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Time gated separation of lanthanide luminescence

The distinctive luminescence of lanthanide ions has long been a challenge for the spectroscopist. Methods to counter the extremely low absorption coefficients and luminescence quantum yields have concentrated on
Protective ligands to reduce vibrational quenching effects by XH moieties (X = O, N,).
Deuterated solvents (also to reduce quenching effects of XH).
Antenna chromophores for enhanced absorption (with later energy-transfer to, and luminescence by, the lanthanide).

Luminescence from these systems comprises a number of sharp and well-resolved bands with characteristically long lifetimes, roughly 106 103 s, depending upon the degree of quenching. With an integrated phosphorimeter, the SpexFluoroMax-P allows convenient time-gated measurement of lanthanide systems. In this Application Note, two lanthanide complexes comprising an encapsulating complex with benzophenone antenna chromophore (L, see Fig. 1) and europium (Eu) or terbium (Tb) ions are investigated using time-gated spectroscopy with the FluoroMax-P.

Experiment & Results
An excitation source (xenon lamp) flashes (~ 3 s pulse-width). Detection of the samples luminescence occurs within a time-window after the flash, according to a chosen delay time and detection-window size. Acquisition of a spectrum corresponding to any desired time-slice during the luminescence decay is possible. Luminescence lifetime measurements are made by recording the intensity of a specific wavelength with a chosen detection-window size and incremented delay time (Fig. 2).

The individual spectra of Tb-L and Eu-L complexes are shown in Figs. 3 and 4. The resulting luminescence spectrum from a mixture of Tb-L and Eu-L (Fig. 5) combines features from both Tb-L and Eu-L, acquired with a delay time of 0.01 ms, and sample window of 10 ms. Thus results from detection of all the luminescence from both species appear in Fig. 5. Time-gating allows a mixture of the two species to be spectrally separated, based on their different lifetimes (~ 0.60 ms for Eu-L, and ~ 1.1 ms for Tb-L).

Careful choice of gating parameters allows spectra from this mixture to be obtained selectively from an early time (to minimize Tb-L and maximize Eu-L) and a late time (with negligible Eu-L, approximating pure Tb-L), as shown in Fig. 6. A time-gated matrix scan (Fig. 7) shows the dependence of the spectral profile with time. A number of complete emission spectra were recorded while automatically incrementing the delay time between each run. The result is a 3-D array, with intensity, wavelength, and time information.

The Spex FluoroMax-P spectrofluorometer with integral flash-lamp can perform time-gated spectroscopic separation of mixtures of chemical species.

We thank Drs. Andrew Beeby and Gareth Williams of the Chemistry Department, University of Durham, UK, for useful discussion and preparation of the compounds studied herein.

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