Narrow Results

The synthesis of a new terephthalate-strapped porphyrin is described. This porphyrin, prepared in the form of the free base and corresponding zinc(II) complex, was designed to act as the photo-donor portion of a novel, non-covalent artificial light-harvesting complex, the other, acceptor component of which is comprised of a covalently linked sapphyrin dimer. UV-vis and ¹H NMR spectroscopic analyses were used to establish that strong complexes are formed between the porphyrin terephthalate and sapphyrin dimer components (affinity constants on the order of 10⁵ M⁻¹ in dichloromethane). In these complexes the dicarboxylate portion of the strapped porphyrin is sandwiched between two sapphyrins of an individual dimer. Steady state fluorescence studies of the resulting ensembles support the conclusion that, upon photoexcitation of the porphyrin subunit, energy transfer from the porphyrin to the sapphyrin dimer occurs efficiently. This intra-ensemble, anion-chelation-based energy transfer can be inhibited by the addition of fluoride anion.

A series of derivatives M₂P₂ (M₂ ≡ H₄, Co₂, Ni₂, Cu₂, Zn₂, Pd₂, Pt₂, Co/Ni, Ni/Cu, Ni/Zn) of the ligand meso,meso′-bis(octaethylporphyrinyl)butadiyne has been prepared and characterized by ¹H NMR, FT Raman and visible absorption spectroscopies as well as by cyclic and a.c. voltammetry in CH₂Cl₂ solution at 20 and −40 °C. The electronic spectra exhibit multiple Soret bands and the voltammetry reveals successive one-electron reductions indicating the accessibility of ‘mixed valence’ π-radical anions and π-dianions. Using in situ thin layer spectroelectrochemistry, the UV to near-IR spectra of [M₂P₂]¹⁻ and [M₂P₂]²⁻ (M as above) were recorded at ≤ −40 °C. Apart from the Co complexes (reduced at the metal ion), the bis(porphyrin) anions have spectra which include sharp, intense near-IR bands (ε = 50 000–200 000 M⁻¹cm⁻¹) at c. 4500 and 11 500 cm⁻¹([M₂P₂]¹⁻) and 9500 cm⁻¹([M₂P₂]²⁻). An empirically constructed semiquantitative frontier orbital model explains the observed electronic absorption bands. Inter-porphyrin conjugation, mediated by the butadiyne bridge, is responsible for the Soret band multiplicity. The near-IR bands of the anions are assigned to long-axis polarized π → π* transitions within the newly occupied upper manifold of the two-porphyrin eight-orbital framework. The small comproportionation constants found for the diporphyrin monoanions contradict the usual assumption that electronically coupled dimers should have a large voltammetric separation between the first and second redox steps.

Absorption and MCD spectra of the two-electron oxidized lutetium phthalocyanine trimer [(Pc)Lu(CRPc)Lu(Pc)]²⁺ (Pc ≡ phthalocyanine;CRPc ≡ 15-crown-5-substituted Pc) as well as the MCD spectrum of the oxidized lutetium Pc dimer [Lu(Pc)₂]⁺ are reported. These ‘two-electron-deficient’ Pc stacks show absorption bands in the NIR region whose intensities are comparable with those of the Q bands. The band of [Lu(Pc)₂]⁺ showed only an MCD B term with no A term contribution, from which we assign the band to a non-degenerate transition whose transition moments are perpendicular to the Pc planes. In both trimer and dimer cases the excitation energy is higher than that of the corresponding NIR band in the ‘one-electron-deficient’ Pc stack [(Pc)Lu(CRPc)Lu(Pc)]⁺ or [Lu(Pc)₂], but the magnitudes of the shift were significantly different (1500 cm⁻¹ in the trimer and 4500 cm⁻¹ in the dimer). The energy shifts are shown to be represented by a pair of two-electron repulsion integrals in a localized orbital basis. The model reproduced well the direction and magnitude of the shifts using numerical values of the integrals.

