Pyrene‐fused Acenes and Azaacenes: Synthesis and Applications

In this account, the synthesis and applications of pyrene‐fused acenes, as well as pyrene‐fused azaacenes, have been carefully reviewed. Moreover, the synthetic methods involving two key synthons (different lengths of dienes and ynes) have been included. Furthermore, the “clean reaction” strategy has been introduced for the preparation of pyrene‐fused acenes with a single terminal‐pyrene unit from tetracene to octacene, as well as for the synthesis of pyrene‐fused octatwistacenes and nonatwistacenes with double terminal‐pyrene units. Similarly, the synthons and the synthetic methods for pyrene‐fused azaacenes have also been summarized. The applications of pyrene‐fused acenes and pyrene‐fused azaacenes have been included.     

Crystalline Diradical Dianions of Pyrene‐Fused Azaacenes

Although diradicals and azaacenes have been greatly attractive in fundamental chemistry and functional materials, the isolable diradical dianions of azaacenes are still unknown. Herein, we describe the first isolation of pyrene‐fused azaacene diradical dianion salts [(18‐c‐6)K(THF)₂]⁺[(18‐c‐6)K]⁺? 1 ^2?.. and [(18‐c‐6)K(THF)]²⁺? 2 ^2?.. by reduction of the neutral pyrene‐fused azaacene derivatives 1 and 2 with excess potassium graphite in THF in the presence of 18‐crown‐6. Their electronic structures were investigated by various experiments, in conjunction with theoretical calculations. It was found that both dianions are open‐shell singlets in the ground state and their triplet states are thermally readily accessible owing to the small singlet–triplet energy gap. This work provides the first examples of crystalline diradical dianions of azaacenes with considerable diradical character.     

Versatile 2,7-substituted pyrene synthons for the synthesis of pyrene-fused azaacenes

A new synthetic route to prepare a series of versatile 2,7-substituted pyrene synthons for the synthesis of pyrene-fused azaacenes is described. By using such synthons, a library of eight pyrene-fused azacenes with different electronic structures and in most cases with enhanced solubility has been synthesized and characterized.     

n‐Type Azaacenes Containing B←N Units

We disclose a novel strategy to design n‐type acenes through the introduction of boron–nitrogen coordination bonds (B←N). We synthesized two azaacenes composed of two B←N units and six/eight linearly annelated rings. The B←N unit significantly perturbed the electronic structures of the azaacenes: Unique LUMOs delocalized over the entire acene skeletons and decreased aromaticity of the B←N‐adjacent rings. Most importantly, these B←N‐containing azaacenes exhibited low‐lying LUMO energy levels and high electron affinities, thus leading to n‐type character. The solution‐processed organic field‐effect transistor based on one such azaacene exhibited unipolar n‐type characteristics with an electron mobility of 0.21 cm² V^?1 s^?1.     

Synthesis of Triptycene‐Substituted Azapentacene and Azahexacene Derivatives

We describe the synthesis and characterization of novel iptycene‐substituted azaacenes by using either a classic condensation route (diamine plus ortho‐quinone) and/or a Pd‐catalyzed coupling of an aromatic diamine with an aromatic dihalide. The attachment of an iptycene unit leads to a significant blueshift (15 nm) in the UV/Vis spectra of these azaacenes. The iptycene unit stabilizes a hexaazahexacene with a λ_{max abs} of 833 nm. By employing 5,6‐diamino(benzothiadiazole) as a synthon for tetraaminobenzene, we could prepare the symmetrical bis‐triptycene‐substituted tetraazapentacene in high yields.     

Substituted Tetraaza‐ and Hexaazahexacenes and their N,N′‐Dihydro Derivatives: Syntheses,Properties, and Structures

The palladium‐catalyzed coupling of a substituted o‐diaminoanthracene and a substituted o‐diaminophenazine to substituted 2,3‐dichloroquinoxalines furnishes 10 differently substituted N,N′‐dihydrotetraaza‐ or ‐hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N′‐dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N′‐dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO₂. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single‐crystal X‐ray structures have been obtained for the parent tetraazahexacenes and their difluoro‐substituted derivatives. The di‐ and tetrachloro derivatives of the N,N′‐dihydrohexaazahexacene have also been structurally characterized.     

Large N‐Heteroacenes: New Tricks for Very Old Dogs?

