Solvent dependence of the emission intensities in photoluminescent mononuclear europium(III) complexes with tetradentate Schiff base ligands

Mononuclear tetradentate Schiff base-europium(III) complexes with different counter cations, X[Eu(3,5-Clsalen)(2)] {X = (C2H5)(3)NH+, (C2H5)(4)N+, K+; H-2(3,5-Clsalen): N,N'-bis-3,5-dichlorosalicylideneethanediamine}, were prepared and photoluminescence properties of the europium(III) complexes in organic solvents (CH3CN, DMSO, DMF, (CH3)(2)CO, CH3OH, N-methylformamide (NMF), CH2Cl2) were investigated. All the emission spectra of the complexes displayed similar emission spectral patterns based on the f-f transitions by excitation with 365 nm in the solvents. In contrast, the emission intensities of the complexes varied greatly in the different solvents. All the complexes X[Eu(3,5-Clsalen)(2)] {X = (C2H5)(3)NH+, (C2H5)(4)N+, K+} displayed strong emission intensities in polar aprotic solvents (phi = 0.24-0.42 in CH3CN, DMF, DMSO, and (CH3)(2)CO), and weak emission intensities in polar protic solvents (phi = 0.052-0.10 in CH3OH and NMF) at 298 K. The emission intensities of (C(2)H(5))3NH[Eu(3,5-Clsalen)(2)] and (C2H5)(4)N[Eu(3,5-Clsalen)(2)] in CH2Cl2 at 298 K are significantly different (phi = 0.016 for (C2H5)(3)NH[Eu(3,5-Clsalen)(2)] and phi = 0.19 for (C2H5)(4)N[Eu(3,5-Clsalen)(2)]). Mononuclear yttrium(III) and gadolinium(III) complexes X[Y(3,5-Clsalen)(2)] and X[Gd(3,5-Clsalen)(2)] {X = (C2H5)(3)NH+, (C2H5)(4)N+} were prepared. X-ray crystal structure analysis of (C2H5)(3)NH[Gd(3,5-Clsalen)(2)] was performed. Measurements of the H-1 and C-13 NMR spectra of the yttrium(III) complexes were performed to elucidate the structure of the complexes in the solvents. Measurement of the luminescence spectrum of the gadolinium(III) complex in ethanol/methanol (1:1) glass at 77 K was performed to estimate the energy level of the triplet state. The solvent effect on the emission intensities of the complexes is explained by the vibrational nonradiative deactivation caused by the OH, NH, CH vibrations of the solvent molecules, or alkylammonium cations, around the mononuclear europium(III) complex anion. (C) 2018 Elsevier Ltd. All rights reserved.