Structurally isomeric ditopic 2-mercaptobenzoxazole and 2-hydroxybenzothiazole as ligands for design of 2D sodium-based luminescent coordination polymers

Abstract

The novel ditopic centrosymmetric soft-base ligands 3,7-dihydrobenzo[1,2-d:4,5-d′]bis(oxazole)-2,6-dithione (H₂L¹) and 3,7-dihydrobenzo[1,2-d:4,5-d′]bis(thiazole)-2,6-dione (H₂L²) were obtained via a one-pot method. Both compounds form 2D coordination polymers (CPs) through the reaction of free ligands with sodium bis(trismethylsilyl)amide in various solvent media. H₂L¹ containing products were [Na₂L¹(DME)₂]_n (1) and [Na₂L¹(DMSO)₄]_n (2), and for H₂L², [Na₆L²₃(DMSO)₁₂]_n (3) was formed. X-ray analysis revealed that compound 1 is a 2D CP, in which deprotonated H₂L¹ acts as a tetratopic linker, linking four sodium atoms via nitrogen and bridging sulfur. Unlike 1, in CP 2, deprotonated H₂L¹ acts as a ditopic linker coordinating sodium atoms in an unprecedented Na₄O₄ metallocenter formed by the oxygens of DMSO molecules. Compound 3 is a 2D CP featuring rare Na₃O₆ secondary building unit (SBU), whereas H₂L² exhibits both bridging and mixed chelating/bridging mode, linking four sodium atoms via nitrogen and oxygen atoms. Despite the fact that free ligands H₂L¹ and H₂L² are non-luminescent in solution and in the solid state, their deprotonated forms at 298 K in DME media demonstrate moderate photoluminescence (PL) in DME solutions with broad bands in the range of 360–500 nm. At 77 K, the same solutions show broadened and intense PL consisting of bands in the 360–670 nm region corresponding to ligand fluorescence and phosphorescence. CPs 1–3 in the solid state exhibit PL at 298 and 77 K. At 298 K, CPs 1 and 3 exhibit intense fluorescence in the range of 370–450 nm and moderate microsecond timescale phosphorescence in the range of 500–670 nm, while CP 2 exhibits only fluorescence. Upon cooling to 77 K, all three CPs demonstrate both fluorescence and phosphorescence. Based on the low-temperature phosphorescence spectra of 1 and 3 in DME solutions, the triplet energy levels of H₂L¹ and H₂L² were established at 21 000 and 20 800 cm⁻¹, respectively.