Carborane-containing poly(fluorene): response to solvent vapors and amines

Expanded Porphyrin-Anion Supramolecular Assemblies: Environmentally Responsive Sensors for Organic Solvents and Anions

Porphyrins have been used frequently to construct supramolecular assemblies. In contrast, noncovalent ensembles derived from expanded porphyrins, larger congeners of naturally occurring tetrapyrrole macrocycles, are all but unknown. Here we report a series of expanded porphyrin-anion supramolecular assemblies. These systems display unique environmentally responsive behavior. Addition of polar organic solvents or common anions to the ensembles leads to either a visible color change, a change in the fluorescence emission features, or differences in solubility. The actual response, which could be followed easily by the naked eye, was found to depend on the specifics of the assembly, as well as the choice of analyte. Using the ensembles of this study, it proved possible to differentiate between common solvents, such as diethyl ether, THF, ethyl acetate, acetone, alcohol, acetonitrile, DMF, and DMSO, identify complex solvent systems, as well as distinguish between the fluoride, chloride, bromide, nitrate, and sulfate anions.

Carboranes in the chemist's toolbox

Once seldom encountered outside of a few laboratories, carboranes are now everywhere, playing a role in the development of a broad range of technologies encompassing organic synthesis, radionuclide handling, drug design, heat-resistant polymers, cancer therapy, nanomaterials, catalysis, metal-organic frameworks, molecular machines, batteries, electronic devices, and more. This perspective highlights selected examples in which the special attributes of carboranes and metallacarboranes are being exploited for targeted purposes in the laboratory and in the wider world.

Carborane dyads for photoinduced electron transfer: photophysical studies on carbazole and phenyl-o-carborane molecular assemblies

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

Preparation of fluorescent conjugated polymer fibrous membranes for rapid recognition of aromatic solvents

Fluorescent poly(phenylenevinylene) (PPV)/poly(vinyl alcohol) (PVA) fibrous membrane was prepared via electrospinning of PPV precursor and PVA aqueous solution followed by thermal elimination. Further cross-linking produced the cross-linked membrane PPV/CPVA. Both PPV/PVA and PPV/CPVA membranes were found to have similar morphology and photophysics. These membranes showed a great fluorescence quenching response to aromatic solvents and a much smaller response to other organic solvents. Water also effectively quenched the fluorescence of PPV/PVA but not that of PPV/CPVA. This was attributed to un-cross-linked PVA being able to dissolve in water and the cross-linking improving the resistance of the membrane toward water. The sensing behavior was found to have good reversibility. The contact angle study showed that addition of only about 1% of PPV into the matrix reduced the hydrophilicity of the membrane significantly, suggesting that the PPV chains would be located at the surface of the fibers. X-ray photoelectron spectroscopy (XPS) investigation further confirmed such surface enrichment of PPV in the binary polymer blends. The PPV chain on the surface facilitated the π-π interaction between the polymer backbones and the aromatic molecules, thus leading to good selectivity and fast response of the two fibrous membranes toward aromatic solvents.

BODIPY functionalized o-carborane dyads for low-energy photosensitization

Bodipy-o-carborane dyads undergo a photo-induced electron transfer (PET) process under low energy visible light sensitization and their PET efficiencies are in the 63–71% range.

Syntheses and reductions of C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes

An investigation ofC-dimesitylboryl-ortho-carboranes, 1-(BMes₂)-2-R-1,2-C₂B₁₀H₁₀(1and2), reveals that the carborane is the electron-acceptor and the mesityl group is the electron-donor in these dyads.

Fluorescence of new o-carborane compounds with different fluorophores: can it be tuned?

