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.

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.

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.

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.

meso-(4-(N,N-dialkylamino)phenyl)-substituted subporphyrins: remarkably perturbed absorption spectra and enhanced fluorescence by intramolecular charge transfer interactions.

A series of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subporphyrins was synthesized by means of Buchwald-Hartwig amination protocol. Substitution of the amino group at the 4-position of the meso-phenyl substituent resulted in a remarkable red shift in the absorption spectra and drastic enhancement of fluorescence intensity probably as a consequence of intramolecular CT interaction. These characteristics have been utilized to construct a cation-sensing system by appending a 1-aza-15-crown-5 unit to subporphyrin that displays large spectral changes upon cation binding.

Nonlinear optical properties as a guide to aromaticity in congeneric pentapyrrolic expanded porphyrins: pentaphyrin, sapphyrin, isosmaragdyrin, and orangarin.

On the basis of two-photon absorption and time-resolved spectroscopic measurements, as supported by theoretical calculations of quantitative aromaticity, a relationship between the nonlinear optical properties and aromaticity index has been established for a series of four fully conjugated pentapyrrolic expanded porphyrins, namely pentaphyrin (1.1.1.1.1), sapphyrin (1.1.1.1.0), isosmaragdyrin (1.1.1.0.0), and orangarin (1.0.1.0.0), all of which proved amenable to study in dichloromethane.

Excitation energy migration processes in cyclic porphyrin arrays probed by single molecule spectroscopy.

By using single molecule fluorescence spectroscopy we have investigated the excitation energy migration processes occurring in a series of cyclic porphyrin arrays bearing a close proximity in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. We have revealed that the conformational heterogeneity induced by the structural flexibility in large cyclic porphyrin arrays, which provides the nonradiative deactivation channels as an energy sink or trap, reduces significantly the energy migration efficiency. Our study provides detailed information on the energy migration efficiency of the artificial light-harvesting arrays at the single molecule level, which will be a guideline for future applications in single molecular photonic devices in the solid state.

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.

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.

Structure-property relationship for two-photon absorbing multiporphyrins: supramolecular assembly of highly-conjugated multiporphyrinic ladders and prisms.

Two-photon absorption (TPA) phenomena of a series of single-strand as well as supramolecular self-assembled ladders and prisms of highly conjugated ethyne bridged multiporphyrin dimer, trimer, and star shaped pentamer have been investigated. The ligand mediated self-assembled supramolecular structures were characterized by UV-visible spectroscopy and small- and wide-angle X-ray scattering (SAXS/WAXS) analysis. The TPA cross section values of multiporphyrins increase nonlinearly from approximately 100 to approximately 18000 GM with an increased number of porphyrin units and elongated pi-conjugation length by virtue of charge transfer and excited-state cumulenic configurations. The observed opposite TPA behavior between their supramolecular ladder and prism configurations necessitates the importance of interstrand interactions between the multiporphyrinic units and the overall shape of the assembly. Furthermore, the diminished TPA cross section of the pentamer, despite the increased pi-conjugation resulting from duplex formation suggests that destabilizing the essential functional configurations at the cost of elongation of pi-delocalization pathway must cause unfavorable effects. We have also shown that one- and two-photon allowed energy-levels of linear multiporphyrins are nearly isoenergetic and the latter transition originates exclusively from the extent of pi-delocalization within the molecule. The identical TPA maximum position of the trimer and pentamer indicates that the TPA of the pentamer arises only from its basic trimer unit in spite of its extended two-dimensional pi-conjugation pathway involving five porphyrinic units.

Evaluation of planarity and aromaticity in sapphyrin and inverted sapphyrin using a bidirectional NICS (Nucleus-Independent Chemical Shift) scan method?

Using the bidirectional NICS scan method in conjunction with two-photon absorption (TPA) measurements, it has proved possible to determine the relationship between pi-conjugation and aromaticity in two structurally related expanded porphyrin systems, sapphyrin and inverted sapphyrin, and establish that differences in these defining factors depend on the presence or absence of a key sp3 hybrid molecular orbital within the macrocyclic periphery.

Excitation energy migration in a dodecameric porphyrin wheel.

