Synthesis, characterization, and spectroscopic analysis of antiaromatic benzofused metalloporphyrins.

A two-electron oxidation of the Cu(II) (9) and Zn(II) (12) complexes of tetraphenyltetrabenzoporphyrin (TPTBP) results in the formation of stable antiaromatic [(TPTBP)Cu(II)(H(2)O)](2+)⋅2 [SbF(6)](-) (10) and [(TPTBP)Zn(II)(H(2)O)(2)](2+)⋅2 [SbF(6)](-) (13) with 16π electrons on the inner ligand perimeter. X-ray structures of the parent TPTBP complexes, the dications, and singly oxidized species [(TPTBP)Cu(II)](⋅+)[SbF(6)](-) (11) reveal that the use of TPTBP rather than a porphyrin ligand reduces the degree of nonplanarity in the 16π-electron species relative to the parent 18π complex. Significant high-field shifts of the (1)H NMR signals of the outer ring protons and large positive values in calculations of nucleus-independent chemical shifts on the central cavity of the porphyrin ring provide unambiguous evidence for the antiaromatic character of the 16π Zn(II) species. A combination of magnetic circular dichroism spectroscopic studies and TD-DFT calculations on both the Zn(II) and Cu(II) species demonstrates that the main electronic bands of the dicationic species can be readily assigned by using Michl's 4N perimeter model. Femtosecond transient absorption studies clearly demonstrated that the number of π electrons on the inner ligand perimeter and the configuration of the central metal ion play a critical role in the excited-state relaxation dynamics. Redox potentials for conversion between the 16π, 17π, and 18π systems were measured by cyclic voltammetry in dichloromethane and benzonitrile, and UV/Vis spectra of each oxidation/reduction product were monitored by thin-layer spectroelectrochemistry.

Electron delocalization in various triply linked zinc(II) porphyrin arrays: role of antiaromatic junctions between aromatic porphyrins.

A series of meso-meso, ß-ß, ß-ß triply linked linear, radial and square-type zinc(II) porphyrin arrays consist of the constituent porphyrin units and naphthalene junctions. To understand the unique nature of triply linked porphyrin arrays, numerous research activities have been focused on the electronic structures of the constituent porphyrin units. In this study, however, we have paid attention to the role of the naphthalene junction in the electronic delocalization of various triply linked porphyrin arrays. On the basis of our study, we have unveiled that unique π-conjugation behaviors in triply linked porphyrin arrays are induced by their intrinsic molecular orbital interactions and subsequently by antiaromatic junctions. Furthermore, the structural deformation by triple linkages gives rise to a deteriorative effect on the electronic delocalization between inner and outer porphyrin units. Finally, we propose a different type of electron delocalization in linear multichromophoric systems by alternating aromatic and antiaromatic units.

Investigation of aromaticity and photophysical properties in [18]/[20]π porphycene derivatives.

In this study, we have investigated the relationship between aromaticity and photophysical properties of trifluoromethyl-substituted [18]/[20]π porphycenes by using theoretical calculations and various spectroscopic methodologies. Interestingly, we have found that the HOMO-LUMO gap of [20]π porphycene is larger than that of [18]π porphycene, which is in a sharp contrast with those of typical [4n]/[4n+2]π porphyrinoids. Based on our observations, we demonstrate that the origin of this contrasting feature of [20]π porphycene arises from the uniquely large energy splitting between LUMO and LUMO+1 of [18]π porphycene compared with other aromatic [4n+2]π porphyrinoids with nearly degenerate LUMO/LUMO+1. Consequently, we can propose that the energy difference between LUMO and LUMO+1 levels of aromatic [4n+2]π porphyrinoids is an important factor in determining the electronic nature of their corresponding antiaromatic [4n]π porphyrinoids. Moreover, to the best of our knowledge, this is the first study to illustrate the photophysical properties of porphycenes with [4n]π electronic circuits.

Multistimuli two-color luminescence switching via different slip-stacking of highly fluorescent molecular sheets.

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H···N and C-H···O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.

Aromaticity and photophysical properties of various topology-controlled expanded porphyrins.

