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

Abstract

Predictable structure-property relationships for the near-infrared (NIR)-emitting chromophores and sustainable photothermal agents remain challenging. Herein, two series of indolenine-based squaraines are synthesized to investigate the core-wing structural modulation corresponding to the photophysical properties. Dicyanovinylene modification on the squaraine carbonyl core induces substantial bathochromic shift and enriches the de-activation pathways, while further wing functionalization strengthens the intramolecular charge transfer (ICT) efficiency and enhances the nonlinear two-photon absorption (2PA). Intrinsic zwitterionic stabilization and bent-shaped molecular skeleton of the bis(dicyanovinylene)-cored squaraines are accompanied theoretically. Optimal benzindolenine-winged squaraine dye SQ2-4CN demonstrates a strong two-photon absorption cross section (δ2PA) 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.