A series of near-infrared absorbing and emitting indolizine squaraine fluorophores were designed and synthesized. The dyes were probed for their photophysical and electrochemical properties and observed to exhibit very high viscosity sensitivity.

Abstract

Changes in the viscosity of intracellular microenvironments may indicate the onset of diseases like diabetes, blood-based illnesses, hypertension, and Alzheimer's. To date, monitoring viscosity changes in the intracellular environment remains a challenge with prior work focusing primarily on visible light-absorbing viscosity sensing fluorophores. Herein, a series of near-infrared (NIR, 700–1000 nm) absorbing and emitting indolizine squaraine fluorophores (1PhSQ, 2PhSQ, SO₃SQ, 1DMASQ, 7DMASQ, and 1,7DMASQ) are synthesized and studied for NIR viscosity sensitivity. 2PhSQ exhibits a very high slope in its Forster-Hoffmann plot at 0.75 which indicates this dye is a potent viscosity sensor. The properties of the squaraine fluorophores are studied computationally via density functional theory (DFT) and time-dependent (TD)-DFT. Experimentally, both steady-state and time-resolved emission spectroscopy, absorption spectroscopy, and electrochemical characterization are conducted on the dyes. Precise photophysical tuning is observed within the series with emission maxima wavelengths as long as 881 nm for 1,7DMASQ and fluorescence quantum yields as high as 39.5 and 72.0 % for 1PhSQ in DCM and THF, respectively. The high tunability of this molecular scaffold renders indolizine squaraine fluorophores excellent prospects as viscosity-sensitive biological imaging agents with 2PhSQ giving a dramatically higher fluorescence quantum yield (from 0.3 to 37.1 %) as viscosity increases.