This paper reviews the studies on the electronic structures and spectroscopic properties of sandwich-type complexes M(Pc)₂ and M₂(Pc)₃. The subjects discussed are as follows. (1) Electronic spectra of closed-shell Pc dimers and trimers. The complexes with closed-shell systems, such as [Lu(Pc)₂]⁻, Sn(Pc)₂ and Lu₂(Pc)₃, can be thought of as stacked systems composed only of Pc²⁻. The excited states of these complexes can be described by locally excited and charge transfer configurations. The coupling terms of the configurations are written using orbitals localized on each Pc ring. Assignments of the observed absorption bands are discussed. Computational studies on the band assignments were carried out using a localized molecular orbital (LO) basis which maximizes orbital populations on one of the Pc rings. (2) Electronic structures of πelectron-deficient Pc dimers and trimers. Oxidation of [Lu(Pc)₂]⁻ or Lu₂(Pc)₃ yields systems with π-electron deficiency or π-hole(s) residing on multiple Pc sites. The delocalized nature of the π-hole in Lu(Pc)₂ is elucidated by comparison of the electronic spectra of symmetric and asymmetric dimers composed of Pc and Nc(H₂Nc ≡ naphthalocyanine). The band assignments of the dimer radicals are discussed. The Pc trimer radical shows an intense absorption band at about 5000 cm⁻¹, which is 2000 cm⁻¹ lower than the valence resonance band of Lu(Pc)₂. The two-electron-deficient complexes [Lu(Pc)₂]⁺ and [Lu₂(Pc)₃]²⁺ also show intense near-IR bands at higher energy than the corresponding monoradical species. The interactions that determine the excitation energies of the near-IR bands of the π-electron-deficient species are elucidated.

New complexes of ytterbium with porphyrins containing crown- and other oxygen-containing cyclic substituents were obtained. The absorption, excitation, ligand luminescence and 4f-luminescence spectra have been investigated. It has been shown that 4f-luminescence of ytterbium Yb(III) ions was a result of the intramolecular energy transfer from the triplet level of the ligands to the resonance ²F_5/2 level of Yb(III). It has been also established that in presence of alkali metal Na, K and Cs the dimer complexes (sandwich-type) of ytterbium-crown-porphyrin were formed. Molecular luminescence of the porphyrin matrix was quenched completely in these dimers, but at the same time the quantum yield and lifetime of 4f-luminescence increase in the complexes.

The ¹H NMR spectra of the five-coordinated rhodium phthalocyanine complex Me(RPc)Rh(III) (1, RPc = dianion of 1,4,8,11,15,18,22,25-octa-n-pentylphthalocyanine) in toluene indicate that it exists as a discrete monomer at low concentrations. The complex forms a dimer aggregate reversibly in the concentration range of 2.6–10.2 mM at -90 < T < 22 °C. The dimerization constants at -60, -80, and -90 °C are 2.0 × 10², 5.7 × 10², and 1.0 × 10³M^-1, respectively. In chloroform, aggregation of the complex occurs at ~0.3 mM, reflecting the lower solvating power of chloroform for this complex. The ¹H NMR spectra of the chloroform solutions indicate that at concentrations greater than 1.0 mM, two discrete dimers in the ratio of 87:13 are reversibly formed. The ¹H NMR spectra of the major isomer, 2a, suggest that it is the 45 °-staggered cofacial dimer, a dimer with a D_4d symmetry. The ¹H NMR spectra of the minor isomer, 2b, suggest that it is a slipped cofacial dimer with a C_2h symmetry, which is formed by slipping a (RPc)Rh plane of the eclipsed dimer (a D_4h dimer) along a line connecting two opposing meso nitrogens of (RPc)Rh. Interestingly, the solution conformation of 2b is similar to that of the basic stacking unit found in the crystal structure of 1.

