Going beyond Red with a Tri- and Tetracoordinate Boron Conjugate: Intriguing Near-IR Optical Properties and Applications in Anion Sensing

An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Modulation of the Naked-Eye and Fluorescence Color of a Protonated Boron-Doped Thiazolothiazole by Anion-Dependent Hydrogen Bonding

The reaction of a cyclic alkyl(amino)carbene (CAAC)-stabilized thiazaborolo[5,4-d]thiazaborole (TzbTzb) with strong Brønsted acids, such as HCl, HOTf (Tf=O2 SCF3 ) and [H(OEt2 )2 ][BArF 4 ] (ArF =3,5-(CF3 )2 C6 H3 ), results in the protonation of both TzbTzb nitrogen atoms. In each case X-ray crystallographic data show coordination of the counteranions (Cl- , OTf- , BArF 4 - ) or solvent molecules (OEt2 ) to the doubly protonated fused heterocycle via hydrogen-bonding interactions, the strength of which strongly influences the 1 H NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb-to-CAAC charge transfer character involved in the S0 →S1 transition, ultimately enabling fine-tuning of their absorption and emission spectral features.

Triarylborane-Appended Anils and Boranils: Solid-State Emission, Mechanofluorochromism, and Phosphorescence

Herein, the design, synthesis, optical properties, and mechanofluorochromism characteristics of a series of conjugates having covalently linked triarylborane (TAB) and anil/boranil units (TAB-anil: 1 a-3 a and TAB-boranil: 1-3) are reported. The electronic interactions between TAB and anil/boranil in 1 a-3 a and 1-3 were fine-tuned by changing the boryl moiety's position on the phenyl spacer connecting the BMes₂ (Mes=mesityl) and anil/boranil units. A boryl moiety at the meta position (1 a) of the phenyl spacer stabilizes the enolic form (E-OH), whereas a boryl moiety at the para position (2 a and 3 a) stabilizes the keto form (Z-NH) in the solid state. However, in solution 1 a, 2 a, and 3 a exhibit keto-enol tautomerism in both ground and excited states. Compounds 1 a-3 a and 1-3 show red-shifted absorption compared with 4 a and 4, which are devoid of TAB moieties, which indicate effective participation of an empty p orbital on the boron center in 1 a-3 a and 1-3. Compounds 1 and 2 showed fluorescence variations in response to external stimuli such as mechanical grinding. Long phosphorescence lifetimes of 18-46 ms were observed for compounds 1-3. The observed optical properties of 1 a-3 a and 1-3 are rationalized in the context of quantum mechanical calculations.

Molecular Conformational Effect on Optical Properties and Fluoride Induced Color Changes in Triarylborane-Vinylbithiophene-BODIPY Conjugates

Three new triads [bis(mesityl)boryl (Mes₂B)-vinylbithiophene-BODIPY] bearing zero (1), two (2), and four (3) methyl groups on the BODIPY core are synthesized, and their optical properties are reported. The vinyl linker between the thiophene rings in the spacer moiety improved the electronic communication between the boryl and BODIPY units. It displayed a bathochromic shift in the absorption and emission spectra compared to the Mes₂B-bithiophene-BODIPY triad reported elsewhere. These compounds exhibit intriguing multiple-emission features due to an incomplete energy transfer from donor borane to the acceptor BODIPY unit. These compounds' photoluminescence color can be conveniently fine-tuned by fluoride binding at the coordinatively unsaturated tricoordinate boron center. Triad 3, with a rigid molecular structure, showed a sharp emission band, whereas triads 1 and 2, with flexible molecular structures, displayed broad emission bands with a robust bathochromic shift, ascribed to their excited state structural reorganizations. These triads selectively bind fluoride ions and show colorimetric responses, which can be seen with an unaided eye. DFT computational studies were performed to rationalize the optical signatures of these compounds.

A benzimidazole-based non-symmetrical tripodal receptor for the ratiometric fluorescence sensing of fluoride ions and solid state recognition of sulfate ions

A novel tripodal receptor has been reported as a fluorescent chemosensor for fluoride ions. Moreover, in solid state, seven units of the protonated receptors form a pseudo-capsular cavity to encapsulate two pairs of sulphate anions.

Photo-Induced Charge Separation vs. Degradation of a BODIPY-Based Photosensitizer Assessed by TDDFT and RASPT2

A meso-mesityl-2,6-iodine substituted boron dipyrromethene (BODIPY) dye is investigated using a suite of computational methods addressing its functionality as photosensitizer, i.e., in the scope of light-driven hydrogen evolution in a two-component approach. Earlier reports on the performance of the present iodinated BODIPY dye proposed a significantly improved catalytic turn-over compared to its unsubstituted parent compound based on the population of long-lived charge-separated triplet states, accessible due to an enhanced spin-orbit coupling (SOC) introduced by the iodine atoms. The present quantum chemical study aims at elucidating the mechanisms of both the higher catalytic performance and the degradation pathways. Time-dependent density functional theory (TDDFT) and multi-state restricted active space perturbation theory through second-order (MS-RASPT2) simulations allowed identifying excited-state channels correlated to iodine dissociation. No evidence for an improved catalytic activity via enhanced SOCs among the low-lying states could be determined. However, the computational analysis reveals that the activation of the dye proceeds via pathways of the (prior chemically) singly-reduced species, featuring a pronounced stabilization of charge-separated species, while low barriers for carbon-iodine bond breaking determine the photostability of the BODIPY dye.