Azaacenes have been known for a very long time, either as N,N′‐dihydro compounds or in their oxidized form as 4 n+2π systems, but only recently have processable and charcterizable derivatives been sought. In the last three years synthetic routes to large N‐heteroacenes have been developed. In particular, the Pd‐catalyzed coupling of aromatic diamines with activated aromatic dihalogenides has enabled simple access to numerous new azaacenes. Since 2010, azapentacene and stabile oligoazahexacene have been synthesized, as well as a symmetrical tetraazapentacene, which acts as an excellent electron‐transport material for thin‐film transistors.     

The Palladium Way to N‐Heteroacenes

Novel synthetic methodologies allow increasingly efficient access to known organic materials, as well as the preparation of otherwise inaccessible species. Pd‐catalyzed coupling of aromatic dihalides to ortho‐diaminoarenes furnishes embedded stable N,N′‐dihydropyrazines expediently and in often excellent yields. The embedded N,N′‐dihydropyrazines can then be oxidized by MnO₂ to give substituted azatetracenes, azapentacenes, azahexacenes, and azaheptacenes, which are soluble, processable, and stable. This powerful Pd‐catalyzed methodology allows the preparation of azaacenes, including diaza‐, tetraaza‐ and hexaazaacenes. In combination with a suitable Pd precursor, Buchwald‐type biarylphosphines have been shown to give excellent results. Activated dihalides such as 2,3‐dihaloquinoxalines are coupled easily under simplified conditions, whereas 2,3‐dibromoacenes require more stringent conditions and advanced catalyst precursors. Pd catalysts effect the assembly of azaacenes with otherwise difficult to obtain substitution patterns. High yields and flexibility make this method most attractive.     

Intramolecular energy transfer in pyrene-bodipy molecular dyads and triads

Molecules bearing a 4,4‐difluoro‐8‐(aryl)‐1,3,5,7‐tetramethyl‐2,6‐diethyl‐4‐bora‐3a,4a‐diaza‐s‐indacene (bodipy) core and 1‐pyrenyl‐1‐phenyl‐4‐(1‐ethynylpyrene), or 1‐phenyl‐4‐[1‐ethynyl‐(6‐ethynylpyrene)pyrene] units were constructed in a step‐by‐step procedure based on palladium(0)‐promoted cross‐coupling reactions with the required preconstructed modules. X‐ray structures of single crystals reveal a twisted arrangement of the two chromophores. In one case, an almost perfect orthogonal arrangement is found. These dyes are strongly luminescent in solution and display rich electrochemistry in which all redox processes of the bodipy and pyrene fragments are clearly resolved. The absorption spectra indicate that the bodipy and pyrene chromophores are spectrally isolated, thereby inducing a large “virtual” Stokes shift. The latter is realised by efficient transfer of intramolecular excitation energy by the Förster dipole–dipole mechanism. The rate of energy transfer depends on the structure of the dual‐dye system and decreases as the centre‐to‐centre separation increases. The energy transfer efficiency, however, exceeds 90 % in all cases. The linkage of two pyrene residues by an ethyne group leads to a decrease in the energy‐transfer efficiency, with the two polycycles acting as a single chromophore. The directly linked bodipy–pyrene dual dye binds to DNA and operates as an efficient solar concentrator when dispersed in plastic.     

Protease Probes that Enable Excimer Signaling upon Scission

Peptide‐based probes that fluoresce upon proteolytic cleavage are invaluable tools for monitoring protease activity. The read‐out of protease activity through pyrene excimer signaling would be a valuable asset because the large Stokes shift and the long lifetime of the excimer emission facilitate measurements in autofluorescent media such as blood serum. However, proteolytic cleavage abolishes rather than installs the proximity relationships required for excimer signaling. Herein, we introduce a new probe architecture to enable the switching on of pyrene excimer emission upon proteolytic scission. The method relies on hairpin‐structured peptide nucleic acid (PNA)/peptide hybrids with pyrene units and anthraquinone‐based quencher residues positioned in a zipper‐like arrangement within the PNA stem. The excimer hairpin peptide beacons afforded up to a 50‐fold enhancement of the pyrene excimer emission. Time‐resolved measurements allowed the detection of matrix metalloprotease 7 in human blood serum.     