Recently, expanded porphyrins have come to the forefront in the research field of aromaticity, and been recognized as the most appropriate molecular system to study both Hückel and Möbius aromaticity because their molecular topologies can be easily changed and controlled by various methods. Along with this advantage, many efforts have been devoted to the exploration of the aromaticity-molecular topology relationship based on electronic structures in expanded porphyrins so that further insight into the aromaticity--a very attractive field for chemists--can be provided. In this tutorial review, we describe the recent developments of various topology-controlled expanded porphyrins and their photophysical properties, in conjunction with the topological transformation between Hückel and Möbius aromaticity by various conformational control methods, such as synthetic methods, temperature control, and protonation.

A stable non-Kekulé singlet biradicaloid from meso-free 5,10,20,25-tetrakis(pentafluorophenyl)-substituted [26]hexaphyrin(1.1.1.1.1.1).

A meso,meso-diketohexaphyrin was isolated and characterized as a chemically stable non-Kekulé singlet biradicaloid. Two unpaired electrons are seemingly delocalized on two tripyrrolic units separated by C=O bonds. These results underscore the potential of expanded porphyrins to achieve unique electronic states.

Highly pure synthesis, spectral assignments, and two-photon properties of cruciform porphyrin pentamers fused with benzene units.

Tetrameric porphyrin formation of 2-hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene-fused porphyrin pentamers. Thermal conversion of the pentamers gave fully pi-conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully pi-conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD-DFT calculations, the HOMO level is 0.49 eV higher than the HOMO-1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single-electron transitions (683 nm: 86 %, HOMO-->LUMO; 680 nm: 86 %, HOMO-->LUMO+1). The two-photon absorption (TPA) cross section value (sigma((2))) of the benzene-fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional pi-conjugation pathways.

Facile formation of a benzopyrane-fused [28]hexaphyrin that exhibits distinct Möbius aromaticity.

A benzopyrane-fused [28]hexaphyrin, 2, was prepared by simple heating of [26]hexaphyrin 1 in acetic acid. Fused [28]hexaphyrin 2 features a molecular twist, a distinct diatropic ring current, a large HOMA value, a large negative NICS value, and a large two-photon absorption (TPA) cross section even at room temperature, all of which support the Möbius aromaticity of 2. To the best of our knowledge, 2 is the first macrocycle that acquires distinct Möbius aromaticity without any assistance from metal coordination, temperature control, or protonation.

Conformational changes of meso-aryl substituted expanded porphyrins upon protonation: effects on photophysical properties and aromaticity.

meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) and meso-hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) have been investigated with a particular focus on their photophysical properties affected by protonation with acids using steady-state and time-resolved spectroscopic measurements along with femtosecond Z-scan method. It was found that the smaller Stokes shift and longer excited singlet/triplet state lifetimes of protonated [30]heptaphyrin and [38]nonaphyrin compared to their distorted neutral counterparts are strongly associated with the rigid and planar molecular structures. Much larger two photon absorption cross-section values of protonated [30]heptaphyrin and [38]nonaphyrin (6300 and 6040 GM) than those of their neutral forms (1350 and 1300 GM) also reflect the enhanced rigidity and planarity as well as aromaticity. In parallel with this, the nucleus-independent chemical shift (NICS) values of protonated forms exhibit large negative values, -14.3 and -11.5 ppm for [30]heptaphyrin and [38]nonaphyrin, respectively, at central positions. Thus we have demonstrated the structure-property relationships between molecular planarity, photophysical properties, and aromaticity of expanded porphyrins upon protonation based on our experimental and theoretical investigations. This study also promises a possibility of structural control of expanded porphyrins through protonation in which the molecular flexibilities of expanded porphyrins lead to distorted structures especially as the number of pyrrole rings increases.

Temperature-dependent conformational change of meso-hexakis(pentafluorophenyl) [28]Hexaphyrins(1.1.1.1.1.1) into Möbius structures.

At room temperature, meso-hexaaryl-substituted [28]hexaphyrins(1.1.1.1.1.1) in solution exist largely as an equilibrium between planar antiaromatic and distorted Möbius aromatic conformers. As the temperature decreases, the molecular structure changes into the distorted Möbius topology that commonly occurs in [28]hexaphyrins, which gives rise to longer excited singlet and triplet state lifetimes than planar antiaromatic [28]hexaphyrins. Temperature-dependent two-photon absorption measurements of [28]hexaphyrin indicate that the degree of aromaticity of Möbius [28]hexaphyrin is large, comparable to that of Hückel aromatic planar [26]hexaphyrin. Through our spectroscopic investigations, we have demonstrated that a subtle balance between the strains induced by the size of the [28]hexaphyrin macrocyclic ring and the energy stabilization contributed by pi-electron delocalization in the formation of distorted Möbius [28]hexaphyrin leads to the molecular structure change into the Möbius topology as the temperature decreases.

