Oral 23rd International Society of Magnetic Resonance Conference 2023

EPR investigation of coherent delocalization and dynamic electron transfer in porphyrin radical cations (#88)

Sebastian M. Kopp 1 2 , Ashley J. Redman 1 , Jie-Ren Deng 2 , Harry L. Anderson 2 , Christiane R. Timmel 1
  1. Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, Oxford, UK
  2. Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford, UK

A detailed understanding of the interplay of coherent radical delocalization and dynamic processes such as electron transfer or polaron migration is of vital importance in the study of open-shell molecules. To understand the conductance of pi-conjugated oligomers and utilize them as building blocks in single molecule electronic devices, it is important to determine the distribution of unpaired electrons along their backbones and the mechanism that governs the radical distribution.

In this work, we highlight different continuous wave and pulse EPR approaches to investigate the electronic structure of two families of porphyrin radical cations in the limits of strong and weak electronic coupling along their backbones:

Edge-fused porphyrin tape radical cations f-PN•+ with efficient pi-conjugation between neighboring porphyrin units display coherent electron delocalization over more than ten porphyrin units (~ 8.5 nm). This marks the longest polaron delocalization reported for organic materials to date and highlights the promise of f-PN for applications as molecular wires. In contrast, the porphyrin units connected by a twisted biphenyl-linker in dimer P2 lack global electronic conjugation and coherent radical delocalization. Instead, P2•+ displays dynamic electron transfer between its two redox sites with a solvent-dependent electron transfer barrier that can be determined by variable temperature cw-EPR. This makes P2 a promising building block for a single-molecule spin valve. These results are expected to inform the design of new molecules for application in single-molecule devices.

643907a0a2058-Abstract_Picture_Kopp.png

Figure 1. Schematic visualization of coherent radical cation delocalization in edge-fused porphyrin hexamer f-P6•+(left) and the dynamic electron transfer in biphenyl-linked dimer P2•+ (right).

 

  1. G. Moise et al., J. Phys. Chem. Lett. 10, 5708 (2019)
  2. M. D. Peeks et al., J. Am. Chem. Soc. 139, 10461 (2017)
  3. A. Tsuda et al., Science 79, 293 (2001)