The origin of global and macrocyclic aromaticity in porphyrinoids

The global and macrocyclic aromaticity of porphyrinoids was characterized using our graph theory of aromaticity. The sequential line plots of topological resonance energy (TRE) against the number of π-electrons (N(π)) for different porphyrinoids are similar with four major extrema to those for five-membered heterocycles. This supports the view that five-membered rings are the main origin of global aromaticity in porphyrinoids. Macrocyclic circuits contribute significantly to macrocyclic π-circulation but modestly to global aromaticity. Macrocyclic aromaticity/antiaromaticity in oligopyrrolic macrocycles can be predicted by formally applying Hückel's [4n + 2] rule to an annulene-like main macrocyclic conjugation pathway (MMCP). This bridged annulene model can be justified by examining the contribution of individual macrocyclic circuits to macrocyclic aromaticity. A Hückel-like rule of macrocyclic aromaticity was found for porphyrinoid species.     

From Open‐Shell Singlet Diradicaloid to Closed‐Shell Global Antiaromatic Macrocycles

A dithieno[a,h]‐s‐indacene‐ (DTI‐) based diradicaloid DTI‐2Br was synthesized and its open‐shell singlet diradical character was validated by magnetic measurements. On the other hand, its macrocyclic trimer DTI‐MC3 and tetramer DTI‐MC4 turned out to be closed‐shell compounds with global antiaromaticity, which was supported by X‐ray crystallographic analysis and NMR spectroscopy, assisted by ACID and 2D‐ICSS calculations. Such change can be explained by a subtle balance between two types of antiferromagnetic spin–spin coupling along the π‐conjugated macrocycles. The dications of DTI‐MC3 and DTI‐MC4 turned out to be open‐shell singlet diradical dications, with a singlet–triplet energy gap of ?2.90 and ?2.60 kcal mol^?1, respectively. At the same time, they are both global aromatic. Our studies show that intramolecular spin–spin interactions play important roles on electronic properties of π‐conjugated macrocycles.     

Diamagnetic and paramagnetic ring currents in expanded porphyrins

Ipsocentric current density maps are computed at the coupled Hartree-Fock level in the 6-31G** basis set for the planar C(2v) B3LYP geometries of the expanded porphyrins, sapphyrin and orangarin. Both give clearly dominant global macrocyclic ring currents, but with opposite senses of circulation: in 22[small pi] sapphyrin, a diatropic current runs, with some bifurcation, around the conventional 22-centre delocalisation pathway; in 20[small pi] orangarin, a paratropic current runs around the inner 17-atom pathway. In agreement with the annulene analogy for these macrocycles, analysis of orbital contributions shows that in each case topology, energy and symmetry of the frontier orbitals together determine the macrocyclic ring current. In sapphryrin, 4-electron diamagnetism (aromaticity) arises from translationally allowed HOMO-LUMO excitations as in benzene itself; in orangarin, 2-electron paramagnetism (antiaromaticity) arises from rotationally allowed HOMO-LUMO excitations as in planarised cyclooctatetraene. The active orbitals invoked in the explanation of ring currents are those involved in the longstanding four-orbital model of porphyrin electronic spectra.     

A Macrocyclic Furan with Accessible Oxidation States: Switching Between Aromatic and Antiaromatic Global Ring Currents

Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of −0.23 vs. Fc/Fc⁺, and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30πe⁻ aromatic state, as indicated by the formation of a strong diatropic current observed in the ¹H NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.     

Global Aromaticity and Antiaromaticity in Porphyrin Nanoring Anions

Doping, through oxidation or reduction, is often used to modify the properties of π‐conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Herein we show that reduction of a six‐porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6? state; [90 π]) and antiaromaticity (in the 4? state; [88 π]), consistent with the Hückel rules. Aromaticity is assigned by NMR spectroscopy and density‐functional theory calculations.     

Effect on the aromaticity of heterocyclic ligands by coordination with ruthenium electron-withdrawing metal centers

Density functional theory calculations of polypyridyl ruthenium complexes with polyaromatic ligands have been performed to understand the metal fragment effect on the modulation of their electronic properties and the influence on the aromatic character. The change of positions of the nitrogen atoms in the ligand structure, as well as the metal moiety, seems to influence the electronic behavior of the π-extended structure and the aromatic character of the complexes at both the ground and excited states. In this framework, structural, electronic, and magnetic-based aromaticity indices were used to understand the aromaticity of the free and coordinated ligands. The aromaticity character of the ligands is highly influenced by the metal fragment, and the aromaticity/antiaromaticity is achieved according to both the electron-withdrawing capability of the ligand and the metal fragment. The electronic distribution observed on the aromatic ligand determines their π-stacking ability; thus, it is proposed that the control of the π-stacking ability is modulated according to the electronic nature of the ruthenium moiety.     

