Sodium-based Donor-Acceptor Assemblies Featuring Thermally Activated Delayed Fluorescence Enabled by Highly Efficient Through-Space Charge Transfer

Abstract

Exploring more economical and innovative alternatives to precious 4d and 5d metal complexes used in photocatalysis, OLEDs, or energy conversion has primarily focused on 3d metals, leaving highly abundant alkali metals underexplored. We show that under stoichiometry control of 1,10-phenanthroline-5-carbonitrile (^CNphen) and sodium 2,6-bis(trimethylsilyl)benzenethiolate Na(^TMSBT), sodium complexes can be obtained that either form the neutral linear assembly [Na(THF)(^CNphen)(^TMSBT)]_N (Na1D), or {[Na(^CNphen)₄](^TMSBT)}_N (Na2D) as a 2D-polymeric arrangement with octahedrally coordinated Na, similar to 18 valence electron transition metal complexes, and the thiolate acting as counter anion. Na1D and Na2D feature bright visible light absorption and thermally activated delayed fluorescence (TADF) from intra-and intermolecular through-space charge-transfer (^1/3TSCT) excited states, respectively, with high radiative rates k_TADF up to 4.5 × 10⁵ s⁻¹, unprecedented for alkali metal-based luminophores. Solution studies revealed extensive dynamic behavior, including reversible metal−ligand bond dissociation. However, the Na assemblies show ^1/3TSCT emission (λ_em,max = 555 nm) in solution and for the first time we have successfully employed Na-based compounds as visible light photosensitizers in Dexter energy transfer catalysis. This study demonstrates the critical role of TSCT states in constructing photoactive coordination complexes and indicates an underexplored, yet significant potential of sodium-based luminophores as TADF emitters and earth-abundant photocatalysts.