Electron spin polarization in an Al(III) porphyrin complex with an axially bound nitroxide radical

Prashanth K Poddutoori; Yuri E Kandrashkin; Paul Karr; Art van der Est

doi:10.1063/1.5127760

Electron spin polarization in an Al(III) porphyrin complex with an axially bound nitroxide radical

J Chem Phys. 2019 Nov 28;151(20):204303. doi: 10.1063/1.5127760.

Authors

Prashanth K Poddutoori¹, Yuri E Kandrashkin², Paul Karr³, Art van der Est⁴

Affiliations

¹ Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, USA.
² Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Sibirsky Tract 10/7, Kazan 420029, Russia.
³ Department of Physical Sciences and Mathematics, Wayne State College, Wayne, Nebraska 68787, USA.
⁴ Department of Chemistry Brock University, St. Catharines Ontario L2S 3A1, Canada.

PMID: 31779322
DOI: 10.1063/1.5127760

Abstract

The generation and transfer of electron spin polarization and coherence plays an important role in quantum information technologies and spintronics. In this context, the excited state spin dynamics of molecular systems in which a stable free radical is attached to a chromophore are of particular interest. In such complexes, the coupling between the electron spin on the free radical with those on the chromophore generates excited states referred to as sing-doublet, trip-doublet, and trip-quartet. Here, we study the light-induced electron spin polarization in an aluminum(iii) porphyrin (AlPor) complex in which the nitroxide (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) is covalently bound to the Al center via an ester linkage. In addition to the covalent bond to the TEMPO moiety, the Al center in AlPor also acts as a Lewis acid and can coordinate Lewis bases such as pyridine. Without pyridine bound, the spin polarized transient electron paramagnetic resonance spectrum of the complex at 80 K is unusual and displays a strong absorptive pattern with a Lorentzian lineshape. Coordination of pyridine to AlPor-TEMPO results in a dramatic change in the spin polarization pattern and a spectrum typical of the quartet state with broad wings from the m_s = ±1/2 ↔ ±3/2 transitions with emissive/absorptive polarization and a narrow absorptive peak from the m_s = ±1/2 ↔ ±1/2 transitions. At later times, the pattern evolves to a purely absorptive spectrum similar to that observed without pyridine. These changes are discussed in terms of a model in which back and forth transitions between the nearly degenerate lowest trip-doublet and trip-quartet states occur. It is argued that these transitions lead to the observed net polarization in AlPor-TEMPO and are fast enough that the outer lines are broadened. Density functional theory computations and the UV/Vis spectra suggest that the exchange interaction between TEMPO and the triplet state of AlPor increases when pyridine is bound, and the near degeneracy of the lowest excited states is lifted. It is argued that this slows the back and forth transitions which results in the strong change in the polarization pattern.