Effects of Axle‐Core,Macrocycle, and Side‐Station Structures on the Threading and Hydrolysis Processes of Imine‐Bridged Rotaxanes

Imine‐bridged rotaxanes are a new type of rotaxane in which the axle and macrocyclic ring are connected by imine bonds. We have previously reported that in imine‐bridged rotaxane 5 , the shuttling motion of the macrocycle could be controlled by changing the temperature. In this study, we investigated how the axle and macrocycle structures affect the construction of the imine‐bridged rotaxane as well as the dynamic equilibrium between imine‐bridged rotaxane 5 and [2]rotaxane 7 by using various combinations of axles ( 1 A , B ), macrocycles ( 2 a – e ), and side‐stations (XYL and TEG). In the threading process, the flexibility of the macrocycle and the substituent groups at the para position of the aniline moieties affect the preparation of the threaded imines. The size of the imine‐bridging station and the macrocyclic tether affects the hydrolysis of the imine bonds under acidic conditions.     

Recognizing Through‐Bond and Through‐Space Self‐Exchange Charge/Spin Transfer Pathways in Bis(triarylamine) Radical Cations with Similar Geometrical Arrangements

Radical cations of bis(triarylamine)s, 3 and 4 , in which the triarylamine redox centers are bridged by an ortho ‐phenylene and ortho ‐carborane cluster, respectively, have been prepared to elucidate the difference in intramolecular charge/spin‐transfer (ICT/IST) pathway owing to the two different bridging units affording similar geometrical arrangements between the redox centers. Electrochemistry, absorption spectroscopy, VT‐ESR spectroscopy, and DFT calculations reveal that 3 ^.+ and 4 ^.+ are classified into class II and class I mixed‐valence systems, respectively, and therefore, through‐bond and through‐space mechanisms are dominant for the ICT/IST phenomena in 3 ^.+ and 4 ^.+, respectively. Moreover, SQUID measurements for dicationic species provide the fact that virtually no spin‐exchange interaction is observed for spins in 4 ²⁺, while the antiferromagnetic interaction for spins in 3 ²⁺, in accordance with the existence of a conjugation pathway for the ortho ‐phenylene bridge.     

Controlling Two‐Step Multimode Switching of Dihydroazulene Photoswitches

We present the synthesis and switching studies of systems with two photochromic dihydroazulene (DHA) units connected by a phenylene bridge at either para or meta positions, which correspond to a linear or cross‐conjugated pathway between the photochromes. According to UV/Vis absorption and NMR spectroscopic measurements, the meta‐phenylene‐bridged DHA–DHA exhibited sequential light‐induced ring openings of the two DHA units to their corresponding vinylheptafulvenes (VHFs). Initially, the VHF–DHA species was generated, and, ultimately, after continued irradiation, the VHF–VHF species. Studies in different solvents and quantum chemical calculations indicate that the excitation of DHA–VHF is no longer a local DHA excitation but a charge‐transfer transition that involves the neighboring VHF unit. For the linearly conjugated para‐phenylene‐bridged dimer, electronic communication between the two units is so efficient that the photoactivity is reduced for both the DHA–DHA and DHA–VHF species, and DHA–DHA, DHA–VHF, and VHF–VHF were all present during irradiation. In all, by changing the bridging unit, we can control the degree of stepwise photoswitching.     

“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal‐ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α‐α or α‐meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso ‐bridges; introducing a para ‐quinodimethane bridge; employing a neo‐confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.     

Regiocontrolled Synthesis of Ethene‐Bridged para‐Phenylene Oligomers Based on PtII‐ and RuII‐Catalyzed Aromatization

We report the regiocontrolled syntheses of ethene‐bridged para‐phenylene oligomers in three distinct classes by using Pt^II‐ and Ru^II‐catalyzed aromatization. This synthetic approach has been developed based on twofold aromatization of the 1‐aryl‐2‐alkynylbenzene functionality, which proceeds by distinct regioselectivity for platinum and ruthenium catalysts. Variable‐temperature NMR spectra provide evidence that large arrays of these oligomers are prone to twist from planarity. The UV/Vis and photoluminescence (PL) spectra as well as the band gaps of these regularly growing arrays show a pattern of extensive π conjugation with increasing array sizes, except for in one instance.     

