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113. Helimeric Porphyrinoids: Stereostructure and Chiral Resolution of meso-Tetraarylmorpholinochlorins

Author

Brückner, Christian, Götz, Daniel C. G., Fox, Simon P., Ryppa, Claudia, McCarthy, Jason R., Bruhn, Torsten, Akhigbe, Joshua, Banerjee, Subhadeep, Daddario, Pedro, Daniell, Heather W., Zeller, Matthias, Boyle, Ross W., and Bringmann, Gerhard

Abstract

The synthesis and chiral resolution of free-base and Ni(II) complexes of a number of derivatives of meso-tetraphenylmorpholinochlorins, with and without direct β-carbon-to-o-phenyl linkages to the flanking phenyl groups, is described. The morpholinochlorins, a class of stable chlorin analogues, were synthesized in two to three steps from meso-tetraphenylporphyrin. The conformations and the relative stereostructures of a variety of free-base and Ni(II) complexes of these morpholinochlorins were elucidated by X-ray diffractometry. Steric and stereoelectronic arguments explain the relative stereoarray of the morpholino-substituents, which differ in the free-base and Ni(II) complexes, and in the monoalkoxy, β-carbon-to-o-phenyl linked morpholinochlorins, and the dialkoxy derivatives. The Ni(II) complexes were all found to be severely ruffled whereas the free-base chromophores are more planar. As a result of the helimeric distortion of their porphyrinoid chromophores, the ruffled macrocycles possess a stable inherent element of chirality. Most significantly, resolution of the racemic mixtures was achieved, both by classical methods via diastereomers and by HPLC on a chiral phase. Full CD spectra were recorded and modeled using quantum-chemical computational methods, permitting, for the first time, an assignment of the absolute configurations of the chromophores. The report expands the range of known pyrrole-modified porphyrins. Beyond this, it introduces large chiral porphyrinoid π-systems that exist in the form of two enantiomeric, stereochemically stable helimers that can be resolved. This forms the basis for possible future applications, for example, in molecular-recognition systems or in materials with chiroptic properties.

Published
2024
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114. Conformational Landscapes of Metal(II) Porphyrinato, Chlorinato, and Morpholinochlorinato Complexes

Author

Sharma, Meenakshi, Ticho, Alexander L., Samankumara, Lalith, Zeller, Matthias, and Brückner, Christian

Abstract

The macrocycle conformation of [meso-tetraarylporphyrinato]metal complexes is metal-dependent. Furthermore, hydroporphyrins and some of their analogues are known to be more conformationally flexible than the parent porphyrins, but the extent to which this is reflected in their metal-dependent conformations was much less studied. meso-Tetraarylmorpholinochlorins are intrinsically nonplanar chlorin analogues in which the five-membered pyrroline moiety was replaced by a six-membered morpholine moiety. The metal complexes (M = Ni2+, Cu2+, Zn2+, Pd2+, Ag2+) of meso-aryl-2,3-dimethoxychlorins and meso-arylmorpholinochlorins were prepared. Their conformations were determined using X-ray crystal structure diffractometry and compared against those of their free bases, as well as against the conformations of the corresponding metalloporphyrins. Out-of-plane displacement plots visualized and quantified the conformational changes upon stepwise conversion of a pyrrole moiety to a dimethoxypyrroline moiety and to a dialkoxymorpholine moiety, respectively. The generally nonplanar macrocycle conformations were found to be central-metal-dependent, with the smaller ions showing more nonplanar conformations and with the metallomorpholinochlorins generally showing a much larger conformational range than the corresponding metallochlorins, which, in turn, were more nonplanar than the corresponding porphyrins. This attests to the larger conformational flexibility of the morpholinochlorin macrocycle compared to that of a chlorin or even a porphyrin macrocycle. The degree of nonplanarity affects the electronic structure of the metal complexes, as can also be seen in a comparison of their UV–vis spectra. We thus further define the conformational and electronic effects governing pyrrole-modified porphyrins.

