Sensitisation of Eu(iii)- and Tb(iii)-based luminescence by Ir(iii) units in Ir/lanthanide dyads: evidence for parallel energy-transfer and electron-transfer based mechanisms

Sykes, Daniel and Cankut, Ahmet J. and Ali, Noorshida Mohd and Stephenson, Andrew and Spall, Steven J. P. and Parker, Simon C. and Weinstein, Julia A. and Ward, Michael D. (2014) Sensitisation of Eu(iii)- and Tb(iii)-based luminescence by Ir(iii) units in Ir/lanthanide dyads: evidence for parallel energy-transfer and electron-transfer based mechanisms. Dalton Transactions, 43 (17). p. 6414. ISSN 1477-9226

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Official URL: http://dx.doi.org/10.1039/c4dt00292J

Abstract

A series of blue-luminescent Ir(iii) complexes with a pendant binding site for lanthanide(iii) ions has been synthesized and used to prepare Ir(iii)/Ln(iii) dyads (Ln = Eu, Tb, Gd). Photophysical studies were used to establish mechanisms of Ir→Ln (Ln = Tb, Eu) energy-transfer. In the Ir/Gd dyads, where direct Ir→Gd energy-transfer is not possible, significant quenching of Ir-based luminescence nonetheless occurred; this can be ascribed to photoinduced electron-transfer from the photo-excited Ir unit (*Ir, 3 MLCT/3LC excited state) to the pendant pyrazolyl-pyridine site which becomes a good electron-acceptor when coordinated to an electropositive Gd(iii) centre. This electron transfer quenches the Ir-based luminescence, leading to formation of a charge-separated {Ir4+} •—(pyrazolyl-pyridine)•− state, which is short-lived possibly due to fast back electron-transfer (<20 ns). In the Ir/Tb and Ir/Eu dyads this electron-transfer pathway is again operative and leads to sensitisation of Eu-based and Tb-based emission using the energy liberated from the back electron-transfer process. In addition direct Dexter-type Ir→Ln (Ln = Tb, Eu) energy-transfer occurs on a similar timescale, meaning that there are two parallel mechanisms by which excitation energy can be transferred from *Ir to the Eu/Tb centre. Time-resolved luminescence measurements on the sensitised Eu-based emission showed both fast and slow rise-time components, associated with the PET-based and Dexter-based energy-transfer mechanisms respectively. In the Ir/Tb dyads, the Ir→Tb energy-transfer is only just thermodynamically favourable, leading to rapid Tb→Ir thermally-activated back energy-transfer and non-radiative deactivation to an extent that depends on the precise energy gap between the *Ir and Tb-based 5 D4 states. Thus, the sensitised Tb(iii)-based emission is weak and unusually short-lived due to back energy transfer, but nonetheless represents rare examples of Tb(iii) sensitisation by a energy donor that could be excited using visible light as opposed to the usually required UV excitation.

Item Type: Article
Subjects: 500 Natural Sciences and Mathemetics > 540 Chemistry & allied sciences
Department: School of Human Sciences
Depositing User: Daniel Sykes
Date Deposited: 21 Jun 2016 12:00
Last Modified: 21 Jun 2016 12:00
URI: http://repository.londonmet.ac.uk/id/eprint/1028

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