NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

An Unusually Small Singlet–Triplet Gap in a Quinoidal 1,6‐Methano[10]annulene Resulting from Baird’s 4n π‐Electron Triplet Stabilization

Within the continuum of π‐extended quinoidal electronic structures exist molecules that by design can support open‐shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open‐shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open‐shell structure is accompanied by structural reorganization from a contorted Möbius aromatic‐like shape in S₀ to a more planar shape in the Hückel aromatic‐like T₁. This stability was attributed to Baird’s Rule which dictates the aromaticity of 4n π‐electron triplet excited states.     

Cyclobis(7,8‐(para‐quinodimethane)‐4,4′‐triphenylamine) and Its Cationic Species Showing Annulene‐Like Global (Anti)Aromaticity

π‐Conjugated macrocycles containing all‐benzenoid rings usually show local aromaticity, but reported herein is the macrocycle CBQT , containing alternating para‐quinodimethane and triphenylamine units displaying annulene‐like anti‐aromaticity at low temperatures as a result of structural rigidity and participation of the bridging nitrogen atoms in π‐conjugation. It was easily synthesized by intermolecular Friedel–Crafts alkylation followed by oxidative dehydrogenation. X‐ray crystallographic structures of CBQT , as well as those of its dication, trication, and tetracation were obtained. The dication and tetracation exhibited global aromaticity and antiaromaticity, respectively, as confirmed by NMR measurements and theoretical calculations. Both the dication and tetracation possess open‐shell singlet ground states, with a small singlet–triplet gap.     

Azulene‐1‐azo‐2′‐thiazoles. Synthesis and properties

Several derivatives belonging to a new compound class, namely azulene‐1‐azo‐2′‐thiazoles, were prepared by the diazotization of 2‐aminothiazoles in the presence of HNO₃/H₃PO₄ followed by the coupling of diazonium salts with azulenes in buffered medium. The reactions proved to be general for this class, the yields are, however, considerably influenced by the substituents at thiazole moiety. For the first time a N‐oxide provided from an amino substituted five‐member nitrogenous heterocycle was diazotized and coupled. The structure of the obtained compounds was assigned and their physico‐chemical properties were discussed. The new azulene azo derivatives exhibit a strong bathochromic shift in UV‐Vis due to the intense push‐pull effect of aromatic system and to the intrinsic properties of thiazole moiety.     

Substituent effects in trans‐p,p′‐disubstituted azobenzenes: X‐ray structures at 100 K and DFT‐calculated structures

The crystal and molecular structures of two para‐substituted azobenzenes with π‐electron‐donating –NEt₂ and π‐electron‐withdrawing –COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the –NEt₂ group in N,N,N′,N′‐tetraethyl‐4,4′‐(diazenediyl)dianiline, C₂₀H₂₈N₄, (I), may be caused by steric hindrance and the π‐electron‐donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp²‐hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4′‐(diazenediyl)dibenzoate, C₁₈H₁₈N₂O₄, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N—C—C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).     

Photo‐physical Properties of N‐methyl‐3,4‐fulleropyrrolidine and Its Derivatives: A DFT and TD‐DFT Investigation

A series of N‐methyl‐3,4‐fulleropyrrolidine (NMFP) derivatives were designed by selecting different π‐conjugated linkers and electron‐donating groups as D‐π‐A and D‐A systems. The optimised structures and photo‐physical properties of NMFP and its derivatives have been determined using density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods with the B3LYP functional and the 6‐31G basis set. According to the computation analysis, both the π‐conjugated linkers and the electron‐donating groups can influence the electronic and photo‐physical properties of the NMFP derivatives. Our calculated results demonstrated that the electron‐donating groups, with significant electron‐donating ability, had the tendency to increase the highest occupied molecular orbital (HOMO) energy. The π‐conjugated linkers with lower resonance energy decreased the lowest occupied molecular orbital (LUMO) energy and caused a significant decrease in the energy gap (E_g) between the E_HOMO and E_LUMO. A Natural Bond Orbital (NBO) analysis examines the effect of the electron‐donating group, π conjugated linker, and electron‐withdrawing group for these NMFP derivatives. For the NMFP derivatives, a projected density of state (PDOS) analysis demonstrated that the electron density of HOMO and LUMO are concentrated on the electron‐donating group and the π‐conjugated linker, respectively. A TD‐DFT/B3LYP calculation was performed to calculate the electronic absorption spectra of these NMFP derivatives. Both the electron‐donating group and the π‐conjugated linker contribute to the major absorption peaks, which are assigned as HOMO to LUMO transitions and are red‐shifted relative to those of non‐substituted NMFP.     

