Stimuli-Responsive Photoluminescence of Eu(III) Complexes with Triazine-Based Luminophores

The interaction of europium nitrate with triazine-based luminophores, namely 3-(pyridin-2-yl)triimidazo[1,2-a:1′,2′-c:1″,2′′-e][1,3,5]triazine (L1), 3-(pyridin-4-yl)triimidazo[1,2-a:1′,2′-c:1″,2′′-e][1,3,5]triazine (L2), and 3,7,11-tri(pyridin-4-yl)triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine (L3), resulted in three Eu(III) compounds with the compositions (L1H)[Eu(NO3)4(H2O)2] (1), (L2HL2)2[Eu(NO3)5(H2O)] (CH3CN)2 (2), and [Eu(NO3)3(H2O)2(L3)] (3) whose structures were disclosed by single crystal X-ray diffraction analysis. In the organic–inorganic hybrid salts 1 and 2, the direct Eu(III)–triazinic ligand coordination is lacking, while in the neutral complex 3 the direct Eu(III)–L3 coordination occurs through one of the luminophore's N-pyridine binding sites. The solid-state photoluminescence behavior of 2 and 3 was studied. At room temperature, 2 revealed predominantly the ligand-based photoluminescence in the cyan-blue region of the spectrum when excited at 337 nm; the orange-red Eu(III)-based emission dominated the spectrum when excited at 405 nm. On the other hand, irrespective of the excitation wavelength, compound 3 displayed the characteristic emission of Eu(III), suggesting that the coordinated L3 ligand effectively played an antenna effect toward the metal. The Eu(III)-based contribution in photoluminescence in 2 and 3 displayed an opposite thermochromic behavior when monitored in the 12–300 K range. Specifically, the emission intensity decreases for 2 and increases for 3 with an increase in the temperature. The unusual thermochromic effect for 3 was hypothesized from a density of states (DOS) viewpoint.