Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses.

Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

Excited-State Vibrational Coherence in Perylene Bisimide Probed by Femtosecond Broadband Pump-Probe Spectroscopy.

Broadband laser pulses with ultrashort duration are capable of triggering impulsive excitation of the superposition of vibrational eigenstates, giving rise to quantum beating signals originating from coherent wave packet motions along the potential energy surface. In this work, coherent vibrational wave packet dynamics of an N,N'-bis(2,6-dimethylphenyl)perylene bisimide (DMP-PBI) were investigated by femtosecond broadband pump-probe spectroscopy which features fast and balanced data acquisition with a wide spectral coverage of >200 nm. Clear modulations were observed in the envelope of the stimulated emission decay profiles of DMP-PBI with the oscillation frequencies of 140 and 275 cm(-1). Fast Fourier transform analysis of each oscillatory mode revealed characteristic phase jumps near the maxima of the steady-state fluorescence, indicating that the observed vibrational coherence originates from an excited-state wave packet motion. Quantum calculations of the normal modes at the low-frequency region suggest that low-frequency C-C (C═C) stretching motions accompanied by deformation of the dimethylphenyl substituents are responsible for the manifestation of such coherent wave packet dynamics.

Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule.

The reversal of (anti)aromaticity in a molecule's triplet excited state compared with its closed-shell singlet ground state is known as Baird's rule and has attracted the interest of synthetic, physical organic chemists and theorists because of the potential to modulate the fundamental properties of highly conjugated molecules. Here we show that two closely related bis-rhodium hexaphyrins (R26H and R28H) containing [26] and [28] π-electron peripheries, respectively, exhibit properties consistent with Baird's rule. In the ground state, R26H exhibits a sharp Soret-like band and distinct Q-like bands characteristic of an aromatic porphyrinoid, whereas R28H exhibits a broad absorption spectrum without Q-like bands, which is typical of an antiaromatic porphyrinoid. In contrast, the T-T absorption of R26H is broad, weak and featureless, whereas that of R28H displays an intense and sharp Soret-like band. These spectral signatures, in combination with quantum chemical calculations, are in line with qualitative expectations based on Baird's rule.

Efficacy of rhBMP-2 loaded PCL/PLGA/ß-TCP guided bone regeneration membrane fabricated by 3D printing technology for reconstruction of calvaria defects in rabbit.

We successfully fabricated a three-dimensional (3D) printing-based PCL/PLGA/ß-TCP guided bone regeneration (GBR) membrane that slowly released rhBMP-2. To impregnate the GBR membrane with intact rhBMP-2, collagen solution encapsulating rhBMP-2 (5 µg ml(-1)) was infused into pores of a PCL/PLGA/ß-TCP membrane constructed using a 3D printing system with four dispensing heads. In a release profile test, sustained release of rhBMP-2 was observed for up to 28 d. To investigate the efficacy of the GBR membrane on bone regeneration, PCL/PLGA/ß-TCP membranes with or without rhBMP-2 were implanted in an 8 mm calvaria defect of rabbits. Bone formation was evaluated at weeks 4 and 8 histologically and histomorphometrically. A space making ability of the GBR membrane was successfully maintained in both groups, and significantly more new bone was formed at post-implantation weeks 4 and 8 by rhBMP-2 loaded GBR membranes. Interestingly, implantation with rhBMP-2 loaded GBR membranes led to almost entire healing of calvaria defects within 8 weeks.

Deprotonation-induced aromaticity enhancement and new conjugated networks in meso-Hexakis(pentafluorophenyl)[26]hexaphyrin.

meso-Hexakis(pentafluorophenyl)-substituted neutral hexaphyrin with a 26π-electronic circuit can be regarded as a real homolog of porphyrin with an 18π-electronic circuit with respect to a quite flat molecular structure and strong aromaticity. We have investigated additional aromaticity enhancement of meso-hexakis(pentafluorophenyl)[26]hexaphyrin(1.1.1.1.1.1) by deprotonation of the inner N-H groups in the macrocyclic molecular cavity to try to induce further structural planarization. Deprotonated mono- and dianions of [26]hexaphyrin display sharp B-like bands, remarkably strong fluorescence, and long-lived singlet and triplet excited-states, which indicate enhanced aromaticity. Structural, spectroscopic, and computational studies have revealed that deprotonation induces structural deformations, which lead to a change in the main conjugated π-electronic circuit and cause enhanced aromaticity.

