Photophysical properties of Pr(iii) and Er(iii) complexes of poly(pyrazolyl)borates

Abstract

The complexes [M(L¹)₂(NO₃)] and [M(L²)(NO₃)₂] (M = Pr, Er; L¹ = the tetradentate ligand dihydrobis-[3-(2-pyridyl)pyrazolyl]borate; L² = the hexadentate ligand hydrotris-[3-(2-pyridyl)pyrazolyl]borate) were prepared and their structural and photophysical properties studied. All complexes are 10-coordinate. Crystallographic analysis of [M(L¹)₂(NO₃)] (M = Pr, Er) showed that for the smaller Er(III) ions steric congestion at the metal centre results in two of the Er–N(pyridyl) distances being particularly long, which does not occur with the larger Pr(III) ion that is better able to accommodate 10-fold coordination. On UV irradiation, both Pr(III) complexes show, in the visible region of their luminescence spectra, transitions originating from both the ³P₀ level (at ca. 21 000 cm⁻¹) and the ¹D₂ level (at ca. 17 000 cm⁻¹), a consequence of the fact that the lowest triplet state of the coordinated pyrazolylborate ligands lies at ca. 24 000 cm⁻¹ in each case so is high enough in energy to populate both levels. This contrasts with Pr(III) complexes based on diketonate ligands in which the lower triplet energies of the ligands result in emission from the ¹D₂ level only. At longer wavelengths, near-infrared luminescence arising from the ¹D₂ emissive level is observed with lifetimes (in both the solid state and solution) being in the range 50–110 ns. For both Er(III) complexes, luminescence at 1530 nm occurs following UV excitation of ligand-centred transitions. In CH₂Cl₂ both complexes gave dual-exponential luminescence, with the major component having a lifetime characteristic of an intact Er(III) complex (≈ 1.5 µs) and the minor component being much shorter lived (0.2–0.5 µs), suggestive of a species in which a ligand is partially detached and the metal is solvated, with the two forms interconverting slowly. This behaviour is consistent with the steric congestion and long M–N(pyridyl) bonds that were observed in [Er(L¹)₂(NO₃)]. In the solid state both Er(III) complexes gave very weak luminescence, which could be fitted to a single exponential decay with a lifetime similar to the longer-lived of the solution components.