A DFT study on 1,4‐dihydro‐1,4‐azaborinine annulated linear polyacenes: Absorption spectra,singlet‐triplet energy gap,aromaticity, and HOMO–LUMO energy modulation

Linear polyacene (LPA) mimics containing multiple heterocycles have been computationally designed by annulating 1,4‐dihydro‐1,4‐azaborinine moieties to benzene (aB₁–aB₅), naphthalene (aN₁–aN₅), anthracene (aA₁–aA₅), and tetracene (aT₁–aT₅) cores. DFT studies conducted on them using M06L/6‐311++G(d,p) method reveal a perfect planar structure for all and suggest the utilization of nitrogen lone pairs for aromatic π‐electron delocalization. The computed values of aromaticity indices such as HOMA, NICS, and dehydrogenation energy (E _dh) of heterocycles support strong aromatic character for each six‐membered ring in the LPA mimics. On the basis of the minimum value of the molecular electrostatic potential (V _min) observed on each LPA unit in the LPA mimics, the extended delocalization of π‐electrons is verified. The energetic parameter E _dh showed strong linear correlation with HOMA, NICS and V _min parameters, which strongly supports the multidimensional character of aromaticity in LPA mimics. The electronic property modification is shown by the theoretical absorption spectra data and singlet‐triplet energy gap (ΔE _ST). The bandgap and ΔE _ST tunings are achieved for LPA mimics by selecting appropriate number of azaborinine type units and the size of LPA core used for annulation. © 2017 Wiley Periodicals, Inc.     

σ‐Aromaticity in an Unsaturated Ring: Osmapentalene Derivatives Containing a Metallacyclopropene Unit

In general, aromaticity can be clarified as π‐ and σ‐aromaticity according to the type of electrons with major contributions. The traditional π‐aromaticity generally describes the π‐conjugation in fully unsaturated rings whereas σ‐aromaticity may stabilize fully saturated rings with delocalization caused by σ‐electron conjugation. Reported herein is an example of σ‐aromaticity in an unsaturated three‐membered ring (3 MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3 MR in cyclopropaosmapentalene is cleaved by ethane through two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0 kcal mol^?1). These positive values are in sharp contrast to the expected exothermicity, thus indicating aromaticity in the 3 MR. Further nucleus‐independent chemical shift and anisotropy of the current‐induced density calculations reveal the nature of σ‐aromaticity in the unsaturated 3 MR.     

σ Aromaticity Dominates in the Unsaturated Three‐Membered Ring of Cyclopropametallapentalenes from Groups 7–9: A DFT Study

Aromaticity, an old but still fantastic topic, has long attracted considerable interest of chemists. Generally, π aromaticity is described by π‐electron delocalization in closed circuits of unsaturated compounds whereas σ‐electron delocalization in saturated rings leads to σ aromaticity. Interestingly, our recent study shows that σ aromaticity can be dominating in an unsaturated three‐membered ring (3MR) of cyclopropaosmapentalene. An interesting question is raised: Can the σ aromaticity, which is dominant in the unsaturated 3MR, be extended to other cyclopropametallapentalenes? If so, how could the metal centers, ligands, and substituents affect the σ aromaticity? Here, we report a thorough theoretical study on these issues. The nucleus‐independent chemical shift calculations and the anisotropy of the current‐induced density plots reveal the dominant σ aromaticity in these unsaturated 3MRs. In addition, our calculations show that substituents on the 3MRs have significant effects on the σ aromaticity, whereas the ligand effect is particularly small.     

Germacyclobutenes: Generation by 1,1‐Carbalumination or 1,1‐Carbagallation and Their Photophysical Properties

Aluminium‐ and gallium‐functionalised alkenylalkynylgermanes, R¹₂Ge(C?C?R²)[C{E(CMe₃)₂}?C(H)?R²] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α‐C atoms of the intact ethynyl groups. These interactions activate the Ge?C(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the E?C(vinyl) bonds by means of 1,1‐carbalumination or 1,1‐carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes. Germacyclobutenes with an unsaturated GeC₃ heterocycle and endo‐ and exocyclic C?C bonds resulted from concomitant Ge?C bond formation to the β‐C atoms of the alkynyl groups. These heterocyclic compounds show an interesting photoluminescence behaviour with Stokes shifts of >110 nm. The fascinating properties are based on extended π‐delocalisation including σ*‐orbitals localised at Ge and Al. High‐level quantum chemical DFT and TD‐DFT calculations for an Al compound were applied to elucidate their absorption and emission properties. They revealed a biradical excited state with the transfer of a π‐electron into the empty p‐orbital at Al and a pyramidalisation of the metal atom.     

