Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

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Abstract

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor-acceptor chromophoreviatwo differentmeta-phenylene bridges in(bpy)Pt(CAT-m-Ph-NN)(mPh-Pt) and(bpy)Pt(CAT-6-Me-m-Ph-NN)(6-Me-mPh-Pt) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate,m-Ph =meta-phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙−, CAT+˙ = semiquinone (SQ), and NN), charge-separated doublet2S1(S = chromophore excited spin singlet configuration) excited state. Following excitation, the2S1state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversionviathe2T1(T = chromophore excited spin triplet configuration) state. This process generates emissive ground state ESP in6-Me-mPh-Ptwhile formPh-Ptthe ESP is absorptive. It is proposed that the emissive polarization in6-Me-mPh-Ptresults from zero-field splitting induced transitions between the chromophoric2T1and4T1states, whereas predominant spin-orbit induced transitions between2T1and low-energy NN-based states give rise to the absorptive polarization observed formPh-Pt. The difference in the sign of the ESP for these molecules is consistent with a smaller excited state2T1-4T1gap for6-Me-mPh-Ptthat derives from steric interactions with the 6-methyl group. These steric interactions reduce the excited state pairwise SQ-NN exchange coupling compared to that inmPh-Pt.

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Kirk, M. L., Shultz, D. A., Hewitt, P., Stasiw, D. E., Chen, J., & van der Est, A. (2021). Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization. Chemical Science, 12(41), 13704–13710. https://doi.org/10.1039/d1sc02965g

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