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.     

Novel chiral macrocycles containing two electronically interacting arylene chromophores

Novel chiral macrocycles consisting of two rigid oligoarylene rods and two chiral spiroindane clips have been synthesized by condensation of spiroindane diols and CF3-activated alpha-omega-difluorooligoaryls. Since a broad variety of planar aromatic macrocycles is known, our non-planar, chiral rings represent a new class of macrocyclic compounds. The first two examples, which contain quaterphenylene and diphenylbithiophene rods, are presented in this communication; for one of them a crystal structure is given. The chiroptical properties of the macrocycles can be interpreted as an interplay of the "intra-rod" helicity of individual oligoarylene rods and the "inter-rod" helicity between both chromophores of the macrocycle. The macrocycles can act as chiral dopands of commercially available, and novel, polymeric nematic liquid crystals (emissive polyfluorenes). The "intra-rod" helicity of individual oligoarylene rods is the main feature in determining the resulting helical twisting power (HTP). The cholestric induction in mesogenic, emissive polyfluorenes is of special interest for a realization of electronic devices that have a circularly polarized electroluminescence. The results are also important for an understanding of larger ensembles of chiral rodlike molecules, especially their pi-pi interactions.     

Aromatic Fused [30] Heteroannulenes with NIR Absorption and NIR Emission: Synthesis,Characterization, and Excited‐State Dynamics

Two hitherto unknown planar aromatic [30] fused heterocyclic macrocycles (1.1.0.1.1.0), with NIR absorption in free‐base form and protonation‐induced enhanced NIR emission, have been synthesized from easy to make precursors. The induced correspondence of fusion on the macrocyclic structure, electronic absorption, and emission spectra have been highlighted.     

Aromaticity of planar boron clusters confirmed

Low-energy boron clusters are characterized by two-dimensional geometry. Aromaticity of these planar boron clusters was established in terms of topological resonance energy (TRE). All planar boron clusters were found to be highly aromatic with large positive TREs even if they have 4n pi-electrons. Aromaticity must therefore be the origin of unusual planar or quasi-planar geometry. Thus, the aromaticity concept is as useful in boron chemistry as it is in general organic chemistry. It is evident that the Hückel 4n + 2 rule of aromaticity should not be applied to such polycyclic pi-systems. Some of the boron clusters are in the triplet electronic state to attain higher aromaticity. Multivalency and electron deficiency of boron atoms are responsible for lowering the energies of low-lying pi molecular orbitals and then for enhancing aromaticity. For polycyclic pi-systems, paratropicity does not always indicate antiaromaticity.     

Interatomic magnetizability: a QTAIM-based approach toward deciphering magnetic aromaticity

Interatomic magnetizability provides insight into the extent of electronic current density between two adjacent atomic basins. By studying a number of well-known aromatic, nonaromatic, and antiaromatic molecules, it is demonstrated that interatomic magnetizability (bond magnetizability) not only is able to verify the exact nature of aromaticity/antiaromaticity among different molecules, but also can distinguish the correct aromaticity order among sets of aromatic/antiaromatic molecules. The interatomic magnetizability is a direct measure of the current flux between two adjacent atomic basins and is the first QTAIM-derived index that evaluates aromaticity based on a response property, that is, magnetizability. Bond magnetizability is easy to compute, straightforward to interpret, and can be employed to evaluate the pure π- or σ-orbital contributions to magnetic aromaticity.     

Dianion and dication of tetrabenzo[5.7]fulvalene. Greater antiaromaticity than aromaticity in comparable systems

The dianion, 5(2-), and dication, 5(2+), of tetrabenzo[5.7]fulvalene represent an aromaticity/antiaromaticity continuum in which the fluorenyl system changes from aromatic in 5(2-) to antiaromatic in 5(2+). Conversely, the antiaromatic dibenzotropylium system of 5(2-) becomes an aromatic system in 5(2+), allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 5(2-) was prepared and characterized by (1)H NMR spectroscopy. The fluorenyl system of 5(2-) showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from (1)H NMR spectroscopy were supported by NICS(1) zz calculations for each system. Comparison of the (1)H NMR spectrum and NICS(1) zz of 5(2-) with those of 5(2+) supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 5(2-) and 5(2+) with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 5(2+) was greater than the magnitude of the aromaticity in the fluorenyl system of 5(2-), with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion.     

