Linked Porphyrin Arrays Research Articles

Triply Linked Porphyrinoids.

Triply linked porphyrin arrays, so called porphyrin tapes, are intriguing molecules in terms of their planar structures, far red-shifted absorption bands, multi-charge storage abilities, and high conductivities. Recently, porphyrin tape variants such as porphyrin-expanded porphyrin hybrid tapes, porphyrin arch-tapes, corrole tapes, and a subporphyrin tape have been explored as novel π-conjugated materials. These new molecules exhibit electronic and structural characteristics owing to direct triple linkages. This Minireview first discusses the fundamental properties of the parent porphyrin tapes and the peripherally functionalized porphyrin tapes. Then, a focus has been placed on the syntheses, structures, and unique properties of the recently explored porphyrin tape variants.

Investigation and Control of Single Molecular Structures of Meso- Meso Linked Long Porphyrin Arrays.

We have investigated conformational structures of meso- meso linked porphyrin arrays (Z n) by single molecule fluorescence spectroscopy. Modulation depths ( M values) were measured by excitation polarization fluorescence spectroscopy. The M value decreases from 0.85 to 0.46 as the number of porphyrin units increases from 3 to 128, indicating that longer arrays exhibit coiled structures. Such conformational changes depending on the length have been confirmed by coarse-grained simulation. The histograms of M values and traces of centroid position of emitting sites by localization microscopy showed that the structures of longer arrays changed to more stretched after solvent vapor annealing with tetrahydrofuran.

Synthesis of Direct β‐to‐β Linked Porphyrin Arrays with Large Electronic Interactions: Branched and Cyclic Oligomers

AbstractDirect β‐to‐β linked branched and cyclic porphyrin trimers and pentamers have been synthesized by the Suzuki–Miyaura coupling of β‐borylporphyrins and β‐bromoporphyrins. The cyclic porphyrin trimer, the smallest directly linked cyclic porphyrin wheel to date, and its twined pentamer, exhibit small electrochemical HOMO–LUMO gaps, broad nonsplit Soret bands, and red‐shifted Q‐bands, thus indicating large electronic interactions between the constituent porphyrin units.

Synthesis of direct β-to-β linked porphyrin arrays with large electronic interactions: branched and cyclic oligomers.

Direct β-to-β linked branched and cyclic porphyrin trimers and pentamers have been synthesized by the Suzuki-Miyaura coupling of β-borylporphyrins and β-bromoporphyrins. The cyclic porphyrin trimer, the smallest directly linked cyclic porphyrin wheel to date, and its twined pentamer, exhibit small electrochemical HOMO-LUMO gaps, broad nonsplit Soret bands, and red-shifted Q-bands, thus indicating large electronic interactions between the constituent porphyrin units.

Fused porphyrinoids as promising near-infrared absorbing dyes

Various aromatic segments have been fused onto the porphyrin periphery to create porphyrinoids that have expanded π-conjugated networks and thus exhibit red-shifted absorption spectra. Fused coplanar porphyrin oligomers, represented by meso–meso, β–β, β–β triply linked porphyrin arrays (porphyrin tapes), are endued with more red-shifted absorption spectra and better nonlinear optical properties. These fused porphyrinoids have emerged as promising near-infrared (NIR) absorbing dyes, pointing to future applications such as conducting wire, NIR emitter, photovoltaics, and nonlinear optical materials. Developments of the fused porphyrinoid chemistry are reviewed in this feature article with focus on the synthesis, the relationships between their absorption spectra and molecular structures, and the applications in materials science.

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

A series of meso-meso, β-β, β-β 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.

Synthesis of Extremely π‐Extended Porphyrin Tapes from Hybrid meso‐meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length

Directly meso-meso, beta-beta, beta-beta triply linked porphyrin arrays are exceptional pi-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12-mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso-meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso-meso linked porphyrin arrays BOn up to 24-mer, which have two different aryl groups, a 2,4,6-tris(3,5-di-tert-butylphenoxy) phenyl group (Ar(1)) and a 3,5-dioctyloxy phenyl group (Ar(2)). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ-Sc(OTf)(3). Importantly, the low energy Q-band-like absorption bands of TBOn are progressively red-shifted with an increase in the number of porphyrins n until 16 but the red-shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14-16. The regularly introduced meso-aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of pi-pi interactions as well as improvement of the chemical stabilities of TBOn.

