showed a broad absorbance spectrum with no fluorescence in sodium phosphate buffer (100 mM; pH 7.4) (Figures )). The observed complete quenching of fluorescence was considered to be due to ultrafast cis/trans conformational change of the azo bond of T-azoJSiR640 in the excited state, as in the case of our previously reported probes. On the other hand, T-JSiR640 showed marked fluorescence with a peak at 662 nm in the NIR range (Figure )), and its fluorescence quantum yield was 0.12 (Table ). We also confirmed that T-JSiR640 showed essentially the same fluorescence spectra over the pH range from 5.0 to 9.0, indicating that fluorescence signal of T-JSiR640 is not affected by pH (Figure ). These results suggested that T-azoJSiR640 is able to generate NIR fluorescence upon cleavage of the azo group under hypoxia.