Carlos

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Let′s not get together: Frustrated Lewis pairs (FLP) are combinations of Lewis acids and Lewis bases which are prevented from forming a stable adduct by steric or electronic hindrance. The growth of the field of FLP chemistry during the last four years has been rapid, including small-molecule activation, applications in catalysis, and the development of unprecedented reactions.

Heptalene, a nonaromatic, bicyclic 12 π-electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two-electron oxidized or reduced forms. The synthesis of thiophene-fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4, and its solid-state structure, which was confirmed by X-ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂5²⁻. The X-ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5. ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5²⁻, which was corroborated by nucleus-independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π-electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.

Evolution: Reductive transannular cyclization of an arene-fused bisdehydro[12]annulene granted access to a π-extended heptalene skeleton. The tetrathieno-fused heptalene exhibits a highly twisted, nonaromatic saddle structure. X-ray crystallography shows that chemical reduction transforms this nonplanar system into a more planar heptalene dianion with lesser degrees of bond alternation, reflecting high levels of Hückel aromaticity.

New bis- and tris(iminopyrrole)-functionalized linear (1,2-(HNC₄H₃-C(H)N)₂-C₆H₄ (2), 1,3-(HNC₄H₃-C(H)N)₂-C₆H₄ (3), 1,4-(HNC₄H₃-C(H)N)₂-C₆H₄ (4), 4,4′-(HNC₄H₃-C(H)N)₂-(C₆H₄-C₆H₄) (5), 1,5-(HNC₄H₃C-(H)N)₂-C₁₀H₆ (6), 2,6-(HNC₄H₃C-(H)N)₂-C₁₀H₆ (7), 2,6-(HNC₄H₃C-(H)N)₂-C₁₄H₈ (8)) and star-shaped (1,3,5-(HNC₄H₃-C(H)N-1,4-C₆H₄)₃-C₆H₃ (9)) π-conjugated molecules were synthesized by the condensation reactions of 2-formylpyrrole (1) with several aromatic di- and triamines. The corresponding linear diboron chelate complexes (Ph₂B[1,3-bis(iminopyrrolyl)-phenyl]BPh₂ (10), Ph₂B[1,4-bis(iminopyrrolyl)-phenyl]BPh₂ (11), Ph₂B[4,4′-bis(iminopyrrolyl)-biphenyl]BPh₂ (12), Ph₂B[1,5-bis(iminopyrrolyl)-naphthyl]BPh₂ (13), Ph₂B[2,6-bis(iminopyrrolyl)-naphthyl]BPh₂ (14), Ph₂B[2,6-bis(iminopyrrolyl)-anthracenyl]BPh₂ (15)) and the star-shaped triboron complex ([4′,4′′,4′′′-tris(iminopyrrolyl)-1,3,5-triphenylbenzene](BPh₂)₃ (16)) were obtained in moderate to good yields, by the treatment of 3–9 with B(C₆H₅)₃. The ligand precursors are non-emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π-conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time-dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13, in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples (11, 12, 14, and 15), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π-system, to afford green to yellow emissions. Organic light-emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m⁻², for single layer devices, increasing to 4400 cd m⁻² when a hole-transporting layer is used.

Bridge the gap: Luminescent linear and star-shaped polynuclear organoboron complexes containing iminopyrrolyl ligands linked by aryl bridges were synthesized, showing good luminescent properties depending on π-conjugation lengths of the molecules (see figure). Organic light-emitting diodes (OLEDs) were successfully fabricated with the new boron complexes, achieving luminance values of up to 4400 cd m⁻².

New BN-embedded, thiophene-fused, polycyclic aromatic compounds with planar geometry were designed and synthesized. The molecules showed excellent stability and chemical robustness. Postfunctionalization on this skeleton was demonstrated with a series of electrophilic bromination, palladium-catalyzed cross-coupling, and Knoevenagel condensation reactions. The π skeleton remained intact during these late-stage transformations. The optical and electronic properties have been well tuned through incorporation of electron-rich and -deficient groups on the backbone. This work shows the great advantage of the postfunctionalization strategy on BN-containing polycyclic aromatic compounds for fast diversification and materials screening.

Simple diversification: Postfunctionalization on a new planar BN-embedded thiophene-fused polycyclic aromatic compound was demonstrated with a series of electrophilic bromination, palladium-catalyzed cross-coupling, and Knoevenagel condensation reactions to provide opportunities for fast diversifications of BN-containing compounds and fine-tuning of their molecular properties (see scheme).

The synthesis, optoelectronic, and photovoltaic properties of novel acceptor–donor–acceptor (A–D–A) based π-conjugated functional molecules 1–3, comprising a planar S,N-heteropentacene as central donor substituted with various terminal acceptor units, such as 1,1-dicyanovinylene (DCV) and 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC), are reported. The structural variation of the end groups provides molecules 1–3 with gradually increased π-conjugation due to a rising number of double bonds, which comes from the DCC unit(s). From optoelectronic investigation, structure–property relationships are deduced and the novel A–D–A heteropentacenes 1–3 are implemented as photoactive donor component in solution-processed bulk heterojunction solar cells together with [6,6]-phenyl-C₆₁-butyric acid methyl ester as acceptor. The structural variation in the S,N-heteropentacenes leads to clear trends in the photovoltaic performance and power conversion efficiencies of up to 4.9% are achieved. Furthermore, due to extension of the double bonds a clear trade-off between the open circuit voltage (V _OC) and the short circuit current density (J _SC) values is observed. The role of additives on the optimization of the nanoscale morphology and device performance is investigated. The findings presented herein demonstrate that depending on the types of materials the additive may have significantly different effects on the active layer morphology and the device performance.