Intramolecular excitation energy hopping (EEH) time within a dodecameric porphyrin wheel C6ZA, in which six meso-meso linked zinc(II) diporphyrin (Z2) subunits are bridged by 1,3-phenylene spacers, is deduced by a Förster energy hopping model based on S(1)-S(1) exciton-exciton annihilation and anisotropy depolarization. Under the assumption that the energy hopping sites are six Z2 subunits, two different observables (e.g., exciton-exciton annihilation and anisotropy depolarization times) consistently give the EEH time of 4.0 +/- 0.4 ps via 1,3-phenylene spacer of C6ZA, which is faster than 9.4 ps of linear 2Z2 (1,3-phenylene-linked zinc(II) tetraporphyrin). As a consequence, C6ZA serves as a well-defined two-dimensional model for a light-harvesting complex.

Effect of conformational heterogeneity on excitation energy transfer efficiency in directly meso-meso linked Zn(II) porphyrin arrays.

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.

Ultrafast time-resolved spectroscopy of self-assembled cyclic Fe(II)-bisterpyridine complexes.

Femtosecond time-resolved absorption spectroscopy has been used to study the excited-state charge transfer dynamics in a set of self-assembled cyclic Fe(II)-bisterpyridine compounds with different π-conjugated ligands. By analyzing the dynamics, the internal conversion process involving a ligand-centered π-π* state to a lower lying metal-to-ligand charge transfer (MLCT) state was investigated. This is followed by intersystem crossing to the lowest MLCT state, which was found to occur at the ∼100 fs time scale. Vibrational cooling in the lowest MLCT state was found to occur on the 10s of femtoseconds time scale. The lowest MLCT state had an excited-state lifetime longer than 5 ns, indicating the possibility of light-induced excited-state spin trapping (LIESST).

Möbius aromaticity and antiaromaticity in expanded porphyrins.

Aromaticity is a key concept in chemistry, dating back to Faraday's discovery of benzene in 1825 and Kekulé's famous alternating-double-bond structure of 1865. In 1858, the Möbius strip was discovered by Möbius and Listing. The Hückel rules for predicting aromaticity, stating that [4n + 2] π electrons result in an aromatic system, work for planar molecules. Although molecules with Möbius geometry are not found in nature, chemists have tried to synthesize such molecules since the first theoretical prediction by Heilbronner in 1964 and the prediction of Möbius aromaticity for suitable compounds with [4n] π electrons. However, Möbius-aromatic molecules have proved difficult to synthesize, and sometimes even to identify. Here we summarize recent contributions of several research groups that have succeeded in synthesizing Möbius-type molecules. The results of this survey lead us to suggest that the generation of Möbius topologies in expanded porphyrins is easier than hitherto appreciated.

Synthesis of nanometer-scale porphyrin wheels of variable size.

Starting from 1,3-phenylene linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag I-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays, the Ag I-promoted intramolecular cyclization reaction of 2 nZA (n=5, 6, 8, 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2nZA (n=5, 6, 8, 9, 12, and 16), whereas large arrays 2nZA (n=24, 32, and 48) did not provide cyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.

meso-Aryl-substituted subporphyrins: synthesis, structures, and large substituent effects on their electronic properties.

Two synthetic methods of meso-aryl-substituted subporphyrins have been developed by means of the reaction of pyridine-tri-N-pyrrolylborane with a series of aryl aldehydes. One method relies on the condensation under Adler conditions with chloroacetic acid in refluxing 1,2-dichlorobenzene to afford subporphyrins in 1.1-3.2%, and the other is a two-step reaction consisting of the initial treatment of the two substrates with trifluoroacetic acid at 0 degrees C followed by air-oxidation in refluxing 1,2-dichlorobenzene to provide subporphyrins in up to 5.6% yield. 1H NMR studies indicate that phenyl and sterically unhindered substituents at the meso position of subporphyrins rotate rather freely even at -90 degrees C, whereas the rotation of meso-2,4,6-trimethoxyphenyl substituents is strictly prohibited even at 130 degrees C. The structures of six subporphyins have been revealed by X-ray crystallographic analysis to be all cone-shaped tripyrrolic macrocycles. Dihedral angles of meso-phenyl and sterically unhindered aryl substituents to the subporphyrinic core are rather small (38.3-55.7 degrees ) compared to those of porphyrin analogues, whereas those of meso-2,4,6-trimethoxy-substituted subporphyrins are large (68.7-75.7 degrees ). These rotational features of the meso-aryl substituents lead to their large influences on the electronic properties of subporphyrins, as seen for 4-nitrophenyl-substituted subporphyrin 14e that exhibits perturbed absorption and fluorescence spectra, depending upon solvents. Large solvent-polarity dependence of the fluorescence of 14e suggests the charge-transfer character for its excited state. Electrochemical and theoretical studies are performed to understand the electronic properties. Overall, meso-aryl-substituted subporphyrins are promising chromophores in future functional devices.