β‐Octamethoxy‐Substituted 22π and 26π Stretched Porphycenes: Synthesis,Characterization, Photodynamics,and Nonlinear Optical Studies

Three meso‐expanded tetrapyrrolic aromatic macrocycles, including 22π and 26π acetylene–cumulene bridged stretched octamethoxyporphycenes and octamethoxy[22]porphyrin‐(2.2.2.2), are reported, for the first time, by modification of previously reported synthetic methods. This strategy led to an enhancement in the overall yield of their corresponding octaethyl analogues. The methoxy‐substituted expanded porphycenes display slightly blueshifted absorption relative to their ethyl analogues, along with very weak fluorescence, probably due to efficient intramolecular charge transfer (ICT). Additionally, the two‐photon absorption (TPA) cross sections of these macrocycles were evaluated; these are strongly related to core expansion of the porphyrin aromaticity through increased meso‐bridging carbon atoms as well as conformational flexibility and substitution effects at the macrocyclic periphery. In particular, the octamethoxy stretched porphycenes display strong TPA compared with the octaethyl analogues due to the dominant ICT character of methoxy groups with a maximum TPA cross section of 830 GM at 1700 nm observed for 26π‐octamethoxyacetylene–cumuleneporphycene.     

Macrocyclic conjugation pathways in porphyrins

Macrocyclic aromaticity is the most important concept in porphyrinoid chemistry. Bond resonance energy (BRE) for any pi-bond linking adjacent pyrrolic or other rings represents the stabilization energy due to macrocyclic aromaticity. We found that a main conjugation pathway associated with macrocyclic aromaticity can be traced by choosing a pi-bond with a larger BRE at every bifurcation of the pi-network. All pi-bonds located along the main conjugation pathway are intensified with large positive BREs compared with those located along the bypasses. On the other hand, a main destabilization pathway associated with macrocyclic antiaromaticity can be traced by choosing a pi-bond with a smaller BRE at every bifurcation of the pi-network. Macrocyclic conjugation pathways thus determined are fully consistent with the chemical shifts of protons attached to the macrocycle.     

Thiophene-Fused 1,4-Diazapentalene: A Stable C=N-Containing π-Conjugated System with Restored Antiaromaticity

A thiophene-fused 1,4-diazapentalene (TAP) was rationally designed and synthesized as a C=N-containing 4n π-electron system that exhibits restored antiaromaticity impaired by the doping with C=N bonds. X-ray crystallographic analysis and quantum chemical calculations revealed that the annulation of thiophene rings with the 1,4-diazapentalene moiety resulted in a much higher antiaromaticity than the pristine 1,4-diazapentalene. These effects can be ascribed to the reduced bond alternation of the eight-membered-ring periphery caused by stabilization of the less-stable bond-shifted resonance structure upon increasing the degree of substitution of imine moieties. Consequently, TAP underwent facile hydrogenation even under mild conditions because of its pronounced antiaromaticity and the high aromaticity of the corresponding hydrogenated product H₂-TAP. In addition, the electrophilic C=N moieties in TAP led to the formation of a dense π-stacked structure. These results highlight the effect of partial replacement of C=C bonds with C=N bonds in antiaromatic π-electron systems.     

Planar macrocyclic fluoropentamers as supramolecular organogelators

Despite their great diversities, 2D-shaped macrocycles that can serve as the organogelators have been surprisingly rare; two planar macrocyclic fluoropentamers designed by us were highly able to gelate organic solvents, largely derived from their strong tendency to form 1D stacked fibrillar structures stabilized by both interplanar H-bonds and π-π stacking forces.     