Dimesitylboryl-functionalised cyanostilbene derivatives of phenothiazine: distinctive polymorphism-dependent emission and mechanofluorochromism

Dimesitylborane-functionalized cyanostilbene derivatives of phenothiazine exhibit polymorphism with distinct solid-state emissions.

Anion Sensing with a Blue Fluorescent Triarylboron-Functionalized Bisbenzimidazole and Its Bisbenzimidazolium Salt

A blue fluorescent p-dimesitylboryl-phenyl-functionalized 1,3-bisbenzimidazolyl benzene molecule (1) has been synthesized in high yield by Stille coupling of bisbenzimidazolyl bromobenzene with p-BMes₂-SnBu₃-benzene. Methylation of 1 led to the formation of the bisbenzimidazolium salt (2). The utility of both 1 and 2 in sensing CN^- and halide (F^-, Cl^-, Br^-, and I^-) was examined, and it was found that only the small fluoride and cyanide anions were able to bind to the boron atom with binding constants in the range of 2.9 × 10⁴ to 5 × 10⁵ M^-1. Computational studies provided insight into the photophysical properties of the molecules and verified that a charge-transfer process is quenched in these "turn-off" molecular sensors.

Recent developments in and perspectives on three-coordinate boron materials: a bright future

The empty p z -orbital of a three-coordinate organoboron compound leads to its electron-deficient properties, which make it an excellent π-acceptor in conjugated organic chromophores. The empty p-orbital in such Lewis acids can be attacked by nucleophiles, so bulky groups are often employed to provide air-stable materials. However, many of these can still bind fluoride and cyanide anions leading to applications as anion-selective sensors. One electron reduction generates radical anions. The π-acceptor strength can be easily tuned by varying the organic substituents. Many of these compounds show strong two-photon absorption (TPA) and two-photon excited fluorescence (TPEF) behaviour, which can be applied for e.g. biological imaging. Furthermore, these chromophores can be used as emitters and electron transporters in OLEDs, and examples have recently been found to exhibit efficient thermally activated delayed fluorescence (TADF). The three-coordinate organoboron unit can also be incorporated into polycyclic aromatic hydrocarbons. Such boron-doped compounds exhibit very interesting properties, distinct from their all-carbon analogues. Significant developments have been made in all of these areas in recent years and new applications are rapidly emerging for this class of boron compounds.

A smart ratiometric red fluorescent chemodosimeter for fluoride based on anthraquinone nosylate

A smart ratiometric red fluorescent chemodosimeter (AH) has been explored for specific detection of F⁻ with a lowest detection limit of 3.45 × 10⁻¹⁰ M. The sensing mechanism was worked out as fluoride-triggered deprotonation of imidazole accompanied by deprotection of the nosylate group.

Why do TD-DFT excitation energies of BODIPY/Aza-BODIPY families largely deviate from experiment? Answers from electron correlated and multireference methods

The vertical excitation energies of 17 boron-dipyrromethene (BODIPY) core structures with a variety of substituents and ring sizes are benchmarked using time-dependent density functional theory (TD-DFT) with nine different functionals combined with the cc-pVTZ basis set. When compared to experimental measurements, all functionals provide mean absolute errors (mean AEs) greater than 0.3 eV, larger than the 0.1-0.3 eV differences typically expected from TD-DFT. Due to the high linear correlation of TD-DFT results with experiment, most functionals can be used to predict excitation energies if corrected empirically. Using the CAM-B3LYP functional, 0-0 transition energies are determined, and while the absolute difference is improved (mean AE = 0.478 eV compared to 0.579 eV), the correlation diminishes substantially (R(2) = 0.961 to 0.862). Two very recently introduced charge transfer (CT) indices, q(CT) and d(CT), and electron density difference (EDD) plots demonstrate that CT does not play a significant role for most of the BODIPYs examined and, thus, cannot be the source of error in TD-DFT. To assess TD-DFT methods, vertical excitation energies are determined utilizing TD-HF, configuration interaction CIS and CIS(D), equation of motion EOM-CCSD, SAC-CI, and Laplace-transform based local coupled-cluster singles and approximate doubles LCC2* methods. Moreover, multireference CASSCF and CASPT2 vertical excitation energies were also obtained for all species (except CASPT2 was not feasible for the four largest systems). The SAC-CI/cc-pVDZ, LCC2*/cc-pVDZ, and CASPT2/cc-pVDZ approaches are shown to have the smallest mean AEs of 0.154, 0.109, and 0.100 eV, respectively; the utility of the LCC2* approach is demonstrated for eight extended BODIPYs and aza-BODIPYs. We found that the problems with TD-DFT arise from difficulties in dealing with the differential electron correlation (as assessed by comparing CCS, CC2, LR-CCSD, CCSDR(T), and CCSDR(3) vertical excitation energies for five compounds) and from contributions of multireference character and double excitations (from analysis of the CASSCF wave functions).