We present the number of dimer coverings N_d(n) on the Tower of Hanoi graph TH_d(n) at n stage with dimension 2 $\le$d$\le$ 5. When the number of vertices v(n) is even, N_d(n) gives the number of close-packed dimers; when the number of vertices is odd, it is impossible to have a close-packed configurations and one of the outmost vertices is allowed to be unoccupied. We define the entropy of absorption of diatomic molecules per vertex as S${}_{{\mathrm{\text{TH}}}_d}$=${lim}_{n\to \infty }ln$N_d(n)/v(n), that can be shown exactly for TH₂, while its lower and upper bounds can be derived in terms of the results at a certain n for TH_d(n) with 3 $\le$d$\le$ 5. We find that the difference between the lower and upper bounds converges rapidly to zero as n increases, such that the value of S${}_{{\mathrm{\text{TH}}}_d}$ with d=3 and 5 can be calculated with at least 100 correct digits.

The geometries, structural stability, electrical and magnetic characteristics of pure and multiple palladium (Pd)-adsorbed graphene, followed by hydrogen adsorption, are investigated using first-principles calculations with the density functional theory. In the DFT-D2 technique, first-principles computations with the van der Waals interaction are done using the generalized gradient approximation. In a $4\times 4$ supercell, the adsorption energy per Pd atom is found to be 1.20 eV in the optimal adsorption shape. The bandgap of 51 meV has opened in multiple Pd-decorated graphene, according to band calculations. This band’s opening is ascribed to a symmetry break. The binding energy for hydrogen adsorption in optimal double Pd-decorated graphene was determined to be in the range of (0.14–0.73) eV per hydrogen molecule, indicating that Pd-decorated graphene might be used as a hydrogen storage material.

Porphyrinic molecules have been shown to be viable candidates for a molecular-based information storage medium on the basis of redox activity. An optimal redox-based information storage medium requires a large charge density in the molecular footprint on the anchoring substrate. The use of dimeric versus monomeric architectures affords one route to achieving increased charge density without sacrificing surface cross sectional area. Towards this goal, a series of zinc and cobalt containing porphyrin dimers has been prepared and characterized. The interporphyrin linkages in the dimers include p-phenylene, ethynyl, 1,4-butadiynyl, and ethynylphenylethynyl joining porphyrin meso-positions; Crossley-type fusion bridging porphyrin β-positions, and Osuka-type triple fusions bridging one meso- and two β-positions. The electrochemical features of each dimer have been evaluated.

1,2-bis[10,15-di(3,5-di-tert-butyl)phenylporphyrinatonickel(II)-5-yl]diazene was synthesised via copper-catalyzed coupling of aminated nickel(II) 5,10-diarylporphyrin ("corner porphyrin") and its X-ray crystal structure was determined. Two different crystals yielded different structures, one with the free meso-positions in a trans-like orientation, and the other with a cis-like disposition. The free meso-positions of the obtained dimer have been further functionalized while the synthesis of a zinc analog has so far been unsuccessful. The X-ray crystal structure of the dinitro derivative of the dinickel(II) azoporphyrin was determined, and the structure showed a cis-like disposition of the nitro groups.

The QCA paradigm is one of the approaches to decrease the size scale of computing devices. When molecules are used as QCA cells, they may be able to perform computing at room temperature. This paper describes a novel molecular QCA cell candidate which is a side-by-side iron phthalocyanine dimer, and an investigation of its optical and redox properties. The new dodeca(pentyloxy) substituted side-by-side iron phthalocyanine dimer, along with the octa(pentyloxy) iron phthalocyanine monomer, are soluble in non-polar organic solvents. These compounds were isolated by gel permeation chromatography (GPC) and high-performance liquid chromatography (HPLC) to final purities of 98% and 99%, respectively. The NMR spectra of both compounds in CDCl₃ are broad due to aggregation, but become well resolved after the addition of the coordinating solvent pyridine-d₅. Addition of pyridine also gives changes in the UV-vis spectra and electrochemical peaks of both monomer and dimer in dichloromethane indicative of axial iron coordination. The electrochemical data indicates the loss of pyridine ligands from the oxidized products of both monomer and dimer. The comproportionation constant of side-by-side phthalocyanine dimer shows that its oxidized and reduced mixed-valence complexes are fairly stable. The dimer is thus a candidate for molecular QCA systems.

