Dyes that fluoresce beyond the visible wavelength region are ideal for biological imaging. Because there are few endogenous chromophores capable of absorbing at wavelengths over 700 nm, there is less attenuation of the optical signal, lower phototoxicity, and less autofluorescence background. In order to achieve absorption and fluorescence at these longer wavelengths, correspondingly extended p-systems are typically required. With these larger dyes come limitations, such as increased hydrophobic surface area and non-radiative relaxation pathways. Recently, the applicable strategies to achieve large red-shifts in fluorescent dyes changed dramatically, with an idea borrowed from silole organic electronic materials. Si- rhodamines incorporate a dimethylsilyl group in their bridging position, which red-shifts both absorption and fluorescence by ~100 nm. Accordingly, these dyes have had numerous biomedical applications ranging from single-molecule and super-resolution techniques to in vivo imaging methods. More recently, other second-row elements such as phosphorus and sulfur have shown even larger LUMO-lowering effects. For example, a sulfone bridge, found in thiophene S,S-dioxide optical materials, has been adapted to construct sulfone-rhodamine dyes that absorb and fluoresce over 700 nm. However, like the dimethylsilyl bridging group of Si- rhodamines, the sulfone bridging group has no attachment point for functionalization and no means to further fine-tune emission. We hypothesize that S-imine-bridged dyes will allow facile modulation of the photophysical and solubility properties of photostable near-IR dyes, as well as allow the easy introduction of functional handles for attachment to biomolecules and sensor moieties.
Fluorescent dyes are valuable for studying biology, as they allow us to use light to see where a labeled molecule is located, or when a fluorescent dye is released. However, many of the current dyes used for biological study are large, expensive, lack photostability, and/or suffer from interference from biological components that reduce or eliminate fluorescence. Here we propose a new class of fluorescent dyes to overcome these limitations.