运用密度泛函活性理论的信息论方法研究苯并富烯衍生物的芳香性

文献中已有越来越多的芳香性体系被发现,同时也有越来越多的芳香性指标被提出来,但是如何解释芳香化合物稳定性的起源以及理解芳香性的本质仍然是当今理论化学中一个悬而未决的难题。运用我们新近提出的密度泛函活性理论信息论方法,不久前我们曾对一系列富烯衍生物进行了系统研究并得到了一个全新的认识。本文进一步探讨苯并富烯衍生物的芳香性行为,目的在于考察一个或多个苯环与富烯连接之后其芳香性发生变化的情况。运用香农熵,费舍尔信息,Ghosh-Berkowitz-Parr熵,Onicescu信息能,信息增益,以及相对Rényi熵六个信息量,和四种芳香指标,ASE,HOMA,FLU和NICS,我们系统地研究了信息量和芳香性指标在单、双、三苯并富烯衍生物中的相关性。我们发现,不管是否有苯环与富烯相连,芳香指标和信息量的交叉相关性都是一样的。这表明,虽然苯环本身具有芳香性,但苯环与富烯相连并不能改变富烯的芳香性与反芳香性本质。苯并富烯衍生物与富烯衍生物的芳香性和反芳香性一致。苯并富烯衍生物的芳香性和反芳香性完全取决于富烯本身的芳香性和反芳香性。这些结果为认识和理解复杂体系芳香性和反芳香性起源和本质将提供有益的启示。     

Effect of transition state aromaticity and antiaromaticity on intrinsic barriers of proton transfers in aromatic and antiaromatic heterocyclic systems; an ab initio study

An ab initio study of two series of carbon-to-carbon proton transfer reactions is reported. The first series refers to the heterocyclic C(4)H(5)X(+)/C(4)H(4)X (X = CH(-), NH, S, O, PH, CH(2), AlH, BH) systems, and the second to the linear [Formula: see text] (X = CH(-), NH, S, PH, O, CH(2), AlH, BH) reference systems . The major objective of this study was to examine to what degree the aromaticity of C(4)H(4)X (X = CH(-), NH, S, O, PH) and the antiaromaticity of C(4)H(4)X (X = AlH, BH) is expressed at the transition state of the proton transfer and how this affects the respective intrinsic barriers. From the differences in the barriers between a given cyclic system and the corresponding linear reference system , ΔΔH(++) = ΔH(++)(cyclic) - ΔH(++)(linear), it was inferred that in the cyclic systems both aromaticity and antiaromaticity lower ΔH(++)(cyclic). This conclusion was based on the assumption that the factors not associated with aromaticity or antiaromaticity such as resonance, inductive and polarizability effects in the protonated species, and charge delocalization occurring along the reaction coordinate affect ΔH(++) for the cyclic and linear systems in a similar way and hence offset each other in ΔΔH(++). The extent by which ΔH(++)(cyclic) is lowered in the aromatic systems correlates quite well with the degree of aromaticity of C(4)H(4)X as measured by aromatic stabilization energies as well as the NICS(1) values of the respective C(4)H(4)X. According to the rules of the principle of nonperfect synchronization (PNS), these results imply a disproportionately large degree of aromaticity at the transition state for the aromatic systems and a disproportionately small degree of transition state antiaromaticity for the antiaromatic systems. These conclusions are consistent with the changes in the NICS(1) values along the reaction coordinate. Other points discussed in the paper include the complex interplay of resonance, inductive, and polarizability effects, along with aromaticity and antiaromaticity on the proton affinities of C(4)H(4)X.     

Control of Aromaticity and cis‐/trans‐Isomeric Structure of Non‐Planar Hexaphyrin(2.1.2.1.2.1) and Metal Complexes

Vinylene‐bridged hexaphyrin(2.1.2.1.2.1) was synthesized from dipyrrolyl diphenylethenes by acid‐catalyzed condensation reactions. Freebase hexaphyrin(2.1.2.1.2.1) forms a distorted structure with non‐aromatic characteristics. The aromaticity and molecular configuration of non‐planar hexaphyrin(2.1.2.1.2.1) can be controlled by insertion of metal ions. Freebase and zinc complexes show a distorted structure without macrocyclic aromaticity, whereas copper complexes show a figure‐of‐eight structure with macrocyclic aromaticity. It is the first example of aromaticity conversion of a distorted expanded porphyrin involving vinylene bridges.     

Synthesis of macrocyclic ligands incorporating sulfur and furan subunits

Abstract

Polyammonium macrocycles containing sulfur and furan units in the macrocyclic ring have been synthesized and studied for ATPase activity. The synthetic methodology involved using tosyl protection for the amines and the formation of macrocyclic Lactams, followed by reduction using borane in THF. Deprotection of the tosylated forms of the macrocycle was accomplished using sodium in butanol for the furan macrocycles, and HBr in HOAc for the sulfur containing macrocycle. The macrocycles were found to be poor catalysts for ATP hydrolysis compared to other similar polyammonium macrocycles.