Two sets of o-carborane derivatives incorporating fluorene and anthracene fragments as fluorophore groups have been successfully synthesized and characterized, and their photophysical properties studied. The first set, comprising fluorene-containing carboranes 6-9, was prepared by catalyzed hydrosilylation reactions of ethynylfluorene with appropriate carboranylsilanes. The compound 1-[(9,9-dioctyl-fluorene-2-yl)ethynyl]carborane (11) was synthesized by the reaction of 9,9-dioctyl-2-ethynylfluorene and decaborane (B10H14). Furthermore, reactions of the lithium salt of 11 with 1 equivalent of 4-(chloromethyl)styrene or 9-(chloromethyl)anthracene yielded compounds 12 and 13. Members of the second set of derivatives, comprising anthracene-containing carboranes, were synthesized by reactions of monolithium or dilithium salts of 1-Me-1,2-C2B10H11, 1-Ph-1,2-C2B10H11, and 1,2-C2B10H12 with 1 or 2 equivalents of 9-(chloromethyl)anthracene, respectively, to produce compounds 14-16. In addition, 2 equivalents of the monolithium salts of 1-Me-1,2-C2B10H11 (Me-o-carborane) and 1-Ph-1,2-C2B10H11 (Ph-o-carborane) were reacted with 9,10-bis(chloromethyl)anthracene to produce compounds 17 and 18, respectively. Fluorene derivatives 6-9 exhibit moderate fluorescence quantum yields (32-44 %), whereas 11-13, in which the fluorophore is bonded to the Ccluster (Cc), show very low emission intensity (6 %) or complete fluorescence quenching. The anthracenyl derivatives containing the Me-o-carborane moiety exhibit notably high fluorescence emissions, with ϕF = 82 and 94 %, whereas their Ph-o-carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the Cc-Cc bond length and the fluorescence intensity in CH2Cl2 solution, comparable to that observed for previously reported styrene-containing carboranes. Thus, our hypothesis is that for systems of this type the fluorescence may be tuned and even predicted by changing the substituent on the adjacent Cc.

Crystal structures of the carborane dianions [1,4-(PhCB₁₀H₁₀C)₂C₆H₄]²⁻ and [1,4-(PhCB₁₀H₁₀C)₂C₆F₄]²⁻ and the stabilizing role of the para-phenylene unit on 2n+3 skeletal electron clusters

While carboranes with 2 n+2 and 2 n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two-cage assemblies, where two C-phenyl-ortho-carboranyl groups are linked by a para-phenylene or a para-tetrafluorophenylene bridge, revealed two well separated and reversible two-electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X-ray crystallography. The diamagnetic dianions contain two 2 n+3 SE clusters where each cluster has a notably long carborane C-carborane C distance of ca 2.4 Å. The π conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.

Riboflavin-based fluorogenic sensor for chemo- and enantioselective detection of amine vapors

A novel turn-on fluorogenic chiral sensory system has been developed using a protonated riboflavin and riboflavin-derived cationic polymer as a fluorophore precursor and a specific amine receptor, respectively, which enables the solid-state chemo- and enantioselective fluorogenic visual detection of primary and secondary amine vapors.

Colour-tunable aggregation-induced emission of trifunctional o-carborane dyes

o-Carborane compounds showed a dual-emission property (normal emission and AIE), and the emission intensities could be precisely controlled, resulting in colour-tunable emission from blue to orange via white.

Polythiophenes with vinylene linked ortho, meta and para-carborane sidechains

The influence of the carborane isomer and double bond geometry on the optoelectronic properties for a series of side chain functionalised polythiophenes is reported.

AIE macromolecules: syntheses, structures and functionalities

A comprehensive review of macromolecules with aggregation-induced emission attributes is presented, covering the frontiers of syntheses, structures, functionalities and applications.

The coordination chemistry of two symmetric fluorene-based organic ligands with cuprous chloride

