Rectangular Holes in Porphyrin Isomers Act As Mono- and Binucleating Ligands: Stereochemistry of Mono- and Diboron Porphycenes and Their Protonation Behaviors

The first boron complexes of porphycenes, structural isomers of porphyrin, are reported. They are synthesized in good yields by reacting the free-base porphycene ligands with BF₃·Et₂O through a microwave-assisted method. Depending on the substituent group of porphycenes, two different coordination structures, mono- and diboron porphycenes, are obtained simultaneously. The single crystal structures and DFT calculations suggest that the boron atom of the monoboron porphycene is favorably coordinated on the dipyrroethene site, and the regioisomer of diboron porphycene is of cisoid stereochemistry, which is more stable than transoid. We also investigate the protonation behavior of boron porphycene complexes. Diboron porphycene does not undergo protonation, whereas monoboron porphycene undergoes protonation at the nonboron coordinating pyrroline site, resulting in a red shift in both absorption and emission spectra. Protonation and deprotonation of monoboron porphycene can be reversibly triggered using acids and bases.

Synthesis, Characterization and Protonation Behavior of Quinoxaline-Fused Porphycenes

9,10-Quinoxaline-fused porphycenes 1a-H₂ and 1b-H₂ were synthesized by intramolecular McMurry coupling. As a result of the annulation of the quinoxaline moiety on the porphycene skeleton, 1a-H₂ and 1b-H₂ display absorption and fluorescence in the near infra-red (NIR) region. Additionally, the quinoxaline moieties of 1a-H₂ and 1b-H₂ act as electron-withdrawing groups, introducing lower reduction potentials than for pristine porphycene. The protonation occurred at the nitrogen atoms in the cavity of freebase porphycenes and at the quinoxaline moieties for their nickel complexes to give diprotonic species.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

Protonation-deprotonation equilibria in tetrapyrroles Part 3: Mono- and diprotonations of the trimethyl esters of chlorin e6 and the 71-acetal of rhodin g7 in methanolic hydrochloric acid

Spectrophotometric protonation titrations were performed for the trimethyl esters (TME) of chlorin e6 (31,32-didehydrorhodochlorin-15-acetic acid) and the 71-acetal of rhodin g7 (31,32-didehydrorhodochlorin-71-oxo-15-acetic acid) using HCl as the acid and methanol as the solvent. For rhodin g7 TME, the 71-acetal formation could be clearly detected as the first step in the titration. Only two spectroscopically different protonated species were observed for each chlorin derivative in addition to the neutral forms. The two protonated species were assigned to the monocation and dication of each chlorin derivative. The following p K a values were obtained: p K3 = 4.63 and p K4 = 0.62 for chlorin e6 TME and p K3 = 4.40 and p K4 = 0.60 for the acetal of rhodin g7 TME. The protonation titration for chlorin e6 TME with HCl in acetic acid afforded UV-vis spectra similar to those obtained with HCl in methanol. The UV-vis spectrometric parameters are given for the neutral forms of chlorin e6 TME, rhodin g7 TME and its 71-acetal, as well as for the mono- and diprotonated species of chlorin e6 TME and rhodin g7 TME acetal. The protonation titration results of the chlorin e6 derivatives are compared with those previously obtained for phytyl/methyl pyropheophorbide a.

Protonation-deprotonation equilibria in tetrapyrroles Part 2: Mono- and diprotonation of methyl pyropheophorbide a in methanolic hydrochloric acid as verified by the 1H, 13C HSQC and 1H, 15N HMBC NMR experiments

The two-dimensional 1H, 13C and 1H, 15N heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond correlation (HMBC) NMR techniques were applied to investigate the formation of N-protonated cationic species of methyl pyropheophorbide a in methanolic hydrochloric acid (CD3OH-HCl). The 1H, 13C HSQC and 1H, 15N HMBC NMR spectra, recorded at the temperature of 278 K, verified that the CD3OH-HCl solution with [H]+ = 0.021 M, contained the N22-protonated monocations of methyl pyropheophorbide a, whereas the CD3OH-HCl solution with [H]+ = 5.0 M contained the N22, N24-protonated dications of the phorbin. No further protonations to form the trication or tetracation were observed. Consequently, the two middle electronic spectra, which were previously tentatively interpreted as representing the dication and trication, were now attributed to conformational alterations originating from steric hindrance between the central hydrogens and changes in counter-ion and solvation interactions and/or conformational alterations intimately connected to NH-tautomerism. These possibilities to explain the previously observed middle electronic spectra are briefly discussed.