From para‐Benziporphyrin to Rhodium(III) 21‐Carbaporphyrins: Imprinting Rh⋅⋅⋅η2‐CC,Rh⋅⋅⋅η2‐CO,and Rh⋅⋅⋅η2‐CH Coordination Motifs

Rhodium(III) para‐benziporphyrin alters the fundamental reactivity of the built‐in para‐phenylene moiety. Due to additional macrocyclic stabilization, a sequence of intramolecular rearrangements are triggered to afford rhodium(III) 21‐carbaporphyrin, which incorporates the rhodacyclopropane motif. The peculiar reversible transformations of the bridging methylene unit provide an example of selective and reversible aliphatic C?H bond elimination. Rhodium(III) 21‐carbaporphyrin can be oxygenated to rhodium(III) 21‐oxy‐21‐carbaporphyrin, whereas the metal ion interacts with the C(21)?O(25) fragment in an η² fashion. This species demonstrates a remarkable axial affinity toward alkenes.     

Synthesis and luminescence of yellow/orange‐emitting poly[tris(2,5‐dihexyloxy‐1,4‐phenylenevinylene)‐ alt‐(1,3‐phenylenevinylene)]s

Soluble yellow/orange‐emitting poly[tris(2,5‐dihexyloxy‐1,4‐phenylenevinylene)‐alt‐(1,3‐phenylenevinylene)] derivatives ( 6 ) were synthesized and characterized. These polymers contained oligo(p‐phenylene vinylene) chromophores of equal conjugation length, which were jointed via a common m‐phenylene unit. An optical comparison of 6 and its model compound ( 8 ) at room temperature and low temperatures revealed the similarity in their absorption and fluorescence band structures. The vibronic band structure of 6 was assigned with the aid of the spectroscopic data for 8 at the low temperatures. 6 was electroluminescent and had an emission maximum wavelength at approximately 565 nm. With the device indium tin oxide/PEDOT/ 6 /Ca configuration, the polymer exhibited an external quantum efficiency as high as 0.25%. Simple substitution on m‐phenylene of 6 raised the electroluminescence output by a factor of about 10. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5853–5862, 2004     

Synthesis of a Tetrabenzotetraaza[8]circulene by a “Fold‐In” Oxidative Fusion Reaction

Tetrabenzotetraaza[8]circulene ( 1 ) has been synthesized in good yield by a “fold‐in” oxidative fusion reaction of a 1,2‐phenylene‐bridged cyclic tetrapyrrole. X‐ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctatetraene (COT) core that shows little alternation of the bond lengths. Despite these structural features, 1 shows aromatic‐like character, such as sharp absorption bands, high fluorescence quantum yields (Φ_F=0.55 in THF), and a single exponential fluorescence decay with τ_F=3.8 ns. These observations indicate a dominant contribution of an [8]radialene‐like π conjugation and hence aromatic character of the local aromatic segments in 1 .     

Phenylene‐bridged Porphyrin meso‐Oxy Radical Dimers

Stable meta‐ and para‐phenylene bridged porphyrin meso‐oxy radical dimers and their Ni^II and Zn^II complexes were synthesized. All the dimers exhibited optical and electrochemical properties similar to the corresponding porphyrin meso‐oxy radical monomers, indicating small electronic interaction between the two spins. Intramolecular spin‐spin interaction through the π‐spacer was determined to be J/k_B=?15.9 K for m‐phenylene bridged Zn^II porphyrin dimer. The observed weak antiferromagnetic interaction has been attributed to less effective conjugation between the porphyrin radical and linking π‐spacer due to large dihedral angle. In the case of Zn^II complexes, both para‐ and meta‐phenylene bridged dimers formed 1D‐chain in solutions and in the solid states through Zn‐O coordination.     

Metal‐Ion‐Induced Switch of Liquid‐Crystalline Orientation of Metallomacrocycles

A new series of shape‐persistent imine‐bridged macrocycles were synthesized based on dynamic covalent chemistry. The macrocycles had an alternating sequence of dibenzothiophene and N,N′‐bis(salicylidene)‐ethylenediamine (salen) tethering branched alkyl chains. The macrocycles and tetranuclear metallomacrocycles bearing long and branched alkyl chains exhibited thermotropic columnar liquid‐crystalline phases over a wide temperature range and the metallomacrocycles greatly depended on the characteristics of the coordinated metal ions. The metal‐free macrocycle showed a liquid‐crystalline phase with a lamellar structure and poor birefringence. In sharp contrast, the macrocyclic Ni complex showed a columnar oblique liquid‐crystalline phase, whereas the Pd and Cu complexes showed columnar liquid‐crystalline phases with a lamellar structure. The macroscopic organization and thermal properties of the corresponding liquid‐crystalline metallomacrocycles were significantly dependent on the subtle structural differences among the planar macrocycles, which were revealed by single‐crystal X‐ray crystallographic analysis of the macrocycles with shorter alkyl chains.     

