*3.3.4 Hydrazine (N2H4)*

Hydrazine is used as a common precursor in synthetic chemistry of many polymers, pharmaceutical intermediates, hydrazine fuel cells in power generation sector, and materials science [68, 69]. It is often used in rocket propulsion systems as an important propellant for its flammable and detonable characteristics. Moreover, hydrazine serves as an important metal corrosion inhibitor because of its strong reducing properties; hydrazine scavenges oxygen in water boilers that are used for feed and heating systems. However, hydrazine and its aqueous solutions are highly toxic to all living organisms when inhaled or in contact. It has been shown that hydrazine is mutagenic and carcinogenic which causes serious damage to the human central nervous system, kidneys, liver, and lungs [70]. Therefore, it is of great interest and importance to develop a reliable method for hydrazine detection with selectivity and sensitivity. With a view to develop efficient DCM-based NIR fluorophore for selective detection of hydrazine, a phenyl ring baring *O*-acetyl moiety was introduced onto the into dicyanomethylene-4*H*-benzopyran backbone and synthesized DCBP8 [71]. The absorption and fluorescence properties of DCBP8 were measured in PBS solution (pH = 7.4) containing 50% of ethanol. DCBP8 has absorption in between 300 and 450 nm region with a maximum at 434 nm. After treatment of DCBP8 with N2H4, gradually new absorption peaks started appearing at 551 nm at the expense of 434 nm absorption band. The absorption maximum shifted from 434 nm to 551 nm which indicates the efficiency of DCBP8 for colorimetric detection of N2H4 when absorption intensity ratio (A551/A434) and concentration of N2H4 ranging from 0 to 40 μM are plotted against each other, there is a

**21**

*Photophysical Properties of 4-(Dicyanomethylene)-2-Methyl-6-(4-Dimethylaminostyryl)-4*H*…*

good linearity suggesting a ratiometric response. On the other hand, free DCBP8 is almost no fluorescent upon excitation at 560 nm, which showed a dramatic fluorescence enhancement at 680 nm upon addition of N2H4. The fluorescence enhancement is found to be more than 110-fold. Mass spectrometry data along with photophysical properties revealed that the N2H4-medicated acetyl deprotection of DCBP8 generates **DCBPO−**, a highly fluorescent product because of ICT character.

There is quest for developing molecular probes for rapid, selective, and sensitive detection of the highly toxic thiophenols which are of great importance in both environmental and biological science. James and co-workers have developed a novel near-infrared (NIR) and colorimetric fluorescent molecular probe, **DCBP9**, based on a dicyanomethylene-4H-pyran chromophore for selective detection of glutathione in living cells [72]. The molecular probe **DCBP9** was synthesized by Michael's addition of 2-(2-(4-hydroxystyryl)-4H- chromen-4-ylidene) malononitrile and 2,4-dinitrobenzene-1-sulphonyl chloride (DNBS) in the presence of pyridine at room temperature. Molecular probe has an intense absorption centred at 414 nm in a DMSO-PBS buffer solution; upon the addition of glutathione (GSH), the color of the solution turned to pink from slight yellow and clearly visible to the naked eyes. In addition, a new absorption band emerged at 560 nm with an isosbestic point at 446 nm which is assigned to the specific O–S cleavage, and the generation of phenolate ion with a distinct 146 nm redshift in absorbance is observed. Since the phenolate group is a much stronger electron donor than the sulphonate group, the ICT efficiency is significantly enhanced by the interaction of **DCBP9** with GSH and thus shifts the absorption to a longer wavelength region. Subsequent fluorescence experiments showed that the molecular probe alone is nonemissive (turn-off) in absence of GSH. However, when the probe is excited at 560 nm in presence of GSH, turn-on fluorescence and the intensity at 690 nm were dramatically enhanced. The turn-on fluorescence is due to the release of electron-withdrawing DNBS moiety via a GSH-induced O–S bond cleavage and produces phenolate ion, which possesses a strong ICT character and induces a turn-on NIR fluorescence. Having known chemical properties of thiophenols that are able to cleave sulphonamide selectively and efficiently under mild conditions, a dicyanomethylene-benzopyran-based NIR fluorescent probe **DCBP10** is designed for detection of thiophenols [73]. Upon adding thiophenols to the **DCBP10** solution, the DNBS moiety is cleaved and forms amine (–NH2) functional group at the phenyl ring. Since the amine is an electrondonating group, the ICT of the fluorophore is restored, and as a result, absorption and fluorescence emission properties of the probe were changed. This probe features remarkable large Stokes shift and shows a rapid, highly selective, and sensitive detection process for thiophenols with significant NIR turn-on fluorescent response. Therefore, **DCBP10** was successfully demonstrated as a potential NIR fluorescent probe that can be mitigated not only for quantitative detection of thiophenol in real water samples but also fluorescence imaging of thiophenol in living cells [73]. Slightly similar molecular structure **DCBP11** is redesigned by Li et al. that consists of dicyanomethylene-benzopyran scaffold and 2,4-dinitrophenyl (DNP) connected by ether linkage for probing for thiophenols [74]. It was demonstrated that **DCBP11** shows both colorimetric and rapid turn-on fluorescence sensing process for thiophenols with high selectivity and better sensitivity (DL = 70 nm). Moreover, it should be noted that a dual colorimetric and selective NIR fluorescence sensing phenomenon is also visual to the 'naked eye' without the need of advanced

**3.4 DCM derivatives for sensing biothiols and selecysteine**

*DOI: http://dx.doi.org/10.5772/intechopen.93149*

*3.4.1 Biothiols*

good linearity suggesting a ratiometric response. On the other hand, free DCBP8 is almost no fluorescent upon excitation at 560 nm, which showed a dramatic fluorescence enhancement at 680 nm upon addition of N2H4. The fluorescence enhancement is found to be more than 110-fold. Mass spectrometry data along with photophysical properties revealed that the N2H4-medicated acetyl deprotection of DCBP8 generates **DCBPO−**, a highly fluorescent product because of ICT character.
