Photochromic Thienylpyridine–Bis(alkynyl)borane Complexes: Toward Readily Tunable Fluorescence Dyes and Photoswitchable Materials

Organoboron based photochromic gelator

A series of tetra-coordinate boron-peptide conjugates has been reported. The incorporation of a photochromic organoboron unit into the gelator endows photoactivity to the supramolecular gels. While the structural transformation of the gelator upon UV irradiation minimally impacts the formed self-assembled structures, it indeed influences their rheological properties.

Metal-Free Boron-Mediated ortho-C-H Hydroxylation of N-Benzyl-3,4,5-tribromopyrazoles

A novel route has been reported for C-H hydroxylation of benzyl compounds directed by a 3,4,5-tribromopyrazole auxiliary via boronation/oxidation using BBr₃ and NaBO₃·4H₂O. The strategy exhibits outstanding site selectivity and affords the corresponding phenols in moderate to excellent yields under metal-free conditions. Besides, this protocol can be achieved in one pot, which is highly promising as a practical method for use in a multistep organic synthetic process.

Photochromic dithienylethene-containing four-coordinate boron(III) compounds with a spirocyclic scaffold

A new series of four-coordinate boron compounds bearing a photochromic dithienylethene-containing C^C ligand and an ancillary N^C ligand have been successfully designed and synthesised. These compounds exhibit reversible photochromism upon photoexcitation with percentage conversions of 71-96% and readily tuneable photocycloreversion quantum yields by convenient modification of the ancillary ligand to turn on the thermally activated upconversion from the lower-lying unreactive excited state to the higher-lying photoreactive excited state.

π-Conjugated Materials Derived From Boron-Chalcogenophene Combination. A Brief Description of Synthetic Routes and Optoelectronic Applications

Functional materials composed of Boron-chalcogenophene conjugates have emerged as promising ensemble featuring commendable optoelectronic properties. This review describes the categories, synthetic routes and optoelectronic applications of a range of boron-chalcogenophene conjugates. Conjugation and linking of different types of tri- and tetra-coordinated boron moieties with chalcogenophenes have remained an important strategy for constructing a range of functional materials. Synthetic protocols have been devised to efficiently prepare such chemically robust conjugates, often exhibiting a myriad of photophysical properties, redox capabilities and also solid-state behaviors. Tin-boron and silicon-boron exchange protocols have been efficiently adapted to access these boron-chalcogenophenes. Few other commonly used methods namely, hydroboration of alkynes as well as electrophilic borylations are also mentioned. The chemical and electronic properties of such boron-chalcogenophene conjugates are directly influenced by the strong Lewis acid character of trivalent boranes which can further alter the intra- and inter- molecular Lewis acid-base interactions. Apart from the synthetic protocols, recent advances in the application of these boron-chalcogenophene conjugates towards analyte sensing, organic electronics, molecular switches and several other aspects will be discussed in this review.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the ³MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Recent Advances in π-Conjugated N^C-Chelate Organoboron Materials

Boron-containing π-conjugated materials are archetypical candidates for a variety of molecular scale applications. The incorporation of boron into the π-conjugated frameworks significantly modifies the nature of the parent π-conjugated systems. Several novel boron-bridged π-conjugated materials with intriguing structural, photo-physical and electrochemical properties have been reported over the last few years. In this paper, we review the properties and multi-dimensional applications of the boron-bridged fused-ring π-conjugated systems. We critically highlight the properties of π-conjugated N^C-chelate organoboron materials. This is followed by a discussion on the potential applications of the new materials in opto-electronics (O-E) and other areas. Finally, attempts will be made to predict the future direction/outlook for this class of materials.

Intramolecular (directed) electrophilic C-H borylation

The intramolecular C-H borylation of (hetero)arenes and alkenes using electrophilic boranes is a powerful transition metal free methodology for forming C-B bonds. These C-H borylation reactions are preceded by intermolecular bond (both dative and covalent) formation, with examples proceeding via initial C-B and N-B bond formation dominating this field thus both are discussed in depth herein. Less prevalent intramolecular electrophilic C-H borylation reactions that proceed by intermolecular O-B, S-B and P-B bond formation are also summarised. Mechanistic studies are presented that reveal two mechanisms for C-H borylation, (i) electrophilic aromatic substitution (prevalent with B-X electrophiles); (ii) σ-bond metathesis mediated (prevalent with B-H and B-R electrophiles). To date, intramolecular electrophilic C-H borylation is utilised mainly for accessing boron containing conjugated organic materials, however recent developments, summarized herein alongside early studies, have highlighted the applicability of this methodology for forming synthetically versatile organo-boronate esters and boron containing bioactives. The multitude of synthetic procedures reported for intramolecular electrophilic C-H borylation contain many common features and this enables key requirements for successful C-H borylation and the factors effecting regioselectivity and substrate scope to be identified, discussed and summarized.

