Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches

The
spin delocalization in the radical cations of a series of ethyne-linked
oligoporphyrins was investigated using EPR spectroscopy. The room-temperature
spectral envelope for these oligomers deviates significantly from
the benchmark N–0.5 trend in line
width expected for a completely delocalized spin density, in contrast
to the butadiyne-linked analogues measured previously. Here, we show,
using DFT calculations and complementary low-temperature ENDOR measurements,
that this deviation is primarily driven by a more pronounced inequivalence
of the 14N spins within individual subunits for the ethyne-linked
oligoporphyrins. Once this 14N inequivalence is taken into
consideration, the room-temperature and ENDOR spectra for both butadiyne-linked
and ethyne-linked oligomers, up to N = 5, can be
simulated by similar static delocalization patterns. This work highlights
the importance of EPR in exploring such spin delocalization phenomena
while also demonstrating that the N–0.5 trend should not be interpreted in isolation but only in combination
with careful simulation and theoretical modeling.