Three-Dimensional Fully Conjugated Carbaporphyrin Cage

A fully
conjugated three-dimensional (3D) expanded carbaporphyrin
(2) was prepared in a one-pot procedure that involves
a [2+4] condensation reaction between a dibenzo­[g,p]­chrysene-bearing tetrapyrrole precursor (1) and pentafluorobenzaldehyde, followed by oxidation. Single
crystal X-ray diffraction analysis revealed that 2 possesses
a cage-like structure consisting of four dipyrromethenes and two bridging
dibenzo­[g,p]­chrysene units. As prepared, 2 is nonaromatic as inferred from UV–vis-NIR and 1H NMR spectroscopy and a near-zero (−1.75) nucleus-independent
chemical shift (NICS) value. In contrast, after protonation with trifluoroacetic
acid (TFA), the cage gains global aromatic character as inferred from
the large negative NICS value (−11.63) and diatropic ring current
observed in the anisotropy of the induced current density (ACID) plot,
as well as the ca. 8-fold increase in the excited state lifetime.
In addition, the size of the cavity increases to ca. 143 Å3 upon protonation as deduced from a single crystal X-ray diffraction
analysis. To our knowledge, this is the largest carbaporphyrin prepared
to date and the first with a fully conjugated 3D cage structure whose
size and electronic features may be tuned through protonation.