Anthrathiadiazole Derivatives: Synthesis,Physical Properties and Two-photon Absorption

Anthrathiadiazole is a key synthon for the construction of large azaacenes, however, the attachment of different substituents onto the skeleton of anthrathiadiazole is difficult but highly desirable because it could be easy to enrich the structures of azaacenes. Here, it is demonstrated that anthrathiadiazole derivatives with −Br, −CN, and −OCH₃ groups could be easily constructed through a simple [4+2] cycloaddition reaction between a,a,a’,a’-tetrabromo-o-xylenes derivatives and benzo[c][1,2,5]thiadiazole-4,7-dione. The structures of the as-prepared compounds with different substituents were carefully characterized. Moreover, the basic physical properties of the as-prepared anthrathiadiazole derivatives were fully investigated, where the cyano-substituted derivative (BTH - CN) has the highest stability and the methoxy-substituted derivative ( BTH - OCH₃) is easy to be oxidized. Moreover, the two-photon absorption (TPA) characteristics of different anthrathiadiazoles are also studied by using the femtosecond Z-scan technique. The results show that the fused anthrathiadiazole skeletons possess large TPA cross-section values δ₂ in the range of 3000–5000 GM, where the nature, position and strength of the substituted groups have strong effect on these values.     

Preparation of polystyrene nanoparticles including fluorescent and hydrophobic materials by polymerization at phase inversion temperature

The polystyrene nanoparticles incorporating hydrophobic and fluorescent pyrene was successfully prepared by a direct polymerization at phase inversion temperature. The average diameter of the prepared polystyrene nanoparticles is around 30 nm or less: 22 nm. This preparation does not require any strong agitation, which is necessary for the usual mini‐emulsion polymerization, and does not need the large amount of surfactant as much as the usual microemulsion polymerization. The incorporation of pyrene in the prepared polystyrene particles was confirmed by fluorescence spectra. This developed method is superior to the addition of pyrene to polystyrene particles prepared, which causes large coagulum and gives polystyrene particles with lower weight fraction of pyrene. The developed method is expected to be applicable for preparation of polymer particles including various hydrophobic materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis,Physical Properties,and Anion Recognition of Two Novel Larger Azaacenes: Benzannelated Hexazaheptacene and Benzannelated N,N′‐Dihydrohexazaheptacene

Two novel larger azaacenes with six or ten N atoms in their backbones, benzannelated 9,11,13,22,24,26‐hexazatetrabenzo[a,c,l,n]heptacene ( HATBH , 1 ) and benzannelated 9,26‐dihydro‐9,11,13,22,24,26‐hexaza‐tetrapyrido[3,2‐a: 2′,3′‐c: 3′′,2′′‐l: 2′′′,3′′′‐n]heptacene ( DHATPH, 2 ), have been successfully synthesized in two steps. The theoretical band gaps estimated through DFT calculations for HATBH ( 1 ) and DHATPH ( 2 ) are 1.949 eV and 2.278 eV, which are close to the experimentally obtained optical band gaps (2.14 eV and 2.39 eV). Interestingly, HATBH ( 1 ) can act as efficient anion sensor for F^? and H₂PO₄^?, while DHATPH ( 2 ) selectively responds to F^? among the ten different anions used (F^?, Cl^?, Br^?, I^?, PF₆^?, HSO₄^?, NO₃^?, BF₄^?, AcO^?, and H₂PO₄^?). Our synthetic strategy could offer a promising and easy way to obtain even larger azaacenes.     

An Azaacene Derivative as Promising Electron‐Transport Layer for Inverted Perovskite Solar Cells

It is highly desirable to develop novel n‐type organic small molecules as an efficient electron‐transport layer (ETL) for the replacement of PCBM to obtain high‐performance metal‐oxide‐free, solution‐processed inverted perovskite solar cells (PSCs) because this type of solar cells with a low‐temperature and solution‐based process would make their fabrication more feasible and practical. In this research, the new azaacene QCAPZ has been synthesized and employed as non‐fullerene ETL material for inverted PSCs through a solution‐based process without the need for additional dopants or additives. The as‐fabricated inverted PSCs show a power conversion efficiency up to 10.26 %. Our results clearly suggest that larger azaacenes could be promising electron‐transport materials to achieve high‐performance solution‐processed inverted PSCs.     