Published
2024
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119. Chromene-Annulated Bacteriochlorins

Author

Hyland, Michael A., primary, Hewage, Nisansala, additional, Panther, Kimberly, additional, Nimthong-Roldán, Arunpatcha, additional, Zeller, Matthias, additional, Samaraweera, Milinda, additional, Gascon, José A., additional, and Brückner, Christian, additional

Published
2016
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136. Surprising Outcomes of Classic Ring-Expansion Conditions Applied to Octaethyloxochlorin, 3.[‡] Schmidt-Reaction Conditions.

Author

Li, Ruoshi, Zeller, Mathias, Bruhn, Torsten, and Brückner, Christian

Subjects
KETONES, RING formation (Chemistry), SCHMIDT reaction, SODIUM azide, CHEMICAL derivatives
Abstract

The Schmidt reaction (treatment of a ketone with sodium azide and a mineral acid) is an alternative to the Beckmann rearrangement to expand a cyclic ketone to a lactam. However, when applied toward the conversion of a synthetic porphyrin into a derivative containing a nonpyrrolic building block, this approach failed to generate the expected lactam. Instead, by using sulfuric acid as a catalyst, a novel heptaethyl-2-hydroxy-chlorin-3-one was formed, structurally characterized, and its mechanism of formation deduced. The work demonstrates how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring-expansion reactions. By using hydrochloric acid as a catalyst, a somewhat regioselective chlorination of the meso-positions of the oxochlorin was observed. The halogenation sites were determined spectroscopically and by X-ray crystallography of selected derivatives. The regioselectivity of the halogenation was computationally rationalized. This method is superior to alternative halogenation methods for the regioselective generation of 5-chloro-, 10-chloro-, and 5,10-dichlorooxochlorins. [ABSTRACT FROM AUTHOR]

Published
2017
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137. Surprising Outcomes of Classic Ring-Expansion Conditions Applied to Octaethyloxochlorin, 2.[‡] Beckmann-Rearrangement Conditions.

Author

Li, Ruoshi, Meehan, Eileen, Zeller, Mathias, and Brückner, Christian

Subjects
RING formation (Chemistry), BECKMANN rearrangement, PORPHYRIN synthesis, HYDROLYSIS, CHEMICAL reactions
Abstract

The step-wise conversion of synthetic porphyrins to derivatives that contain a non-pyrrolic building block is an appealing method to generate functionalized porphyrinoids. Applied to octaethyloxochlorin oxime, a Beckmann rearrangement is potentially suited to generate a nitrogen-expanded porphyrinoid. Unexpectedly, this reaction led to a ring-expansion by an oxygen atom. The mechanism – an abnormal Beckmann reaction, followed by an intramolecular ring-closing reaction and hydrolysis – was conclusively derived by trapping and structural characterization of the key secochlorin intermediate. The structural and UV/Vis spectral changes observed upon ring-expansion of the oxochlorin are discussed. Other Beckmann-rearrangement conditions failed to produce ring-expanded products altogether. This work demonstrates in many ways how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring-expansion reactions, which outlines the limits of the “breaking and mending of porphyrins” approach toward pyrrole-modified porphyrins. [ABSTRACT FROM AUTHOR]

Published
2017
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138. Surprising Outcomes of Classic Ring-Expansion Conditions Applied to Octaethyloxochlorin, 1.[‡] Baeyer–Villiger-Oxidation Conditions.

Author

Li, Ruoshi, Zeller, Matthias, and Brückner, Christian

Subjects
CHEMICAL derivatives, PORPHYRINS, BAEYER-Villiger rearrangement, FUNCTIONAL groups, ELECTRIC properties of materials, CHROMOPHORES
Abstract

The conversion of synthetic porphyrins to derivatives that contain a nonpyrrolic building block is an appealing method to generate porphyrinoids with functional groups at their periphery for sensing applications or chromophores with fine-tuned electronic properties for a number of technical and biomedical applications. Baeyer–Villiger oxidations are potentially suited to generate such pyrrole-expanded porphyrinoids from known octaethyloxochlorin. However, the application of different Baeyer–Villiger-oxidation conditions to this ketone merely led to the formation of novel and known compounds, such as porphyrinic meso-OH and N-oxide derivatives. This work demonstrates how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring-expansion reactions, which outlines some limits of the methods to convert a pyrrole in porphyrins into a nonpyrrolic building block. [ABSTRACT FROM AUTHOR]

