Aggregation in methanol and formation of molecular glasses for europium(III) N-acylaminocarboxylates: effects of alkyl chain length and head group

Europium(III) complexes of N-acyl-DL-alaninates (acyl = acetyl, butanoyl, hexanoyl, octanoyl, decanoyl, dodecanoyl and tetradecanoyl), N-octanoyl-DL-phenylalaninate, and N-octanoyl-L-serinate were prepared to understand the effects of alkyl chain length and the type of head group on the formation of glassy states and on the aggregation behaviour in solutions. The acylalaninate complexes had a tendency to form a transparent glass, whereas Eu(ala)3 (ala = DL-alaninate) was easily crystallized. Of the C2(acetyl)–C14(tetradecanoyl) chains in the ligands, the C4–C8 chains were the most favourable to assume a stable glassy state by solvent vaporization. The europium(III) complexes having an acyl chain of C6 (hexanoyl) or longer exhibited a peak below 2θ = 5° due to the presence of a bilayer structure in the glassy state. The octanoylserinate complex easily formed an anisotropic glass by a solvent-cast method, while the octanoylphenylalaninate complex transformed from a transparent glass to an anisotropic glass by an annealing treatment. The trend of glass formation was related with the aggregation behaviour of the complexes in methanol detected by self-diffusion and luminescence properties.