Three-Dimensional\nFully Conjugated Carbaporphyrin\nCage

Abstract

A fully\nconjugated three-dimensional (3D) expanded carbaporphyrin\n(<b>2</b>) was prepared in a one-pot procedure that involves\na [2+4] condensation reaction between a dibenzo­[<i>g</i>,<i>p</i>]­chrysene-bearing tetrapyrrole precursor (<b>1</b>) and pentafluorobenzaldehyde, followed by oxidation. Single\ncrystal X-ray diffraction analysis revealed that <b>2</b> possesses\na cage-like structure consisting of four dipyrromethenes and two bridging\ndibenzo­[<i>g</i>,<i>p</i>]­chrysene units. As prepared, <b>2</b> is nonaromatic as inferred from UV–vis-NIR and ¹H NMR spectroscopy and a near-zero (−1.75) nucleus-independent\nchemical shift (NICS) value. In contrast, after protonation with trifluoroacetic\nacid (TFA), the cage gains global aromatic character as inferred from\nthe large negative NICS value (−11.63) and diatropic ring current\nobserved in the anisotropy of the induced current density (ACID) plot,\nas well as the ca. 8-fold increase in the excited state lifetime.\nIn addition, the size of the cavity increases to ca. 143 ų upon protonation as deduced from a single crystal X-ray diffraction\nanalysis. To our knowledge, this is the largest carbaporphyrin prepared\nto date and the first with a fully conjugated 3D cage structure whose\nsize and electronic features may be tuned through protonation.