Cofacial Dimers Research Articles

Dynamic Molecular Invasion into Multiply Interlocked Catenane Composed of Porphyrins and Phthalocyanine

Commonly, in molecular inclusion phenomena or molecular recognition processes, flexibility on the molecular size or shape of a host molecule is incompatible with a high affinity of association and accuracy of selectivity. However, if the host has a narrow access for incorporating the guest, the transition process during the complexation requires a higher activation energy. A molecular elevator, which is a molecular machine composed of two planar molecular components joined with more than two interlock linkages, could solve the dilemma through dynamic molecular invasion of a guest molecule into the cage surrounded by the planar components. The movable component with macrocyclic rings fused with a planar core can migrate between two competitive recognition sites along the linear threads attached on the other planar component. However, although the macrocyclic rings are attracted to one of the recognition sites, the planar core with the rings still has the ability to move along the threads. This is because the affinity between the ring and recognition point does not help to anchor the ring at this position, but only increases the probability of the position. Therefore, this flexibility of the interlock linkage satisfies the requirement of the dynamic invasion of a guest molecule. The molecular system permits the invasion of a large guest molecule by expanding the access and stabilizing the host–guest interaction by induced fit based on the interactions between the ring and recognition point. A multiply interlocked catenane with a novel molecular topology was synthesized; a phthalocyanine bearing four peripheral crown ethers was quadruply interlocked with a cofacial porphyrin dimer bridged with four alkylammonium chains (Figure). The supramolecular conjugate has two nanospaces surrounded by a porphyrin, a phthalocyanine, and four alkyl chains to accommodate guest molecules. Because the phthalocyanine is movable along the alkyl chains, it acts as an adjustable wall, permitting the invasion of large molecules to the nanospaces without spoiling the affinity of the association. The dynamic molecular invasion allowed the intercalation of dianionic porphyrins into both the nanospaces with a high affinity. Figure 1

Singlet Fission from Upper Excited Electronic States of Cofacial Perylene Dimer.

Singlet fission directly from the upper excited vibrational and electronic states of cofacial perylene dimers, bypassing the relaxed state S1, was detected within 50 fs. This process competes well with vibrational cooling in S1 (4.7-7.0 ps) and S2 → S1 internal conversion (380 fs). The singlet fission has the energy threshold E = 3.06 eV. Other competitive relaxation processes are excimer and dimer cation formation on an ultrafast time scale. Excitation to higher energy levels (4.96 eV) leads to a higher efficiency of singlet fission.

The Importance of Electronic Dimensionality in Multiorbital Radical Conductors.

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σRT ≈ 1 × 10-4 and ∼1 × 10-3 S cm-1, respectively) and higher thermal activation energies ( Eact = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σRT ≈ 1 × 10-2 S cm-1 and Eact = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

Synthesis and characterization of new asymmetric thieno[3,4-b]pyrazine-based D−π−A−A type small molecular donors with near-infrared absorption and their photovoltaic applications

Abstract Two new asymmetric D−π−A1−A2 configured small molecules named PTP and TTP bearing thieno[3,4-b]pyrazine (TP) unit as the A1 group have been designed and synthesized for solution-processed bulk-heterojunction organic solar cells (OSCs). These new dipolar molecular donors not only exhibit strong near-IR absorption but pack into antiparallel dimers, in which PTP shows a coexistence of coplanar and cofacial antiparallel dimers configuration, while TTP reveals a coplanar centrosymmetric dimer structure. Solution-processed OSCs for PTP blended with pristine C70 achieves a promising power conversion efficiency (PCE) of 3.40% along with an open-circuit voltage (VOC) of 0.64 V, a short-circuit current density (JSC) of 9.83 mA cm−2 and a fill factor (FF) of 54.0%, which is the highest efficiency to date among the TP-based materials. Conversely, the optimized TTP:C70 solar cell delivers a VOC of 0.48 V, a JSC of 6.02 mA cm−2 and a FF of 43.3%, corresponding to a moderate PCE of 1.25%.

Experimental and Computational Evidence for "Double Pancake Bonds": The Role of Dispersion-Corrected DFT Methods in Strongly Dimerized 5-Aryl-1λ2,3λ2-dithia-2,4,6-triazines.

