Synthesis, characterization and photothermal conversion performance of three xanthene-functionalized dicopper complexes

Abstract For further elucidating the molecular absorption mechanisms in the photothermal conversion performance, three xanthene-functionalized dicopper coordination compounds have been isolated and structurally characterized by IR, Powder XRD, Solid-state UV, and TGA. They were prepared by solvothermal treatment the mixture of 9H-xanthene-9-carboxylic acid with Cu(CH3CN)4BF4 with/without the assistance of the auxiliary ligands {2,2′-bipyridine (bpy) or bis(diphenylphosphino)methane (dppm)}, namely, [Cu2(X9A)4(CH3CN)2] (1, X9A = xanthene-9-carboxylate), {[Cu2(X9A)(dppm)2(CH3CN)](BF4)∙(CH3CN)} (2), and {[Cu2(X9A)3(bpy)2](BF4)∙0.5(H2O)}2 (3). Single-crystal X-ray structural analysis and mass spectra revealed that the copper atoms in 2 were Cu(I), while in 1 and 3 were Cu(II) coordination compounds. The X9A ligands arranged in two types of coordination modes in 3: one in a bidentate bridging mode (μ2-η1, η1 mode) and two in an arrangement involving chelation and bridging mode (μ2-η1, η2 mode), while only simple bridging mode presented in 1 and 2. With the further stabilization of bpy ligands, compound 3 displayed excellent thermal stability, superior absorption ability as well as the best photothermal conversion performance (the highest temperature increment ~ 19 ℃ and equilibrium temperature ~ 32 ℃).