Reverse
Lyotropic Liquid Crystals from Europium Nitrate
and P123 with Enhanced Luminescence Efficiency
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Abstract
Fabrication
of lyotropic aggregates containing the lanthanide ions
is becoming a preferable way to prepare novel functional materials.
Here, the lyotropic liquid crystals (LLCs) of reverse hexagonal, reverse
bicontinuous cubic, and lamellar phases have been constructed in sequence
directly from the mixtures of Eu颅(NO<sub>3</sub>)<sub>3</sub>路6H<sub>2</sub>O and Pluronic P123 amphiphilc block copolymer with increasing
the salt proportion. Their phase types and structural characteristics
were analyzed using polarized optical microscopy (POM) and small-angle
X-ray scattering (SAXS) measurements. The driving forces of reverse
LLC phase formation were investigated using Fourier-transformed infrared
spectroscopy (FTIR) and rheological measurements. The hydrated europium
salt was found to act not only as a solvent here, but also as the
bridge to form hydrogen bonding between coordinated water molecules
and PEO blocks, which played a key role in the reverse LLCs formation.
Compared to those in aqueous solutions and solid state, the enhanced
luminescence quantum yields and prolonged excited state lifetimes
were observed in two europium containing reverse mesophases. The luminescence
quenching effect of lanthanide ions was efficiently suppressed, probably
due to the substitution of coordinated water molecules by oxyethyl
groups of P123 and ordered phase structures of LLCs, where the coordinated
europium ions were confined and isolated by PEO blocks. The optimum
luminescence performance was then found to exist in the reverse hexagonal
phase. The obtained results on such lanthanide-induced reverse LLCs
should be referable for designing new luminescent soft materials construction
to expand their application fields