Relationship between the optical properties and the structure of a new complex: Nd(III)-isothiocyanate-2,2′-bipyridine

Abstract

A new compound, [Nd(NCS)3 (2,2′-bipyridine)2(H2O)] (C28H24N8NdO2S3), was obtained and investigated for its photophysical properties. Coordination systems of lanthanide-organic compounds are promising materials with applications as efficient luminophores. Organic ligands with chromophore groups (for example double bonds or aromatic rings) can intensify luminescence efficiency (antenna effect). The title compound crystallizes in a triclinic system with the space group P-1 and the following unit cell parameters: a = 9.698 (3), b = 13.000 (4), c = 13.219 (4) Å, α = 110.36 (3), β = 94.12 (2) γ = 97.53 (2)°. The coordination number of the Nd3+ ion is 8. The molecules of 2,2′-bipyridine coordinate to the Nd3+ ion via N atoms. Thus, the neodymium complex, based on this ligand, might have different optical properties from complexes with coordinating Nd–ligand bonds via O atoms bonds only. Additionally, we used another ligand – isothiocyanate ion – inorganic ion, which also coordinates via N atoms. In the absorption and emission spectra of the title compounds at low temperatures, the splitting of the 4f–4f transitions indicates a single low symmetry of the Nd3+ ion in the title compound. This fact confirms our X-ray structural results. The lower nephelauxetic parameter (β), positive bonding parameter (b1/2), and Sinha parameter (δ) indicate a somewhat covalent character of the bond between the metal and the ligand but the NdN,O coordination bonds of [Nd(NCS)3 (2,2′-bipyridine)2(H2O)] indicate a more covalent character than NdO-only bonds but less covalent than NdN-only bonds. The experimental data of the decay time of the 4F3/2 state of the Nd3+ ion for the title compound were fitted using the biexponential function. These values are small and amount to 0.65 and 0.16 μs