3,2-HOPO Complexes of Near Infra-Red (NIR) Emitting Lanthanides: Sensitization of Ho and Pr in Aqueous Solution

There is a growing interest in Near Infra-Red (NIR) emission originating from organic complexes of Ln cations. As a major impetus, biological tissues are considerably more transparent at these low energy wavelengths when compared to visible radiation, which facilitates deeper penetration of incident and emitted light. Furthermore, the long luminescence lifetimes of Ln complexes (eg. Yb, τrad ~ 1 ms) when compared to typical organic molecules can be utilized to vastly improve signal to noise ratios by employing time-gating techniques. While the improved quantum yield of Yb complexes when compared to other NIR emitters favours their use for bioimaging applications, there has also been significant interest in the sensitized emission from other 4f metals such as Ln = Nd, Ho, Pr and Er which have well recognised applications as solid state laser materials (eg. Nd ~ 1.06 μm, Ho ~ 2.09 μm), and in telecommunications (eg. Er ~ 1.54 μm) where they can be used for amplification of optical signals. As a result of their weak (Laporte forbidden) f-f absorptions, the direct excitation of Ln cations is inefficient, and sensitization of the metal emission is more effectively achieved using the so-called antenna effect. We have previously examined the properties of several Eu complexes which feature 1-hydroxypyridin-2-one (Fig. 1) as the light harvesting chromophore. While the 1,2-HOPO isomer was found to strongly sensitize Eu, we noted the analogous Me-3,2-HOPO isomer does not, which prompted further investigation of the properties of this chromophore with other metals.