Metal-free and copper(II) tetra-β-polyethyleneoxide-substituted phthalocyanine derivatives, TDEO6-MPc, where polyethylene oxide derivative is hexaethyleneglycolmonododecylether (DEO6), were synthesized. Those compounds are soluble in both lipophilic and hydrophilic solvents. By taking advantage of this amphiphilic property, solvent effect on the aggregation of the phthalocyanine was studied by means of UV-vis absorption and fluorescence spectroscopies. Dimerization constants of TDEO6-MPc in non-coordinating solvents were determined by the concentration dependence of absorption spectra. Aggregation is suppressed by the increase in the E_T(30) value (acceptor property) of solvents in non-coordinating solvents, whereas it is promoted by the increase in Donor Number (donor property) of solvent in coordinating solvents.

The spectroscopic behavior of octa-t-butyltetra-2,3-triptycenotetraazaporphyrin and some of its metal complexes (Cu²⁺, Zn²⁺ and Co²⁺) were examined. UV-visible and electron paramagnetic resonance spectroscopy indicate that these phthalocyanine derivatives form cofacial dimers in pentane solution. Modeling suggests that the lowest energy configuration of the dimer is a self-complementary embrace in which the two phthalocyanine cores are staggered at an angle of 45° relative to each other. This configuration results in a remarkably intense and sharp absorption band (~635 nm; ε = ~4.0 × 10⁵ M^-1.cm^-1) arising from excitonic coupling within the dimer, a unique property for self-assembled dimers but analogous to the behavior of certain μ-oxo-dimers of silicon phthalocyanine. Introduction of methyl substituents into the bridgehead positions of the triptycene subunits prevents dimer formation.

An efficient route to meso-β doubly connected fused porphyrin dimers was developed. Synthesis of the dimers incorporated two successive C–C bond-forming steps selectively coupling unsubstituted meso- and β-positions. Using Cu(BF₄)₂ as an oxidant in nitromethane solvent, the radical coupling of Cu(II)-porphyrins occurred in high yield and without side-products, allowing chromatography-free purification. Efficient demetalation of the product yielded free-base derivatives and the possibility to incorporate other metals into the macrocycles. The absorption and electrochemical properties vary with the inserted metal, showing broad UV-visible-NIR absorption and multiple one-electron oxidations/reductions in a relatively narrow electrochemical window.

The butadiyne- and acetylene-linked 3,3′-porphycene dimers 1 and 2 were synthesized from the common intermediate, 3-ethynyl-2,7,12,17-tetrahexylporphycene 4. The butadiyne-linked dimer 1 was prepared from 4 by copper mediated Glaser-type homo-coupling. The acetylene-linked dimer 2 was synthesized by Sonogashira coupling of 4 and 2,7,12,17-tetrahexyl-3-iodoporphycene 5. These porphycene dimers were characterized by ¹H and ¹³C NMR spectroscopies, mass spectroscopy, X-ray diffraction analysis, UV-vis absorption and fluorescence spectra, cyclic voltammetry and differential pulse voltammetry. Crystal structure of 1showed a coplanar structure of two porphycene units. The absorption spectra in CH₂Cl₂ indicated the small interaction between the porphycene units through the linkage at 3,3′-positions. The electrochemical measurement showed two one-electron oxidation potentials and four one-electron reduction potentials indicating the electric interaction between the porphycene units.

A series of meso-aryloxy linked BODIPY monomers, dimers and trimer were synthesized by nucleophilic aromatic substitution (S_NAr) reaction from phenols with meso-chloro BODIPY and their photophysical properties were systematically studied by UV-vis and fluorescence spectroscopy. The relationship between their photophysical properties and the spatial arrangement of meso-aryloxy linked BODIPYs has been discussed. The monomers exhibited different extent solvent-dependent fluorescence, and fluorescence quenching in polar solvents were found relative to the HOMO energies of the donor (meso-phenols), indicating possible PET effect from meso-phenols to the BODIPY fluorophore. Ortho-dimer showed unusual broad red-shifted emission bands centered at 550 nm with a larger Stokes shifts at the range of 2900–3400 cm${}^{-1}$, and low fluorescence quantum yields, which was in sharp contrast to those of other dimers and trimer, indicating of possible excimeric species formation due to slipped cofacial arrangement ofortho-dimer.

