| Summary: | The photoexcited triplet states of porphyrin architectures are of significant interest
in a wide range of fields including molecular wires, non-linear 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 sign
of 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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