An oxacalix[2]arene[2]pyrimidine-bis(Zn-porphyrin) tweezer as a selective receptor towards fullerene C70

Photophysical Properties of Naphthalene-oxacalix[m]arene and Recognition of Fullerene C60

Three different pore sizes of oxacalix[m]arene[n]pyrimidines modified with a naphthalene substituent were synthesized and characterized by HRMS, ¹H NMR, and single-crystal analysis (8OA and 8OA-N). Steady-state spectroscopy indicates these naphthalene-oxacalix[m]arenes exhibit good fluorescence properties, which isattributed to the locally excited (LE) state emission, and electrochemical results show that the photoinduced electron transfer (PET) process occurs from the naphthalene substituent to the linked pyrimidine. Nanosecond transient absorption spectra, singlet oxygen quantum yields (Φ_Δ4OA-N = 45.1%, Φ_Δ6OA-N = 56.6%, and Φ_Δ8OA-N = 65.7%) and theoretical calculations demonstrate that the torsion angle between the donor (naphthalene) and the acceptor (pyrimidine) promotes intersystem crossing (ISC), and the lifetime of the triplet state reaches ca. 8 ms. Interestingly, all three host molecules (4OA-N, 6OA-N, and 8OA-N) showed a high affinity for fullerene C₆₀, and significant binding constants in the range of 4.10-6.68 × 10⁴ M^-1 were obtained by fluorescence titration; in contrast, previous reports indicated that the similar oxacalix[m]arene[n]pyrimidine scaffold could not efficiently complex with C₆₀. In the frontier molecular orbital theory calculations of the supramolecular system of 4OA-N@C ₆₀ , the HOMO is distributed on 4OA-N and the LUMO is localized on fullerene. The calculation results further demonstrated that there are strong interactions between the host and the fullerene guest, which is consistent with the result of the experiments. The characteristic photophysical properties of these novel naphthyl-decorated oxacalix[m]arene[n]pyrimidines broaden their application field, and the stable host-guest system with fullerene can be applied to supramolecular chemistry.

Macrocyclic Receptors for Identification and Selective Binding of Substrates of Different Nature

Molecular recognition of host/guest molecules represents the basis of many biological processes and phenomena. Enzymatic catalysis and inhibition, immunological response, reproduction of genetic information, biological regulatory functions, the effects of drugs, and ion transfer-all these processes include the stage of structure recognition during complexation. The goal of this review is to solicit and publish the latest advances in the design and sensing and binding abilities of porphyrin-based heterotopic receptors with well-defined geometries, the recognition ability of which is realized due to ionic, H-bridge, charge transfer, hydrophobic, and hydrophilic interactions. The dissection of the considered low-energy processes at the molecular scale expands our capabilities in the development of effective systems for controlled recognition, selective delivery, and prolonged release of substrates of different natures (including drugs) to their sites of functioning.

Synthesis and Structure of Functionalized Homo Heteracalix[2]arene[2]triazines: Effect of All Heteroatom Bridges on Macrocyclic Conformation

A number of unprecedented homo heteracalix[2]arene[2]triazines were synthesized by means of a fragment coupling approach. Two directional nucleophilic substitution reactions of N-Boc-protected 1,3-dihydrazobenzene with cyanuric acid chloride and 2-butoxy-4,6-dichloro-1,3,5-triazine led to hydrazo-linked trimers, which underwent an efficient macrocyclic condensation reaction with functionalized resorcinol derivatives to afford (NHNBoc)₂,O₂-calix[2]arene[2]triazine macrocycles, which contain a functional group either on the upper rim or the lower rim. The use of 1,3-phenylenediamines instead of resorcinol in the reaction produced (NR)₂,(NHNBoc)₂-calix[2]arene[2]triazines. Postmacrocyclization modifications such as a nucleophilic substitution reaction of chloro on triazine by amines and the removal of Boc from hydrazo moieties produced homo calix[2]arene[2]triazine derivatives. In the solid state, (NHNR)₂,O₂-bridged calix[2]arene[2]triazines with and without a substituent on the upper rim position and (NMe)₂,(NHNBoc)₂-calix[2]arene[2]triazine adopted a typical partial cone conformation while the heavily twisted 1,3-alternate conformational structures were observed for both (NHNBoc)₂,O₂-calix[2]arene[2]triazines bearing a functional group on the lower rim position and (NH)₂,(NHNBoc)₂-calix[2]arene[2]triazine. In solution, all synthesized homo heteracalix[2]arene[2]triazines existed as the mixture of different macrocyclic conformers, which underwent slow interconversions at room temperature relative to the NMR time scale.