Fine Tuning the Performance of DSSCs by Variation of the π‐Spacers in Organic Dyes that Contain a 2,7‐Diaminofluorene Donor

Organic dyes that contain a 2,7‐diaminofluorene‐based donor, a cyanoacrylic‐acid acceptor, and various aromatic conjugation segments, which are composed of benzene, fluorene, carbazole, and thiophene units, as a π‐bridge have been synthesized and characterized by optical, electrochemical, and theoretical investigations. The trends in the absorption and electrochemical properties of these dyes are in accordance with the electron‐donating ability of the conjugating segment. Consequently, the dyes that contained a 2,7‐carbazole unit in the π‐spacer exhibited red‐shifted absorption and lower oxidation potentials than their corresponding fluorene‐ and phenylene‐bridged dyes. However, the enhanced power‐conversion efficiency that was exhibited by the fluorene‐bridged dyes in the DSSCs was attributed to their broader and intense absorption. Despite the longer‐wavelength absorption and reasonable optical density, carbazole‐bridged dyes exhibited lower power‐conversion efficiencies, which were ascribed to the poor alignment of the LUMO level in these dyes, thereby leading to the inhibition of electron injection into the TiO₂ conduction band.     

Spectroscopic characterization and molecular structure of 3,14‐dimethyl‐2,6,13,17‐tetraazapentacyclo[16.4.0.12,17.16,13.07,12]tetracosane

Constrained cyclam derivatives have been found to exhibit anti‐HIV effects. The strength of binding to the CXCR4 receptor correlates with anti‐HIV activity. The conformation of the macrocyclic compound is very important for co‐receptor recognition. Therefore, knowledge of the conformation and crystal packing of macrocycles has become important in developing new highly effective anti‐HIV drugs. Structural modifications of N‐functionalized polyaza macrocyclic compounds have been achieved using various methods. A new synthesis affording single crystals of the title tetraazapentacyclo[16.4.0.1^2,17.1^6,13.0^7,12]tetracosane macrocycle, C₂₂H₄₀N₄, is reported. Formaldehyde reacts readily at room temperature with the tetraazatricyclo[16.4.0.0^2,17]docosane precursor to yield a macropolycycle containing two five‐membered rings. Characterization by elemental, spectroscopic and single‐crystal X‐ray diffraction analyses shows that the asymmetric unit contains half of a centrosymmetric molecule. The molecular structure shows a trans conformation for the two methylene bridges owing to molecular symmetry. The crystal structure is stabilized by intramolecular C—H…N hydrogen bonds. NMR and IR spectroscopic properties support the methylene‐bridged macrocyclic structure.     

An Obtuse‐angled Corner Unit for Fluctuating Carbon Nanohoops

The invention of corner units was the key factor that allowed the synthesis of cyclo‐para ‐phenylenes with strained curved π‐systems. Despite only a few scarce instances of the development of corner units to date, a variety of structural congeners have been synthesized. These preceding corner units commonly possessed directing angles of ≤90°, which enabled the macrocyclization of multiple units, up to six. In this study, we introduce an obtuse‐angled corner unit for the synthesis of cyclo‐para ‐phenylene congeners. The corner unit with oxanorbornadiene possessed a directing angle of 126° and thus allowed for the macrocyclization of larger structures with up to seven units. Reductive aromatization was applicable to complete the cyclo‐para ‐phenylene structures and afforded the congeners with multiple anthracenylene panels. Structural studies with experimental and theoretical methods revealed a fluctuating structure with an intrinsic non‐belt shape.     