Crystal structures are reported for bicyclic 3-CF3C6H4CN5S3 and monocyclic 3-CF3C6H4CN3S2, the latter of which is strongly dimerized in a cis-cofacial geometry [3-CF3C6H4CN3S2]2. The title compounds have previously been characterized in solution by NMR, displaying spectra that are consistent with the structure of [3-CF3C6H4CN3S2]2 in the crystal with anti-oriented CF3 substituents. The interannular binding was investigated using density functional theory (DFT) methods. However, the DFT-optimized geometry spreads the aryl rings too far apart (centroid–centroid distances of ≥4.353 Å versus experimental distance of 3.850 Å). Significant improvements are obtained with dispersion-corrected DFT functionals B3LYP-D3, B3LYP-D3BJ, M062X, and APFD using the 6-311+G(2d,p) basis set. However, all of these overbind the aryl rings with centroid–centroid distances of 3.612, 3.570, 3.526, and 3.511 Å, respectively. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; however, anti cofacial-4 (matching the structure in the solid state) is the most stable. Computed energies of the remainder are as follows: +7.0 kJ mol–1 (syn-cofacial-5), +26.7 kJ mol–1 (anti-cofacial-64), +27.0 kJ mol–1 (syn-cofacial-150), +102.0 kJ mol–1 (S,S-antarafacial), and +103.7 kJ mol–1 (S,N-antarafacial), where the suffixes are torsional angles around the CN3S2 thiazyl ring centroids. The binding in the four most stable cofacial dimers may be described by “double pancake bonding”.

Spectrophotometric study of the cation-induced dimerization of heteroleptic terbium(III) tetra-15-crown-5-bisphthalocyaninate

We proposed a new approach to the synthesis of heteroleptic crown-substituted terbium(III) bisphthalocyaninate, [(15C5)4Pc]Tb(Pc) (1) based on the reaction of unsubstituted lanthanum bisphthalocyaninate (La(Pc)2) with tetra-15-crown-5-phthalocyanine (H2[(15C5)4Pc]) and terbium(III) acetate in a mixture of octanol and chloronaphthalene in the presence of 1,8-diazabicyclo[ 5.4.0]undec-7-ene as an organic base. For the first time, we carried out a comparative spectrophotometric titration of compound 1 with MBPh4 solutions (M = K, Rb, Cs) and demonstrated the formation of isostructural cofacial supramolecular dimers. Spectral-structural correlations were developed for evaluation of the distances between phthalocyanine ligands in new supramolecular assemblies, which can be used for the development of polynuclear suprasingle-molecule magnetic materials.

Theoretical Study on Open-Shell Singlet Character and Second Hyperpolarizabilities in Cofacial π-Stacked Dimers Composed of Weak Open-Shell Antiaromatic Porphyrins.

From the analysis based on the broken-symmetry density functional theory (DFT) calculations, we in this study propose a strategy to enhance the open-shell characters and third-order nonlinear optical (NLO) properties of π-stacked dimers composed of antiaromatic molecules with weak open-shell characters. For this purpose, we here constructed cofacial π-stacked dimer models composed of aromatic and antiaromatic NiII porphyrins in order to examine the π-π stacking distance (R) dependence of the diradical characters (y) and static second hyperpolarizabilities (γ). The antiaromatic porphyrin dimers are found to have intermediate y around R∼3.3 Å, the result of which originates in the unique intermolecular interactions between the antiaromatic monomers. Static γ along the stacking direction of such antiaromatic porphyrin dimers with intermediate diradical characters are shown to be enhanced significantly as compared to those of the isolated monomers and the aromatic porphyrin dimers.

H-Aggregation Effects between π-Conjugated Chromophores in Cofacial Dimers and Trimers: Comparison of Theory and Single-Molecule Experiment.

Excited-state interchromophoric couplings in π-conjugated polymers present a daunting challenge to study as their spectroscopic signatures are difficult to separate from structure-dependent intrachromophoric spectral characteristics. Using custom-designed molecular model systems in combination with single-molecule spectroscopy, a controlled coupling of the excited states between cofacially arranged π-conjugated oligomers is shown to be possible. Multiscale molecular dynamics simulations allow us to generate a representative ensemble of molecular structures of the model molecule embedded in a polymer matrix and examine the connection between structural fluctuations of the molecule with theoretically predicted and measured spectral signatures. The single molecules in the embedding matrix polymer can be assigned to specific conformational features with the help of computer-based "virtual spectroscopy". By combining a quantum chemical approach with an analytical approach, we show that the coupling between the chromophores is well-described by transition dipole coupling above an interchromophoric separation of ∼4.5 Å. Even for aligned chromophores, however, twisting between repeat units of the π-system and bending of the individual π-systems can lead to a decoupling of the chromophores to a degree far beyond what their equilibrium structures would suggest: tiny displacements of the molecular constituents can dramatically impact excited-state interactions. This observation has profound implications for the design of future tunable organic optoelectronic materials.

