Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications

Abstract

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) oligomers (<b>PZn</b>_{<b><i>n</i></b>} compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the <b>PZn</b>_{<b><i>n</i></b>} hole polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b>) and electron polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>}) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b> and <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>} spin–spin (<i>T</i>₂) and spin–lattice (<i>T</i>₁) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin–spin and spin–lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal–dithiol-<b>PZn</b>_{<b><i>n</i></b>}–metal junctions, present <i>meso</i>-to-<i>meso</i> ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications