From Stimuli-Responsive Polymorphic Organic Dye Crystals to Photoluminescent Cationic Open-Framework Metal Phosphate

Four photoluminescent dye crystals, TPB-n (n = 1–3) and TPCH, have been synthesized
in isolation from carbon–carbon coupling of 4,4′-trimethylene-dipyridine
(tmdp) in the absence of organic solvent and catalyst via in situ
one-pot metal/ligand oxidative-dehydrogenation reactions. Large crystals
have been obtained and readily separated from reaction products without
the need for purification. Their structures were characterized: TPB
and TPCH represent two dissimilar oxidized forms of tmdp dimers; TPCH
was less oxidized but accompanied with partial hydrolysis with molecular
formula further confirmed by FAB-MASS analysis. They exhibit distinct
photoluminescence (PL) with quantum efficiency measured up to 42%.
TPB-2 and TPB-3 are two polymorphic dihydrates, stimuli-responsive
toward photoluminescence color changes; both can transform to TPB-1
upon gentle heating. Importantly, the three TPB-n crystals establish the first tetrapyridyl-type ligands adapted by
metal phosphates to create a cationic luminescent framework, NTHU-12.
This study serves as an inspiring route beyond conventional C–C
bond formation reactions and has generated four tmdp dimers in isolation.
The facile syntheses have resulted in ample production of a rare type
of tetrapyridyl ligands, leading to the discovery of the first positively
charged hybrid topology in nanoporous solids. The features of the
four luminescent tmdp dimers and NTHU-12 embodied in synthesis, structure,
and optical property are reported.