Noncovalent Intermolecular Interaction in Cofacially Stacked 24π Antiaromatic Hexaphyrin Dimer.

π-π Stacking is omnipresent not only in nature but in a wide variety of practical fields applied to our lives. Because of its importance in a performance of natural and artificial systems, such as light harvesting system and working layer in device, many researchers have put intensive effort into identifying its underlying nature. However, for the case of π-π stacked systems composed of antiaromatic units, the understanding of the fundamental mechanisms is still unclear. Herein, we synthesized a new type of planar ß,ß'-phenylene-bridged hexaphyrin (1.0.1.0.1.0), referred as naphthorosarin which possesses the 24π-electron conjugated pathway. Especially, the corresponding antiaromatic porphyrinoid shows the unique property to form dimeric species adopting the face-to-face geometry which is unprecedented in cases of known annulated naphthorosarins. In order to elucidate the intriguing properties derived from the stacked dimer, the current study focuses on the experimental support to rationalize the observed π-π interactions between the two subunits.

Trapping of Stable [4n+1] π-Electron Species from Peripherally Substituted, Conformationally Rigid, Antiaromatic Hexaphyrins.

Peripherally substituted antiaromatic naphthorosarins have been synthesized for the first time. The synthesis was accomplished by acid-catalyzed condensation of naphthobipyrrole building blocks with aromatic aldehydes. The naphthobipyrrole building blocks were synthesized by simple oxidative coupling of the corresponding pyrrole substituted aromatics. Solid-state structural analyses of the synthesized naphthorosarins revealed that the presence of meso-2,6-dichlorophenyl- and 5,6-difluoro-substitution substantially alter the geometry and properties of the naphthorosarins. The substituents affect the redox potentials as well and, in turn, the proton-coupled electron-transfer processes leading to the formation of one- and two-electron reduced forms of the corresponding naphthorosarins. One particular naphthorosarin that bears both peripheral fluorine and meso-2,6-dichlorophenyl substituents forms a stable 25 π-electron species upon treating with TFA that was characterized by single-crystal X-ray diffraction analysis. The current study underscores how structural modifications can be used to fine-tune the electronic features of naphthorosarins, including stabilization of odd electron species.

Vertically-expanded imidazo[1,2-a]pyridines and imidazo[1,5-a]pyridine via dehydrogenative coupling.

The anion-radical coupling of structurally diverse series of aromatic compounds possessing biaryl linkages led to seven fused, polycyclic heterocycles in reasonable yields. The yield of the key step (K, toluene, O2) depends on both electronic and steric factors. The whole strategy consists of just two steps starting from an unsubstituted imidazo[1,2-a]pyridine, giving target compounds in an overall yield of 4-34%. The same strategy also works for derivatives of imidazo[1,5-a]pyridine. A new process has been discovered for such vertically-expanded imidazo[1,2-a]pyridines, consisting of a sequential Diels-Alder reaction followed by a retro-Diels-Alder reaction. The optical properties of the library of π-expanded imidazo[1,2-a]pyridines were for the first time fully characterized, showing that fluorescence quantum yields are significantly lower than for the singly-linked compounds.

Why vertically π-expanded imidazo[1,2-a]pyridines are weak fluorescence emitters: experimental and computational studies.

Photophysics of π-expanded analogs of imidazo[1,2-a]pyridine, a well-known heterocyclic compound, has been experimentally and theoretically studied. Two regioisomeric systems differing only in the arrangement of the benzene ring have shown different optical properties (electronic transition energies, fluorescence quantum yields and decay times). DFT calculations have been in agreement with experimental results. Low fluorescence quantum yields have been attributed to the S1→ T2 intersystem crossing channel. Additional computations predict architectures which should possess higher fluorescence quantum yields.

Cyclic diarylheptanoids from Myrica javanica: isolation, transformation, and bioactivity evaluation.

Three cyclic diarylheptanoids, myricanol (1), myricanone (2), and porson (3), were isolated from Myrica javanica (Myricaceae). As a major component, myricanol (1) was obtained from dry powdered bark and twigs (up to 1.6%). Transformation of myricanol (1) afforded 5-prenylmyricanol (4) and 5-benzylmyricanol (5) in 84.5% and 65% yields, respectively. The bioactivities of the isolated cyclic diarylheptanoids and their derivatives were investigated to determine their cytotoxicity and DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging activities. The cytotoxicity assay against murine leukaemia P-388 cells demonstrated that compounds 4 and 5 showed an almost two-fold increase in the activity of their parent molecule (1), with an IC50 value of 12 µM. Furthermore, the free radical scavenging assay showed that myricanol (1) had the highest radical scavenging activity, revealing the importance of the free phenolic group (IC50 39.3 µM).