Multiple emissive triarylborane-A2H2 and triarylborane-Zn-A2H2 porphyrin conjugates

Triarylborane-A2H2 (1) and triarylborane-Zn-A2H2 porphyrins (2) have been synthesized by acid catalyzed condensation of 4-dimesitylboryl-benzaldehyde and dipyrromethane under ambient conditions. Compounds 1 and 2 showed multiple emission bands upon excitation at the triarylborane dominated absorption region (350 nm). Detailed experimental and computational studies show that the multiple emission features of 1 and 2 arise as a result of a partial energy transfer from the donor (triarylborane) to the acceptor (porphyrin) moieties. Compounds 1 and 2 showed very high selectivities towards fluoride ions compared to other competing anions.

In vivo detection of fluoride at trace levels and its removal from raw water at neutral pH utilizing a cyanobacterium pigment as a luminescent probe

A selective method has been developed for trace level (0.01 mg L⁻¹) fluoride detection in HEPES buffer in the presence of interfering ions (such as Cl⁻, I⁻, Br⁻, SO₄²⁻, ClO₄⁻, and CH₃COO⁻) using a cyanobacterium as a luminescent probe.

White light emissive molecular siblings

White-light emission from boron based molecular siblings.

Benzoindolium–triarylborane conjugates: a ratiometric fluorescent chemodosimeter for the detection of cyanide ions in aqueous medium

A new ratiometric fluorescent chemodosimeter has been synthesized and characterized that exhibits high selectivity and sensitivity toward CN⁻ ions in aqueous medium.

Triarylborane conjugated dicyanovinyl chromophores: intriguing optical properties and colorimetric anion discrimination

Three new triarylborane conjugated dicyanovinyl chromophores (Mes2B-π-donor-DCV); donor: N-methyldiphenylamine () and triphenylamine ( and [with two BMes2 substitutions]) of type A-D-A (acceptor-donor-acceptor) are reported. Compounds exhibit intense charge transfer (CT) absorption bands in the visible region. These absorption peaks are combination CT bands of the amine donor to both the BMes2 and DCV units. This inference was supported by theoretical studies. Compound shows weak fluorescence compared to and . The discrimination of fluoride and cyanide ions is essential in the case of triarylborane (TAB) based anion sensors as a similar response is given towards both the anions. Anion binding studies of , and showed that fluoride ions bind selectively to the boron centre and block the corresponding CT transition (donor to BMes2) leaving the other CT transition to be red shifted. On the other hand, cyanide ions bind with both the receptor sites and stop both the CT transition processes and hence a different colorimetric response was noted. The binding of F(-)/CN(-) induces colour changes in the visible region of the electronic spectra of and , which allows for the naked-eye detection of F(-) and CN(-) ions. The anion binding mechanisms are established using NMR titration experiments.

Near-infrared-induced electron transfer of an uranyl macrocyclic complex without energy transfer to dioxygen

Photoexcitation of dichloromethane solutions of an uranyl macrocyclic complex with cyclo[1]furan[1]pyridine[4]-pyrrole () at the near-infrared (NIR) band (1177 nm) in the presence of electron donors and acceptors resulted in NIR-induced electron transfer without producing singlet oxygen via energy transfer.

Colorimetric and ratiometric fluorescent chemodosimeter for selective sensing of fluoride and cyanide ions: tuning selectivity in proton transfer and C–Si bond cleavage

An anthraimidazolyldione based colorimetric and ratiometric fluorescent chemodosimeter (LHSi) was designed and synthesized for fluoride and cyanide ion sensing.

D-π-A triarylboron compounds with tunable push-pull character achieved by modification of both the donor and acceptor moieties

The push-pull character of a series of donor-bithienyl-acceptor compounds has been tuned by adopting triphenylamine or 1,1,7,7-tetramethyljulolidine as a donor and B(2,6-Me2 -4-RC6 H2)2 (R=Me, C6 F5 or 3,5-(CF3)2 C6 H3) or B[2,4,6-(CF3 )3 C6 H2]2 as an acceptor. Ir-catalyzed C-H borylation was utilized in the derivatization of the boryl acceptors and the tetramethyljulolidine donor. The donor and acceptor strengths were evaluated by electrochemical and photophysical measurements. In solution, the compound with the strongest acceptor, B[2,4,6-(CF3)3 C6 H2]2 ((FMes)2 B), has strongly quenched emission, while all other compounds show efficient green to red (ΦF =0.80-1.00) or near-IR (NIR; ΦF =0.27-0.48) emission, depending on solvent. Notably, this study presents the first examples of efficient NIR emission from three-coordinate boron compounds. Efficient solid-state red emission was observed for some derivatives, and interesting aggregation-induced emission of the (FMes)2 B-containing compound was studied. Moreover, each compound showed a strong and clearly visible response to fluoride addition, with either a large emission-color change or turn-on fluorescence.