Phenylene-bridged core-modified planar aromatic octaphyrin: Aromaticity, photophysical and anion receptor properties

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. Crazy eights: A "figure-eight" nonaromatic octaphyrin became a planar aromatic molecule upon the introduction of a phenylene bridge between two opposing meso-carbon atoms. This structural change not only made the molecule planar, but also rigid, as revealed by its enhanced photophysical parameters.