Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings

Aromaticity
can be a useful concept for predicting the behavior
of excited states. Here we show that π-conjugated porphyrin
nanorings exhibit size-dependent excited-state global aromaticity
and antiaromaticity for rings containing up to eight porphyrin subunits,
although they have no significant global aromaticity in their neutral
singlet ground states. Applying Baird’s rule, even rings
([4n] π-electrons) are aromatic in their lowest
excited states, whereas the lowest excited states of odd rings
([4n + 2] π-electrons) are antiaromatic. These
predictions are borne out by density functional theory (DFT) studies
of the nucleus-independent chemical shift (NICS) in the T1 triplet state of each ring, which reveal the critical importance
of the triplet delocalization to the emergence of excited-state aromaticity.
The singlet excited states (S1) are explored by measurements
of the radiative rate and fluorescence peak wavelength, revealing
a subtle odd–even alternation as a function of ring size, consistent
with symmetry breaking in antiaromatic excited states.