Excitation energy transfer in multiporphyrin arrays with cyclic architectures: towards artificial light-harvesting antenna complexes.

Since highly symmetric cyclic architecture of light-harvesting antenna complex LH2 in purple bacteria was revealed in 1995, there has been a renaissance in developing cyclic porphyrin arrays to duplicate natural systems in terms of high efficiency, in particular, in transferring excitation energy. This tutorial review highlights the mechanisms and rates of excitation energy transfer (EET) in a variety of synthetic cyclic porphyrin arrays on the basis of time-resolved spectroscopic measurements performed at both ensemble and single-molecule levels. Subtle change in structural parameters such as connectivity, distance, and orientation between neighboring porphyrin moieties exquisitely modulates not only the nature of interchromophoric interactions but also the rates and efficiencies of EET. The relationship between the structure and EET dynamics described here should assist a rational design of novel cyclic porphyrin arrays, more contiguous to real applications in artificial photosynthesis.

A non-fused mono-meso-free pentaphyrin and its rhodium(I) complex.

5,10,15,20-Tetrakis(pentafluorophenyl) [22]pentaphyrin(1.1.1.1.1) 7 was synthesised and its bis-rhodium(I) complex 12 has been revealed to be a non-fused, yet planar pentaphyrin with an inverted pyrrole. Both 7 and 12 are aromatic, showing sharp Soret-like bands and diatropic ring currents.

Ensemble and single-molecule spectroscopic study on excitation energy transfer processes in 1,3-phenylene-linked perylenebisimide oligomers.

1,3-Phenylene-bridged perylenebisimide dimer (PBI2) and trimer (PBI3) were prepared along with monomer reference (PBI1) using perylene imide-anhydride 5 as a key precursor. 3,3-Dimethylbut-1-yl substituents were introduced at the 2,5-positions of perylenebisimide (PBI) to improve the solubilities of PBI oligomers. Actually, no serious aggregation of PBI2 and PBI3 was detected in their dilute CH(2)Cl(2) solutions. Under these conditions, intramolecular electronic interactions among PBI chromophores have been revealed by measuring the photophysical properties at their ensemble and single-molecule levels. The excitation energy transfer times of PBI2 (0.16 ps) and PBI3 (0.60 ps) were determined from the two different observables, anisotropy depolarization, and singlet-singlet annihilation, respectively, which are considered as the incoherent Förster-type energy hopping (EEH) times as compared with the EEH time constant (1.97 ps) calculated on the basis of the Förster mechanism. The relatively short EEH times compared to similar PBI oligomers can be attributed to 1,3-phenylene linker, which assures a short distance between the chromophores and, as a consequence, makes it hard to treat the PBI unit as a point dipole. The limitation of point-dipole approximation to describe the PBI oligomers and additional through-bond type interactions can be attributed as the causes of the discrepancies in excitation energy transfer times. Considering these photophysical properties, we can suggest that 1,3-phenylene-linked PBI oligomers have potentials as molecular photonic devices including the artificial light-harvesting system.

Synthesis and properties of hybrid porphyrin tapes.

Hybrid porphyrin tapes 3 and 4, consisting of a mixture of 3,5-di-tert-butylphenyl-substituted donor-type Zn(II)-porphyrins and pentafluorophenyl-substituted acceptor-type Zn(II)-porphyrins, were prepared by a synthetic route involving cross-condensation reaction of a Ni(II)-porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring-closure reaction. The Ni(II)-substituted porphyrin tapes 5 (Ni-Zn-Ni) and 6 (Ni-H(2)-Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor-type Zn(II)-porphyrins. The solid-state and crystal packing structures of 3, 4, and 5 were elucidated by single-crystal X-ray diffraction analysis. Singly meso-meso-linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two-photon absorption (TPA) values of 1-6 were measured by using a wavelength-scanning open aperture Z-scan method and found to be 1900, 21,000, 2200, 27,000, 24,000, and 26,000 GM, respectively. These results illustrate an important effect of elongation of π-electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.