[2](1,3)Adamantano[2](2,7)pyrenophane: A Hydrocarbon with a Large Dipole Moment

The fusion of the sp³‐hybridized parent diamondoid adamantane with the sp²‐hybridized pyrene results in a hybrid structure with a very large dipole moment which arises from bending the pyrene moiety. Presented herein is the synthesis, study of the electronic and optical properties, as well as the dynamic behavior of this new hydrocarbon.     

Tunable Crystallinity and Charge Transfer in Two‐Dimensional G‐Quadruplex Organic Frameworks

DNA G‐quadruplex structures were recently discovered to provide reliable scaffolding for two‐dimensional organic frameworks due to the strong hydrogen‐bonding ability of guanine. Herein, 2,7‐diaryl pyrene building blocks with high HOMO energies and large optical gaps are incorporated into G‐quadruplex organic frameworks. The adjustable substitution on the aryl groups provides an opportunity to elucidate the framework formation mechanism; molecular non‐planarity is found to be beneficial for restricting interlayer slippage, and the framework crystallinity is highest when intermolecular interaction and non‐planarity strike a fine balance. When guanine‐functionalized pyrenes are co‐crystallized with naphthalene diimide, charge‐transfer (CT) complexes are obtained. The photophysical properties of the pyrene‐only and CT frameworks are characterized by UV/Vis and steady‐state and time‐resolved photoluminescence spectroscopies, and by EPR spectroscopy for the CT complex frameworks.     

Quinoidal Azaacenes: 99 % Diradical Character

Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low‐energy gap ΔE_ST′ of 0.58 to 1.0 kcal mol^?1. The species are persistent in solution (half‐life≈14–21 h) and in the solid state they are stable for weeks.     

Competition between Arene–Perfluoroarene and Charge‐Transfer Interactions in Organic Light‐Harvesting Systems

Two typical types of luminescent organic cocrystals comprising pyrene–octafluoronaphthalene (pyrene–OFN) and pyrene–1,2,4,5‐tetracyanobezene (pyrene–TCNB) were developed by a simple supramolecular assembly strategy. The cocrystals exhibit distinct optical properties because of their different intermolecular interaction modes; that is, arene–perfluoroarene (AP) and charge‐transfer (CT) interactions. Unexpectedly, a pyrene–TCNB system with strong CT interactions was incorporated into a pyrene–OFN host as a robust guest to generate white‐light emission (WLE). In the supramolecular cocrystal system, an efficient energy‐transfer process from pyrene–OFN to pyrene–TCNB occurred because of the well‐matched spectra of the constituents and a desirable energy donor/acceptor (D/A) distance. The present competitive intermolecular interaction strategy could be applied to the fabrication of more complicated organic light‐harvesting systems.     

A Cobalt Pyrenylnitronylnitroxide Single‐Chain Magnet with High Coercivity and Record Blocking Temperature

Coordination of a [Co(hfac)₂] moiety (hfac=hexafluoroacetylacetonate) with a nitronylnitroxide radical linked to bulky, rigid pyrene (PyrNN) gives a helical 1:1 chain complex, in which both oxygen atoms of the radical NO^. groups are bonded to Co^II ions with strong antiferromagnetic exchange. The complex shows single‐chain magnet (SCM) behavior with frequency‐dependent magnetic susceptibility, field‐cooled and zero‐field‐cooled susceptibility divergence with a high blocking temperature of around 14 K (a record among SCMs), and hysteresis with a very large coercivity of 32 kOe at 8 K. The magnetic behavior is partly related to good chain isolation induced by the large pyrene units. Two magnetic relaxation processes have been observed, a slower one attributable to longer, and a faster one attributable to short chains. No evidence of magnetic ordering has been found.     

Cata-Annulated Azaacene Bisimides

Ultra-electron-deficient azaacenes were synthesized via Buchwald-Hartwig coupling of ortho-diaminoarenes with chlorinated mellophanic diimide followed by oxidation of the intermediate N,N’-dihydro compounds with MnO₂ or PbO₂. The resulting cata-annulated bisimide azaacenes have ultrahigh electron affinities with first reduction potentials as low as −0.35 V recorded for a tetraazapentacene. Attempts to prepare a tetrakis(dicarboximide)tetraazaheptacene resulted in the formation of a symmetric butterfly dimer.