Synthesis and conformational structure of hydrazo-bridged homo calix[2]pyridine[2]triazines

Presented in this paper are the practical synthesis and conformational structure of hydrazo-bridged homo calix[4]arenes.

A bis-corannulene based molecular tweezer with highly sensitive and selective complexation of C70 over C60

A corannulene-based tweezer-like receptor was conveniently synthesized, which showed highly sensitive and selective complexation of C₇₀ over C₆₀.

Complexation between a triptycene-derived oxacalixarene and π-extended viologens: linker-length-dependent orientation of the macrocycles in pseudo[3]rotaxanes

With an expanded electron-rich cavity and a fixed conformation, macrocycle H was found to encapsulate π-extended viologens G1-G4 to form the first case of pseudo[3]rotaxanes based on oxacalixarenes. The complexation was investigated in detail both in solution and in the solid state using NMR spectroscopy and X-ray crystallography. Due to the three-dimensional nonsymmetric structure of H, three orientational isomers of the pseudorotaxanes could be expected theoretically. However, as the crystal structure analysis revealed, only one of the three isomers was obtained with either G1 or G3. Moreover, with regard to the different lengths of the linkers in the guest molecules, completely opposite orientations of the macrocycles on the axles were observed, which could be explained by different complexation modes between the components in the pseudo[3]rotaxanes. Additionally, the complexation between the host and the guest could be reversibly switched on and off using a suitable acid and base. These results will provide us with the opportunity to design and elaborately regulate high-order molecular devices.

Selective Formylation of Azacalixpyridine Macrocycles and Their Transformation to Molecular Semicages

The aromatic electrophilic formylation reaction of azacalix[2]arene[2]pyridine and azacalix[4]pyridine were systematically studied. By simply controlling the ratio of reactants and the reaction temperature, the Vilsmeier-Haack reaction selectively afforded mono-, di-, and tetra-formylated azacalix[2]arene[2]pyridines and azacalix[4]pyridines. The preferential and selective functionalization reactions of macrocycles were discussed in terms of their conformational structure and conjugation effect between aromatic subunits and bridging nitrogen atoms. All resulting functionalized azacalix[2]arene[2]pyridines and azacalix[4]pyridines adopted a 1,3-alternate conformation both in the crystalline state and in solution. Taking advantage of the close proximity of aldehyde groups in 1,3-alternate di- and tetra-formylated azacalixpyridine macrocycles, the McMurry reductive coupling reaction of carbonyls was accomplished to yield unique semicage molecules.

Altering synthetic fragments to tune the AIE properties and self-assemble grid-like structures of TPE-based oxacalixarenes

The AIE properties and self-assemble structures in the solid state of TPE-based oxacalixarenes can be tuned effectively by utilization different building blocks in synthetic process.

Porphyrin-based sensor nanoarchitectonics in diverse physical detection modes

Porphyrins and related families of molecules are important organic modules as has been reflected in the award of the Nobel Prizes in Chemistry in 1915, 1930, 1961, 1962, 1965, and 1988 for work on porphyrin-related biological functionalities. The porphyrin core can be synthetically modified by introduction of various functional groups and other elements, allowing creation of numerous types of porphyrin derivatives. This feature makes porphyrins extremely useful molecules especially in combination with their other interesting photonic, electronic and magnetic properties, which in turn is reflected in their diverse signal input-output functionalities based on interactions with other molecules and external stimuli. Therefore, porphyrins and related macrocycles play a preeminent role in sensing applications involving chromophores. In this review, we discuss recent developments in porphyrin-based sensing applications in conjunction with the new advanced concept of nanoarchitectonics, which creates functional nanostructures based on a profound understanding of mutual interactions between the individual nanostructures and their arbitrary arrangements. Following a brief explanation of the basics of porphyrin chemistry and physics, recent examples in the corresponding fields are discussed according to a classification based on physical modes of detection including optical detection (absorption/photoluminescence spectroscopy and energy and electron transfer processes), other spectral modes (circular dichroism, plasmon and nuclear magnetic resonance), electronic and electrochemical modes, and other sensing modes.

