Talal F. Al-Azemi

Molecular Assemblies of Porphyrins and Macrocyclic Receptors: Recent Developments in Their Synthesis and Applications

Metalloporphyrins which form the core of many bioenzymes and natural light harvesting or electron transport systems, exhibit a variety of selective functional properties depending on the state and surroundings with which they exist in biological systems. The specificity and ease with which they function in each of their bio-functions appear to be largely governed by the nature and disposition of the protein globule around the porphyrin reaction center. Synthetic porphyrin frameworks confined within or around a pre-organized molecular entity like the protein network in natural systems have attracted considerable attraction, especially in the field of biomimetic reactions. At the same time a large number of macrocyclic oligomers such as calixarenes, resorcinarenes, spherands, cyclodextrins and crown ethers have been investigated in detail as efficient molecular receptors. These molecular receptors are synthetic host molecules with enclosed interiors, which are designed three dimensionally to ensure strong and precise molecular encapsulation/recognition. Due to their complex structures, enclosed guest molecules reside in an environment isolated from the outside and as a consequence, physical properties and chemical reactions specific to that environment in these guest species can be identified. The facile incorporation of such molecular receptors into the highly photoactive and catalytically efficient porphyrin framework allows for convenient design of useful molecular systems with unique structural and functional properties. Such systems have provided over the years attractive model systems for the study of various biological and chemical processes, and the design of new materials and molecular devices. This review focuses on the recent developments in the synthesis of porphyrin assemblies associated with cyclodextrins, calixarenes and resorcinarenes and their potential applications in the fields of molecular encapsulation/recognition, and chemical catalysis.

A new approach for the synthesis of mono- and A1/A2-dihydroxy-substituted pillar[5]arenes and their complexation with alkyl alcohols in solution and in the solid state

A new approach was employed for the synthesis of mono- and A1/A2-dihydroxy-functionalized pillar[5]arenes by the removal of the pillar[5]arene-bearing benzyl group(s) using catalytic hydrogenation. Host–guest complexation between a mono-hydroxy-pillar[5]arene with long-chain alkyl alcohol guests was studied. The encapsulation characteristics of the pillar[5]arene was affected by the presence of a hydroxy group, resulting in the formation of a 1 : 2 complex with long-chain alkyl alcohols in solution and in the solid state. For comparison, analog experiments were conducted with permethylated pillar[5]arene (DMP5) and long-chain alkyl alcohol guests. The complexation experiments revealed that the absence of a hydroxyl group on the pillar[5]arene frame resulted in the formation of a 1 : 1 complex. The formed complexes were confirmed by proton nuclear magnetic resonance spectroscopy and single-crystal X-ray analysis.

Constitutional Isomers of Pentahydroxy-Functionalized Pillar[5]arenes: Synthesis, Characterization, and Crystal Structures

We herein report the preparation of constitutional isomers of pentahydroxy-functionalized pillar[5]arenes via the deprotection of their benzylated derivatives by catalytic hydrogenation. The structures of the obtained isomers were then established using single crystal X-ray diffraction. We also found that the yield distribution of the different constitutional isomers was dependent on the nature of the substitution, as revealed by HPLC analysis of the crude mixture. Finally, further characterization of the separated constitutional isomers indicated that they possess different melting points, NMR spectra, crystal structures, and stacking patterns in the solid state.

Effect of the resorcin[4]arene host on the catalytic epoxidation of a Mn(III)-based resorcin[4]arene–metalloporphyrin conjugate

The single-crystal X-ray diffraction data, binding behavior, and epoxidation reactions of the cavitand resorcin[4]arene–porphyrin conjugate are presented. Polar and nonpolar organic molecules such as pyridine and styrene were included in the cavity of resorcin[4]arene to form a 1 : 1 complex in the solid state, as demonstrated by single-crystal X-ray diffraction analysis. Binding studies revealed that the presence of resorcin[4]arene enhanced the association constant, obtained from UV titration, by 50% compared to the original porphyrin. The epoxidation reaction of alkenes with the Mn(III)-based resorcin[4]arene–porphyrin conjugate showed enhanced activity and depended on the olefins and axial ligands used.

Single-crystal X-ray diffraction study of a host–guest system comprising monofunctionalized-hydroxy pillar[5]arene and 1-octa­namine

The crystal structure and supramolecular interactions between a hydroxy-functionalized pillarene and 1-octanamine guest molecule, which forms an interesting host–guest system, are reported.

Bis-resorcin[4]arene–bridged porphyrin conjugates: synthesis, fluorescence and binding studies

In this study, three bis-resorcin[4]arene–bridged porphyrin conjugates were synthesised from 5-15-di(3-hydroxyphenyl)-10,20-di(4-toluyl)porphyrin (PT), 5,10-di(3-hydroxyphenyl)-15,20-di(4-toluyl)porphyrin (PC) and 5-(3,5-dihydroxyphenyl)-10,15,20-tri(4-toluyl)porphyrin (PH), respectively; porphyrins PT, PC and PH were synthesised by the condensation of their corresponding aldehydes with pyrrole in propionic acid. In addition, the single-crystal X-ray diffraction analysis of the resorcin[4]arene host as well as the binding behaviour and fluorescence decay studies of the bisresorcin[4]arene–porphyrin conjugate are reported. Moreover, zinc derivatives of the three resorcin[4]arene–porphyrin conjugates, are synthesised. Binding studies conducted via UV-vis titration revealed that compared to the association constant (Ka) for free zinc-porphyrin (ZnTTP), those for bis-resorcin[4]arene–bridged porphyrin conjugates are higher, with an increase of approximately 50%. The fluorescence quenching of porphyrin is considerably enhanced by the attachment of resorcin[4]arene to the photoactive subunit; however, the efficiency of fluorescence quenching is affected by the structure type of porphyrin bis-conjugates.