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-.

中文翻译：

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结合先进的EPR和理论方法，对游离碱5,10,15,20-四（4-磺酰基苯基）卟啉的三重态电子结构进行了全面研究。

通过先进的电子顺磁共振（EPR）技术结合量子化学计算研究了游离碱5,10,15,20-四（4-磺酰基苯基）卟啉（H2TPPS4-）的光激发三重态的性质。零场分裂（ZFS）参数D和E，ZFS张量框架中跃迁偶极矩的方向以及质子超精细耦合已通过磁光选择-EPR和脉冲电子-核双共振光谱法确定。时间分辨和脉冲实验都利用了光激发三重态的电子自旋极化。磁性观测物与计算结果（包括ZFS相互作用张量的CASSCF计算）的比较，提供了三重态电子结构的准确图片。理论研究已与对母体游离碱卟啉分子的系统分析相结合，以评估磺酰基苯基取代基对磁张量的影响。另外，在H2TPPS4-的激发单重态下的互变异构方面讨论了磁光选择效应。