Parent,Unsubstituted Hemiporphycene: Synthesis and Properties

Among seven possible nitrogen‐in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene ( HPc ), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner‐cavity dimensions, orbital‐energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH‐stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol^?1. Tautomerization from the higher‐ to the lower‐energy form is detected in the lowest‐excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H‐bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N ??? N distance and the N?H?N angle.     

Vibrational gating of double hydrogen tunneling in porphycene

A procedure that enables determining the reaction rate from the analysis of fluorescence anisotropy is described and applied to the investigation of double hydrogen transfer between inner-cavity nitrogen atoms in electronically excited porphycene. Tautomerization proceeds as a thermally activated synchronous double hydrogen tunneling. The barrier to the reaction is dynamically modulated by a vibration that simultaneously changes the strength of two intramolecular hydrogen bonds. Different mechanisms of tautomerization in porphycene and its parent isomer, porphyrin, can be understood by analyzing the potentials for hydrogen transfer.     

First synthesis of dodecasubstituted porphycenes

The synthesis of dodecasubstituted porphycenes has not been reported, to date. Herein, the preparation of tetramethyloctaethylporphycene by a McMurry‐type coupling of 3,3′,4,4′‐tetraethyl‐5,5′‐diformyl‐2,2′‐bipyrrole was attempted at first, but dodecasubstituted porphycene was not successfully obtained and only pyrrolocyclophene was obtained. The structure of the pyrrolocyclophene was determined by ¹H NMR spectroscopy, FAB MS, and X‐ray crystal‐structure analysis. The pyrrolocyclophene was not successfully oxidized to porphycene. Then, the McMurry‐type coupling of bicyclo[2.2.2]octadiene (BCOD)‐fused 5,5′‐diacyl‐2,2′‐bipyrroles was performed and tetra‐meso‐octa‐β‐substituted (dodecasubstituted) porphycenes were successfully obtained for the first time. The structures were determined by ¹H NMR spectroscopy and X‐ray crystal‐structure analysis. The crystal structures and NMR spectra were compared carefully with octasubstituted porphycenes, and there was a good correlation between the position of the substituents, the N1? N2 and N1? N4 distances of the porphycene inner nitrogen atoms, and NMR chemical shifts of the inner NH protons, which expressed the strength of N? H???N hydrogen bonding between N1 and N2. These results suggested that the BCOD structure was relatively compact compared with common alkyl groups and that was why the dodecasubstituted porphycenes were available this time. UV/Vis absorption and fluorescence properties are also discussed.     

Porphycenes: synthesis and derivatives

Porphycene is an aromatic macrocycle and a constitutional isomer of porphyrin. It and its derivatives display unique physical and optical properties, including strong absorptions in the red region of the UV-vis spectrum. These features have made porphycene and porphycene analogues appealing molecules for use in biomedical applications and in the design of new materials. This critical review provides a concise overview of the most important syntheses of porphycenes, as well as those of various functionalized derivatives and heteroatom containing analogues. (95 references.)     

Hydrazine-hydrazone tautomerism in phenylhydrazinopyrimidones

4-Phenylhydrazino-2-pyrimidone and its 1-methyl analog have been shown by ¹H nmr to exist in intercon-verting hydrazine and hydrazone tautomers in deuteriodimethylsulfoxide solution. Solvent effects indicate that increasing solvent polarity favors the hydrazine forms. In contrast, the 3-methyl analog occurs exclusively as the hydrazone. The hydrazone forms of the parent and 1-methyl derivatives appear to adopt the syn rotamers as a result of intramolecular hydrogen bonding. Variable temperature studies showed relatively high free energy barriers to tautomerization in these compounds, resulting both from solvation and intramolecular hydrogen bonding. The ¹H nmr spectra of 1-methyl-4-hydrazino-2-pyrimidone suggest that it exists predominately as the hydrazone in deuteriodimethylsulfoxiderdeuteriochloroform solution, although the barrier to tautomerization is similar to those for the phenylhydrazino compounds.     

