Abstract
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.
Highlights
Porphyrin molecules are well established as ideal building blocks for molecular wires in nanoscale electronic devices, spintronics, and photovoltaic cells.[1−10] Despite the extensive research dedicated to the understanding of their ground and excited-state properties and their potential applications, porphyrin systems remain an elusive class of organic compounds, especially when it comes to their magnetic properties
Previous electron paramagnetic resonance (EPR) studies have been successful in revealing the extensive electronic communication between the subunits of linear and cyclic oligoporphyrin wires in their radical cation[23−26] or anion[27] states, as well as their photoexcited triplet states.[28−32] In this contribution, we focus on the magnetic fine-structure parameters of a series of perfluoroalkyl meso-substituted porphyrin monomers
The EPR and computational results discussed here are consequential on a number of fronts, as they (1) highlight the complexities that might be encountered in interpreting EPR spectra of photogenerated triplet states, (2) provide guidance for future synthetic efforts in the field of porphyrin chemistry, and (3) demonstrate the urgency for developing more sophisticated theoretical frameworks to keep up with synthetic and experimental EPR advances
Summary
Porphyrin molecules are well established as ideal building blocks for molecular wires in nanoscale electronic devices, spintronics, and photovoltaic cells.[1−10] Despite the extensive research dedicated to the understanding of their ground and excited-state properties and their potential applications, porphyrin systems remain an elusive class of organic compounds, especially when it comes to their magnetic properties As a result, they represent an ideal testing ground for numerous computational and quantum chemical studies in conjunction with spectroscopic techniques such as transient and pulse electron paramagnetic resonance (EPR), electronic absorption, and fluorescence.[11−19]. The experimental findings are reconciled with the results of quantum chemical calculations employing DFT and complete active space self-consistent field (CASSCF) methodologies
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