Two novel symmetric fluorene-based ligands, namely, 2,7-bis(1H-imidazol-1-yl)-9,9-dimethyl-9H-fluorene [L1 or (I), C21H18N4] and 2,7-bis(1H-imidazol-1-yl)-9,9-dipropyl-9H-fluorene (L2), have been used to construct the coordination polymers catena-poly[[dichloridodicopper(I)(Cu-Cu)]-μ-2,7-bis(1H-imidazol-1-yl)-9,9-dimethyl-9H-fluorene], [Cu2Cl2(C21H18N4)]n, (II), and catena-poly[[tetra-μ2-chlorido-tetracopper(I)]-bis[μ-2,7-bis(1H-imidazol-1-yl)-9,9-dipropyl-9H-fluorene]], [Cu4Cl4(C25H26N4)2]n, (III). There are three types of C-H···N hydrogen bonds in (I), resulting a two-dimensional network in the ab plane, including a chiral helical chain along the b axis. Compounds (II) and (III) are related one-dimensional polymers. In both, Cu(I) atoms connect the symmetric ligands (L1 or L2) into a one-dimensional chain. In (II), the {[Cu(I)Cl2](-)} unit, acting as a co-anion, adheres to the one-dimensional chain through a weak Cu···Cu interaction. However, in (III), the {[Cu(I)2Cl4](2-)} unit links two different chains into a one-dimensional rope-ladder-type chain. In addition, there are C-H···Cl hydrogen bonds and π-π interactions in the extended structures of (II) and (III), the difference is that the chains in (II) are linked into a two-dimensional network while the chains in (III) are stacked into a three-dimensional framework.

Synthesis, characterization, and thermal behavior of carboranyl-styrene decorated octasilsesquioxanes: influence of the carborane clusters on photoluminescence

Novel polyhedral oligomeric silsesquioxanes (POSS) or octasilsesquioxanes with carboranyl-styrene fragments attached to each corner are described. These compounds have been synthesized by olefin-metathesis reactions between octavinylsilsesquioxane and carboranyl-styrene compounds that possess different substituents (Ph, Me, or H). In all cases, these reactions, which were catalyzed by the Grubbs catalyst, are highly regioselective and yield exclusively the E isomers. The existence of the carborane cage in the POSS structure induces a remarkable thermal stability in these compounds. After combustion at 1000 °C, these carboranyl-POSS compounds exhibit a mass loss lower than 10%. The UV/Vis absorption data of these carboranyl-POSS compounds shows a slight bathochromic shift with respect to the carboranyl-styrene monomers, with an absorption maximum around 262 nm. Nevertheless, important differences in the emission spectra of the carboranyl-POSS compounds with regard to their carboranyl-styrene precursors are observed; the phenyl-o-carborane-containing POSS compound exhibits the highest fluorescence intensity (Φ(F)=44%), whereas for the POSS compound bearing the methyl substituent, and for the unsubstituted o-carborane clusters, the fluorescence intensity is much lower (Φ(F)=9 and 2%, respectively). This is precisely the reverse of what occurs with the monomers, in which the unsubstituted o-carboranyl-styrene compound exhibits the highest Φ(F), and a quenching of the fluorescence is observed in the phenyl-o-carboranyl-styrene compound. In addition, a large red shift of around 100 nm is observed for the POSS compounds with respect to their precursors. These experimental results can only be accounted for by the spatial ordering induced by the POSS core that eases interactions, which otherwise would not occur. These results have been confirmed by time-dependent density functional theory (TDDFT) calculations that exclude a photoinduced electron transfer (PET) process in the POSS compounds.

C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes: synthesis, structures, photophysics and electrochemistry

Six new C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes, 1,A-R2-1,A-C2B10H10, where R represents the group 2-(1,3-Et2-1,3,2-N2BC6H4) or 2-(1,3-Ph2-1,3,2-N2BC6H4) and A is 2, 7 or 12, were synthesized from o-, m-, and p-dicarbadodecaboranes (carboranes) by lithiation and subsequent treatment with the respective 2-bromo-1,3,2-benzodiazaboroles. UV-visible and fluorescence spectra of all carboranes display low energy charge transfer emissions. While such emissions with Stokes shifts between 17,330 and 21,290 cm(-1) are typical for C,C'-bis(aryl)-ortho-carboranes, the observed low-energy emissions with Stokes shifts between 8320 and 15,170 cm(-1) for the meta- and para-isomers are unusual as high-energy emissions are typical for meta- and para-dicarbadodecaboranes. Fluorescence quantum yields (φF) for the novel 1,7- and 1,12-bis(benzodiazaborolyl)-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane with N-ethyl substituents 1,12-(1',3'-Et2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 has a φF value of 41% in cyclohexane solution and only of 9% in the solid state, whereas the analogous 1,12-(1',3'-Ph2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 shows quantum yields of 3% in cyclohexane solution and 72% in the solid state. X-ray crystallographic, computational and cyclic voltammetry studies for these carboranes are also presented.

