A near-IR luminescent ratiometric ytterbium pH probe.

The hypersensitive 2F5/2 to 2F7/2 transition of Yb3+ can be used to monitor perturbations of the coordination sphere in ytterbium(iii) complexes. An envelope of Stark components gives rise to a relatively broad and asymmetric emission band, whilst changes in their relative intensity and energy enable a ratiometric response. We report a new ytterbium complex with a sulphonamide arm that binds reversibly to Yb3+ as a function of pH, giving rise to significant pH dependent changes in the Yb emission spectrum.