Cinnamoyl Dipyrromethenes as Fluorescence Zinc(II) Ion Sensor.

Friedel-Crafts acylation of dipyrromethane with cinnamoyl chloride was conducted to obtain dicinnamoyl dipyrromethane compounds 3 and 4. Both compounds were subsequently oxidized by DDQ to produce dicinnamoyl dipyrromethene ligands (DC-1 and DC-2). A large bathochromic shift compared to dipyromethene (D) was observed at 95 nm for DC-1 and 67 nm for DC-2. Both compounds showed remarkable chelation-enhanced fluorescence (CHEF) upon addition of zinc(II) ions. Similar to the quadrupolar system, DC-1 exhibited absorption and emission near the optical windows of the tissue. However, asymmetrical DC-2 had a better "turn-on" CHEF, with a fluorescence intensity that was 22 times higher than that of compound D. The DC-2 ligand also showed a limit of detection (LOD) of up to 3.0×10-8  M and selectivity toward zinc(II) ions compared to alkali and alkaline earth metal ions.

Antimicrobial activities of fungus comb extracts isolated from Indomalayan termite (Macrotermes gilvus Hagen) mound.

Incorporating antimicrobial components into food packaging materials can prevent microbial contamination. Fungus combs could be an alternative source of natural antimicrobial agents. In this study, n-hexane, ethyl acetate, methanol, and water extracts were obtained from fungus combs isolated from Indomalayan termite (Macrotermes gilvus Hagen) mound. Their antibacterial and antifungal activities against food spoilage microorganisms including Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 25923, Aspergillus flavus, and Aspergillus niger were evaluated by Kirby-Bauer disc diffusion and microdilution. Results showed that ethyl acetate extract formed the largest diameter inhibition zone for all tested bacteria and fungi, exhibited antibacterial activity against all tested bacteria with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of 0.39 and 0.78 mg/mL, respectively, and suppressed A. flavus and A. niger with an MIC value of 0.78 mg/mL. This extract contained guaiacol and syringol, which were predicted as the main antimicrobial components in fungus comb. n-Hexane extract only inhibited Gram-positive bacteria. S. aureus ATCC 25923 was the most sensitive to all the extracts, and A. flavus was more sensitive than A. niger. All these fungus comb extracts exhibited antimicrobial activity against E. coli ATCC 25922, P. aeruginosa ATCC 27853, S. aureus ATCC 25923, A. flavus, and A. niger. This study revealed that fungus comb extracts, especially ethyl acetate, could be considered as a new antimicrobial agent.

Unexpected halide anion binding modes in meso-bis-ethynyl picket-calix[4]pyrroles: effects of meso-π (ethynyl) extension.

meso-Ethynyl extended aryl-picket calix[4]pyrroles 2 and 3 are designed and synthesized by directly anchoring arylethynyl groups at diametrically opposed meso-positions. The critical roles of direct ethynyl linkers are manifested through the isolation of unexpected host-anion conformers of meso-arylethynyl calix[4]pyrroles and a significant enhancement in halide binding affinities.

Double head-to-tail direct arylation as a viable strategy towards the synthesis of the aza-analog of dihydrocyclopenta[hi]aceanthrylene--an intriguing antiaromatic heterocycle.

The first case of double head-to-tail direct arylation of aromatic compounds and the unusual photophysical properties of the resulting 2,2a(1,5)b(1,7)-tetraazacyclopenta[hi]aceanthrylene are reported. This molecule, comprising of two imidazo[1,2-a]pyridine units, is antiaromatic due to the changes in the efficiency of π-electron ring current and it belongs to a class of seldom encountered compounds with a dark lowest electronically excited singlet state.

Vertically π-expanded imidazo[1,2-a]pyridine: the missing link of the puzzle.

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpol'skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.

Bright, fluorescent dyes based on imidazo[1,2-a]pyridines that are capable of two-photon absorption.

A library of imidazo[1,2-a]pyridines was synthesized by using the Gevorgyan method and their linear and non-linear optical properties were studied. Derivatives that contained both electron-donating and electron-withdrawing groups at the 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2-0.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron-donating substituents improved the luminescence performance of these compounds, whereas electron-withdrawing substituents led to a more erratic behavior. Substitution on the six-membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2-a]pyridine units on its periphery and a 1,4-dicyanobenzene unit at its center. This system exhibited a large Stokes-shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge-transfer character. This quadrupolar feature also led to an acceptable two-photon absorption response in the NIR region.