Core–wing modulated squaraines with enhanced two-photon absorption and efficient photothermal eradication of bacteria

Abstract

Predictable structure–property relationships for near-infrared (NIR)-emitting chromophores and sustainable photothermal agents remain challenging. Herein, two series of indolenine-based squaraines are synthesized to investigate how core–wing structural modulation influences photophysical properties. Dicyanovinylene modification of the squaraine carbonyl core induces a substantial bathochromic shift and enriches de-activation pathways, while further wing functionalization strengthens intramolecular charge transfer (ICT) efficiency and enhances nonlinear two-photon absorption (2PA). Intrinsic zwitterionic stabilization and the bent-shaped molecular skeleton of bis(dicyanovinylene)-cored squaraines are also supported by theoretical calculations. Optimal benzindolenine-winged squaraine dye SQ2-4CN demonstrates a strong two-photon absorption cross-section (δ_2PA) of 2140 GM at 890 nm within the NIR biological spectral window, approximately 8.95-fold enhancement over that of the indolenine-winged counterpart SQ1-2O. Femtosecond transient absorption spectroscopies further reveal the ICT character and plausible excited-state dynamics. Moreover, efficient bio-staining capability and photothermal eradication of bacteria with a significant photothermal conversion efficiency of 42.4% under 808 nm laser irradiation are initially validated for SQ2-4CN. The present work opens a pathway for designing core/wing-modified squaraines with superior nonlinear optical properties for potential bioimaging and photothermal applications.