Turn-on fluorescence sensors based on dynamic intramolecular N→B-coordination

A series of ten aryl-triazole-functionalized boranes bearing BMes₂-groups and capable of forming intramolecular five-membered N→B-coordinated heterocycles, has been prepared by 1,3-dipolar cycloaddition.

8.78% Efficient All-Polymer Solar Cells Enabled by Polymer Acceptors Based on a B←N Embedded Electron-Deficient Unit

In the field of all-polymer solar cells (all-PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron-deficient units to build high-performance polymer acceptors is critical. A novel electron-deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single-crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4-difluorothiophene, two novel conjugated polymers named BN-T and BN-2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350-800 nm, low-lying energy levels, and ambipolar film-transistor characteristics. Using PBDB-T as the donor and BN-2fT as the acceptor, all-PSCs afford an encouraging efficiency of 8.78%, which is the highest for all-PSCs excluding the devices based on imide and dicyanoethylene-type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron-deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

Four-Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices

A new class of four-coordinate donor-acceptor fluoroboron-containing thermally activated delayed fluorescence (TADF) compounds bearing a tridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H-spiro[acridine-9,9'-fluorene] to 9,9-dimethyl-9,10-dihydroacridine, these boron derivatives exhibit a wide range of emission colors spanning from blue to yellow with a large spectral shift of 2746 cm^-1 , with high PLQYs of up to 96 % in the doped thin film. Notably, vacuum-deposited organic light-emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A^-1 , power efficiencies of 58.4 lm W^-1 and external quantum efficiencies of 18.0 %. More importantly, long operational stabilities of the green-emitting OLEDs based on 2 with half-lifetimes of up to 12 733 hours at an initial luminance of 100 cd m^-2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy-chelating four-coordinate boron TADF compounds for long operational stabilities, suggesting great promises for the development of stable boron-containing TADF emitters.

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.

Borylated perylenediimide: self-assembly, photophysics and sensing application

A red emissive perylenediimide-triarylboron conjugate (PDI-TAB) with interesting 1D self-assembled nanowires, concentration dependent emission colors and potential fluoride ion sensing is reported.

Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: new substituents and alternative substrates

A series of new boranes capable of forming intramolecular N → B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations.

BN Embedded Polycyclic π-Conjugated Systems: Synthesis, Optoelectronic Properties, and Photovoltaic Applications

In the periodic table of elements, boron (B, atomic number, 5) and nitrogen (N, atomic number, 7) are neighboring to the carbon (C, atomic number, 6). Thus, the total electronic number of two carbons (12) is equal to the electronic sum of one boron (5) and one nitrogen (7). Accordingly, replacing two carbons with one boron and one nitrogen in a π-conjugated structure gives an isoelectronic system, i.e., the BN perturbed π-conjugated system, comparing to their all-carbon analogs. The BN embedded π-conjugated systems have unique properties, e.g., optical absorption, emission, energy levels, bandgaps, and packing order in contrast to their all-carbon analogs and have been intensively studied in terms of novel synthesis, photophysical characterizations, and electronic applications in recent years. In this review, we try to summarize the synthesis methods, optoelectronic properties, and progress in organic photovoltaic (OPV) applications of the representative BN embedded polycyclic π-conjugated systems. Firstly, the narrative will be commenced with a general introduction to the BN units, i.e., B←N coordination bond, B-N covalent bond, and N-B←N group. Then, the representative synthesis strategies toward π-conjugated systems containing B←N coordination bond, B-N covalent bond, and N-B←N group will be summarized. Afterwards, the frontier orbital energy levels, optical absorption, packing order in solid state, charge transportation ability, and photovoltaic performances of typical BN embedded π-conjugated systems will be discussed. Finally, a prospect will be proposed on the OPV materials of BN doped π-conjugated systems, especially their potential applications to the small molecules organic solar cells.

