Phenylene‐Bridged Core‐Modified Planar Aromatic Octaphyrin: Aromaticity, Photophysical and Anion Receptor Properties

Abstract

The synthesis of a planar expanded meso porphyrin with an intramolecular para-phenylene-bridged core is reported. The planarity of the octaphyrin macrocycle was confirmed by single-crystal X-ray structural analysis, in which the bridged para-phenylene unit deviated by 27° from the mean macrocyclic plane. Spectroscopic analyses and theoretical calculations suggested that the macrocycle was Hückel aromatic and followed a major [34 π] single-conjugation pathway, which indicated that the bridging para-phenylene unit was not involved in the macrocyclic conjugation. Analysis of the photophysical properties of this system by steady-state absorption/fluorescence spectroscopy and transient absorption spectroscopy revealed moderate enhancement in the parameters of the octaphyrin as compared to its non-bridged octaphyrin congeners, which was attributed to the planarity and rigidity of the macrocycle as imposed by the bridging para-phenylene unit. Preliminary anion-binding studies revealed that the protonated macrocycle bound selectively with chloride ions through N-H⋅⋅⋅Cl hydrogen-bonding interactions.

Flexible Porphyrinoids

“…Such a strategy was addressed previously in the case of hexaphyrins. 295 The macrocycle 110-1 itself is slightly folded, as shown by X-ray analysis, yet still fully conjugated (Figure 27). p-Phenylene is tilted by 27°in respect to the mean plane.…”

“…Additionally, in solution, 110-1 adopts the planar conformation, demonstrating significant diatropicity consistent with a 34-π-electron pathway. 295 Accordingly, the protons of the p-phenylene bridge are internally situated and, thus, strongly upfield relocated to 1.0 ppm. An attempt to obtain an all-pyrrole-bridged octaphyrin resulted in the synthesis of the doubly meso-reduced species 110-2 (Scheme 110, Figure 27).…”

“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

“…[25] In an analogous synthetic method, bithiophened iol 54 was condensed with 1,4-phenylbis(dipyrromethane) (28), followed by oxidation with DDQ (Scheme 21). [37] The resulting macrocycle was identified as 34 p octaphyrin 55,w hichc ontained ap henylene bridge at the center of the macrocycle. However,t he phenylene ring in the macrocyclic cavity enabled the octaphyrin to maintain ap lanar configuration and ah igherl evel of symmetry,t herebyl eadingt oa ltered electronic properties.…”

“…This approach was previously explored by Osuka and co‐workers to maintain a planar macrocycle ( 30 ), but it fell short of a completely conjugated system . In an analogous synthetic method, bithiophene diol 54 was condensed with 1,4‐phenyl‐bis(dipyrromethane) ( 28 ), followed by oxidation with DDQ (Scheme ) …”

Two non-identical twins in one unit cell: characterization of 34π aromatic core-modified octaphyrins, their structural isomers and anion bound complexes

“…Later Osuka and co-workers followed a bridging approach to tie a bridge across the meso carbons to avoid twisting and were successful in synthesizing 2a which turned out to be non-aromatic 7. Later, we used a similar bridging approach to synthesize 34π aromatic bridged octaphyrin 2b 8 …”