**5. Transition metal based singlet oxygen probe**

Though lanthanide based fluorescent probes possess many advantages including low background interference, widely applicable pH range, and excellent water solubility; these probes can be expected to utilise for visualising spatial and temporal distribution of 1 O2 in aqueous system but a major drawback of these probes is that they need ultraviolet light for excitation which might cause cell damage, thus limiting their application in biological system. Liu et al. demonstrated a Re<sup>I</sup> complex, Re(CO)3Cl(aeip){aeip = 2-(anthracen-9-yl)-1-ethyl-imidazo[4,5-f] [1, 10], phenanthroline, which can be excited via visible light of 410 nm wavelength in aqueous medium thus minimising the effect of cell damage (shown in **Figure 11**) [31]. The Re<sup>I</sup> complex is non-luminescent in the native state probably due to quenching of luminescence of Re → aeip (M → L) charge transfer transition in the excited state through exchange triplet-triplet intramolecular energy transfer by anthryl moiety

**Figure 11.** *The ReI complex and the formation of endoperoxide via reaction with 1 O2 (adapted from [37]).*

#### **Figure 12.**

*Changes in the luminescence intensity of ReI complex (*λ*ex = 410 nm) with increasing concentration of 1 O2 in (a) in neutral and (b) in alkaline medium. The inset of these figures show that the sharp change in luminescence intensity at 565 nm with gradually increasing concentration of 1 O2 (adapted from [40]).*

as mentioned in similar type ReI complexes [37]. The strong enhancement of luminescence of Re<sup>I</sup> complex due to the formation of endoperoxide in both neutral and alkaline medium is perceived, that due to the termination of electronic coupling between the anthryl and the parent Re<sup>I</sup> complex (**Figure 11**). In comparison to other fluorescent probes of Eu3+ and Tb3+ complexes, ReI complex exhibits higher molar absorption at the visible wavelength of 410 nm and fluorescence can also be initiated with this wavelength of light [31].

With increasing concentration of <sup>1</sup> O2 the luminescence intensity increases as shown in **Figure 12** and the luminescence quantum yield changes from 8.9 × 10−5 to 7.1 × 10−4 and 4.7 × 10−5 to 8.7 × 10−4 in neutral and alkaline media [37]. The limit of detection obtained using ReI complex is found to be 4.9 nM/L and 10.5 nM/L in neutral and alkaline medium respectively, which are very much comparable with that obtained for Eu3+ and Tb3+ complex [31, 37, 39].

The additional benefit of visible light excitation and long lifetime enable Re<sup>I</sup> complex to be used in biological systems.
