D–π–A–π–D Dyes with a 1,3,2‐Dioxaborine Cycle in the Polymethine Chain: Efficient Long‐Wavelength Fluorophores

Abstract

The nitrile group in newly synthesized 5‐cyano‐2,2‐difluoro‐4,6‐dimethyl‐1,3,2‐dioxaborine is shown to significantly increase the acidity of the methyl groups. This allows the cyanine condensation with both methyl groups to form D–π–A–π–D dyes in which the dioxaborine cycle is a part of the polymethine chain. There, the reactivity of the remaining methyl group in the semi‐product is reduced enough to perform the reaction stepwise, giving access to nonsymmetric derivatives. The synthesis of dyes with various electron donor termini [indole, benzothiazole, pyran, 4‐(dialkylamino)phenyl] and with different polymethine chain lengths is reported. The obtained compounds show intense absorption and fluorescence in the red and NIR region, their fluorescence brightness (the product of the molar extinction and the fluorescence quantum yield) attaining a value of 200,000 m–1 cm–1. They are characterized by small‐scale positive solvatochromism, yet their fluorescence quantum yield is much more sensitive to ambient polarity. Time‐dependent (TD)DFT calculations have been performed to study the electronic structure of the new dioxaborines and reveal the nature of the long‐wavelength electronic transitions in those molecules.

Photophysical and Photoacoustic Properties of π‐Extended Curcumin Dyes. Effects of the Terminal Dimethylamino Electron‐donor and the Bridging Aryl Ring

“…For in vivo PA imaging applications it is preferred to employ water‐soluble chromophores with a strong absorption in the biological window (680–980 nm). Even with significant interest in curcuminoid dyes for biological imaging applications, and many efforts to structurally tune their photophysical properties, NIR absorbing curcuminoid dyes are rare . As such it is important to highlight the fundamental focus of this study and recognize that there is still work to be done to transition this technology to in vivo relevant platforms.…”

“…The therapeutic use of turmeric and its easy extraction have made curcumin a highly investigated therapeutic compound. Indeed, interest in the development of synthetic curcumins (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33) inspired by the natural pigment has been increasing with applications in molecular devices (22), nonlinear optics (20,24), photovoltaics (29), light emitting diodes (32) and photocatalysis (33). However, curcumin derivatives may also serve the medical field from a diagnostic perspective as a molecular contrast agent for fluorescence imaging (25) and even photoacoustics (PA) imaging (30,34,35).…”

Effect of the electron donating group on the excited-state electronic nature and epsilon-near-zero properties of curcuminoid-borondifluoride dyes

“…It is to be noted that a similar conclusion was drawn theoretically by Brédas et al in literature on a dye of the same family as 2 21 and on a dye containing a cyano unit in meso position of curcuminoid. 18 To support further this statement, Stokes shi of a polymethine model squaraine (see Fig. S2 † for molecular structure compound 7) was examined.…”

“…For this purpose, donor-acceptordonor (D-A-D) curcuminoid borondiuoride derivatives represent excellent candidates since their polymethine character can be chemically tuned by the donor strength. 18 In fact, this research work addresses two important issues that are particularly relevant for the future development of organic ENZ materials. First, in order to establish the structureproperty relationship of donor strength and polymethine character, six new curcuminoid borondiuoride (Curc) dyes are synthesized.…”

“…For this purpose, donor–acceptor–donor (D–A–D) curcuminoid borondifluoride derivatives represent excellent candidates since their polymethine character can be chemically tuned by the donor strength. 18 …”