Counterintuitive Fluorescence Blue Shift in Symmetry Breaking Dicationic Bis(indolium) with Two-Photon Absorption Properties for NIR Living Cell Imaging

Abstract

Two-photon absorption (2PA) has become a cornerstone for bioimaging, enabling deeper tissue penetration while minimizing photodamage. Recently, the correlation between 2PA and excited-state symmetry breaking (ES-SB) has been probed. Typically, the first hallmark of ES-SB is a red shift in fluorescence with increasing solvent polarity. Herein, water-soluble bis(indolium) dyes, synthesized in only one step are reported, that exhibit remarkable 2PA performance, coupled to the ES-SB process, displaying a counterintuitive fluorescence blue shift in polar solvents. The relationship between their chemical structure and the extent of the ES-SB and 2PA properties is investigated via steady-state optical spectroscopies and transient absorption spectroscopy (TAS). The unusual blue shift is ascribed to their dicationic nature, leading to an uneven stabilization of the ground and ES-SB excited states in polar environments due to inefficient excited state solvation and positive charge confinement. These compounds also offer specific mitochondrial staining of living HeLa cells, performing at the level of more complex commercial mitochondria markers. Herein, a novel family of dicationic dyes are introduced with robust 2PA properties and practical use in bioimaging and provide connections between their dication quadrupolar nature, which imparts larger water solubility, 2PA performance, and ES-SB, which improving fluorescence properties and bioimaging microscopy detection and resolution.