Dynamic Molecular Invasion into Multiply Interlocked Catenane Composed of Porphyrin and Phthalocyanine

Commonly, in molecular inclusion phenomena or molecular recognition processes, flexibility on the molecular size or shape of a host molecule is incompatible with a high affinity of association and accuracy of selectivity. However, if the host has a narrow access for incorporating the guest, the transition process during the complexation requires a higher activation energy. A molecular elevator, which is a molecular machine composed of two planar molecular components joined with more than two interlock linkages, could solve the dilemma through dynamic molecular invasion of a guest molecule into the cage surrounded by the planar components. The movable component with macrocyclic rings fused with a planar core can migrate between two competitive recognition sites along the linear threads attached on the other planar component. However, although the macrocyclic rings are attracted to one of the recognition sites, the planar core with the rings still has the ability to move along the threads. This is because the affinity between the ring and recognition point does not help to anchor the ring at this position, but only increases the probability of the position. Therefore, this flexibility of the interlock linkage satisfies the requirement of the dynamic invasion of a guest molecule. The molecular system permits the invasion of a large guest molecule by expanding the access and stabilizing the host–guest interaction by induced fit based on the interactions between the ring and recognition point. A multiply interlocked catenane with a novel molecular topology was synthesized; a phthalocyanine bearing four peripheral crown ethers was quadruply interlocked with a cofacial porphyrin dimer bridged with four alkylammonium chains (Figure). The supramolecular conjugate has two nanospaces surrounded by a porphyrin, a phthalocyanine, and four alkyl chains to accommodate guest molecules. Because the phthalocyanine is movable along the alkyl chains, it acts as an adjustable wall, permitting the invasion of large molecules to the nanospaces without spoiling the affinity of the association. The dynamic molecular invasion allowed the intercalation of dianionic porphyrins into both the nanospaces with a high affinity. Figure 1

Cofacial porphyrin dimers assembled from N-heterocyclic carbene–metal bonds

A porphyrin bearing four imidazolium rings on the meso positions was used as an N-heterocyclic carbene (NHC) precursor for the synthesis of porphyrin dimers with face-to-face orientations. The porphyrins are connected through the formation of eight M-CNHC bonds, with M = AgI or AuI.

Dynamic Molecular Invasion into a Multiply Interlocked Catenane

AbstractA multiply interlocked catenane with a novel molecular topology was synthesized; a phthalocyanine bearing four peripheral crown ethers was quadruply interlocked with a cofacial porphyrin dimer bridged with four alkylammonium chains. The supramolecular conjugate has two nanospaces surrounded by a porphyrin, a phthalocyanine, and four alkyl chains to accommodate guest molecules. Because the phthalocyanine can move along the alkyl chains, it acts as an adjustable wall, thus permitting the invasion of large molecules into the nanospaces without spoiling the affinity of the association. The dynamic molecular invasion allowed the intercalation of dianionic porphyrins into both nanospaces with high affinity. A photometric titration experiment revealed the two‐step inclusion phenomenon. The multiply interlocked catenane complexed with three Cu2+ ions, and the spin–spin interaction was switched off by the intercalation of dianionic porphyrins.

Dynamic Molecular Invasion into a Multiply Interlocked Catenane.

A multiply interlocked catenane with a novel molecular topology was synthesized; a phthalocyanine bearing four peripheral crown ethers was quadruply interlocked with a cofacial porphyrin dimer bridged with four alkylammonium chains. The supramolecular conjugate has two nanospaces surrounded by a porphyrin, a phthalocyanine, and four alkyl chains to accommodate guest molecules. Because the phthalocyanine can move along the alkyl chains, it acts as an adjustable wall, thus permitting the invasion of large molecules into the nanospaces without spoiling the affinity of the association. The dynamic molecular invasion allowed the intercalation of dianionic porphyrins into both nanospaces with high affinity. A photometric titration experiment revealed the two-step inclusion phenomenon. The multiply interlocked catenane complexed with three Cu2+ ions, and the spin-spin interaction was switched off by the intercalation of dianionic porphyrins.

