Abstract
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.
Highlights
C arbocyclic π-systems with circuits of [4n + 2] and [4n] πelectrons are expected to be aromatic and antiaromatic, respectively, according to modern formulations of Hückel’s rule.[1]
The three main computational methods for investigatingaromaticity involve calculating: (1) bond length alternation using the harmonic oscillator model (HOMA); (2) the aromatic stabilization energy (ASE); and (3) the magnetic effects ofaromaticity using the nucleusindependent chemical shift (NICS).[15−17] It is generally accepted that the magnetic criterion is the least ambiguous, for large molecules comprising several potentialaromatic electron pathways, for which the HOMA and ASE can be unsuitable
Kim and co-workers assigned excited-statearomaticity on the basis of the shape of the excited-state absorption spectrum.[6]. They found that the antiaromatic excited states of hexaphyrins and other expanded porphyrins exhibit broad and featureless absorption spectra, whereas the aromatic excited-state spectra are sharper and more structured, qualitatively resembling the ground-state absorption spectra of ground-state aromatic analogues
Summary
The. NICS(0)zz values in the S0 states were approximately zero for all rings (Table 1 and Figure S1), confirming their groundstate global nonaromaticity, whereas NICS(0)iso depicts shielding above and below the plane of each porphyrin subunit, consistent with local aromaticity. The NICS values indicate the presence of substantial global aromatic and antiaromatic ring currents in the triplet states of c-P6 and c-P5, respectively, whereas the effect is more subtle in c-P7 and c-P8. In our previous studies of c-P6 in its oxidized states, we found that oxidation to the 4+ or 6+ state results in loss of and S2) suggest that the local aromaticity of each porphyrin subunit persists in the triplet states (cf negative NICS above and below each porphyrin), except in the case of the porphyrin unit with the greatest spin density For this porphyrin, the NICS(0)iso is consistent with weak local antiaromaticity.
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