY₅, and PY₆). Utilizing state-of-the-art quadratic response theory (RI-CC2), we systematically examined how these fusion strategies and polar substitutions modulate their optical properties. Our findings indicate that these structural modifications effectively tune both the linear and nonlinear optical properties. Notably, a substantial enhancement in two-photon absorption (TPA) strength was observed across all investigated systems, with those featuring extended conjugation lengths exhibiting markedly higher TPA efficiencies. Certain systems exhibited strong TPA responses in the near-infrared region (∼1200 nm), rendering them promising candidates for several nonlinear optical applications. To gain deeper insight into the key factors influencing TPA strength, we further employed a generalized few-state model to analyze the dominant transition dipole moments and their relative orientations in the most TP-active systems from each fusion scheme. Our investigation contributes a useful starting point for further exploration of these kinds of fused bdp-sq systems that can be TP active and also provides a valuable foundation for future experimental research and development.