Thermally induced structural transitions and temperature memory effect in a luminescent vitrified film of an anisometric europium(III) β-diketonate complex

Abstract

• Vitrified Eu(DK12-14)3phen film (sandwiched between quartz plates) has been fabricated. • Performed weeklong monitoring revealed temperature memory effect. • Proposed model indicates thermally-induced transitions between two local states of film. • These two states are most likely close to crystalline and amorphous local structures. • Effect could be utilized in designing photonic materials equipped with memory function. We report on a temperature memory effect, which is highly unusual for known film-type materials based on lanthanide(III) β-diketonate complexes. It was observed in a 20 µm thick vitrified film fabricated by cooling the melt of a powder of an anisometric europium(III) β-diketonate complex between two quartz plates. The excitation of the film with light in the wavelength range of 280-425 nm causes a bright orange-red emission of Eu 3+ ions (due to efficient ligand-to-metal energy transfer). The memory effect shows itself as ability of the film to be in different states with their own luminescence decay time at the same temperature. In order to study the effect we carried out a weeklong experiment to monitor the evolution of the luminescence decay time in series of temperature treatments (alternating keeping the film at 370 K and storage in the dark at room temperature). We proposed a simple two-state model to simulate the results of the spectroscopic measurements. This made it possible to reveal the memory effect mechanism, which is thermally induced transitions between two different local states of the film. They are most likely in their nature close to crystalline and amorphous local structures. The obtained and analyzed data clearly indicate that the memory effect observed in the film opens broad perspectives in designing of new type luminescent photonic materials having memory function.