π‐Conjugation Enlargement Toward the Creation of Multi‐Porphyrinic Systems with Large Two‐Photon Absorption Properties

Recent progress in the synthesis of covalently linked porphyrin arrays with large two-Photon absorption (TPA) cross-section values has been reviewed with a particular focus on the relation of TPA properties with molecular structures. Covalently linked porphyrin arrays continue to be important and useful for the creation of functional materials owing to their chemical robustness, fine-tuning, and easy manipulation. More importantly, the porphyrin electronic systems are quite susceptible to periphery conjugative perturbations, hence allowing facile fabrications to extensively delocalized systems. This property has been used for exploration of porphyrin-based molecular systems with large TPA values, demonstrating a general trend that enhancement in electronic interactions leads to large TPA cross-section values. As a representative example, the porphyrin tapes exhibit larger TPA values owing to the fully delocalized nature of the pi-electrons. This Focus Review will help understand the structural requirements of porphyrin arrays with large TPA values, which will be useful for future applications in optical communication in the IR region.

Post‐Modification of meso–meso‐Linked Porphyrin Arrays by Iridium and Rhodium Catalyses for Tuning of Energy Gap

meso-meso-Linked diporphyrins fabricated with multiple unsaturated carboxylic acid groups are efficiently synthesized by the use of transition-metal catalysis. Iridium-catalyzed direct borylation of meso-meso-linked diporphyrins furnish multi-borylated diporphyrins with high regioselectivity. Then, the introduction of alpha,beta- or alpha,beta,gamma,delta-unsaturated ester functions to the diporphyrins is achieved through rhodium-catalyzed Heck-type addition of the borylated diporphyrins to acrylate or 2,4-pentadienoate esters. Saponification of the esters to the corresponding acids is accomplished in high yields. The post-modification of diporphyrin 6 smoothly provides a water-soluble multiporphyrin array 1 in 84% total yield in 3 steps. This functionalization significantly affects the electronic properties of the porphyrins, resulting in different energy levels of each porphyrin unit. The absorption and fluorescence spectra indicate efficient energy transfer in the porphyrin dimers.

Determination of the Superradiance Coherence Length of Directly Linked Linear Porphyrin Arrays at the Single‐Molecule Level

Down to the wire: Photobleaching dynamics show the exciton delocalization length of directly linked porphyrin arrays (see picture) to be about four or five porphyrin units at the single-molecule level. This result provides a better understanding of how light-signal transmission occurs in the solid state and gives a perspective for the porphyrin arrays to be used as single-molecule photonic wires.

Synthesis of Doubly Strappedmeso–meso‐Linked Porphyrin Arrays and Triply Linked Conjugated Porphyrin Tapes

Abstract1,10‐Dioxydecamethylene doubly strapped ZnII‐porphyrinS1was prepared and treated with AgPF6to givemeso–meso‐linked porphyrin oligomersSn(n= 2, 3, 4, 6, 8, and 12), which were converted to triply linked porphyrin tapesTSnbymeso,meso′‐dibromomeso–meso‐linked porphyrin arraysBSnandmeso,meso′‐diphenylmeso–meso‐linked porphyrin arraysPSn. The structures ofS1andS2have been determined by single‐crystal X‐ray diffraction analysis. Characteristically,Snexhibit sharp Q(0,0) absorption and fluorescence bands. Low energy Q‐band‐like absorption bands ofTSnare progressively red‐shifted with an increase in the number of porphyrins without saturation behavior of conjugation. The double straps suppress π–π stacking to some extent as seen from partial preservation of vibration structures in the Q‐band‐like bands ofTS4andTS6and improve the chemical stabilities of longer tapes such asTS8andTS12. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Quantum-Chemical Investigation of the Electroabsorption Spectra of Directly Meso−Meso-Linked Porphyrin Arrays: Essential Role of Charge-Transfer Excited States Accidentally Overlapping with Soret Bands

The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.

