**4.1 [4**′**-(9-Anthryl)-2,2′:6′,2″-terpyridine-6,6″-diyl]bis (methylenenitrilo) tetrakis (acetate)-Eu3+ (ATTA-Eu3+)**

Bo Song et al. first synthesised the Eu3+ cheleate based phosphorescence probe, [4′ -(9-anthryl)-2,2′:6′,2″-terpyridine-6,6″-diyl] bis (methylenenitrilo) tetrakis (acetate)-Eu3+ (ATTA-Eu3+) (shown in **Figure 9**) for the time resolved luminescence study. The probe is highly sensitive, selective for 1 O2 and water soluble for time resolved luminescence detection of 1 O2 with a detection limit of 2.8 nM/L [16]. The almost non luminescent ATTA-Eu3+ specifically reacts with 1 O2 to yield its endoperoxide (**Figure 10**) (EP- ATTA-Eu3+) resulting a great enhancement of luminescence intensity (luminescence quantum yield of EP- ATTA-Eu3+ is 17 fold greater than that of ATTA-Eu3+) as the population of the excited state of Eu3+ was increased enormously after formation of endoperoxide [16, 27]. The endoperoxide compound EP-ATTA-Eu3+ exhibit favourable chemical stability with a conditional stability constant was measured to be 1020 level. Apart from that no decrease in phosphorescence intensity was observed even after storage of EP-ATTA-Eu3+ for several days at room temperature.

Another advantage of EP-ATTA-Eu3+ is that the phosphorescence intensity of EP-ATTA-Eu3+ is stable even at very low pH of 3 whereas in case of fluorescein based probe (DPAXs and DMAXs) the rapid decrease of fluorescence intensity is reported below pH 7 [16, 27].

#### **Figure 10.**

*The chemical structure and the reaction scheme of (a) ATTA-Eu3+, (b) PATA-Tb3+ and (c) MTTA-Eu3+ with singlet oxygen (adapted from [16]).*

### **4.2 N,N,N´,N´-[2,6-Bis-(3′-aminomethyl-1′-pyrazolyl)-4-(9″-anthryl)pyridine] tetrakis(acetate)-Tb3+ (PATA-Tb3+)**

Mingqian Tan et al. developed another chelate complex of Tb3+, N,N,N´,N´-[2,6 bis-(3′-aminomethyl-1′-pyrazolyl)-4-(9″-anthryl)pyridine] tetrakis(acetate) Tb3+ (PATA-Tb3+), (shown in **Figure 10b**) is known to be effective fluorescent probe for the detection of 1 O2 [38]. Because of the presence of 9-anthryl moiety within the ligand the compound, PATA-Tb3+ is almost non fluorescent but once it reacts with 1 O2 the corresponding endoperoxide, EP-PATA-Tb3+ becomes strongly fluorescent with almost 23 fold enhancement of fluorescent quantum yield [39]. PATA- Tb3+ is known to be an excellent fluorescent probe for 1 O2 due to the characteristics, including high water solubility, wide applicable pH range and long fluorescence lifetime of *Photophysical Detection of Singlet Oxygen DOI: http://dx.doi.org/10.5772/intechopen.99902*

endoperoxide (2.76 ms) which makes it suitable for time resolved fluorescent detection with a detection limit as low as 10.8 nM/L [27, 38]. The specificity of PATA- Tb3+ towards 1 O2 is also confirmed upon reaction with some other reactive oxygen species, including hydroxyl radical, superoxide ion, peroxynitrite and hydrogen peroxide, the no significant change of fluorescence intensity support its specificity for <sup>1</sup> O2.