A new class of A–D–A-type molecular donor materials based on S,N-heteropentacene is developed for incorporation into solution-processed bulk heterojunction solar cells providing promising power conversion efficiencies of 3.1–4.9%. The extension of the π-system and increase in the highest occupied molecular orbital energy level provide direct correlation for the improved short circuit current (J _SC) and decreased open circuit voltage (V _OC) values.

In this work, two rigid, multiple tetraphenylethene (TPE)-substituted, π-conjugated, snowflake-shaped luminophores BT and BPT were facilely synthesized by using a 6-fold Suzuki coupling reaction. These molecules are constructed based on the nonplanar structure of propeller-shaped hexaphenylbenzene (HPB) or benzene as core groups and TPE as end groups. As a result, they reserve the intrinsic aggregation-induced emission (AIE) property of the TPE moiety. Meanwhile, both fluorescence quantum yield and piezochromic behavior in the solid state can be tuned or switched by inserting the phenyl bridges through changing the twisting conformation. The more extended structure BPT showed a much stronger AIE effect and higher Φ_F,f in the solid state in comparison with that of BT. Furthermore, an excellent optical waveguide application of these molecules was achieved. However, the revisable piezofluorochromic behavior has only appeared when BT was ground using a pestle and treated with solvent.

Snowflake-shaped luminophores: Two new rigid, multiple tetraphenylethene (TPE) substituted, π-conjugated, snowflake-shaped luminophores BT and BPT (see figure) were facilely synthesized and exhibited unique aggregation-induced emission (AIE) properties. Both fluorescence quantum yield and piezochromic behavior in the solid state can be tuned or switched by inserting the phenyl bridges through changing the twisting conformation.

A highly luminescent Zn₄L₆ tetrahedron is reported with 3.8 nm perylene bisimide edges and hexadentate Zn^II–imine chelate vertices. Replacing Fe^II and monoamines commonly utilized in subcomponent self-assembly with Zn^II and tris(2-aminoethyl)amine provides access to a metallosupramolecular host with the rare combination of structural integrity at concentrations <10⁻⁷ mol L⁻¹ and an exceptionally high fluorescence quantum yield of Φ_em=0.67. Encapsulation of multiple perylene or coronene guest molecules is accompanied by strong luminescence quenching. We anticipate this self-assembly strategy may be generalized to improve access to brightly fluorescent coordination cages tailored for host–guest light-harvesting, photocatalysis, and sensing.

Fe^II-free and fluorescent: A highly luminescent (Φ_em=0.67) Zn₄L₆ tetrahedron with perylene bisimide edges and hexadentate zinc–imine chelate vertices is accessed by dynamic imine–metal coordination self-assembly. Encapsulation of multiple perylene or coronene guest molecules is detected by quenching of the host emission. The chelate may be generalized to other dyes, improving access to brightly luminescent metallosupramolecular cages.

A novel white-light-emitting organic molecule, which consists of carbazolyl- and phenothiazinyl-substituted benzophenone (OPC) and exhibits aggregation-induced emission-delayed fluorescence (AIE-DF) and mechanofluorochromic properties was synthesized. The CIE color coordinates of OPC were directly measured with a non-doped powder, which presented white-emission coordinates (0.33, 0.33) at 244 K to 252 K and (0.35, 0.35) at 298 K. The asymmetric donor–acceptor–donor′ (D-A-D′) type of OPC exhibits an accurate inherited relationship from dicarbazolyl-substituted benzophenone (O2C, D-A-D) and diphenothiazinyl-substituted benzophenone (O2P, D′-A-D′). By purposefully selecting the two parent molecules, that is, O2C (blue) and O2P (yellow), the white-light emission of OPC can be achieved in a single molecule. This finding provides a feasible molecular strategy to design new AIE-DF white-light-emitting organic molecules.

Like fluorescent butterflies: A single white-light-emitting organic compound with aggregation-induced emission-delayed fluorescence (AIE-DF) was achieved by selection of the parent molecules. This novel design principle is a form of molecular inheritance of the tuning properties for a targeted molecule.

A novel nitrogen-doped corannulene derivative, 8-tert-butyl-6b²-azapentabenzo[bc,ef,hi,kl,no]corannulene, was synthesized by 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide with a diarylethyne and subsequent palladium-catalyzed intramolecular cyclization. This molecule represents the first example of a corannulene derivative bearing an internal heteroatom, and exhibits unique structural and physical properties caused by the introduction of the nitrogen atom and extended π-conjugation, as compared to the parent corannulene.

Bowled over: A novel benzene-fused azacorannulene (left) was synthesized by the 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide with a diarylethyne and subsequent palladium-catalyzed intramolecular cyclization. This molecule represents the first example of a heteroatom-doped corannulene derivative and in the solid state bowl-in-bowl columnar π-stacking is observed (right).

A discotic liquid-crystalline (LC) material, consisting of a planarized triphenylborane mesogen, was synthesized. X-ray diffraction analysis confirmed that this compound forms a hexagonal columnar LC phase with an interfacial distance of 3.57 Å between the discs. At ambient temperature, this boron-centered discotic liquid crystal exhibited ambipolar carrier transport properties with electron and hole mobility values of approximately 10⁻³ and 3×10⁻⁵ cm² V⁻¹ s⁻¹, respectively.

B planar, B aligned: A planarized triphenylborane, bearing three 3,4,5-tridodecyloxyphenyl groups, forms a hexagonal columnar liquid-crystalline phase at ambient temperature. It has a columnar π-stacked structure and ambipolar carrier-transport properties with hole- and electron-mobility values of 3×10⁻⁵ cm² V⁻¹ s⁻¹ and approximately 10⁻³ cm² V⁻¹ s⁻¹, respectively.