Molecular dynamics and density functional theory simulations of matrix deposition. II. Absolute site structure assignment for porphyrin in xenon

Molecular dynamics calculations reveal that the main trapping site for porphyrin embedded in a xenon matrix corresponds to a hexagonal cavity formed after removal of seven host atoms. Tautomerization involving two inner hydrogen atoms leads to two trans forms that interact differently with the matrix cage. Therefore, both electronic and infrared spectra are split into doublets. Comparison of the experimentally observed splitting patterns with the results of density functional theory calculations that explicitly include the nearest xenon atoms allows assigning each spectral feature to one of two different configurations of the chromophore inside the xenon cavity. The main factor responsible for the splittings is a distortion of the molecular skeleton from a squarelike towards rectangular geometry.     

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.     

A Hexagonal Covalent Porphyrin Framework as an Efficient Support for Gold Nanoparticles toward Catalytic Reduction of 4‐Nitrophenol

A hexagonal porphyrin‐based porous organic polymer, namely, CPF‐1, was constructed by 3+2 ketoenamine condensation of the C₂‐symmetric porphyrin diamine 5,15‐bis(4‐aminophenyl)‐10,20‐diphenylporphyrin and 1,3,5‐triformylphloroglucinol. This material exhibits permanent porosity and excellent thermal and chemical stability. CPF‐1 can be employed as a superior supporting substrate to immobilize Au nanoparticles (NPs) as a result of the strong interactions between Au NPs and the CPF support. An Au@CPF‐1 hybrid was synthesized by an interfacial solution infiltration method with NaBH₄ as reducing agent. Au NPs (5 nm) grew on CPF‐1 and were distributed without aggregation. Moreover, Au@CPF‐1 exhibits superior catalytic activity compared to many other reported Au‐based catalysts for the reduction of 4‐nitrophenol in the presence of NaBH₄. In addition, Au@CPF‐1 has excellent stability and recyclability, and it can be reused for three successive reaction cycles without loss of activity. The dense distribution of phenyl rings on the channel walls of the CPF support can reasonably be regarded as the active sites that adsorb the 4‐nitrophenol molecule through hydrogen‐bonding and C?H ??? π interactions, as was confirmed by the X‐ray structure of model compound DAPP‐Benz.     

Conformational Re‐engineering of Porphyrins as Receptors with Switchable N−H⋅⋅⋅X‐Type Binding Modes

The selectivity and functional variability of porphyrin cofactors are typically based on substrate binding of metalloporphyrins wherein the pyrrole nitrogen units only serve to chelate the metal ions. Yet, using the porphyrin inner core system for other functions is possible through conformational engineering. As a first step towards porphyrin “enzyme‐like” active centers, a structural and spectroscopic study of substrate binding to the inner core porphyrin system shows that a highly saddle‐distorted porphyrin with peripheral amino receptor groups ( 1 , 2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetrakis(2‐aminophenyl)porphyrin) coordinates analytes in a switchable manner dependent on the acidity of the solution. The supramolecular ensemble exhibits exceptionally high affinity to and selectivity for the pyrophosphate anion (2.26±0.021)×10⁹ m ^?1. ¹H NMR spectroscopic studies provided insight into the likely mode of binding and the characterization of atropisomers, all four of which were also studied by X‐ray crystallography.     

