Abstract
The relative sensitivities of structurally related Eu(III) complexes to quenching by electron and energy transfer processes have been compared. In two sets of 9-coordinate complexes based on 1,4,7-triazacyclononane, the Eu emission lifetime decreased as the number of conjugated sensitising groups and the number of unbound ligand N atoms increased, consistent with photoinduced electron transfer to the excited Eu(III) ion that is suppressed by N-protonation. Quenching of the Eu 5D0 excited state may also occur by electronic energy transfer, and the quenching of a variety of 9-coordinate complexes by a cyanine dye with optimal spectral overlap occurs by an efficient FRET process, defined by a Förster radius (R0) value of 68 Å and characterised by second rate constants in the order of 109 M-1 s-1; these values were insensitive to changes in the ligand structure and to the overall complex hydrophilicity. Quenching of the Eu and Tb excited states by energy transfer to Mn(II) and Cu(II) aqua ions occurred over much shorter distances, with rate constants of around 106 M-1 s-1, owing to the much lower spectral overlap integral. The calculated R0 values were estimated to be between 2.5 to 4 Å in the former case, suggesting the presence of a Dexter energy transfer mechanism that requires much closer contact, consistent with the enhanced sensitivity of the rate of quenching to the degree of steric shielding of the lanthanide ion provided by the ligand.
Highlights
The sensitised emission of lanthanide luminescence has been studied in detail because of the wide range of applications that have arisen in developing luminescence bioassays, in devising targeted optical imaging agents and in the creation of selective analytical probes and sensors for a wide range of analytes. 1-4 The salient properties of the sensitising chromophore have been considered in depth, and key design criteria have emerged. 5-7 In particular, careful consideration has been given to the photophysical properties of the sensitiser, notably its molar absorptivity at the excitation wavelength, the energies of its internal charge transfer (ICT), singlet and triplet states and the size of the energy gap that determines the facility of inter-system crossing
The lanthanide excited state may be subject to quenching by electron transfer and by vibrational and electronic energy transfer processes, leading to a reduction in its lifetime
8 More commonly, fast electron transfer occurs from a ligand HOMO, such as a π or lone pair orbital, exemplified here with a set of six Eu(III) complexes where the lifetime was shortest for the complexes with the most high lying π orbitals or the most N atoms
Summary
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. Authors can make their results available to the community, in citable form, before we publish the edited article We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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