Spectroscopic and quantum chemical investigation on non-covalent interaction in chromophore appended fullerene complexes of calix[4]arene

The present paper describes the spectroscopic and theoretical insights on non-covalent interaction of a calix[4]arene molecule, namely, 4-iso-propyl-calix[4]arene (1) with chromophore appended fullerenes, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (2) and [6,6]-phenyl-C(71)- butyric acid methyl ester (3) in solvents having varying polarity, viz., toluene and benzonitrile. Absorption spectrophotometric studies reveal appreciable ground state interaction between fullerenes and 1. The most fascinating feature of the present study is that 1 binds very effectively with both 2 and 3 as obtained from binding constant (K) data of such complexes; i.e., K(2-1) and K(3-1) exhibit value of 4.53 × 10(5) dm(3) mol(-1) (7.95 × 10(5) dm(3) mol(-1)) and 13.35 × 10(5) dm(3) mol(-1) (27.62 × 10(5) dm(3) mol(-1)) in toluene (benzonitrile), respectively. The effect of solvent over the complexation between fullerenes and 1 is clearly observed from the trend in the K values. Estimation of solvent reorganization energy (R(S)) evokes that both 2-1 and 3-1 complexes are stabilized more in toluene compared to benzonitrile. Molecular mechanics force field (MMMF) calculations in vacuo evoke geometrical structures of the 2-1 and 3-1 complexes and reveal interesting feature regarding binding pattern of fullerenes toward 1 in terms of heat of formation value of the respective complexes.

Corrole NH tautomers: spectral features and individual protonation

Protonation of a free-base meso-pyrimidinyl-substituted AB(2)-corrole (H(3)AB(2)) in ethanol solution by stepwise addition of sulfuric acid has been studied in the temperature range from 293 to 333 K. The formation rate of protonated species was found to depend profoundly on the temperature at which the titration was undertaken. Two steps in the titration curve were identified at temperatures around 293-298 K, whereas one-step formation of protonated species was found to occur at temperatures above 308 K. The protonation product was the same in both cases, i.e., H(4)AB(2)(+) corrole, protonated at the macrocycle core nitrogen atoms. The two steps in the protonation kinetics at lower temperatures were attributed to protonation of individual tautomers of the free-base H(3)AB(2) corrole. To the best of our knowledge, this is the first well-illustrated (spectrophotometric) observation of individual properties of corrole NH tautomers in fluid solution. Concomitant increase in the NH tautomerization rate with increasing temperature is proposed to account for the one-step protonation. Evidences for the role of individual corrole NH tautomers in the protonation process as well as their optical features are discussed based on spectroscopic results and simulation data.

Molecular assemblies of porphyrins and macrocyclic receptors: recent developments in their synthesis and applications

Metalloporphyrins which form the core of many bioenzymes and natural light harvesting or electron transport systems, exhibit a variety of selective functional properties depending on the state and surroundings with which they exist in biological systems. The specificity and ease with which they function in each of their bio-functions appear to be largely governed by the nature and disposition of the protein globule around the porphyrin reaction center. Synthetic porphyrin frameworks confined within or around a pre-organized molecular entity like the protein network in natural systems have attracted considerable attraction, especially in the field of biomimetic reactions. At the same time a large number of macrocyclic oligomers such as calixarenes, resorcinarenes, spherands, cyclodextrins and crown ethers have been investigated in detail as efficient molecular receptors. These molecular receptors are synthetic host molecules with enclosed interiors, which are designed three dimensionally to ensure strong and precise molecular encapsulation/recognition. Due to their complex structures, enclosed guest molecules reside in an environment isolated from the outside and as a consequence, physical properties and chemical reactions specific to that environment in these guest species can be identified. The facile incorporation of such molecular receptors into the highly photoactive and catalytically efficient porphyrin framework allows for convenient design of useful molecular systems with unique structural and functional properties. Such systems have provided over the years attractive model systems for the study of various biological and chemical processes, and the design of new materials and molecular devices. This review focuses on the recent developments in the synthesis of porphyrin assemblies associated with cyclodextrins, calixarenes and resorcinarenes and their potential applications in the fields of molecular encapsulation/recognition, and chemical catalysis.

Bisporphyrin connected by pyrimidine: synthesis and photophysical properties

We report the synthesis of a series of zinc porphyrins conjugated with pyrimidine derivatives. V shaped porphyrin dimer with a pyrimidine central core exhibits a planar geometry according to DFT computational studies, presents an internal charge transfer upon excitation proved by typical solvatochromic behavior and moderate two-photon absorption properties. Such porphyrin conjugates can afford valuable advantages in applications of TPA materials such as in photodynamic therapy and imaging of biological entities.