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

Exploring more economical and innovative alternatives to precious second- and third-row metal complexes widely used in photocatalysis, electroluminescent devices, or energy conversion has primarily focused on 3d metals, leaving abundant representatives of alkali metals underexplored. Here, we introduce novel sodium-based coordination systems comprising 1,10-phenanthroline-5-carbonitrile (CNphen) and sodium 2,6-bis(trimethylsilyl)benzenethiolate (NaTMSBT). Under the control of the stoichiometry of Na(TMSBT) and CNphen, two molecular assemblies can be obtained in the solid state. [Na(THF)(CNphen)(TMSBT)]N (Na1D) represents an unprecedented neutral linear assembly, whereas {[Na(CNphen)4](TMSBT)}N (Na2D) is a two-dimensional polymeric arrangement with the sodium ion being octahedrally coordinated similar to 18 valence electron transition metal complexes, and the thiolate acting as counter anion. Due to the high electron-rich character of TMSBT and electron-deficient CNphen, Na1D and Na2D feature bright low-energy emissions from intra- and intermolecular through-space charge transfer (TSCT) excited states, respectively. An exceptionally short C–H···π distance between CNphen and TMSBT, enabled by the weak covalent character of the Na–ligand bonds, leads to a highly efficient TSCT process. Consequently, the 1/3TSCT electronically excited states are nearly isoenergetic. Both assemblies are thus unique sodium TSCT-TADF emitters, featuring high radiative rate constants involving triplet states of krTADF up to 4.5×105 s-1 that are unprecedented for alkali metal-based luminophores. Detailed solution studies have revealed extensive dynamic behavior, including reversible metal-ligand bond dissociation. However, the novel systems show weak emission (λem,max = 555 nm) in tetrahydrofuran (THF), also associated with the TSCT process. Furthermore, for the first time, we have successfully employed sodium-based compounds as visible light photosensitizers in triplet energy transfer, as evidenced by the photoisomerization of trans-stilbene. This study demonstrates the critical role of TSCT states in constructing photoactive coordination complexes and highlights the significant potential of sodium-based luminophores as TADF emitters and earth-abundant photocatalysts.