Receptor-free poly(phenylenevinylene) fibrous membranes for cation sensing: high sensitivity and good selectivity achieved by choosing the appropriate polymer matrix

Poly(phenylenevinylene)/polyimide (PPV/PI) and poly(phenylenevinylene)/ polymethylmethacrylate (PPV/PMMA) fibrous membranes without any deliberately introduced receptors were prepared as fluorescence sensing materials through electrospinning, followed by thermal treatment. Both of these membranes displayed higher sensitivity toward most cations compared to the corresponding spin-coated films. PPV/PMMA membranes were more sensitive than PPV/PI membranes toward Cu(2+) and Fe(3+). About 4.5 fold of intensity enhancement upon 20 nM of Cu(2+), 80% of quenching upon 20 nM of Fe(3+) with fast response and simple regeneration were realized for PPV/PMMA membrane. The preliminary investigation into the mechanism revealed that the properties of the polymer matrix and thermal treatment of the membrane played important roles in the sensing performance.

Carborane-based optoelectronically active organic molecules: wide band gap host materials for blue phosphorescence

Carborane-based host materials were prepared to fabricate deep blue phosphorescence organic light-emitting diodes (PHOLEDs), which constituted three distinctive geometrical structures stemming from the corresponding three different isomeric forms of carboranes, namely, ortho-, meta-, and para-carboranes. These materials consist of two carbazolyl phenyl (CzPh) groups as photoactive units on each side of the carborane carbons to be bis[4-(N-carbazolyl)phenyl]carboranes, o-Cb, m-Cb, and p-Cb. To elaborate on the role of the carboranes, comparative analogous benzene series (o-Bz, m-Bz, and p-Bz) were prepared, and their photophysical properties were compared to show that advantageous photophysical properties were originated from the carborane structures: high triplet energy. Unlike m-Bz and p-Bz, carborane-based m-Cb and p-Cb showed an unconjugated nature between two CzPh units, which is essential for the blue phosphorescent materials. Also, the carborane hosts showed high glass transition temperatures (T(g)) of 132 and 164 °C for m-Cb and p-Cb, respectively. Albeit p-Cb exhibited slightly lower hole mobility when compared to p-Bz, it still lies at the high end hole mobility with a value of 1.1 × 10(-3) cm(2)/(V s) at an electric field of 5 × 10(5) V/cm. Density functional theory (DFT) calculations revealed that triplet wave functions were effectively confined and mostly located at either side of the carbazolyl units for m-Cb and p-Cb. Low-temperature PL spectra indeed provided unequivocal data with higher triplet energy (T(1)) of 3.1 eV for both m-Cb and p-Cb. p-Cb was successfully used as a host in deep blue PHOLEDs to provide a high external quantum efficiency of 15.3% and commission internationale de l'elcairage (CIE) coordinates of (0.15, 0.24).

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.

Effect of o-Carborane on the Optoelectronic and Device-Level Properties of Poly(fluorene)s

Carboranes have been previously noted to distinctively affect the luminescent properties of semiconducting polymers when incorporated into the conjugated backbone. In this report, we use carborane-based poly(fluorene) derivatives as active materials for polymer light-emitting diodes and transistors. Optoelectronic analysis unequivocally shows that carborane does not participate in the π-conjugated network, yet their presence causes major red-shifting in device electroluminescence as well as in thin film photoluminescence. In field effect transistors, they also improve charge carrier mobility by an order of magnitude despite disrupting π-conjugation. This use of carborane-containing conjugated polymers in active devices holds promise as new responsive materials in electronic polymer applications.

Carborane photochemistry triggered by aryl substitution: carborane-based dyads with phenyl carbazoles

A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.