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

Theodore Lazarides, Mohammed A H Alamiry, Harry Adams, Simon J A Pope, Stephen Faulkner, Julia A. Weinstein, Michael D. Ward

    Research output: Contribution to journalArticlepeer-review

    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. © The Royal Society of Chemistry 2007.
    Original languageEnglish
    Pages (from-to)1484-1491
    Number of pages7
    JournalDalton Transactions
    Issue number15
    DOIs
    Publication statusPublished - 2007

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