Synthesis of 7,8‐Dehydropurpurin Dimers and Their Conversion into Conformationally Constrained β‐to‐β Vinylene‐Bridged Porphyrin Dimers

7,8‐Dehydropurpurin has attracted much attention owing to the dual 18π‐ and 20π‐electron circuits in its macrocyclic conjugation. The two‐fold Pd‐catalyzed [3+2] annulation of meso‐bromoporphyrin with 1,4‐diphenylbutadiyne furnished 7,8‐dehydropurpurin dimers. The 8^a,8^a‐linked dimer displays a red‐shifted and enhanced absorption band in the NIR region and a small electrochemical HOMO–LUMO band gap as a consequence of efficient conjugation between the two coplanar 7,8‐dehydropurpurin units. Treatment of this dimer with N‐bromosuccinimide in chloroform and ethanol gave β‐to‐β vinylene‐bridged porphyrin dimers. Owing to the highly constrained conformations, these dimers exhibit perturbed absorption spectra, small Stokes shifts, and high fluorescence quantum yields.     

Acetylene and trans‐Ethylene Bridged BIII‐Subporphyrin Dimers

Acetylene and trans‐ethylene bridged B^III‐subporphyrin dimers were synthesized by cross‐coupling reactions of meso‐bromo B^III subporphyrin. These dimers display perturbed and red‐shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico‐second time‐resolved transient absorption spectra indicated that the S₁‐states of the dimers relax with structural changes, which are larger for the trans‐ethylene bridged dimer.     

Bridge‐Localized HOMO‐Binding Character of Divinylanthracene‐Bridged Dinuclear Ruthenium Carbonyl Complexes: Spectroscopic,Spectroelectrochemical, and Computational Studies

The electronic properties of four divinylanthracene‐bridged diruthenium carbonyl complexes [{RuCl(CO)(PMe₃)₃}₂(μ? CH?CHArCH?CH)] (Ar=9,10‐anthracene ( 1 ), 1,5‐anthracene ( 2 ), 2,6‐anthracene ( 3 ), 1,8‐anthracene ( 4 )) obtained by molecular spectroscopic methods (IR, UV/Vis/near‐IR, and EPR spectroscopy) and DFT calculations are reported. IR spectroelectrochemical studies have revealed that these complexes are first oxidized at the noninnocent bridging ligand, which is in line with the very small ν(C?O) wavenumber shift that accompanies this process and also supported by DFT calculations. Because of poor conjugation in complex 1 , except oxidized 1⁺ , the electronic absorption spectra of complexes 2⁺ , 3⁺ , and 4⁺ all display the characteristic near‐IR band envelopes that have been deconvoluted into three Gaussian sub‐bands. Two of the sub‐bands belong mainly to metal‐to‐ligand charge‐transfer (MLCT) transitions according to results from time‐dependent DFT calculations. EPR spectroscopy of chemically generated 1⁺ – 4⁺ proves largely ligand‐centered spin density, again in accordance with IR spectra and DFT calculations results.     

Controllable Electronic Structures and Photoinduced Processes of Bay‐Linked Perylenediimide Dimers and a Ferrocene‐Linked Triad

A series of perylene‐3,4,9,10‐bis(dicarboximide) (PDI) dimers linked through the bay regions was systematically synthesized to examine the electronic structures and photophysical properties in dependence on the distance and orientation between the two PDI units. The spectroscopic and electrochemical measurements suggested that the coupling value of a directly linked PDI dimer (PDI)₂ is much larger than those of para‐ and meta‐phenylene‐bridged PDI dimers p‐(PDI)₂ and m‐(PDI)₂. The width of Davydov splitting was quantitatively evaluated to compare the coupling values between the two PDI units in these dimers by absorption spectroscopy in frozen 2‐methyl‐THF. Excimer formation of PDI dimers induced the strong fluorescence quenching and large red‐shifts. Femtosecond transient absorption revealed a broad absorption derived from an excimer in the range from about 600 nm to the near‐IR region. The rate constants of formation and decay of the excimer are strongly dependent on the coupling values. Time‐resolved measurements on ferrocene‐linked p‐(PDI)₂ revealed a competition between the photoinduced processes of electron transfer and excimer formation in PhCN, which is in sharp contrast with the sole electron‐transfer process in toluene.     