Two‐Photon‐Induced Fluorescence in New π‐Expanded Diketopyrrolopyrroles

Structurally unique π‐expanded diketopyrrolopyrroles (EDPP) were designed and synthesized. Strategic placement of a fluorene scaffold at the periphery of a diketopyrrolopyrrole through tandem Friedel–Crafts‐dehydration reactions resulted in dyes with supreme solubility. The structure of the dyes was confirmed by X‐ray crystallography verifying a nearly flattened arrangement of the ten fused rings. Despite the extended ring system, the dye still preserved good solubility and was further functionalized by using Pd‐catalyzed coupling reactions, such as the Buchwald–Hartwig amination. Photophysical studies of these new functional dyes revealed that they possess enhanced properties when compared with expanded DPPs in terms of two‐photon absorption cross‐section. It is further demonstrated that in addition to the initial diacetals, the final electrophilic cyclization step can also be applied to diketones. By placing two amine groups at peripheral positions of the resulting dyes, values of two‐photon absorption cross‐section on the level of 2000 GM around 1000 nm were achieved, which in combination with high fluorescence quantum yield (Φ_fl), generated a two‐photon brightness of approximately 1600 GM. These characteristics in combination with strong red emission (665 nm) make these new π‐expanded diketopyrrolopyrroles of major promise as two‐photon dyes for bioimaging applications. Finally, the corresponding N‐alkylated DPPs displayed a solid‐state fluorescence.     

Photomagnetic and nonlinear optical properties in cis‐trans green fluoroprotein chromophore coupled Bis‐imino nitroxide diradicals

The current study extends an earlier investigation (Bhattacharya, et al., Phys. Chem. Chem. Phys. 2012, 14, 6905) to further explore various photomagnetic and optical properties of bis‐imino nitroxide, that is, (IN)₂‐based green fluorescent protein (GFP) chromophore coupled diradicals revealing new significant features. The conversion mechanisms of selected trans‐isomers into their corresponding cis‐conformers are discussed in detailed using a number of recently‐developed density functional theory (DFT) functionals based on the Minnesota suite of DFT‐models as well as using some other DFT functionals developed earlier. To provide a more in‐depth analysis of variations in magnetic properties as trans‐conformers (singlet ground‐state) convert into their cis‐analogues (triplet ground‐state), the changes in exchange magnetic coupling constants J are compared with the variation of the selected aromaticity indices. The aromaticity indices include the nuclear independent chemical shift [NICS(0)] values calculated at the center of ring structures and the harmonic oscillator model of aromaticity. Furthermore, the investigation of static nonlinear optical response properties in the (IN)₂‐based GFP chromophore coupled diradicals reveal unusually large static first hyperpolarizabilities for these systems which is highly significant for practical applications in optics and optoelectronics. © 2015 Wiley Periodicals, Inc.     

Formation of 6,10‐Diphenyl‐Substituted Heptalene‐4,5‐dicarboxylates

It is shown that 4,8‐diphenylazulene ( 1 ) can be easily prepared from azulene by two consecutive phenylation reactions with PhLi, followed by dehydrogenation with chloranil. Similarly, a Me group can subsequently be introduced with MeLi at C(6) of 1 (Scheme 2). This methylation led not only to the expected main product, azulene 2 , but also to small amounts of product 3 , the structure of which has been determined by X‐ray crystal‐structure analysis (cf. Fig. 1). As expected, the latter product reacts with chloranil at 40° in Et₂O to give 2 in quantitative yields. Vilsmeier formylation of 1 and 2 led to the formation of the corresponding azulene‐1‐carbaldehydes 4 and 5 . Reduction of 4 and 5 with NaBH₄/BF₃ ? OEt₂ in diglyme/Et₂O 1 : 1 and BF₃ ? OEt₂, gave the 1‐methylazulenes 6 and 7 , respectively. In the same way was azulene 9 available from 6 via Vilsmeier formylation, followed by reduction of azulene‐1‐carbaldehyde 8 (Scheme 3). The thermal reactions of azulenes 1, 6 , and 7 with excess dimethyl acetylenedicarboxylate (ADM) in MeCN at 100° during 72 h afforded the corresponding heptalene‐4,5‐dicarboxylates 11, 12 , and 13 , respectively (Scheme 4). On the other hand, the highly substituted azulene 9 gave hardly any heptalene‐4,5‐dicarboxylate.     

