Absorption peculiarities for DEA-CuCl₄ nanoparticles into the polymer matrices

M.Piasecki, S.Tkaczyk, P.Bragiel, I.V.Kityk K.Ozga

Poland

M.G.Brik

Estonia

V.Kapustyanyk

Ukraine

 

Single crystals of diethylammonium tetrachlorcuprate NH₂(C₂H₅)₂CuCl₄ (DEA-CuCl₄) (space group P2₁ /n; lattice constants a = 7.36 Å; b = 15.025 Å; c = 45.193 Å) and their derivatives attract considerable interest of researchers due to the substantial sensitivity of the CuCl₄ complexes to the surrounding environment, which allows for changing optical properties of the DEA-CuCl₄ host. At room temperature these crystals are bright green and, since they possess a dozen of phase transitions, their colour depends on temperature. Following, phase transitions observed at 311-330 K depend on local environment of the Cu²⁺ ions, which is manifested by about 50 nm spectral shift of the principal spectral bands. Their principal structure consists of the slightly deformed [CuCl₄]^2- tetrahedrons, the shape and size of which are very sensitive to the external field. As a consequence, slight variations of the corresponding local structure (induced by the external temperature) can produce significant changes in the soft mode condensation, responsible for phase transitions.

Following the general consideration, one can expect the properties of the [CuCl₄]^2- units to be sensitive to the very low sizes (about 10 nm) of these nanocrystallites (NC), on one hand, and to the surrounding matrices (in particular, polymers), into which these NC are embedded, on the other hand. In the present work we focus on the DEA-CuCl₄ absorption spectra behavior depending on the content of the DEA-CuCl₄ NC. It will be shown that changes of the effective nano-sheet surfaces (which occur when the NC concentration is varied) induce noticeable changes of the absorption spectra.

Experimental studies of the absorption spectra of the DEA-CuCl₄ nanoparticles embedded into PMMA were performed together with the DSC temperature measurements. Using DSC temperature study it was shown the substantial influence of the polymer matrix on the phase transition temperatures, reflecting a disturbing effect of principal 3d Cu-3pCl metal-ligand charge transfer bands. It was shown that an increase in the nanocrystallites concentration affects the position of the main absorption bands, shifting the CT band and d-d band to the lower and higher energies, respectively. A model explaining these shifts has been suggested. According to this model, at low concentrations of the NC, CuCl₄ units (forming the absorption spectra) are more isolated, and practically do not interact with each other. With increasing the NC concentration, they tend to form larger aggregates and behave more like bulk crystal. This is accompanied by an enhancement of the overlap effects between Cu²⁺ and Cl^- ions and shifts of the absorption bands. In addition to these experimental studies, crystal field calculations of the Cu²⁺ energy levels in the considered cluster were performed. The agreement between the calculated and observed energy levels is good.