Boron–Diindomethene (BDI) Dyes and Their Tetrahydrobicyclo Precursors—en Route to a New Class of Highly Emissive Fluorophores for the Red Spectral Range

Abstract

The X-ray crystallographic, optical spectroscopic, and electrochemical properties of a newly synthesized class of boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors (bc-BDP) are presented. The BDI chromophore was designed to show intensive absorption and strong fluorescence in an applicationary advantageous spectral range. Its modular architecture permits fusion of a second subunit, for example, a receptor moiety to the dye's core to yield directly linked yet perpendicularly prearranged composite systems. The synthesis was developed to allow facile tuning of the chromophore platform and to thus adjust its redox properties. X-ray analysis revealed a pronounced planarity of the chromophore in the case of the BDIs, which led to a remarkable close packing in the crystal of the simplest derivative. On the other hand, deviation from planarity was found for the diester-substituted bc-BDP benzocrown that exhibits a "butterfly"-like conformation in the crystal. Both families of dyes show charge- or electron-transfer-type fluorescence-quenching characteristics in polar solvents when equipped with a strong donor in the meso-position of the core. These processes can be utilized for signaling purposes if an appropriate receptor is introduced. Further modification of the chromophore can invoke such a guest-responsive intramolecular quenching process, also for receptor groups of low electron density, for example, benzocrowns. In addition to the design of various prototype molecules, a promising fluoroionophore for Na+ was obtained that absorbs and emits in the 650 nm region and shows a strong fluorescence enhancement upon analyte binding. Furthermore, investigation of the remarkable solvatokinetic fluorescence properties of the "butterfly"-like bc-BDP derivatives suggested that a second intrinsic nonradiative deactivation channel can play a role in the photophysics of boron-dipyrromethene dyes.

The design of a novel fluorescent PET sensor for proton and water: A phenylaminonaphtho[1,2-d]oxazol-2-yl-type fluorophore containing proton donor and acceptor groups

“…4 % with increasing polarity from 1,4-dioxane to acetone. Similar spectral changes are generally observed for most of fluorescent dyes whose dipole moments in the excited state are larger than those in the ground state [41,42].…”

A Doubly N‐Fused Benzohexaphyrin and Its Rearrangement to a Fluorescent Macrocycle upon DDQ Oxidation

“…[16] Furthermore, the fluorescence of 11 is quite dependent on solvent polarity and its fluorescent quantum yield decreases with increasing solvent polarity: F F = 0.056 in chlorobenzene, 0.054 in toluene, 0.048 in benzene, 0.033 in ethyl acetate, 0.029 in tetrahydrofuran, 0.013 in pyridine, 0.008 in benzonitrile, 0.004 in acetone, 0.002 in methanol, and less than 10 À3 in nitromethane, N,N-dimethylformamide, and acetonitrile, in parallel to the reported trend of polar diindomethene dyes. [14] The quenching of the fluorescence of 11 is indeed dependent on the polarity of the bulk solvent, but not on specific molecular interaction with polar solvent molecules, as revealed by experiments in mixed solvent systems, CH 2 Cl 2 /acetonitrile and toluene/nitromethane. These results are encouraging for the potential use of 11 as a polarity indicator in complicated environments.…”

“…As shown in Figure 4, the fluorescence emission of 11 is observed at l = 718 nm (l ex = 578 nm), with a large Stokes shift and a quantum yield of F F = 0.034 in CH 2 Cl 2 with respect to that of methylene blue (F F = 0.035 in methanol). [13] The fluorescence of 11 can be thought to originate from a diindomethene subunit, according to a recent report on a series of related boron-diindomethene dyes, [14] but its substantial shift to longer wavelengths is noteworthy and can be accounted for in terms of conjugation with an isoquinolin-5-one subunit. The relatively high quantum yield of fluorescence for 11 is attractive as fluorescence in this near-IR region (650-900 nm) is quite useful for a variety of applications including biochemical fluorescence imaging and sensing, [15] despite a limited number of fluorescent dyes in this region known.…”

New Trends in Near-Infrared Fluorophores for Bioimaging

“…58 Much work has been published on the basis of this concept over the past years (e.g. benzo-fused BODIPY (112 -164), [59][60][61][62][63][64] benzo-fused B,O-chelated BODIPY (165 -167), 65,66 benzo-fused aza-BODIPY (168 -177), 37,67,68 and areno-fused BODIPY (178 -186) 69,70 ), and the following briefly describes the tendency of the effect of modifying the optical properties: about a 50 nm red shift is achieved by introducing one benzene ring, resulting in similar, or a bit lower, optical properties compared to that of the original BODIPY (spectral sharpness, extinction coefficients, quantum yields). The extinction coefficients are for the most part almost identical to, or lower than, those of BODIPY (~80000 M -1 cm -1…”