Solvent- and temperature-dependent conformational changes between Hückel antiaromatic and Möbius aromatic species in meso-trifluoromethyl substituted [28]hexaphyrins.

We investigated the photophysical properties of figure-eight-like meso-hexakis(trifluoromethyl) [26]- and [28]hexaphyrins(1.1.1.1.1.1) denoted as TFM26H and TFM28H, respectively, using steady-state and time-resolved spectroscopy along with theoretical calculations to explore their electronic and magnetic natures depending on their molecular aromaticity. TFM26H exhibited a well-resolved absorption feature and intense fluorescence, both of which were neither solvent- nor temperature-dependent. These optical properties were in agreement with its Hückel's [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S(1)-state lifetime of ~50 ps for TFM26H in solution was shorter than those in planar aromatic hexaphyrins (>100 ps) presumably due to nonplanar figure-eight geometry of TFM26H. However, TFM28H exhibited remarkable changes in solvent- and temperature-dependent absorption spectra as well as excited-state lifetimes indicating that a dynamic equilibrium occurs between the two conformational species. With the help of quantum mechanical geometry optimization and vertical excitation energy calculations, we found that the figure-eight double-sided conformer observed in the solid-state and single-sided distorted one could be the best candidates for the two conformers, which should be Hückel antiaromatic and Möbius aromatic species, respectively, based on their optical characteristics, molecular orbital structures, and excited-state lifetimes. Conformational dynamics between these two conformers of TFM28H was scrutinized in detail by temperature-dependent (1)H NMR spectra in various solvents, which showed that the conformational equilibrium was quite sensitive to solvents and that a conformational change faster than the NMR time-scale occurs even at 173 K.

Neutral radical and singlet biradical forms of meso-free, -keto, and -diketo hexaphyrins(1.1.1.1.1.1): effects on aromaticity and photophysical properties.

We have investigated the electronic structures and photophysical properties of 5,10,20,25-tetrakis(pentafluorophenyl)-substituted hexaphyrin(1.1.1.1.1.1) (1) and its meso-keto (2) and meso-diketo derivatives (3) using various spectroscopic measurements. In conjunction with theoretical calculations, these analyses revealed fundamental structure-property relationships within this series, including unusual ground-state electronic structures with neutral, monoradical, and singlet biradical character. The meso-free species 1 is a representative 26 π-electron aromatic compound and shows characteristic spectroscopic features, including a sharp Soret band, well-defined Q-like bands, and a moderately long excited state lifetime (τ = 138 ps). In contrast, the meso-keto derivative 2 displays features characteristic of a neutral monoradical species at the ground state, including the presence of lower energy absorption bands in the NIR spectral region and a relatively short excited-state lifetime (13.9 ps). The meso-diketo 3 exhibits features similar to those of 2, specifically NIR absorptions and a short excited-state lifetime (9.7 ps). Compound 3 is thus assigned as being a ground-state singlet biradicaloid. Two photon absorption (TPA) measurements revealed comparatively large σ(2) values of 600 GM for 2 and 1600 GM for 3 with excitation at λ(ex) =1600 nm as compared to that observed for 1 (σ(2): 360 GM). The enhanced nonlinear optical properties of 2 and 3 are rationalized in terms of the open-shell electronic configuration allowing a large, field-induced fluctuation in the electron density (i.e., a large polarization). This interpretation is supported by theoretical evaluations of the static second hyperpolarizabilities (γ) and γ density analyses. Furthermore, nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values and anisotropy of the induced current density (AICD) plots revealed a clear distinction in terms of the aromatic character of 1-3. Importantly, the open-shell radicaloid 2 and singlet biradicaloid 3 can be formally regarded as 27 π-electron nonaromatic and 26 π-electron aromatic species, respectively, constrained within a dominant 28 π-electron conjugated network. On the basis of the combined experimental and theoretical evidence, it is concluded that the meso-carbonyl groups of 2 and 3 play an important role in perturbing the macrocyclic π-conjugation of the parent hexaphyrin structure 1. In particular, they lead to the imposition of intrinsic radical and biradical character on the molecule as a whole and thus easy-to-discern modifications of the overall electronic effects.

