The unusual thermochromic NIR luminescence of Cu(i) clusters: tuned by Cu–Cu interactions and packing modes

Two hexanuclear Cu(I) clusters [CuI3(4-ptt)3]2·3DMF·3H2O (1) and [CuI(4-ptt)]6·8DMF·7H2O (2) (4-Hptt = 5-(pyridin-4-yl)-1H-1,2,4-triazole-3-thiol, DMF = N,N-dimethylformamide), were synthesized and characterized. Compounds 1 and 2 with similar coordination environments are isomers, but their detailed structures are different due to the reaction temperature tuning effect. Both 1 and 2 extend from monomers to 3D supramolecules with the help of hydrogen bonding between the triazole and pyridine from the 4-ptt ligands. The Cu6S6 units of 1 pack in a polydirectional array, while the Cu6S6 units in 2 extend in one direction and link the planes of adjacent ligands to enhance the delocalization of π electrons. Their varied Cu–Cu interactions and individual packing modes cause differences in luminescent and thermostable behaviors. Compound 1 exhibits an unusually long wavelength at about 900 nm and a higher thermal stability; while the emission of 2 splits into two bands (high-energy and low-energy emission bands) as the temperature decreases. Therefore, the emissions of 1 originate from a 3CC transition, and those of 2 are from a mixture of 3CC and MLCT.