Dinuclear Macrocyclic Quinoline Bridged Mercury and Silver Bis(N‐heterocyclic Carbene) Complexes: Synthesis,Structure, and Spectroscopic Studies

Quinoline bridged imidazolium precursors 5,8‐bis(N‐R‐imidazolylidenylmethylene)quinoline PF₆^– salts [H₂L](PF₆)₂ [R = Me ( 1a ), R = naphthylmethyl ( 1b )] were prepared by quaternization of N‐methylimidazole and N‐naphthylmethylimidazole with 5,8‐bis(bromomethyl)quinoline, respectively. Reaction of the imidazolium ligands 1a and 1b with Hg(OAc)₂ and Ag₂O in acetonitrile gave the macrocyclic transition metal carbene complexes [Hg₂L₂](PF₆)₄ ( 2a and 2b ) and [Ag₂L₂](PF₆)₂ ( 3a and 3b ), respectively. All the N‐heterocyclic carbene complexes were characterized in detail by NMR, ESI‐MS, and elemental analysis. Structures of complexes 2a and 3a were determined by X‐ray diffraction studies. Structural studies revealed that the coordination arrangement of the central mercury atom in complex 2a displays a tricoordinate mode and the molecular conformation results in a“closed” form with the bridging quinoline functionality in the macrocycle, whereas the silver complex 3a does not show an coordiantion between the bridging quinoline and the Ag^I ion, which results in an “open” conformation of the macrocycle. The Hg^II and Ag^I NHC complexes showed similar UV absorption and luminescence in acetonitrile solutions.     

Metallomacrocycles with a Difference: Macrocyclic Complexes with Exocyclic Ruthenium(II)‐Containing Domains

The templated synthesis of organic macrocycles containing rings of up to 96 atoms and three 2,2′‐bipyridine (bpy) units is described. Starting with the bpy‐centred ligands 5,5′‐bis[3‐(1,4‐dioxahept‐6‐enylphenyl)]‐2,2′‐bipyridine and 5,5′‐bis[3‐(1,4,7‐trioxadec‐9‐enylphenyl)]‐2,2′‐bipyridine, we have applied Grubbs’ methodology to couple the terminal alkene units of the coordinated ligands in [FeL₃]²⁺ complexes. Hydrogenation and demetallation of the iron(II)‐containing macrocyclic complexes results in the isolation of large organic macrocycles. The latter bind {Ru(bpy)₂} units to give macrocyclic complexes with exocyclic ruthenium(II)‐containing domains. The complex [Ru(bpy)₂(L)]²⁺ (isolated as the hexafluorophosphate salt), in which L=5,5′‐bis[3‐(1,4,7,10‐tetraoxatridec‐12‐enylphenyl)]‐2,2′‐bipyridine, undergoes intramolecular ring‐closing metathesis to yield a macrocycle which retains the exocyclic {Ru(bpy)₂} unit. The poly(ethyleneoxy) domains in the latter macrocycle readily scavenge sodium ions, as proven by single‐crystal X‐ray diffraction and atomic absorption spectroscopy data for the bulk sample. In addition to the new compounds, a series of model complexes have been fully characterized, and representative single‐crystal X‐ray structural data are presented for iron(II) and ruthenium(II) acyclic and macrocyclic species.     

Synthesis and Metalation of Doubly o‐Phenylene‐Bridged Cyclic Bis(dipyrrin)s with Highly Bent Skeleton of Dibenzoporphyrin(2.1.2.1)

Facile synthesis of dibenzoporphyrins(2.1.2.1) has been successfully reported by the simple condensation reaction of o‐dipyrrolylbenzene with various aldehydes in the presence of a Lewis acid. This reaction enables the preparation of various dibenzoporphyrin(2.1.2.1) derivatives with p‐substituted phenyl groups, five‐membered heterocycles, and ethynyl groups at the meso‐positions. Dibenzoporphyrins(2.1.2.1) consist of two dipyrrin units that are connected by o‐phenylene bridges, which adopt highly bent saddle‐shaped structures; this was confirmed by X‐ray diffraction analysis. We found that dibenzoporphyrin(2.1.2.1) can be described as a 20π antiaromatic conjugated system, but practically, it is not an antiaromatic macrocycle, which we revealed by ¹H NMR spectroscopy. The redox potentials had good correlations with Hammett substituent constant (σ_p) of the substituents at the meso‐positions. The free‐base dibenzoporphyrin(2.1.2.1) was able to form the metal complexes with nickel(II), copper(II), palladium(II), platinum(II), and tin(IV) ions. These results suggested that dibenzoporphyrin(2.1.2.1) derivatives can be utilized as novel macrocyclic dianionic tetradentate ligands for various metal ions to give complexes with varying optical and electrochemical properties.