The photophysical properties of expanded porphyrins: relationships between aromaticity, molecular geometry and non-linear optical properties.

Porphyrins, which consist of four pyrrolic subunits, are a ubiquitous class of naturally occurring compound with versatile photophysical properties. As an extension of the basic structure of the porphyrin macrocycle, there have been a multitude of approaches to synthesize expanded porphyrins with more than four pyrrole rings, leading to the modification of the macrocyclic ring size, planarity, number of pi-electrons and aromaticity. However, the relationship between the photophysical properties and the structures of expanded porphyrins has not been systematically investigated. The main purpose of this article is to describe the structure-property relationships of a variety of expanded porphyrins based on experimental and theoretical results, which include steady-state and time-resolved spectroscopic characterizations, non-linear absorption ability and nucleus-independent chemical shift calculations.

Comment on 'aggregation-induced phosphorescent emission (AIPE) of iridium(III) complexes': origin of the enhanced phosphorescence.

An investigation of the photophysics of Ir(III) complexes with controlled ligand structures and our quantum chemical calculations attest that the most probable explanation for the reported 'aggregation-induced phosphorescent emission', which was originally claimed to be related to an intermolecular excimer, is restricted intramolecular motion.

Unambiguous identification of Möbius aromaticity for meso-aryl-substituted [28]hexaphyrins(1.1.1.1.1.1).

meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by (1)H, (13)C, and (19)F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25 degrees C exist largely as an equilibrium among several rapidly interconverting twisted Möbius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take either planar or Möbius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100 degrees C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Möbius conformation. Detailed analyses of the solid-state Möbius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic pi-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Möbius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.

Two-dimensionally extended porphyrin tapes: synthesis and shape-dependent two-photon absorption properties.

We report the synthesis and characterization of L- and T-shaped porphyrin tapes as extensible structural motifs of two-dimensionally extended porphyrin tapes. The two-photon absorption (TPA) cross-section values (sigma((2))) for L- and T-shaped porphyrin tapes as well as those for linear trimeric and tetrameric porphyrin tapes were measured by an open-aperture Z-scan method at 2300 nm, a wavelength at which the one-photon absorption contribution is either zero or almost negligible. Under these conditions, the sigma((2)) values for the linear porphyrin tape trimer and tetramer were determined to be 18 500 and 41 200 GM, respectively. The sigma((2)) value for the L-shaped trimer was determined to be 8700 GM, which is only half that of the linear trimer, whereas the sigma((2)) value for the T-shaped tetramer was measured to be 35 700 GM. These results clearly indicate the dependence of the TPA cross-section on the molecular shape, which underscores the importance of directionality in the pi-conjugation pathway for the enhancement of TPA cross- section.

Photophysical properties of core-modified expanded porphyrins: nature of aromaticity and enhancement of ring planarity.

We have investigated the excited-state dynamics and nonlinear optical properties of representative core-modified expanded porphyrins, tetrathiarubyrin, tetraselenarubyrin, pentathiaheptaphyrin, tetrathiaoctaphyrin, and tetraselenaoctaphyrin, containing 26, 30, and 34 pi electrons using steady-state and time-resolved absorption and fluorescence spectroscopic measurements along with femtosecond Z-scan method, with a particular attention to the photophysical properties related to molecular planarity and aromaticity. Core-modification of macrocycles by sulfur and selenium leads to NIR-extended steady-state absorption and fluorescence spectra and short-lived excited-state due to the heavy-atom effect in time-resolved spectroscopic experiments. Large negative nucleus-independent chemical shift values ranging from -13 to -15 ppm indicate that all molecular systems are highly aromatic. The observed enhancement of two-photon absorption cross-section values over 10 (4) GM for core-modified hepta- and octaphyrins is mainly attributable to their rigid and planar structures as well as their aromaticity. Overall, the observed spectroscopic and theoretical results consistently demonstrate the enhanced molecular planarity of core-modified expanded porphyrins compared with their corresponding all-aza expanded porphyrins.