Selective Conversion of P=O‐Bridged Rhodamines into P=O‐Rhodols: Solvatochromic Near‐Infrared Fluorophores

Abstract

The substitution of an oxygen atom in rhodols with a phosphine oxide (P=O) moiety affords P=O-bridged rhodols as a new type of near-infrared (NIR) fluorophore. This compound class can be readily accessed upon exposure of the corresponding rhodamines to aqueous basic conditions. The electron-withdrawing effect of the P=O group facilitates the hydrolytic deamination, and, moreover, prolonged exposure to aqueous basic conditions generates P=O-bridged fluoresceins, that is, a series of three P=O-bridged xanthene dyes is available in one simple operation. The P=O-bridged rhodols show significant bathochromic shifts of the longest-wavelength absorption maximum (Δλ=125 nm; >3600 cm ) upon changing the solvent from toluene to water, whereas the emission is shifted less drastically (Δλ=70 nm; 1600 cm ). The hydrogen bonding between the P=O and C=O groups with protic solvents results in substantial stabilization of the LUMO level, which is responsible for the solvatochromism.

Intramolecular Phosphacyclization: Polyaromatic Phosphonium P‐Heterocycles with Wide‐Tuning Optical Properties

“…In water/DMSO and PBS/DMSO solutions 5 c with a ‐NPh 2 electron‐rich fragment displays a broad emission band at 640 nm although at a rather low Φ em of 0.02. This observation is consistent with the presence of intramolecular charge transfer (ICT), which is also supported by the DFT studies (Figure S6, Supporting Information) . In line with solvatochromism detected in the absorption spectra of 6 c , its emission is more sensitive to changes of the solvent than for 5 c .…”

“…With respect to the predecessor 1 c , the performed changes of the molecular structure resulted in a remarkable 12 103 cm 1 (362 nm) bathochromic shift of fluorescence demonstrated by 6 c . It is important to note that the emission characteristics of 5 c and 6 c in organic solvents place them among the most efficient and red shifted organophosphorus dyes reported to date . The photoinduced intramolecular charge transfer leads in turn to large Stokes shifts of approximately 7600 cm −1 (280 nm, 5 c ) and 4700 cm −1 (230 nm, 6 c Figure ), minimizing the reabsorption, which is crucial for practical imaging applications…”

“…[4f, 12] The larger size of the ring assumes richer opportunities for adjusting the electronic features andp hysicochemical functionality through more diverse structural variations of the heterocycle composition and the lateral substituents. The notable advancement of this class of compoundsa sf luorescent systems is associated with the phosphorus analogues of well-known dyes, such as rhodamine, [13] rhodol, [14] and fluorescein. [15] It is essential that these phospha-derivatives exhibit substantial bathochromic shift of absorption and emission reaching near-IR region along with high photostability in aqueous media that enabled their application in cell imaging.…”

Bridge‐Caging Strategy in Phosphorus‐Substituted Rhodamine for Modular Development of Near‐Infrared Fluorescent Probes

“…The introduction of phosphorus into rhodamine by our group has resulted in a further bathochromic shift without destroying the merits of rhodamine . As a versatile atom, phosphorus can adopt different hybrid orbitals, resulting in large series of organophosphorus analogous . Can substituted phosphorus lead to a unique fluorogenic mechanism, rather than just acting as a spectrum tuner?…”

“…[17] As av ersatile atom, phosphorus can adopt different hybrid orbitals, re-sulting in large series of organophosphorus analogous. [18,19] Can substituted phosphorus lead to au nique fluorogenic mechanism, rather than just acting as as pectrum tuner?…”