Protic Ionic Liquid Promoted One Pot Synthesis of 2‐amino‐4‐(phenyl)‐7‐methyl‐5‐oxo‐4H,5H‐pyrano[4,3‐b]pyran‐3‐carbonitrile Derivatives in Water and Their Antimycobacterial Activity

One pot three component reaction of 4‐hydroxy‐6‐methylpyran‐2‐one, 3‐methoxy benzaldehyde, and malononitrile in water using protic ionic liquid as a catalyst at room temperature afforded pyrano[4,3‐b]pyran derivatives. Protic ionic liquid has been proved to be an efficient and mild catalyst for the synthesis of pyrano[4,3‐b]pyran scaffolds due to their highly polar nature. The notable aspects of protic ionic liquid are easy availability, improved reaction rates, high product yields, simple workup procedure, recyclability, and reusability. Molecules docking studies have been performed on enzyme enoyl‐ACP‐reductase from Mycobacterium tuberculosis. The molecular docking simulation indicated plausible π‐alkyl and alkyl‐alkyl interactions between the amino acids and scaffolds. The synthesized derivatives have been evaluated for their in vitro antituberculotic activity against M. tuberculosis H₃₇RV strain using Microplate Alamar Blue Assay method. Together, biological activity data and docking data showed that the tested scaffolds exhibited excellent antituberculotic activity.     

Steric Control in the π‐Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[2.2.2]octene Units

A Two series of oligothiophenes 2 (nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a – c , annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π‐dimerization and the structure–property relationships of the π‐dimers of oligothiophene radical cations. Their radical‐cation salts were prepared through chemical one‐electron oxidation by using nitrosonium hexafluoroantimonate. From variable‐temperature electron spin resonance and electronic absorption measurements, the π‐dimerization capability was found to vary among the members of the 2 (nT)^{+ .} SbF₆^? series and 3 ^{+ .} SbF₆^? series of compounds. To examine these results, density functional theory (DFT) calculations at the M06‐2X/6‐31G(d) level were conducted for the π‐dimers. This level of theory was found to successfully reproduce the previously reported X‐ray structure of ( 2 (3T))₂²⁺ having a bent π‐dimer structure with cis–cis conformations. The absorption bands obtained by time‐dependent DFT calculations for the π‐dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π‐dimerization were divided into four factors: 1) SOMO–SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain‐length dependence are discussed in detail.     

Pyridyl‐Functionalised 3H‐1,2,3,4‐Triazaphospholes: Synthesis,Coordination Chemistry and Photophysical Properties of Low‐Coordinate Phosphorus Compounds

Novel conjugated, pyridyl‐functionalised triazaphospholes with either tBu or SiMe₃ substituents at the 5‐position of the N₃PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole‐based systems. Photoexcitation of the 2‐pyridyl‐substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all‐nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2‐pyridyl substituted systems have a more rigid and planar structure than their 3‐ and 4‐pyridyl isomers. Time‐dependent (TD) DFT calculations show that only the 2‐pyridyl‐substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N‐hybrid ligand forms a Re^I complex of the type [(N^N)Re(CO)₃Br] through the coordination of nitrogen atom N² to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π‐accepting character of the triazaphosphole, which is again in contrast to that of the all‐nitrogen‐containing triazoles. The synthesis and photophysical properties of a new class of phosphorus‐containing extended π systems are described.     

Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole‐based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl‐substituted 2,2′‐biimidazole using low‐valent titanium, followed by treatment with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone. Imidacene displays a porphyrin‐like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π‐electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high‐level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π‐electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π‐electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π‐ and 22 π‐electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π‐electron five‐membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.     

Redox‐Active Peptide‐Functionalized Quinquethiophene‐Based Electrochromic π‐Gel

An electrochromic system based on a self‐assembled dipeptide‐appended redox‐active quinquethiophene π‐gel is reported. The designed peptide‐quinquethiophene consists of a symmetric bolaamphiphile that has two segments: a redox‐active π‐conjugated quinquethiophene core for electrochromism, and peptide motif for the involvement of molecular self‐assembly. Investigations reveal that self‐assembly and electrochromic properties of the π‐gel are strongly dependent on the relative orientation of peptidic and quinquethiophene scaffolds in the self‐assembly system. The colors of the π‐gel film are very stable with fast and controlled switching speed at room temperature.