Porphycene dimers connected by $m$-phenylene (DPc–mP) and 2,5-thienylene (DPc–T) linkages have been synthesized from 2,7,12,17-tetrahexyl-3-iodoporphycene by palladium-catalyzed cross-couplings. The crystal structures of porphycene dimers revealed that meta-phenylene linkage provides a highly twisted structure with the dihedral angel of 103° between two porphycene units, while thienylene linkage provides a nearly planar structure of two porphycene units. These porphycene dimers showed strong absorption thorough the visible to NIR regions and HOMO and LUMO energy levels of them are suitable as $n$-type materials of organic solar cells (OSCs) in combination with P3HT as a $p$-type material. The hole and electron mobilities of the blended films of the porphycene dimers and P3HT obtained by space-charge-limited-current (SCLC) method were 3.7 × 10${}^{-4}$ and 0.26 × 10${}^{-4}$ cm².V${}^{-1}$.s${}^{-1}$ for DPc–mP :P3HT, and 3.2 × 10${}^{-4}$ and 0.027 × 10${}^{-4}$ cm².V${}^{-1}$.s${}^{-1}$ for DPc–T :P3HT. Atomic force microscopy (AFM) and thin-film X-ray diffraction analysis (XRD) measurements indicated that the blended films of the porphycene dimers and P3HT formed amorphous films with smooth and low-roughness surfaces, whereas the blended film of porphycene monomer and P3HT created the highly crystalline film with huge domain structures. The OSCs composed of porphycene dimers and P3HT showed power conversion efficiencies of 0.08% was twice as high as that of monomer-based OSC.

This work summarizes the achievements in the use of monohydroxyphthalocyanines for the preparation of stable J- and H-type dimers with covalent and non-covalent bonding of the macrocycles. A modified approach for the monohydroxyphthalocyanines preparation is given. Target compounds have a tendency to nanoaggregation with distinguishing optical properties which are useful for developing the next generation of optical materials.

Two β,β′-linked porphyrin-chlorin heterodimers have been successfully synthesized with 4-fluorophenyl or 4-chlorophenyl substituted aldehyde as starting reagents. But those aldehydes with bulkier substituents did not lead to the corresponding heterodimers. These porphyrin-chlorin heterodimers and their metallic complexes have been characterized by X-ray crystallography. In all the structures, the pyrroline group in chlorin moiety and the pyrrole group in porphyrin moiety are directly connected by a single bond. Pyrroline ring has two sp${}^{\mathrm{\text{3}}}$ hybridized carbons. The direct bonding makes the porphyrin and chlorin moieties closely contact with each other, pyrroline group and the pyrrole group forms a dihedral angle of ~70°. If porphyrin-chlorin heterodimers have bulkier substituents, the close contact could cause too much repulsion. That is probably why they can not be synthesized. For nickel complexes, the chlorin planes show large saddling and moderate ruffling conformation. The C–H⋯${}^{}\pi$ interaction could contribute to the saddling conformation. The distorted core makes dihedral angles and metal to metal distances between porphyrin and chlorin plane much smaller than those in their copper complexes. Their NMR, UV-visible and fluorescence spectral data have also been briefly discussed.

Two novel quinoxaline-bridged bisBODIPYs have been synthesized by the condensation of 2,3-bis(5-formylpyrrol-2-yl)quinoxaline with 3-ethyl-2,4-dimethylpyrrole followed by a modification using a Knoevenagel reaction. They were well characterized by X-ray diffraction, NMR, HRMS UV-vis and fluorescence spectroscopy. These two quinoxaline-bridged bisBODIPYs have unusual broad absorption bands, which are different from those of typical BODIPYs They exhibit broad red-shifted emission bands centered at around 610 nm and 730 nm respectively with larger Stokes shifts at the range of 1421–2136 cm${}^{\mathrm{\text{-1}}}$ Both bisBODIPYs show different extent solvent-dependent fluorescence and exhibit fluorescence quenching in polar solvents due to the existence of possible intramolecular charge transfer.