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Magnetically induced current densities and current pathways have been calculated for six tautomers of substituted and nonsubstituted core-modified porphyrinoids with one of the pyrrole rings replaced by a thienopyrrole moiety. The calculations show that the aromatic properties of the porpyrinoid macrocycle are strongly influenced by the ethyl-formate substituent at the pyrrole ring of the thienopyrrole moiety, whereas the alkyl substituents at the β positions of the ordinary pyrrole rings have a much smaller effect on the ring-current strength. The ethyl-formate substitution decreases the strength of the paratropic ring current of the macrocycle rendering the energetically lowest tautomer nonaromatic. The substituted tautomers with both porphyrinoid hydrogens inside the macroring are antiaromatic according to the ring current criterion, whereas the three tautomers with one hydrogen at the outer nitrogen of the thienopyrrole moiety are nonaromatic. Current calculations on the nonsubstituted core-modified porphyrinoids show that they are all antiaromatic. The antiaromatic thienopyrrole-modified porphyrinoids are dominated by paratropic ring currents inside the macroring and a weaker diatropic current along the periphery of the macroring. The nonaromatic porphyrinoid tautomers sustain significant ring currents around the thienopyrrole moiety, whereas the other pyrrole rings are practically nonaromatic.

Abstract: 5,15‐Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(α,α′‐dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the α‐hydroxy‐substituted intermediate. This unprecedented molecule is a 20π‐electron antiaromatic system, in terms of Hückel's rule of aromaticity, because lone pair electrons of oxygen atoms are incorporated into the 18π‐electron conjugated system of the porphyrin. A theoretical analysis based on the gauge‐including magnetically induced current method confirmed its antiaromaticity and a dominant inner ring pathway for the ring current. The unique reactivity of 5,15‐dioxaporphyrin forming a β,β‐linked dimer upon oxidation was also revealed. [ABSTRACT FROM AUTHOR]

Magnetizabilities and magnetically induced ring-current strength susceptibilities have been calculated at the Hartree–Fock, density functional theory and second order Møller–Plesset levels for a number of antiaromatic closed-shell carbaporphyrins, carbathiaporphyrins and isophlorins. The calculations yield a linear relation between magnetizabilities and ring-current strength susceptibilities. The calculations show that the porphyrinoids with the largest ring-current strength susceptibility are closed-shell paramagnetic molecules with positive magnetizabilities. The closed-shell paramagnetism is due to the large paramagnetic contribution to the magnetizability originating from the strong paratropic ring current in the antiaromatic porphyrinoids. [ABSTRACT FROM AUTHOR]

Themagnetically induced current density susceptibility, also calledcurrent density, has been calculated for a recently synthesized octaethylporphyrin(OEP) zinc(II) dication with formally 16 π electrons. Numericalintegration of the current density passing selected chemical bondsyields the current pathway around the porphyrinoid ring and the strengthof the ring current. The current strengths show that the OEP-Zn(II)dication is strongly antiaromatic, as also concluded experimentally.The calculation of the ring current pathway shows that all 24 πelectrons participate in the transport of the ring current becausethe current splits into inner and outer branches of practically equalstrengths at the four pyrrolic rings. The corresponding neutral octaethylporphyrinoidwithout Zn and inner hydrogens is found to be antiaromatic, sustaininga paratropic ring current along the inner pathway with 16 πelectrons. The neutral OEP-Zn(II) molecule with formally 18 πelectrons is found to be almost as aromatic as free-base porphyrin.However, also in this case, all 26 π electrons contribute tothe ring current, as for free-base porphyrin. A comparison of calculatedand measured 1H NMR chemical shifts is presented. The currentstrength susceptibility under experimental conditions has been estimatedby assuming a linear relation between experimental shielding constantsand calculated current strengths. [ABSTRACT FROM AUTHOR]

Magneticallyinduced current densities and current pathways havebeen calculated for carbaporphyrins and carbathiaporphyrins usingthe gauge including magnetically induced current (GIMIC) method. Thearomatic character and current pathways are obtained from the calculatedcurrent density susceptibilities. The current-density calculationsshow that five of the studied carbaporphyrinoids are aromatic, twoare antiaromatic, and one is nonaromatic. The analysis of the currentpathways of the investigated molecules reveals some general trendsfor the current flow in carbaporphyrinoids. Insertion of a CH2group into the all-carbon ring generally cuts or restrictsthe current flow, leading to a stronger current of the alternativepathway of the ring. No obvious trends regarding the current strengthsand pathways of the thiophene and cyclopentadienyl rings were obtained.The present study shows that it is indeed difficult to predict theelectron delocalization pathways of general carbaporphyrinoids. Thus,a careful analysis of the current density is necessary for determiningtheir electron delocalization pathways. [ABSTRACT FROM AUTHOR]