Intermacrocyclic Interaction Triggers Facile One‐Pot Synthesis of a Chlorin–Porphyrin Heterodimer

This work reports a highly facile one-pot synthesis of a new series of fully π-conjugated unsymmetric chlorin-porphyrin heterodimers with quantitative yields by utilizing intermacrocyclic interactions. One-electron oxidations of dicopper(II) and dipalladium(II) porphyrin dimers using mild oxidants such as iodine at room temperature resulted in the formation of a strongly interacting cofacial mixed-valent π-cation radical dimers. The radical, being highly reactive, drives spontaneous and rapid transformation involving a new N=C bond formation, 1,2-ethyl migration, and the generation of a new indolizinium ring that bridges between the two macrocycles. X-ray structural characterization of the heterodimers reveals that the two macrocycles are nearly coplanar and thereby extends the π-conjugation from one end to the other. DFT calculations that reproduce the experimental results are also reported.

Covalent bonds immobilization of cofacial Mn porphyrin dimers on an ITO electrode for efficient water oxidation in aqueous solutions

We previously reported several manganese porphyrin dimers as the first manganese-containing molecular catalysts for water oxidation, however, in non-aqueous CH3CN solution containing 5% H2O. Here, we successfully fabricated these dimers with mono- and hexaphosphonic acid groups, Mn2DP-PO3H2 and Mn2DP-(PO3H2)6, to covalently assemble them on the surface of ITO electrode (ITO=indium-doped tin oxide) and use the Mn2DP-PO3H2 and Mn2DP-(PO3H2)6|ITO assemblies as heterogeneous catalysts for electrochemical water oxidation in aqueous buffer solutions. The mono-phosphonic acid fabricated assemblies showed unprecedent high turnover frequencies (TOFs) (up to 44.9s−1) at a low overpotential (η=0.47V) in a neutral buffer solution. In acidic buffered solutions (pH = 1.5), they showed higher TOFs (up to 47.4s−1) at a very low overpotential (η=0.26V). The robustness of the mono-phosphonic acid fabricated catalysts, Mn2DP-PO3H2|ITO, was tested at a high overpotential (η=0.80V). Although they showed an oxygen evolution with 178.3s−1 TOF, the oxygen evolution completely stopped after 11h electrolysis. UV–vis spectra monitored during the electrolysis clearly indicated the gradual detachment of the catalysts from the ITO surface is likely the main reason of stopping the oxygen evolution. The hexa-phosphonic acid catalyst assembly, Mn2DP-(PO3H2)6|ITO, however, showed a continuous oxygen evolution without stopping even after 23h of electrolysis with 199.3s−1 TOF. Tafel plots in different pHs give insights on the mechanism of H2O oxidation.

A Theoretical Study of Benzene Dimers in the Excited States: Wavefunction Delocalization, Charge‐Transfer Admixture, and Electronic Coupling

Ab initio calculations were carried out to investigate various benzene dimers, including T‐shaped (T), parallel‐displaced (PD), herringbone‐type (Hb), face‐to‐face eclipsed (F2F‐E), and face‐to‐face staggered (F2F‐S) configurations in the lowest singlet (S1) and triplet (T1) excited states. Monomer and dimer geometries in the excited states were optimized at the scaled opposite‐spin (SOS) CIS(D0) level of theory, and their energetics were compared. In PD and edge‐on type dimer configurations (T and Hb), the exciton wavefunctions are localized mainly within single benzene molecules, making intermolecular interactions in the excited states nearly the same as those in the ground state. On the other hand, both the cofacial dimers (F2F‐E and F2F‐S) in the excited states present substantial enhancements in the binding energies, approximately ranging from 0.12 to 0.55 eV, compared to the ground state. However, further analyses of electronic couplings between the frontier molecular orbitals of each molecule demonstrate the significant difference between such dimers; unlike in the F2F‐E dimer, in the F2F‐S configuration, the excimer formation arises from coupling between different states of each monomer, e.g., the S1 state of one molecule and the S2 state of the other. In addition, for these cofacial dimers, the electronic coupling for triplet exciton is much weaker than that for its singlet counterpart, which can be rationalized by the different role of exchange energy in stabilizing respective states.

Diphenylphosphine-Oxide-Fused and Diphenylphosphine-Fused Porphyrins: Synthesis, Tunable Electronic Properties, and Formation of Cofacial Dimers.

