Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores

Abstract

Fluorescence-based biological imaging in the shortwave infrared (SWIR, 1000–1700 nm) is an attractive replacement for modern in vivo imaging techniques currently employed in both medical and research settings. Xanthene-based fluorophores containing heterocycle donors have recently emerged as a way to access deep SWIR emitting fluorophores. A concern for xanthene-based SWIR fluorophores though is chemical stability toward ambient nucleophiles due to the high electrophilicity of the cationic fluorophore core. Herein, a series of SWIR emitting silicon-rosindolizine (SiRos) fluorophores with emission maxima >1300 nm (up to 1550 nm) are synthesized. The SiRos fluorophore photophysical properties and chemical stability toward nucleophiles are examined through systematic derivatization of the silicon-core alkyl groups, indolizine donor substitution, and the use of o-tolyl or o-xylyl groups appended to the fluorophore core. The dyes are studied via absorption spectroscopy, steady-state emission spectroscopy, solution-based cyclic voltammetry, time-dependent density functional theory (TD-DFT) computational analysis, X-ray diffraction crystallography, and relative chemical stability over time. Optimal chemical stability is observed via the incorporation of the 2-ethylhexyl silicon substituent and the o-xylyl group to protect the core of the fluorophore.

Phospha-RosIndolizine Dye with Shortwave Infrared (SWIR) Absorption and Emission

“…Rhodamine and xanthene dyes have been extensively studied for over a century. − These dyes absorb and emit strongly, having extensive use in biological imaging and cellular microscopy. − Furthermore, the photophysical parametersnamely, the peak emission wavelength (λ emis )of these dyes are extremely tunable with both donor and core modifications. ,− Replacing the functional group in the central ring of the xanthene core with electron-donating (e.g., O, N), -neutral (e.g., C, Si), and -withdrawing (e.g., SO 2 , CO, PO) ,,− groups can dramatically shift λ emis to shorter or longer wavelengths. The phosphine oxide (PO) substituted xanthenes, like 1b , display some of the longest λ emis that can be accessed through core modifications alone, and this moiety has also been shown to improve dye stability. ,,,, Core modifications which delete the central atom altogether have also been used to shift peak absorption wavelength (λ abs ) to longer wavelengths; however, these dyes exhibit much less intense absorption (∼1/3rd the molar absorptivity) and emission (low or undetectable). , Our group has previously demonstrated how replacing traditional amine donors with indolizine donors produces a substantial shift of λ emis toward longer wavelengths in O- and Si-substituted xanthenes ( tol RosIndz and SiRos1300 , , Figure ). With this in mind, we hypothesize that replacing the amine donors in a xanthene substituted with an electron-withdrawing PO group in the central ring ( 1b ) with stronger fully conjugated indolizine donors ( PRos1450 ) would pave a direct path toward longer wavelength SWIR emitting small-molecule chromophores (Figure ).…”