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Thermodynamic vs. kinetic control: Case study of Eu(III) molecular and polymer β-diketonate complexes
Crystal engineering of lanthanide coordination compounds plays crucial role in the design of emitters with set properties. Reaction of Eu(OH)3 and 4,4,4-trifluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL) in various media led to the formation of 8 new crystal structures with the general formula [Eu(L)3(Solv1 )x](Solv2 )y, where Solv1 , Solv2 and x, y vary in the choice of solvent (Solv = MeOH, EtOH, i-PrOH, n-PrOH, i-BuOH, n-BuOH, t-BuOH, H2O) and x = 1, 2; y = 0, 1. Moreover, a linear polymer with the formula [Eu(L)3(H2O)]n was isolated from its ethanol solution. The nature of the solvent plays crucial role in the crystal engineering of molecular Eu3+ diketonates. Molecular complexes were confirmed to be kinetic products of the reaction that undergo transformation to thermodynamically stable polymer complex [Eu(L)3(H2O)]n. Variation of the alcohol steric hindrance results in redistribution of Stark subbands and Eu3+ lifetimes, allowing for fine tuning of the luminescent properties of the complexes. Our findings pave the way for selective synthesis of novel materials for optoelectronics and biomedicine.