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Abstract
The synthesis of ethyne-linked porphyrin nanorings has been achieved by template-directed Sonogashira coupling. The cyclic hexamer and octamer are predicted by density functional theory to adopt low symmetry conformations, due to dihedral twists between neighboring porphyrin units, but their symmetries are effectively D6h and D8h, respectively, in solution by 1H NMR. The fluorescence spectra indicate that the singlet excited states of these nanorings are highly delocalized.
Highlights
Π-Conjugated macrocycles are useful for testing the limits of aromaticity and electronic delocalization,[1,2] and they are prototype materials for achieving efficient transport of charge and electronic excitation.[3]
Meso bond causes major steric interactions, forcing the porphyrin units into perpendicular planes and preventing πconjugation;[11] the insertion of a C2 bridge alleviates this clash, steric interactions still destabilize the coplanar conformation and the dihedral angle is about 35° allowing substantial π-overlap;[12,13] the C4 bridge results in a preference for the coplanar conformation.[14]
We report an investigation of π-conjugated nanorings consisting of porphyrins linked by single acetylene (C2) bridges: the cyclic hexamer c-P6 and the cyclic octamer c
Summary
Communication up to the heptamer by palladium-catalyzed Sonogashira coupling, with or without a copper(I) cocatalyst.[18,20] We found that standard conditions using Pd(PPh3)[4] and CuI, in tandem with NBS bromination, gave efficient access to trimer 2 from monomer 1. Vis-NIR titrations, but their stabilities can be determined by competition with pyridine.[10] Previously, we reported that strongly coordinating ligands, such as quinuclidine, are required to displace a hexadentate template from a butadiyne-linked 6porphyrin ring;[10,24] the complexes presented here are dissociated by addition of pyridine. Comparison of the fluorescence spectra of l-P6 and c-P6·T6 illustrates both these points: the fluorescence quantum yield is 1000-fold smaller in the ring compound and the emission band is red-shifted by 164 nm (l-P6: 909 nm; c-P6·T6: 1073 nm) This demonstrates that the first excited state is fully delocalized around the nanoring in c-P6·T6.
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