Evolution of Group 14 Rhodamines as Platforms for Near-Infrared Fluorescence Probes Utilizing Photoinduced Electron Transfer

Abstract

The absorption and emission wavelengths of group 14 pyronines and rhodamines, which contain silicon, germanium, or tin at the 10 position of the xanthene chromophore, showed large bathochromic shifts compared to the original rhodamines, owing to stabilization of the LUMO energy levels by σ*-π* conjugation between group 14 atom-C (methyl) σ* orbitals and a π* orbital of the fluorophore. These group 14 pyronines and rhodamines retain the advantages of the original rhodamines, including high quantum efficiency in aqueous media (Φ(fl) = 0.3-0.45), tolerance to photobleaching, and high water solubility. Group 14 rhodamines have higher values of reduction potential than other NIR light-emitting original rhodamines, and therefore, we speculated their NIR fluorescence could be controlled through the photoinduced electron transfer (PeT) mechanism. Indeed, we found that the fluorescence quantum yield (Φ(fl)) of Si-rhodamine (SiR) and Ge-rhodamine (GeR) could be made nearly equal to zero, and the threshold level for fluorescence on/off switching lies at around 1.3-1.5 V for the SiRs. This is about 0.1 V lower than in the case of TokyoGreens, in which the fluorophore is well established to be effective for PeT-based probes. That is to say, the fluorescence of SiR and GeR can be drastically activated by more than 100-fold through a PeT strategy. To confirm the validity of this strategy for developing NIR fluorescence probes, we employed this approach to design two kinds of novel fluorescence probes emitting in the far-red to NIR region, i.e., a series of pH-sensors for use in acidic environments and a Zn(2+) sensor. We synthesized these probes and confirmed that they work well.

A near-infrared fluorophore for live-cell super-resolution microscopy of cellular proteins

“…1a) [7][8][9] . SiR derivatives were shown to possess excellent spectroscopic properties, and in certain cases to be membrane permeable 8 .…”

Synthesis of a Far‐Red Photoactivatable Silicon‐Containing Rhodamine for Super‐Resolution Microscopy

“…To overcome this problem, we envisioned creating a red‐shifted isologue of caged Rh Q in which the xanthene oxygen is replaced with a (CH 3 ) 2 Si moiety to yield caged Si‐Q‐rhodamine (NVOC 2 ‐SiRh Q , 2 ; Scheme 1). The O‐to‐(CH 3 ) 2 Si modification elicits a desirable 100 nm bathochromic shift 10. Such Si‐rhodamines with an o ‐carboxyl group on the pendant phenyl ring have recently been shown to be excellent labels for microscopy,11 but N ‐acyl or photoactivatable derivatives of these photostable Si‐rhodamines have not been described.…”

New Trends in Near-Infrared Fluorophores for Bioimaging

“…55,56 The same concept is also applicable for 3,5-styryl-substituted BODIPYs. 3,4,4a-Trihydroxanthenefused BODIPY (111), which looks like a styrene-restricted BODIPY, shows NIR fluorescence at 747 nm, which is at over a 100 nm longer wavelength than that of styryl BODIPY. " BODIPY (112 -225, Figs.…”

“…107 Around 50 -60 nm bathochromic shifts were observed, 108 and due to the extreme photostability, some of those compounds are available for super-resolution microscopy, such as stimulated emission depletion (STED) imaging. 109,113,114 Si-pyronins (255) and Si-rhodamines (264, 294 -299), which were first reported by Xiao's group in 2008, 110 and further investigated by Nagano's group in 2011, 111 are among the most attractive fluorophores, because of their NIR absorption/emission spectra (over 650 nm) with high extinction coefficients (ca. 100000 M -1 cm -1 ) and quantum yields (0.3 -0.4 in aqueous buffer).…”

“…103 3·2·3 "10-Replaced" rhodamine (253 -299, Fig. 11) Recently, replacement of an oxygen atom at the 10 position with another atom, such as a different member of group 16 atoms (Se, 104 Te, 105,106 ) or a group 14 atom (C, [107][108][109] Si, 110,111 Ge, 111 Sn 111 ), was found to result in a remarkable change of the photophysical and chemical properties (253 -265). Especially, the noteworthy effect of replacing an oxygen atom with a group 14 atom has been confirmed, resulting in a revolution of rhodamines based on this approach.…”