Molecular structure and spectroscopic properties of octaethylbenzochloracene, the porphycene analog of a benzochlorin

The X-ray structure, electrochemistry, and optical spectroscopy of the porphycene analog of a benzochlorin, octaethylbenzochloracene (OEBzC) are reported. The OEBzC macrocycle is easier to oxidize and harder to reduce than octaethylporphycene (OEPc). The optical spectra of OEBzC and of the diacid salt are also reported. The optical spectrum for the one-electron oxidation of OEBzC is indicative of π-cation radical formation. In porphyrins, saturation of one pyrrole ring and addition of an exocyclic benzene ring progressively red-shifts the first absorption band. For porphycenes, saturation of one pyrrole ring results in a 35 nm blue shift in the first absorption relative to OEPc. Compared to 2,3-dihydroporphycene (DHPc), the first absorption band in OEBzC is red-shifted 24 nm. These studies further illustrate how the fused exocyclic benzene ring influences the macrocycle conformation, chemical and physical properties of porphycenes.

Synthesis of a metal-free texaphyrin

The synthesis of a metal-free form of texaphyrin, an aromatic porphyrin-like macrocycle, is described. Previously, texaphyrins could only be obtained reproducibly in the form of metal complexes. Using ferrocenium cation as the oxidizing agent and starting with a reduced porphyrinogen-like nonaromatic form of texaphyrin, we isolated, in good yield, the metal-free oxidized texaphyrin as its HPF(6) salt. This product was characterized by X-ray diffraction analysis, UV-vis spectroscopy, and cyclic voltammetry. [structure: see text]

Electrochemical studies of corrphycenes and metallocorrphycenes

Corrphycene 3 (Cn) is a structural isomer of porphyrin 1 that was synthesized for the first time 5 years ago. This paper reports on the redox properties of free-base octaethylcorrphycene H2OECn and 16 metal complexes derived therefrom. In CH2Cl2 solution, the free base and the metallo(II) octaethylcorrphycenes, M(II)OECn, typically undergo four distinct one-electron redox steps involving the tetrapyrrolic macrocycle, of which two are reduction steps and two are oxidations. One exception to this general pattern is displayed by the Co(II)OECn complex. In this instance, the first one-electron reduction is metal-centered and produces Co(I)OECn. A comparison of the redox potentials of corrphycenes with those of porphyrins and porphycenes indicates that the first reduction potentials of the free base and of the metallo-octaethylcorrphycenes are between those of the porphycenes-the easiest to reduce molecules in this set of isomeric tetrapyrrolic systems-and those of the porphyrins. The oxidation potentials of corrphycenes and porphyrins are found to be quite similar. On the other hand, porphycenes are oxidized at less positive potentials. The redox gap deltaE1/2 = E1/2Ox1 - E1/2Red1 is equal to 2.15 +/- 0.08 V for the free base corrphycene and the various metallocorrphycenes that were subjected to study. This redox gap is not much different from that observed in porphyrins (deltaE1/2 = 2.25 +/- 0.1 V), whereas if differs significantly from that observed in porphycenes (deltaE1/2 = 1.85 +/- 0.15 V). The sequence of these deltaE1/2 values parallels the lowest energy absorption maxima observed in the UV-vis spectra of these three isomers.

Anion selectivity of a sapphyrin-modified silica gel HPLC support

A sapphyrin-modified silica gel support for use in high-performance liquid chromatography was prepared by attaching a sapphyrin monocarboxylic acid to aminopropyl silica gel through an amide bond. The anion retention characteristics of this modified silica gel were tested by exploring the extent to which a specific anion in the mobile phase would act to affect the rate at which AMP was eluted from an HPLC column containing this functionalized stationary phase. In general, it was found that phosphate and arsenate anions were more effective as eluents than carboxylic acids and halides, a result that was interpreted in terms of these former species binding better to sapphyrin (and hence being more effective in terms of displacing AMP) than other anions tested. Support for the contention that phosphate anions will bind to sapphyrin subunits covalently tethered to the silica gel came from solid state 31P NMR spectroscopic analyses. These revealed that the 31P nucleus undergoes a 5 ppm upfield shift, relative to control, when allowed to interact with the sapphyrin-containing support.