Magnetically induced current densities have been investigated for some recently synthesised thieno-bridged porphyrins. The aim of the study is to understand the influence of the direction of the thieno bridge on the aromatic character of the molecules. The calculated ring-current susceptibilities for two tautomers of 2,3- and 3,4-thieno-bridged porphyrins as well as of the corresponding Zn-containing compounds show that the molecules are all aromatic according to the ring current criterion. The ring-current susceptibilities of 16.2 nA/T and 20.2 nA/T around the porphyrin ring of the two tautomers of 2,3-thieno-bridged porphyrin are somewhat weaker than the ring-current susceptibilities of 22.8 nA/T and 23.8 nA/T obtained for the two tautomers of 3,4-thieno-bridged porphyrin. For 2,3-thieno-bridged porphyrin, the positions of the inner hydrogens affect the ring-current susceptibility more than for 3,4-thieno-bridged porphyrin. The current pathways of the Zn porphyrinoids are a superposition of the pathways of the twotranstautomers of the corresponding free-base porphyrinoids. The current-density calculations explain the different aromatic character of 2,3-thieno-bridged and 3,4-thieno-bridged porphyrins. Current-density calculations on tetra-2,3-thieno-bridged and tetra-3,4-thieno-bridged porphyrins yielded similar current pathways as obtained for mono-substituted thieno-bridged porphyrins. Tetra-2,3-thieno-bridged porphyrin is practically non-aromatic since it sustains a very weak ring current of 2.6 nA/T. The calculated ring-current susceptibilities for tetra-3,4-thieno-bridged porphyrin is 13.2 nA/T, which can be compared to the ring-current susceptibilities of 11.8 and 27.5 nA/T for benzene and free-base porphyrin, respectively. [ABSTRACT FROM AUTHOR]

Magnetically induced current densities have been calculated for free-base porphynoids using the gauge including magnetically induce current (GIMIC) method. Numerical integration of the current density passing selected chemical bonds yields current pathways and the degree of aromaticity according to the magnetic criterion. The ring-current strengths of the porphins, chlorins, and bacteriochlorins are 1.5-2.5 times stronger than for benzene. The calculations show that the 18π [16]annulene inner cross is not the correct picture of the aromatic pathway for porphyrins. All conjugated chemical bonds participate in the current transport independently of the formal number of π electrons. The ring current branches at the pyrrolic rings taking both the outer and the inner route. The NH unit of the pyrrolic rings has a larger resistance and a weaker current strength than the pyrroles without inner hydrogens. The traditional 18π [18]annulene with inactive NH bridges is not how the ring-current flows around the macroring. The porphins have the strongest ring current of ca. 27 nA/T among the investigated porphynoids. The current strengths of the chlorins and bacteriochlorins are 19-24 nA/T depending on whether the ring current is forced to pass an NH unit or not. The current strengths of the 3-fold and 4-fold β-saturated porphynoids are 13-17 nA/T, showing that the inner-cross 18π [16]annulene pathway is not a preferred current route. [ABSTRACT FROM AUTHOR]

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems. Little is known about interactions between two antiaromatic molecules. Here, the authors synthesised a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. [ABSTRACT FROM AUTHOR]

Magnetically induced current densities and ring-current pathways have been calculated at density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) levels of theory for a set of expanded porphyrins consisting of five or six pyrrolic rings. The studied molecules are sapphyrin, cyclo[6]pyrrole, rubyrin, orangarin, rosarin, and amethyrin. Different functionals have been employed to assess the functional dependence of the ring-current strength susceptibility. Vertical singlet and triplet excitation energies have been calculated at the second-order approximate coupled cluster (CC2), expanded multiconfigurational quasi-degenerate perturbation theory (XMC-DPT2), and time-dependent density functional theory levels. The lowest electronic transition of the antiaromatic molecules was found to be pure magnetic transitions providing an explanation for the large paratropic contribution to the total current density. Rate constants for different nonradiative deactivation channels of the lowest excited states have been calculated yielding lifetimes and quantum yields of the lowest excited singlet and triplet states. The calculations show that the spin-orbit interaction between the lowest singlet (S0) and triplet (T1) states of the antiaromatic molecules is strong, whereas for the aromatic molecule the spin-orbit coupling vanishes. The experimentally detected fluorescence from S2 to S0 of amethyrin has been explained. The study shows that there are correlations between the aromatic character and optical properties of the investigated expanded porphyrins. [ABSTRACT FROM AUTHOR]