Preparation and O2 binding study of myoglobin having a cobalt porphycene

Sperm whale myoglobin, an oxygen-storage hemoprotein, was reconstituted with 2,7-diethyl-3,6,12,17-tetramethyl-13,16-bis(carboxyethyl)porphycenatocobalt(II) in order to investigate the reactivities of a cobalt porphycene in a protein matrix. Similar to the previously reported finding for the myoglobin with the iron porphycene, the reconstituted myoglobin with the cobalt porphycene was also found to have an O2 affinity 2 orders of magnitude greater than that of the myoglobin possessing cobalt protoporphyrin IX. The EPR spectra of the deoxy and oxy myoglobins having the cobalt porphycene at 77 K also have features similar to those of the myoglobin with cobalt protoporphyrin IX. These spectra suggest that the porphycene cobalt in the deoxy form is coordinated by one nitrogenous ligand postulated to be the imidazole ring of His93, and that the bond configuration of CoII-O2 is regarded as the CoIII-Omicron2*- species.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

1H/2H NMR studies of geometric H/D isotope effects on the coupled hydrogen bonds in porphycene derivatives

The 1H and 2H NMR spectra of porphycene (1), 2,3,6,7,12,13,16,17-octaethylporphycene (2), 2,7,12,17-tetra-n-propylporphycene (3), and 2,7,12,17-tetra-(tert-butyl)-3,6-13,16-dibenzo[cde;mno]porphycene (4) partially deuterated in the mobile proton sites are reported. These compounds exhibit two intramolecular NHN hydrogen bonds of increasing strength representing models of the concerted HH transfer in the parent compound, porphycene. The 1H chemical shifts of the mobile protons are correlated with the difference of the energies of the amino- and imino-N1s orbitals reported by Ghosh A.; Moulder J.; Br?ring M.; Vogel E. Angew. Chem., Int. Ed. 2001, 113, 445-448. The chemical shifts of 4 indicate a reduced contribution of the aromatic ring current as compared to the other compounds which is associated to the nonplanarity of this molecule. The primary H/D isotope effects on the chemical shifts give information about the primary, secondary, and vicinal geometric isotope effects of the two inner hydrogen bonds of porphycenes. The vicinal effects indicate a cooperative coupling of the two hydrogen bonds which may favor a concerted double proton-transfer mechanism.     

Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole‐based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl‐substituted 2,2′‐biimidazole using low‐valent titanium, followed by treatment with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone. Imidacene displays a porphyrin‐like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π‐electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high‐level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π‐electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π‐electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π‐ and 22 π‐electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π‐electron five‐membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.     

Laser control of double proton transfer in porphycenes: towards an ultrafast switch for photonic molecular wires

Electronic excitation energy transfer along a molecular wire depends on the relative orientation of the electronic transition dipole moments of neighboring chromophores. In porphycenes, this orientation is changed upon double proton transfer in the electronic ground state. We explore the possibility to trigger such a double proton transfer reaction by means of an infrared pump-dump laser control scheme. To this end, a quantum chemical characterization of an asymmetrically substituted porphycene is performed using density functional theory. Ground state geometries, the topology of the potential energy surface for double proton transfer, and \(\hbox{S}_0\rightarrow\hbox{S}_1\) transition energies are compared with the parent compound porphycene and a symmetric derivative. Employing a simple two-dimensional model for the double proton transfer, which incorporates sequential and concerted motions, quantum dynamics simulations of the laser-driven dynamics are performed which demonstrate tautomerization control. Based on the orientation of the transition dipole moments, this tautomerization may lead to an estimated change in the Förster transfer coupling of about 60%.     

Inner-hydrogen tautomerism in some 5,15-unsymmetrically disubstituted and beta-unsubstituted porphyrins

5,15-Unsymmetrically disubstituted and beta-unsubstituted porphyrins such as 5-R, 15-(3,5-dimethoxyphenyl) porphyrins [where R=2-benzyloxy-1-naphthyl (1), 2-(2-naphthylmethoxy)-1-naphthyl (2), anthryl (3), or 2,4,6-triphenylphenyl (4)] and 5-(2-benzyloxy-1-naphthyl), 10,15,20-tri(3,5-dimethoxyphenyl) porphyrin (1') were synthesized and studied by (1)H-NMR spectroscopy. At room temperature, 1, 2, 3 and 4 showed doubling of the inner-hydrogen resonances with equal intensities, whereas the pyrrolic betaH signals were completely averaged. For 1 and 1', variable-temperature (1)H-NMR experiments were also performed. For 1, the two peaks of the inner hydrogen coalesced at about 313 K. In contrast, the pyrrolic betaH signals were only slightly broadened even at 213 K. On the other hand, 1' showed ordinary singlets of the inner hydrogens at room temperature, and the resonances of both the inner hydrogens and the pyrrolic betaH coalesced at about 233 K. We interpret these results as indicating the existence of two distinct paths, one slow and the other fast, leading to NH tautomerization in 1. We discuss the structures and energies of cis-tautomers as transition intermediates in relation to the two paths of NH tautomerization.     

