*3.3.1 Hydrogen sulphide (H2S)*

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

samples and also demonstrated its utility in imaging living cells.

interpreted in terms of ICT variations upon sensing the receptor.

**3.2 DCM derivatives as anion sensors**

successfully applied for the quantitative estimation of Cu2+ in various types of water

**DCBP3** is designed based on the Pd(0)-catalyzed Tsuji-Trost allylic oxidative insertion reaction and dicyanomethylene benzopyran moiety [53]. Photophysical properties revealed that the probe **DCBP3** exhibits high sensitivity and selectivity towards the detection of both Pd(0) and Pd(II) under reducing conditions. The probe **DCBP3** shows a major absorption band with a maximum at 450 nm, and after treating with palladium, another new absorption peak started appearing around 560 nm. On the other hand, **DCBP3** displays no fluorescence at 700 nm when excited at 560 nm. However, upon the addition of palladium, the fluorescence emission peak at 700 nm increases gradually. Additionally, marked color changes were also noticed. All the photophysical properties have been explained based on palladium-triggered cleavage reaction that produced a free **DCBP-OH**. Moreover, the probe **DCBP3** is little affected with pH variation and has low cytotoxicity.

As discussed in Section 3.1, the molecules **DCBP1** and **DCM2** form copper complexes (**DCBP1 Cu2+** and **DCBP1-Cu2+**), and their fluorescence emission quenches drastically [48, 51]. In situ generated **DCBP1 Cu2+** and **DCBP1-Cu2+** recognize PPi anion which can be tracked from spectrophotometrically and fluorescence measurements. Later, the molecule **DCBP1** was modified by decorating with a lithium iminodiacetate group in place of N-aryl group [54]. The synthesized NIR fluorophore, **DCBP4**, selectively binds with Cu2+ ions because of lithium iminodiacetate receptor and found to have very good solubility in aqueous water. The photophysical properties of metallated fluorophore (**DCBP4-Cu2+**) were found to be modified upon interacting selectively with pyrophosphate (PPi) anion. When PPi is gradually added to the solution of **DCBP4-Cu2+**, a new redshifted peak at 503 nm appeared and increased gradually with an isosbestic point at 450 nm. The absorption spectral changes are very much evident to the naked eye where the pale brown color of the **DCBP4-Cu2+** solution changes to red color. On the other hand, simultaneously turned on fluorescence and emission intensity in the NIR region (675 nm) are enhanced gradually and stabilized upon the addition of 15 equiv. of PPi. The fluorescence off–on switching and the colorimetric response of **DCBP4-Cu2+** are

A near-infrared (NIR) fluorescent chemosensor, **DCBP5**, was developed on the basis of dicyanomethylene-4H-benzopyran derivative for detecting fluoride anions [55]. Chemodosimeter **DCBP5** was synthesized by the Knoevenagel condensation of 4-dicyanomethylene-2-methyl-4H-pyran and 4-(tert-butyldiphenylsilyloxy) benzaldehyde. With the addition of F<sup>−</sup> ions to the **DCBP5** sensor, absorption band cantered at 447 nm slowly decreases, and at lower F<sup>−</sup> concentration (<30 μM), a new absorption emerges at 454 nm gradually. When the F<sup>−</sup> concentration was further increased beyond 50 μM, the new absorption band at 454 nm decreases, and a concomitant increase of a new band at 645 nm was observed with an isosbestic point at 510 nm. The large redshift (190 nm) is also noticeable to the naked eye in which the initial pale yellow color of the **DCBP5** solution changes to blue color upon adding fluoride ions. It should also be noted that the sensing process is very fast, and within 30 s the sensing is noticeable to the naked eye. The observed isosbestic point of **DCBP5** sensor upon addition of the F<sup>−</sup> ions clearly indicates formation of a new species which is attributed to phenolate group generation due to Si–O cleavage. Similar supporting results were also observed from fluorescence measurements. The **DCBP5** molecule is non-fluorescent due to the presence of silyl group. However, the sensor **DCBP5** turn-on fluorescence with gradual addition of

**18**

Hydrogen sulphide (H2S) is involved as a signaling molecule in various physiological processes that include modulation of neuronal transmission, regulation of release of insulin, relaxation of the smooth muscle, and reduction of the metabolic rate [56, 57]. From the animal model study of critical illness, it was realized that the H2S donor protect from lethal hypoxia and reperfusion injury and exert antiinflammatory effects [58]. Physiological H2S concentration is estimated to vary from nano- to millimolar levels [59], and once this limit is crossed, the cells release H2S that can cause certain diseases, such as Alzheimer, Down syndrome, diabetes, and other diseases of mental deficiency [60]. Hence, a reliable in vivo study is essential to measure accurately H2S concentration thereby preventing deceases. A NIR probe, **DCBP6**, that comprises dicyanobenzopyran and 4-azidostyryl group as receptor was developed for selective detection of H2S [61]. The probe **DCBP6** selectively reacts with H2S and reduces the azido group (–N3) to amine (–NH2), and the corresponding molecule becomes highly fluorescent than the parent **DCBP6**. Upon H2S detection, the **DCBP6** probe solution changes which is visible to the naked eye and causes a large Stokes shift (>100 nm in different solvents). Besides, the reduced probe **DCBP6NH2** exhibit two-photon absorption (TPA) which is having more advantages than traditional one-photon absorption probes in fluorescence microscopy such as less phototoxicity, better three-dimensional spatial localization, deeper penetration depth, and lower self-absorption. Further, the probe molecule **DCBP6** was successfully used as fluorescent probe for monitoring H2S in living cells and tissues and in vivo in mice via fluorescence bioimaging investigations. More or less at the same time, Xu and coworkers have reported the same molecular probe for in vivo detection of H2S [62].
