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.
The photoexcited triplet states of
porphyrin architectures are
of significant interest in a wide range of fields including molecular
wires, nonlinear optics, and molecular spintronics. Electron paramagnetic
resonance (EPR) is a key spectroscopic tool in the characterization
of these transient paramagnetic states singularly well suited to quantify
spin delocalization. Previous work proposed a means of extracting
the absolute signs of the zero-field splitting (ZFS) parameters,
D
and
E
, and triplet sublevel populations
by transient continuous wave, hyperfine measurements, and magnetophotoselection.
Here, we present challenges of this methodology for a series of
meso
-perfluoroalkyl-substituted zinc porphyrin monomers
with orthorhombic symmetries, where interpretation of experimental
data must proceed with caution and the validity of the assumptions
used in the analysis must be scrutinized. The EPR data are discussed
alongside quantum chemical calculations, employing both DFT and CASSCF
methodologies. Despite some success of the latter in quantifying the
magnitude of the ZFS interaction, the results clearly provide motivation
to develop improved methods for ZFS calculations of highly delocalized
organic triplet states.
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