Comprehensive investigation of the triplet state electronic structure of free-base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin by a combined advanced EPR and theoretical approach

Abstract

The nature of the photoexcited triplet state of free-base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4−) has been investigated by advanced Electron Paramagnetic Resonance (EPR) techniques combined with quantum chemical calculations. The zero-field splitting (ZFS) parameters, D and E, the orientation of the transition dipole moment in the ZFS tensor frame, and the proton hyperfine couplings have been determined by magnetophotoselection-EPR and pulse electron-nuclear double resonance spectroscopy. Both time-resolved and pulse experiments exploit the electron spin polarization of the photoexcited triplet state. Comparison of the magnetic observables with computational results, including CASSCF calculations of the ZFS interaction tensor, provides an accurate picture of the triplet-state electronic structure. The theoretical investigation has been integrated with a systematic analysis on the parent free-base porphyrin molecule to assess the effect of the sulfonatophenyl substituents on the magnetic tensors. Additionally, the magnetophotoselection effects are discussed in terms of tautomerization in the excited singlet state of H2TPPS4−.

EPR of Photoexcited Triplet-State Acceptor Porphyrins

“…The orientation selection effect caused by the polarization of the laser light (parallel/perpendicular to the external field) causes changes to the intensities of the EPR transitions. 29 , 42 , 43 , 50 …”

Neuroglobin Provides a Convenient Scaffold to Investigate the Triplet-State Properties of Porphyrins by Time-Resolved EPR Spectroscopy and Magnetophotoselection

“…The spectral analysis allows to define the relative orientation between the optical TDM and the ZFS principal axes [27]. Magnetophotoselection, applied to porphyrin systems, has been used in a limited number of cases to attain quantitative information on the relative orientation of the ZFS tensor axes with respect to the TDM, if one of the two is known [35,45]. Spectral simulations require the correct formalism to take into account potential dynamic Jahn-Teller effects in the specific case of ZnPP.…”

“…We have demonstrated that magnetophotoselection is a key spectroscopic tool in providing valuable information on the electronic structure of the porphyrin molecule in the excited states and assessing the corresponding symmetry. Combining optical and magnetic selection, the information is not confined to the paramagnetic triplet state, but it is extended to the corresponding excited singlet state connected by the intersystem crossing mechanism, as already highlighted in previous work in the case of free-base porphyrins, where a phototautomerization process is occurring [35].…”

“…Spectra were extracted as averaging 100 ns of the transient signal at about 1.2-1.4 μs after pulsed laser excitation. Simulations of triplet spectra have been performed using a home-written MATLAB program which includes fulldiagonalization of the spin Hamiltonian [27,35]. For the isotropically excited TR-EPR spectrum, the parameters of the simulations include the g and ZFS tensors and the relative triplet sublevel populations (p x , p y , and p z ).…”

Orientation-Selective and Frequency-Correlated Light-Induced Pulsed Dipolar Spectroscopy

“…This likely indicates either that the laser power used was sufficient to saturate the transition and excite both Q x and Q y , or that phototautomerism of the indole protons exchanges the Q x and Q y by a pseudorotation, making the directions of the transition dipole moment indistinguishable in the porphyrin plane. 60 Furthermore, no magneto-photoselection effects were recorded under the conditions of these experiments as minimal differences were seen in both field sweep spectra or ReLaserIMD traces recorded with the laser polarization horizontal and vertical to B 0 (see Figure S3 ). Because the optical transition frame aligns with the ZFS splitting frame generated in the triplet state, as the Q x and Q y are interchangeable, the correlation between the g tensor of the nitroxide and the ZFS frame is lost.…”