Anthracene as a sensitiser for near-infrared luminescence in complexes of Nd(iii), Er(iii) and Yb(iii): an unexpected sensitisation mechanism based on electron transfer

Abstract

The ligand L¹, which contains a chelating 2-(2-pyridyl)benzimidazole (PB) unit with a pendant anthacenyl group An connected via a methylene spacer, (L¹ = PB-An), was used to prepare the 8-coordinate lanthanide(III) complexes [Ln(hfac)₃(L¹)] (Ln = Nd, Gd, Er, Yb) which have been structurally characterised and all have a square antiprismatic N₂O₆ coordination geometry. Whereas free L¹ displays typical anthracene-based fluorescence, this fluorescence is completely quenched in its complexes. The An group in L¹ acts as an antenna unit: in the complexes [Ln(hfac)₃(L¹)] (Ln = Nd, Er, Yb) selective excitation of the anthracene results in sensitised near-infrared luminescence from the lanthanide centres with concomitant quenching of An fluorescence. Surprisingly, the anthracene fluorescence is also quenched even in the Gd(III) complex and in its Zn(II) adduct in which quenching via energy transfer to the metal centre is not possible. It is proposed that the quenching of anthracene fluorescence in coordinated L¹ arises due to intra-ligand photoinduced electron-transfer from the excited anthracene chromophore ¹An* to the coordinated PB unit generating a short-lived charge-separated state [An˙⁺–PB˙⁻] which collapses by back electron-transfer to give ³An*. This electron-transfer step is only possible upon coordination of L¹ to the metal centre, which strongly increases the electron acceptor capability of the PB unit, such that ¹An* → PB PET is endoergonic in free L¹ but exergonic in its complexes. Thus, rather than a conventional set of steps (¹An* → ³An* → Ln), the sensitization mechanism now includes ¹An* → PB photoinduced electron transfer to generate charge-separated [An˙⁺–PB˙⁻], then back electron-transfer to generate ³An* which finally sensitises the Ln(III) centre via energy transfer. The presence of ³An* in L¹ and its complexes is confirmed by nanosecond transient absorption studies, which have also shown that the ³An* lifetime in the Nd(III) complex matches the rise time of Nd-centred near-infrared emission, confirming that the final step of the sequence is ³An* → Ln(III) energy-transfer.