Published
2017
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143. Light-induced energy and charge transfer processes in artificial photosynthetic systems

Author

Röder, Beate, Benson, Oliver, Brückner, Christian, Menting, Raoul, Röder, Beate, Benson, Oliver, Brückner, Christian, and Menting, Raoul

Abstract

Der Gegenstand der vorliegenden Arbeit ist die Untersuchung von photoinduzierten Energietransferprozessen (EET) und Elektronentransferprozessen (ET) in Modellsystemen, die als potentiell geeignet für eine Nutzung in der artifiziellen Photosynthese angesehen werden. Den beiden wesentlichen Zugängen zur Architektur artifizieller Photosynthese-Systeme entsprechend wurden vergleichend kovalente und sich selbst organisierende Systeme untersucht. In beiden Zugängen wurden ähnliche chemische Komponenten als optisch aktive Moleküle eingesetzt, insbesondere Phthalocyanine mit einem Silizium-Zentralatom (SiPc). Durch eine Kombination von stationären und zeitaufgelösten optisch-spektroskopischen Methoden konnten die lichtinduzierten ET- und EET-Prozesse identifiziert und quantifiziert werden. Im ersten Teil der Arbeit wurden mehrere kovalent gebundene Triaden und eine Pentade untersucht. In allen Systemen finden sehr effiziente ET und EET Prozesse statt. Es wurde gezeigt, dass das Lösungsmittel großen Einfluss auf die photophysikalischen Eigenschaften der Systeme ausübt. Die Lebensdauer des ladungsseparierten Zustandes variiert von 1,7 ns in Toluol bis 30 ps in DMF. Im zweiten Teil der Arbeit wurde erstmals gezeigt, dass sich in wässriger Lösung ein supramolekularer Komplex, bestehend aus einem Beta-Cyklodextrin (CD), einem konjugierten Subphthylocyanin (SubPc), einem Porphyrin (Por) und einem SiPc bilden kann. Letzteres wurde über unterschiedliche Ketten an zwei CDs kovalent gebunden. Die Selbstorganisation wird über hydrophobe Wechselwirkungen vermittelt und die Bildung der Komplexe ist sehr effizient. Nach selektiver Anregung von SubPc finden sequenzielle ET- und EET-Prozesse von SubPc zu SiPc statt. Das Por spielt die Rolle einer energetischen und elektronischen Brücke und ermöglicht die ET und EET-Prozesse von SubPc zu SiPc. Die Ladungsrekombination in den Grundzustand geschieht innerhalb von 1,7 ns., The main objective of the present thesis was to conduct investigations of photo-induced electron transfer (ET) and excitation energy transfer (EET) processes in model compounds that are considered potentially appropriate for use in artificial photosynthesis. Two approaches have been used to construct the artificial photosynthetic systems, namely covalent and supramolecular approach. In both systems similar optically active molecules have been employed, particularly silicon-based phthalocyanines (SiPc). A comparative study between the covalently-linked and self-assembled systems had been conducted. For these purposes, thorough spectroscopic measurements in the UV/Vis range had been performed on these conjugates. A combination of steady-state and time-resolved experiments allowed an identification and quantification of the photo-induced ET and EET processes. In the first part of the work several covalently bound triads and a pentad bearing a central SiPc unit were studied. In all systems highly efficient ET and EET processes take place. It was found that the solvent exerts great influence on the photophysical properties of the systems. The lifetime of the charge-separated state varied from 1.7 ns (toluene) down to 30 ps (DMF). In the second part of the thesis, for the first time the formation of ternary supramolecular complexes consisting of a beta-cyclodextrin (CD), a conjugated subphthalocyanine (SubPc), a porphyrin (Por) and a series of SiPcs substituted axially with two CDs via different spacers was shown. These components are held in water by host-guest interactions and the formation of these host-guest complexes was found to be very efficient. Upon excitation of the SubPc-part of the complex sequential ET and EET processes from SubPc to SiPc take place. The Por dye acts as a transfer bridge enabling these processes. The probability of ET is controlled by the linker between CD and SiPc. Charge recombination to the ground state occurs within 1.7 ns.

Published
2013

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