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.

Solvent-dependent aromatic versus antiaromatic conformational switching in meso-(heptakis)pentafluorophenyl [32]heptaphyrin.

We have investigated conformational switching dynamics of meso-heptakis(pentafluorophenyl) [32]heptaphyrin(1.1.1.1.1.1.1) in various solvents using steady-state, time-resolved, and temperature-dependent spectroscopy. Absorption and fluorescence spectra of [32]heptaphyrin are quite sensitive to solvent environments. In nonpolar toluene, the antiaromatic figure-of-eight conformation is dominant, as seen in the X-ray crystallography, based on broad and weak absorption bands without any fluorescence and moderate paratropic ring current. On the other hand, a well-resolved sharp absorption spectrum, strong fluorescence, and diatropic ring current in the 1H NMR spectrum in slightly polar THF indicate that most of [32]heptaphyrin molecules take significantly distorted Möbius conformation with aromatic character. By using transient absorption (TA) spectroscopy, the lowest singlet excited-state lifetimes have been revealed to decay biexponentially with the time constants of 5 and 65 ps for figure-of-eight and Möbius conformations, respectively. Based on these results along with vertical excitation energy calculations, we are able to assign two conformers as Hückel antiaromatic and Möbius aromatic species, respectively; it shoulf be noted that the aromaticity/antiaromaticity does not change with temperature variation. Interestingly, in moderately polar solvent, ethyl ether, we find out that these two conformational isomers coexist with a dynamic equilibrium, as revealed by excitation-wavelength-dependent TA, temperature-dependent absorption and 1H NMR spectra. Through our findings, we have demonstrated that the conformational switching dynamics between Hckel antiaromatic and Möbius aromatic conformers in [32]heptaphyrin(1.1.1.1.1.1.1) are strongly affected by solvent medium as well as temperature.

Excitation energy migration processes in self-assembled porphyrin boxes constructed by conjugated porphyrin dimers.

meso-Pyridine-appended alkynylene-bridged zinc(II) porphyrin dimers D2 and D4 assemble spontaneously, in noncoordinating solvents such as toluene, into tetrameric porphyrin boxes B2 and B4, respectively. Interestingly, the formation of Bn from Dn leads to the two kinds of self-assembled porphyrin boxes constructed by planar and orthogonal conformers, respectively. Excitation energy migration processes within these assemblies have been investigated in detail by using both steady-state and time-resolved spectroscopic methods. The pump-power dependence on the femtosecond transient absorption decay profiles is directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time is well-described in terms of the Foster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates in porphyrin boxes constructed by planar and orthogonal conformers have been estimated to be (approximately 1.2 ps)(-1) and (approximately 1 ps)(-1), respectively. Furthermore, the porphyrin boxes constructed by orthogonal conformers show additional slow excitation energy hopping rates of (approximately 12 ps)(-1). Overall, the dihedral angle in the constituent dimers is a key factor to control the energy transfer efficiency for the fabrication of artificial light-harvesting complexes using conjugated porphyrin dimer systems.

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.

Rapid intramolecular hole hopping in meso-meso and meta-phenylene linked linear and cyclic multiporphyrin arrays.

A series of zinc porphyrin arrays comprised of a meso-meso linked porphyrin dimer, a meta-phenylene linked dimer, gable-like tetramers consisting of the meso-meso linked dimers bridged via a meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins within both dimers and among three porphyrins of the tetramers with rates >10(7) s(-1) at 290 K. Additionally, the hole hops among 8-12 porphyrins in the dodecameric ring with a rate that is >10(7) s(-1) at 290 K, but hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl bridges and direct meso-meso linkages do not provide optimal electronic coupling between the porphyrins within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be employed to tailor the design of long distance charge transport systems.

Structural dependence on excitation energy migration processes in artificial light harvesting cyclic zinc(II) porphyrin arrays.