Trans doubly N-confused porphyrins: Cu(III) complexation and formation of rodlike hydrogen-bonding networks

A trans type of doubly N-confused isomer of NCP (trans-N2CP) was synthesized via N-confused fused porphyrin (NcFP). The aromatic feature of trans-N2CP due to 18pi electronic system is contrasted to the weak aromaticity of cis-derivative. The solid-state structure of trans-N2CP exhibits pi-stacking column, while the Cu(III) complex shows 1-D rodlike hydrogen bonding chain comparable with the zigzag hydrogen-bonding chain of cis-derivatives.     

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.     

Packing forces in dichloridobis(3,5‐diphenyl‐1H‐pyrazole‐κN2)cobalt(II) dichloromethane hemisolvate

The title compound, [CoCl₂(C₁₅H₁₂N₂)₂]·0.5CH₂Cl₂, was crystallized from a binary mixture of dichloromethane and hexane and a dimeric supramolecular structure was isolated. The Co^II centre exhibits a distorted tetrahedral geometry, with two independent pyrazole‐based ligands occupying two coordination sites and two chloride ligands occupying the third and fourth coordination sites. The supramolecular structure is supported by complementary hydrogen bonding between the pyrazole NH group and the chloride ligand of an adjacent molecule. This hydrogen‐bonding motif yields a ten‐membered hydrogen‐bonded ring. Density functional theory (DFT) simulations at the PBE/6‐311G level of theory were used to probe the solid‐state structure. These simulations suggest that the chelate undergoes a degree of conformational distortion from the lowest‐energy geometry to allow for optimal hydrogen bonding in the solid state.     

Structure and photophysics of beta-Octafluoro-meso-tetraarylporphyrins

The structure of THF-coordinated [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]zinc, Zn(F(8)TPP).THF, and photophysical studies of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, F(8)TPP, Zn(F(8)TPP), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, F(28)TPP, and [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphinato]zinc, Zn(F(28)TPP), in benzonitrile, are reported. A key point from these studies is that the octafluorinated F(8)TPP and perfluorinated F(28)TPP porphyrins possess similar absorption spectra, but dissimilar X-ray crystal structures and disparate photophysical characteristics. These data cannot be easily accommodated within currently accepted theories which relate macrocycle distortion and optoelectronic properties.     

H/D isotope effect on porphine and porphycene molecules with multicomponent hybrid density functional theory

To analyze the H/D isotope effect on porphine and porphycene molecules including the protonic/deuteronic quantum nature and electron correlation efficiently, the authors have developed the new scheme of the multicomponent hybrid density functional theory [MC_(HF+DFT)]. The optimized geometries of porphine, porphycene, and these deuterated isotopomers by our MC_(HF+DFT) method are in good agreement with the experimental "high-symmetric" structures, contrary to the "low-symmetric" geometries optimized by pure multicomponent Hartree-Fock method. The optimized geometries for HD-porphine and HD-porphycene molecules, in which an inner hydrogen is replaced to a deuterium, are found to be low symmetric. Such drastic geometrical change induces the electronic polarization, and gives rise to the slight dipole moment values in these HD species. Their results clearly indicate that the difference of the nuclear quantum nature between inner proton and inner deuteron directly influences the molecular geometry and electronic structure.