Directly Linked Porphyrin Arrays with Tunable Excitonic Interactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Directly Linked Porphyrin Arrays with Tunable Excitonic Interactions

On the basis of the Ag(I)-promoted coupling reaction of Zn(II) 5,15-diaryl porphyrin that gave a meso-meso-linked diporphyrin, we developed a variety of directly linked porphyrin arrays including linear, windmill, gridlike, cyclic, and box architectures. Electronic and excitonic interactions are thus fine tuned by placing porphyrin chromophores in well-defined arrangements. Photoexcited-state dynamics of these porphyrin arrays, as revealed by various ultrafast laser-based measurements, are pertinent to photosynthetic light-harvesting antenna in terms of very efficient excitation energy hopping over many porphyrins and lack of a defect that acts as energy sink. The conformational flexibility of a meso-meso-linked diporphyrin has also been used for the fine tuning of excitonic interactions as demonstrated by strapped meso-meso diporphyrins and reversible switching of energy transfer in a triporphyrin. Triply linked porphyrin arrays have also been explored, which exhibit an exceptionally low HOMO-LUMO gap as a result of a fully conjugated pi electronic system over a coplanar platform.

Porphyrin supramolecules by complementary coordination for units constructing photosynthetic systems

Among unique structural arrangements provided by nature, special pair and macroring antenna complexes in bacterial photosynthetic systems have been mimicked by simple organization of choromophores. The special pair was mimicked by imidazolyl-substituted porphyrinatozinc which gave a complementary coordination dimer of slipped cofacial orientation with an extremely large stability constant of 1010 M-1 in CHCl 3. When two imidazolylporphyrinatozinc units were linked directly at the meso positions, linear and continuous growth of the complementary coordination lead to porphyrin arrays of a few hundreds nanometer scale, corresponding to molecular weights of a few 105. At the same time, the porphyrin array could be dissociated into the monomeric unit by competitive coordination of solvents. This provided a way of terminating or initiating the oligomeric porphyrin array with appropriate chain terminals/initiators. Two imidazolylporphyrinatozinc complexes were then linked by a m-phenylene unit with an angle of 120°. The linear and macrocyclic oligomer mixtures initially obtained were converged smoothly by reorganization equilibrium into a mixture of hexameric and pentameric macrocycles under high-dilution conditions. The ring mimicked the structure and function of the light harvesting complexes of bacterial photosynthetic systems. The covalent linking of coordination organized porphyrins was also developed to maintain the structure even in highly coordinating solvents such as pyridine. The linear array formation and the facile introduction of specific terminal/initiator groups by complementary coordination were then applied to introduce antenna function onto solar cell. Through thiolate attachment on a gold surface, imidazolylporphyrinatozinc initiated the growth of meso-meso linked porphyrin arrays by successive complementary coordination. This methodology improved the efficiency of absorption of incident photon and increased significantly the total photocurrent generation efficiency.

Electrical conduction through linear porphyrin arrays.

Electrical conduction measurements were made on two extreme types of directly linked porphyrin arrays by using nanoelectrodes. One type is the directly linked Zn(II)porphyrin arrays, consisting of 48 Zn(II)porphyrin moieties (Z48), and the other type is the completely flat, tape-shaped Zn(II)porphyrin arrays, consisting of eight Zn(II)porphyrin units (T8). The I-V curve for Z48 exhibits the diode-like behavior and the hysteresis depending on the voltage sweep direction presumably due to the conformational heterogeneity arising from the dihedral angle distribution in Z48. On the other hand, the I-V curve for T8 is nearly symmetric without any hysteresis, leading to the higher conductivity and the smaller band gap. These results illustrate that the stronger pi-electron conjugation in T8, as compared with that of Z48, results in better electrical conduction.

Photophysical properties of porphyrin tapes.

