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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, and1,7DMASQ) are synthesized and studied for NIR viscosity sensitivity.2PhSQexhibits 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 computationallyviadensity 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 for1,7DMASQand fluorescence quantum yields as high as 39.5 and 72.0 % for1PhSQin DCM and THF, respectively. The high tunability of this molecular scaffold renders indolizine squaraine fluorophores excellent prospects as viscosity‐sensitive biological imaging agents with2PhSQgiving a dramatically higher fluorescence quantum yield (from 0.3 to 37.1 %) as viscosity increases. 

In vivo fluorescence imaging in the shortwave infrared (SWIR, 1,000–1,700 nm) and extended SWIR (ESWIR, 1,700–2,700 nm) regions has tremendous potential for diagnostic imaging. Although image contrast has been shown to improve as longer wavelengths are accessed, the design and synthesis of organic fluorophores that emit in these regions is challenging. Here we synthesize a series of silicon-RosIndolizine (SiRos) fluorophores that exhibit peak emission wavelengths from 1,300–1,700 nm and emission onsets of 1,800–2,200 nm. We characterize the fluorophores photophysically (both steady-state and time- resolved), electrochemically and computationally using time-dependent density functional theory. Using two of the fluorophores (SiRos1300 and SiRos1550), we formulate nanoemulsions and use them for general systemic circulatory SWIR fluorescence imaging of the cardiovascular system in mice. These studies resulted in high-resolution SWIR images with well-defined vasculature visible throughout the entire circulatory system. This SiRos scaffold establishes design principles for generating long-wavelength emitting SWIR and ESWIR fluorophores. 

A readily accessible dye molecule with potential properties well-beyond the state-of-the-art for dye-sensitized solar cells is realized from extensive quantum chemical characterization of nearly 8000 stochastically-derived novel molecules. 

Shortwave infrared (SWIR, 1000–1700 nm) and extended SWIR (ESWIR, 1700–2700 nm) absorbing materials are valuable for applications including fluorescence based biological imaging, photodetectors, and light emitting diodes.