Triarylboron Anchored Luminescent Probes: Selective Detection and Imaging of Thiophenols in the Intracellular Environment

The advances in boron incorporated organics have captured overwhelming interest on account of their outstanding properties and promising applications in various fields. Mostly, triarylborane compounds (TAB) are exploited as sensors of F^- and CN^- anions at the expense of the intrinsic Lewis acidic nature of boron. New molecular probes 1 and 2 for detection of toxic thiophenol were designed by conjugating highly fluorescent borylanilines with the luminescent quencher 2,4-dinitrobenzene based sulfonamides (DNBS), wherein the electrophilicity of the DNBS moiety has been modulated by fine-tuning the intrinsic Lewis acidity of boron. The interplay between PET (photoinduced electron transfer) and ICT have been employed for developing the TAB tethered turn-on fluorescent sensor for thiophenol with high selectivity for the first time. The newly developed probes showed very fast response toward thiophenol (within ∼5 min) with limits of detection (LOD) lying in the micromolar range, clearly pointing to their potential. Further, compounds 1 and 2 were explored for detecting thiophenol in the intracellular environment by discriminating biothiols. DFT and TD-DFT calculations were performed to support the sensing mechanism.

Coordination Compounds with Photochromic Ligands: Ready Tunability and Visible Light-Sensitized Photochromism

Photochromic compounds are well-known for their promising applications in many areas. In this context, many different photochromic families have been developed. As the early study of these photochromic compounds was mainly focused on the organic system, their photochromic reactivity was mainly derived from the singlet excited state. We hypothesized that the incorporation of the photochromic ligand to the transition metal complex and coordination complex systems would not only render the triplet state of the organic photochromic system more readily accessible due to the large spin-orbit coupling of the heavy metal center but also would lead to ready extension of the excitation wavelength to less destructive longer wavelength low-energy excitation. On the other hand, the long-lived triplet excited states of the metal complexes are also suitable for energy or electron transfer processes, which should lead to new photochromic behavior and photoswitchable functional properties. Through the incorporation of the stilbene-, azo-, spirooxazine-, and dithienylethene-containing ligands to transition metal complex systems with heavy metal centers and suitable excited states, triplet state photosensitized photochromism has been achieved. With the triplet state photosensitization, the photochromism of these compounds could be extended from the high energy UV region to the visible region. In the development of dithienylethene-containing ligands, we have adopted an alternative strategy, which involves the incorporation of nitrogen and sulfur heterocycles that directly form part of the dithienylethene framework as ligands to exert a much stronger perturbation and influence on the excited state properties of the photochromic unit by the metal center. On the basis of the new design, wide ranges of dithienylethene-containing ligands, including phenanthrolines, 2-pyridylimidazoles, N-pyridylimidazol-2-ylidenes, cyclometalating thienylpyridines, β-diketonates, and β-ketoiminates have been designed and incorporated into various coordination systems. Apart from the photosensitization, tuning of the closed form absorption and photochromic behavior based on the perturbation of the metal center, coordination-assisted planarization, modification of the ancillary ligands and introduction of various electronic excited states derived from the coordination system have been successfully demonstrated. This strategy can be used for developing NIR photochromic dithienylethenes. With the above effects observed upon the coordination to different transition metal centers and central atoms, this strategy offers a simple and effective way for the modification of the photochromic characteristics. Moreover, the emission and other functional properties of the coordination systems could also be photoswitched by the photochromic reactions.

Electronic and structural properties of N → B-ladder boranes with high electron affinity

A series of electronically and structurally diverse N → B-ladder boranes has been prepared by hydroboration.

Investigation of NLO Properties of Fluorescent BORICO Dyes: a Comprehensive Experimental and Theoretical Approach

BORICO dyes with N, N-diethyl as a strong donor and BF₂ complexed iminocoumarin six member core as strong acceptor are investigated as an efficient non linear optical chromophores. Extended π-conjugation over iminocoumarin moiety is useful to make ICT character of BORICO dyes more significant and is established on the scale of Generalised Mulliken Hush analysis scale. Bond length alternation and bond order alternation values for three BORICO chromophores estimates the cyanine like framework for optimal non linear optical response. The frontier molecular orbital diagrams obtained from density functional theory calculations shows that there is charge transfer from donor to accepter as well as effective overlap between them making the basis for optimal NLO response of BORICO chromophores. The theoretical values of linear and non linear optical responses for three BORICO NLOphores obtained by using three different functionals B3LYP, CAMB3LYP and BHandHLYP with 6-311+g(d,p) basis set are quite consistent for the values of static dipole moment (μ), linear polarizability (α) and first hyperpolarizability (β). However in case of the γ values calculation, compare to the similar values obtained by CAMB3LYP and BHandHLYP functionals, B3LYP overestimates the same. The vibrational motions play decisive role in the overall non linear optical properties of BORICO chromophores.

Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

A new class of Ar₂B-π-A dyads and A-π-B(Ar)-π-A triads that feature strong organic acceptor moieties (A = pyridalthiadiazole, PT) attached to a central triarylborane were synthesized via Stille cross-coupling of ArB(Th-SnMe₃)₂ (Th = thiophenediyl, Ar = 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethylphenyl) (^FMes)) with one or two equivalents of dibromopyridalthiadiazole. Single-crystal X-ray crystallography data for the triad Mes*B(Th-PT-Br)₂ indicate a highly coplanar conformation, which is ideal for extended π-conjugation and favors intermolecular π-stacking. Despite the presence of Br substituents, these compounds exhibit strong photoluminescence in THF solution with quantum yields reaching up to 52%. Further extension of conjugation by coupling with 2-hexylthiophene leads to additional bathochromic shifts to give a highly soluble and strongly red-emissive material. All these compounds undergo facile reduction, first of the PT substituents and then at more negative potentials for the borane moiety. Upon chemical reduction with in THF, an intramolecular charge transfer (ICT) pathway from the reduced PT moieties to boron is enabled and this results in a change of the color to blue. Theoretical calculations reveal that, due to the electron-withdrawing effect of the PT moieties, not only the PT-centered LUMOs themselves but also the LUMO+1 or LUMO+2, which show contributions from the p orbital of boron, experience a significant decrease in energy; they are much lower in energy than those of typical conjugated triarylboranes. The relatively low energy of both the PT-centered LUMOs and boron-centered LUMO+1 or LUMO+2 opens up multiple pathways for reaction with highly nucleophilic fluoride anions. Evidence for very strong F^- binding to boron is obtained in the case of the more sterically accessible ^FMes derivatives. Fluoride anion binding leads to an electron-rich borate moiety and as such generates an ICT pathway to the electron-deficient PT moieties; the direction of this ICT is opposite to that observed upon chemical reduction. For the Mes* derivatives, F^- binding is hindered, resulting in competing reduction of the PT acceptors. Finally, the electron acceptor character of the hexylthiophene derivative is exploited in electron-only diodes that show an average electron mobility of 6.4 ± 1.6 × 10^-5 cm² V^-1 s^-1.

A photochromic prototype based on difurylperhydrocyclopentene with remarkable photoswitching behavior and in vivo application

A furan-based photochromic prototype with remarkable photoswitching behavior was facilely synthesized and applied in vivo.

Stereoselective photoreaction in P-stereogenic dithiazolylbenzo[b]phosphole chalcogenides

The diastereoselective photoreaction of dithiazolylbenzo[b]phosphole chalcogenides was achieved by means of the conformational control of the photoprecursor state by intramolecular interactions involving the stereogenic phosphorus.

Merging thiophene with boron: new building blocks for conjugated materials

Recent advances on the use of thienylborane chemistry for the development of new functional materials are highlighted.

Luminescent organoboron ladder compounds via directed electrophilic aromatic C–H borylation

Regioselective electrophilic borylation of a pyridyl-flanked fluorene provides highly luminescent, air-stable boron-bridged ladder species.

Facile Arylation of Four-Coordinate Boron Halides by Borenium Cation Mediated Boro-desilylation and -destannylation

The addition of AlCl₃ to four-coordinate boranes of the general formula (C–N-chelate)BCl₂ results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C–N-chelate)BCl]⁺. The latter react with both arylstannanes and arylsilanes by boro-destannylation and -desilylation, respectively, to form arylated boranes. Catalytic quantities of AlCl₃ were sufficient to effect high-yielding arylation of (C–N-chelate)BCl₂. Boro-destannylation is more rapid than boro-desilylation and leads to double arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.

A Solution-Processable Donor-Acceptor Compound Containing Boron(III) Centers for Small-Molecule-Based High-Performance Ternary Electronic Memory Devices

A novel small-molecule boron(III)-containing donor-acceptor compound has been synthesized and employed in the fabrication of solution-processable electronic resistive memory devices. High ternary memory performances with low turn-on (V(Th1)=2.0 V) and distinct threshold voltages (V(Th2)=3.3 V), small reading bias (1.0 V), and long retention time (>10(4) seconds) with a large ON/OFF ratio of each state (current ratio of "OFF", "ON1", and "ON2"=1:10(3):10(6)) have been demonstrated, suggestive of its potential application in high-density data storage. The present design strategy provides new insight in the future design of memory devices with multi-level transition states.