The novel fused Zn(II)porphyrin arrays (Tn, porphyrin tapes) in which the porphyrin macrocycles are triply linked at meso-meso, beta-beta, beta-beta positions have been investigated by steady-state and time-resolved spectroscopic measurements along with theoretical MO calculations. The absorption spectra of the porphyrin tapes show a systematic downshift to the IR region as the number of porphyrin pigments increases in the arrays. The fused porphyrin arrays exhibit a rapid formation of the lowest excited states (for T2, approximately 500 fs) via fast internal conversion processes upon photoexcitation at 400 nm (Soret bands), which is much faster than the internal conversion process of approximately 1.2 ps observed for a monomeric Zn(II)porphyrin. The relaxation dynamics of the lowest excited states of the porphyrin tapes were accelerated from approximately 4.5 ps for the T2 dimer to approximately 0.3 ps for the T6 hexamer as the number of porphyrin units increases, being explained well by the energy gap law. The overall photophysical properties of the porphyrin tapes were observed to be in a sharp contrast to those of the orthogonal porphyrin arrays. The PPP-SCI calculated charge-transfer probability indicates that the lowest excited state of the porphyrin tapes (Tn) resembles a Wannier-type exciton closely, whereas the lowest excited state of the directly linked porphyrin arrays can be considered as a Frenkel-type exciton. Conclusively, these unique photophysical properties of the porphyrin tapes have aroused much interest in the fundamental photophysics of large flat organic molecules as well as in the possible applications as electric wires, IR sensors, and nonlinear optical materials.

Fluorescence from the Highly Excited States and Vibrational Energy Relaxation in Directly Linked Porphyrin Arrays

Abstract The fluorescence decay profiles of zinc(II) 5,15-di-(3,5-di-tert-butylphenyl) porphyrin monomer (Z1) and its meso, meso-linked dimer (Z2), trimer (Z3) and tetramer (Z4) were investigated by using the fluorescence upconversion technique. Fluorescence from the S2 state in Z1 was observed to decay with the time constant of 1.2 ps at 460 nm. Corresponding fluorescence rise due to the internal conversion from S2 to S1 states in Z1 was also observed at 635 nm with the time constant of 1.5 ps. The S2 state lifetime of Z2 was largely reduced (90 fs) due to the participation of exciton-split Soret band as a ladder type deactivation channel. Fluorescence decay profile and time-resolved spectra in 530 – 600 nm region revealed that the intramolecular vibrational relaxation (IVR) rate is retarded as the excess vibrational energy decreases in the porphyrin arrays as well as in Z1. The IVR rate was increased from Z1 to Z2 probably due to the additional normal modes related with the inter-unit bond as an efficient internal bath. Furthermore it was observed that a bottleneck in the bath modes limits the increase of IVR rate with the increased number of internal modes in the arrays over Z2 molecule.

Resonance Raman Spectroscopic Investigation of Directly Linked Zinc(II) Porphyrin Linear Arrays

We have investigated the resonance Raman (RR) spectra of the directly linked porphyrin arrays in order to elucidate the relationship between excitonic interactions and molecular geometry depending on the number of pigments in the arrays. The RR spectra obtained by photoexcitation at the high-energy exciton Soret bands in the arrays are mainly composed of Raman modes localized on the constituent porphyrin monomers. In contrast, the RR spectra of the arrays with photoexcitation at the low-energy exciton split Soret bands reveal some characteristic Raman bands arising from strong excitonic interactions between the adjacent porphyrins in the arrays. Based on the RR measurements of the isotope labeled analogues and the normal-mode analysis of the dimer, it is suggested that the photoexcitation at the high-energy Soret band produces an electronically excited state largely localized within a monomer unit and that at the low-energy exciton split Soret band the excited state is in a way delocalized throughout the ...

Synthesis of meso-meso linked hybrid porphyrin arrays by Pd-catalyzed cross-coupling reaction.

meso-meso linked porphyrin arrays of a variety of compositions and connectivities have been prepared by Pd-catalyzed cross-coupling reaction in a manner complementary to Ag(I)-promoted oxidative coupling. [reaction: see text]