RUI: Charge Transfer Triplet States in Hypervalent Phosphorus(V) and Antimony(V) Porphyrins

In this project funded by the Chemical Mechanism, Function, and Properties Program of the Chemistry Division, Professor Prashanth Poddutoori from the Department of Chemistry at the University of Minnesota Duluth is introducing a new class of group 15 ion-carrying porphyrin sensitizers called hypervalent phosphorus(V) porphyrins (PPor) and antimony(V) porphyrins (SbPor). These sensitizers exhibit intriguing optical and redox properties, including an intrinsic charge transfer state that allows for access to the seldom-studied triplet charge transfer state. Triplet-excited states are crucial for various applications, such as artificial photosynthesis, photodynamic therapy, photovoltaics, and bioimaging. The goal of this research is to establish new design principles for creating hypervalent PPor and SbPor, determine the energetics involved in the formation of the triplet charge transfer state, and investigate the factors that influence charge transfer properties. By developing triplet photosensitizers and exploring their applications, this research aims to advance the understanding in this field while providing hands-on research experience and essential training for the next generation of scientists. This proposal introduces a new class of photosensitizers that, despite their structural simplicity, exhibit interesting photophysical properties and the potential to generate charge transfer triplet states with high efficiency. The study aims to explore the intricate relationship between structure, energetics, and the charge transfer-coupled intersystem crossing mechanism to produce triplet excited states. The specific objectives include the incorporation of P(+5) or Sb(+5) ions into 5,10,15,20-tetraarylporphyrin to create push-pull type hypervalent phosphorus(V) porphyrin (PPor) or antimony(V) porphyrin (SbPor), respectively. This design enables intramolecular charge transfer transitions from the electron-rich peripheral aryl units to the electron-poor porphyrin central ring. The study will employ a combination of experimental and computational approaches. This includes the synthesis of the proposed photosensitizers and the characterization of their structural features using various spectroscopic, analytical, and electrochemical methods. Techniques such as time-dependent density functional theory, ultrafast transient absorption spectroscopy, and time-resolved electron paramagnetic resonance spectroscopy will be employed to elucidate the intricate molecular energetics required for triplet formation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.