A series of covalently linked cyclic porphyrin arrays CNZ that consist of N/2 of meso-meso directly linked zinc(II) porphyrin dimer subunits Z2 bridged by 1,3-phenylene spacers have been prepared by Ag(I)-promoted oxidative coupling reaction. We have investigated the excitation energy migration processes of CNZ in toluene by using femtosecond transient absorption anisotropy decay measurements by taking 2Z2 composed of two Z2 units linked by 1,3-phenylene as a reference molecule. On the basis of the excitation energy transfer rate determined for 2Z2, we have revealed the excitation energy hopping rates in the cyclic arrays CNZ by using a regular polygon model. The number of excitation energy hopping sites N(flat) calculated by using a regular polygon model is close to the observed N(expt) value obtained from the transient absorption anisotropy decays for C12Z-C18Z with circular and well-ordered structures. On the other hand, a large discrepancy between N(flat) and N(expt) was found for smaller or larger arrays (C10Z, C24Z, and C32Z). In the case of C10Z, m-phenylene linked 2Z2 motif with the interchromophoric angle of 120 degrees is not well suited to make a cyclic pentagonal array C10Z based on planar pentagonal structure. This geometrical factor inevitably causes a structural distortion in C10Z, leading to a discrepancy between N(expt) and N(flat) values. On the contrary, the presence of highly distorted conformers such as figure-eight structures reduces the number of effective hopping sites N(expt) in large cyclic arrays C24Z and C32Z. Thus, our study demonstrates that not only the large number of porphyrin chromophores in the cyclic arrays CNZ but the overall rigidity and three-dimensional orientation in molecular architectures is a key factor to be considered in the preparation of artificial light harvesting porphyrin arrays.

Structural factors determining photophysical properties of directly linked zinc(II) porphyrin dimers: linking position, dihedral angle, and linkage length.

We have investigated the relationship between the photophysical properties and structures of a series of directly linked zinc(II) porphyrin dimers (mmZ2, mbZ2, and bbZ2) using time-resolved spectroscopic measurements and theoretical calculations. Their unique characters such as CT nature and torsional motion are caused by interporphyrin interactions and steric effects, respectively, which can be fully understood in terms of three structural factors: linking position, dihedral angle, and linkage length. The orthogonal geometry and heterolinking of mmZ2 and mbZ2 induce the localized MOs and electron unbalance in the constituent porphyrin units, respectively, and consequently lead to distinct CT characters in spite of their different origin. On the other hand, a small interporphyrin steric hindrance in bbZ2 makes a torsional motion possible around the direct beta-beta' linkage in the excited state, which is correlated with the solvent dependence of the fast S(1) fluorescence decay component. On the basis of this work, we can gain further insight into the effect of individual structural factors on the photophysical properties, which provides a firm basis for further understanding of the photophysical properties mainly determined by the structural factors in multiporphyrin systems.

Aromatic versus antiaromatic effect on photophysical properties of conformationally locked trans-vinylene-bridged hexaphyrins.

We have investigated the electronic structures and energy relaxation dynamics of vinylene-bridged hexaphyrins using steady-state and time-resolved spectroscopies along with theoretical calculations in order to reveal their aromaticity-dependent electronic and magnetic properties. Ethynyl-TIPS-substituted planar and rectangular [28]hexaphyrin, regarded as a Huckel antiaromatic compound, tends to adopt a twisted Mobius aromatic topology via structural distortion in order to reduce the total internal energy, in contrast to aromatic [26]hexaphyrin, which maintains a planar conformation in solution. Spectacles-shaped vinylene-bridged [26]- and [28]hexaphyrins represent highly Huckel aromatic and antiaromatic natures, respectively, as revealed by NMR spectroscopy, giving rise to remarkable differences in NICS(0) and HOMA values and shapes of steady-state absorption and emission spectra. In particular, lifetime of the lowest singlet excited state of [28]hexaphyrin (8.6 ps) is 30 times shorter than that of the aromatic congener [26]hexaphyrin (282 ps), as measured by the femtosecond transient absorption technique. Both frontier molecular orbital analyses and vertical excitation energy calculations suggest that vinylene-bridged [28]hexaphyrin has an optically dark lowest singlet state in the NIR region, as observed in the absorption spectrum with a very low oscillator strength, which might act as a ladder state in the excited-state energy relaxation dynamics. Our findings provide further insight into the aromaticity-driven electronic properties of various porphyrinoids as well as of aromatic/antiaromatic hydrocarbon systems.