Shortwave-Infrared (SWIR) Tetrabenzannulated Silicon-Rhodamine

In vivo imaging is the key to the success of transformative biomedical technologies. Currently, robust dyes absorbing and emitting in the shortwave infrared (SWIR) spectral region, i.e., 1000 nm and beyond, as well as a standard protocol to evaluate their performance in in vivo imaging, are sought after. Here, we report the rational design of the tetrabenzannulated silicon-rhodamine dye (ESi7), following the three-step guideline of benzannulation for long wavelength, steric shielding for stability, and deep rigidification for brightness and its convergent synthesis. Compared to its carbon-bridged predecessor (EC7), the highlights of ESi7 are its increased ring strain, reduced vibrational freedom, and improved fluorescence lifetime. ESi7 exhibits a remarkable fluorescence quantum yield of 0.19% in CH2Cl2, the highest for a small-molecule fluorophore absorbing/emitting at 1176/1275 nm to the best of our knowledge. Endowed by its long spectral wavelength, robust photo/chemo-stability, and high fluorescence brightness, in vivo mouse imaging with ESi7 set up a "PEAK" standard, an acronym for "Penetration depth, Exposure time, Acquisition duration, and Contrast".