EPR of Photoexcited Triplet-State Acceptor Porphyrins

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.

Photoreactive CO2 Capture by a Zr-Nanographene MOF

“…This type of polarization pattern is typical for triplet states 51 created from optically excited singlet state by intersystem crossing mechanism. 52 The line shape and zero-field splitting parameters, D≈82 mT, are similar to the triplet state EPR spectra reported previously in porphyrin-type structures 53,54 In addition to the signals associated with the organic triplet state, an EPR signal with a linewidth of 1.1 mT was observed at g≈2.003-2.004. This signal does not demonstrate resolved hyperfine structure, which is indicative of organic radical with highly delocalized electron spin density and no heavier atoms like oxygen or metals involved.…”

Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer

“…Magnetophotoselection TREPR is an excellent method for quantitative determination of the relative orientation of the triplet state ZFS axes and the optical absorption transition moments. [25][26][27] It also allows these axes to be fixed in the molecule if the orientation of one of them is known or can be determined in silico. The results confirm that the triplet state is indeed delocalized over both porphyrins of the dimer and that the excited states have CT character.…”

Time-Resolved Electron Paramagnetic Resonance Spectrometer Based on Ultrawide Single-Sideband Phase-Sensitive Detection

“…Time-resolved electron paramagnetic resonance (TREPR) is a powerful technique that enables direct observation and tracking of short-lived (transient) open shell species, such as free radicals and triplet excited states. [1][2][3] TREPR has been widely applied in the studies of spin dynamics, 4 radical chemistry, 5 , 6 material science 7 , 8 , and biochemistry. 9,10 Time resolution and sensitivity are fundamental factors to determine the power of this technique.…”