Diphenylphosphine-oxide-fused NiII porphyrin 8 was synthesized from 3,5,7-trichloroporphyrin 5 via a reaction sequence of nucleophilic aromatic substitution with lithium diphenylphosphide, oxidation with H2 O2 , and palladium-catalyzed intramolecular cyclization. Reduction of 8 with HSiCl3 gave diphenylphosphine-fused NiII porphyrin 9. The embedded P=O and P moieties serve as a strong electron-accepting and electron-donating group to perturb the optical and electrochemical properties of the NiII porphyrin. NiII porphyrin 9 is diamagnetic with a low-spin NiII center in solution but becomes paramagnetic with a five-coordinated NiII center with high-spin (S=1) state in the solid state. Diphenylphosphine-oxide-fused ZnII porphyrin 10 was also synthesized and shown to form a face-to-face dimer with mutual O-Zn bonds in the crystal and in nonpolar and moderately polar solvents. The dimerization of 10 in CDCl3 has been revealed to be an entropy-driven process with a large entropy gain (ΔSD =207 J K-1 mol-1 ).

Facile Synthesis of Cofacial Porphyrin Oligomers Via the Catalyst-Free Aromatic Nucleophilic Substitution Reactions

Cofacial porphyrin dimers have attracted a considerable attention due to their potential as artificial reaction centers in the photosystem, bimetallic catalysts and function as host molecule. However, their syntheses have required troublesome multi-step reactions. Recently, we have reported a facile and efficient synthetic method for meso-heteroatom-substituted porphyrins from the corresponding meso-bromoporphyrins via the catalyst-free aromatic nucleophilic substitution (SNAr) reactions.1,2,3 In this work, we applied this efficient method to the synthesis of cofacial porphyrin dimers bridged by the dihydroxyarene derivatives. Reaction of resorcinol with an excess of meso-bromoporphyrin in the presence of base afforded the acyclic dimers 1 in good yield. The steric hindrance between porphyrin and substituents on resorcinol determined their formations. For example, 4,6-diethylresorcinol gave cofacial conformation whereas 2-methylresorcinol gave open conformation. Reaction of resorcinol or 2,7-dihydroxynaththalene with meso-dibromoporphyrin afforded cyclic porphyrin dimers 2 and 3 with rigid cofacial conformation in good yield. We will also report the synthesis, structural characterization, and physical properties of the obtained dimer and their metal complexes. References (1) K. Yamashita, K. Kataoka, M. S. Asano, K. Sugiura, Org. Lett. 2012, 14, 190–193. (2) K. Yamashita, S. Sakamoto, A. Suzuki, K. Sugiura, Chem. Asian J. 2016, 11, 1004–1007. (3) K. Yamashita, K. Kataoka, S. Takeuchi, M. S. Asano, K. Sugiura, J. Org. Chem. 2016, 81, 11176–11184. Figure 1

Back Cover: Oxygen Reduction to Water by a Cofacial Dimer of Iron(III)–Porphyrin and Iron(III)–Phthalocyanine Linked through a Highly Flexible Fourfold Rotaxane (Chem. Eur. J. 31/2017)

Back Cover: Oxygen Reduction to Water by a Cofacial Dimer of Iron(III)–Porphyrin and Iron(III)–Phthalocyanine Linked through a Highly Flexible Fourfold Rotaxane (Chem. Eur. J. 31/2017)

Oxygen Reduction to Water by a Cofacial Dimer of Iron(III)-Porphyrin and Iron(III)-Phthalocyanine Linked through a Highly Flexible Fourfold Rotaxane.

A μ-oxo-dinuclear iron complex of a supramolecular porphyrin-phthalocyanine conjugate was synthesized and its catalytic electrochemical oxygen reduction properties were investigated. In the conjugate, porphyrin and phthalocyanine units were connected to form a cofacial dimeric structure through a flexible fourfold rotaxane linkage, which was advantageous for accommodating small substrates between the iron centers. The conjugate showed efficient catalytic properties, at more positive potentials than those of other reported dinuclear porphyrinoid iron complexes, to selectively afford water through a four-electron reduction process.

Programmable Arrangement of Heterometal Ions in a Supramolecular Array of Porphyrin and Phthalocyanine

Abstract Cofacial dimers of porphyrin and/or phthalocyanine are plausible frameworks for functional molecular systems such as molecular catalysts and molecular memories. We recently developed a supramolecular array of a porphyrin and phthalocyanine linked by a four-fold rotaxane. In this study, we demonstrated programmable hetero-ion arrangement in the heterodimer of these tetrapyrrolic ligands. Ni(II) and Fe(II) ions showed kinetically selective binding to the porphyrin, whereas Mn(II) ions selectively bound to the phthalocyanine, in the supramolecular array. After the first metal complexation, subsequent metal insertion into the vacant ligand afforded the programmed heterodinuclear complex. Moreover, the metal arrays were extended to heterotrinuclear complexes via ionic pairing of the cationic four-fold rotaxane with a tetraanionic metalloporphyrin.