**3. DCM derivatives as optical sensors**

A *chemosensor* can be any organic or inorganic complex molecule that is used for sensing of an analyte to produce a detectable change or a signal [40–43]. Similarly, Cambridge defined the chemical sensor as a 'miniaturized device that can deliver

**13**

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

real time and online information on the presence of specific compounds or ions in even complex samples'. The chemical sensors employ specific transduction techniques to obtain analyte information. The chemical sensors are widely developed based on optical absorption, luminescence, and redox potential principles. Moreover, sensors based on other optical parameters, such as refractive index and

Any chemosensor consists of three components: a *chemical receptor* which is capable of recognizing the analyte/guest of interest; a *transducer* or *signaling unit* that converts recognition event into a measureable physical change; and finally a method of measuring change and converting it to useful signal/information. An ideal chemosensor is expected to have high selectivity, sensitivity, prompt response, and low cost. Various approaches have been developed in the recent past years by various groups for designing chemosensors and broadly classified in to three different approaches [45], which only differ in the arrangement of receptor and signaling unit:

These approaches only differ in the arrangement of two units (receptor and signaling) with respect to each other. In the 'binding site-signalling subunit' approach, two parts are linked through a covalent bond. The interaction of the analyte/guest with the binding site induces changes in the electronic properties of the signaling subunit that results sensing of the target anion. The displacement approach is based on the formation of molecular assemblies of binding site-signaling subunit, which in coordination of a certain anion with the binding site results in the release of the signaling subunit into the solution with a concomitant change in their optical properties. In the chemodosimeter approach, a chemical reaction results in an optical signal when a specific anion approaches the receptor. Depending on the type of signals that are produced upon the recognition event, chemosensors are classified into two categories: optical sensors and electronic sensors. While the former sensors change optical signals, the latter change electrochemical properties. Based on the type of optical signal, the optical sensors further can be classified into two categories.

*Chromogenic chemosensors* change the color upon the recognition event (binding of analyte/guest into the receptor subunit) and thus show variation in absorption of signaling unit. Since the color of parent solution is changing after recognition, these

*Fluorogenic chemosensors* change the fluorescence of the signaling unit upon the

It has been demonstrated that the colorimetric sensors are simple and low-cost and offer both qualitative and quantitative information without any need of sophisticated spectroscopic instrumentation, and most often the colorimetric response can be visualized with the naked eye. On the other hand, the fluorescence measurement is a bit expensive but relatively more sensitive and versatile and offers microto nanomolar estimation of guest species. A wide variety of optical chemosensors have been reported for the cation, anion, and neutral molecules. Based on the nature of analyte being detected, irrespective of the photophysical phenomenon the receptors follows, the chemosensors may be broadly classified into three categories:

The ICT mechanism has been exploited quite extensively in ion sensing and molecular switching applications [45, 46]. A fluorosensor is generally designed to

reflectivity are also frequently reported in the literature [44].

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

• Binding site-signaling approach

• Displacement approach

• Chemodosimeter approach

are also known as colorimetric sensors.

recognition event. These are also called fluorosensors.

cations sensors, anions sensors, neutral sensors.

### *Photophysical Properties of 4-(Dicyanomethylene)-2-Methyl-6-(4-Dimethylaminostyryl)-4*H*… DOI: http://dx.doi.org/10.5772/intechopen.93149*

real time and online information on the presence of specific compounds or ions in even complex samples'. The chemical sensors employ specific transduction techniques to obtain analyte information. The chemical sensors are widely developed based on optical absorption, luminescence, and redox potential principles. Moreover, sensors based on other optical parameters, such as refractive index and reflectivity are also frequently reported in the literature [44].

Any chemosensor consists of three components: a *chemical receptor* which is capable of recognizing the analyte/guest of interest; a *transducer* or *signaling unit* that converts recognition event into a measureable physical change; and finally a method of measuring change and converting it to useful signal/information. An ideal chemosensor is expected to have high selectivity, sensitivity, prompt response, and low cost. Various approaches have been developed in the recent past years by various groups for designing chemosensors and broadly classified in to three different approaches [45], which only differ in the arrangement of receptor and signaling unit:


*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

dynamics of DCM in non-polar solvent.

**2.5 What is understood about DCM dye?**

lifetime and relaxation depend on the solvent polarity.

excitation wavelengths [35]. The S1 lifetime was observed to be 9.8 ps which is found to be independent of the excitation wavelengths. The observed S1 lifetime of DCM is less by one order of magnitude as compared to julolidyl DCM dyes DCJT and DCJTB, indicating the significance of the twisting motion of the N,Ndimethylamino group affecting the S1 non-radiative dynamics. Further, TDDFT calculations suggest that an intersystem crossing is responsible for the observed S1

The ground state and dipole moments of DCM are estimated to be very high (5.6 D and 26.6 D) which suggests that the charge is highly polarized even in the ground state. The steady-state absorption and fluorescence spectra of DCM reveal that the molecule exhibit solvatochromic shift and large Stokes shifts depending on the polarity of the solvent [10, 16, 24]. Solvatochromic shift of the electronic absorption is due to high ground-state dipole moment. The dramatic Stokes shift is attributed to the change of the dipole moment upon photoexcitation and fluorescent emitting state to a charge-transfer (CT) state [23, 24]. The fluorescence lifetime of DCM is measured to be of the order of a few nanoseconds, and the solvent relaxation occurs in between sub-picoseconds and picoseconds [9, 10, 23, 28–33]. Both fluorescence

Photoexcitation of DCM to its first absorption band put the excited molecule in the S1/LE state, and subsequently two conformational changes may happen. Firstly, –C=C bond rotation leading to trans and cis isomerization via a phantom singlet state which is a typical photochemical process occurring on trans-stilbene [36] and many olefin molecules [37]. Secondly, twisting of the N,N-dimethylamino group may give rise to a highly polar twisted intramolecular charge-transfer (TICT) state which can be stabilized in polar media like 4-dimethyl-aminobenzonitrile (DMABN) molecule [37, 38]. However, the transition from the LE state to the CT (or TICT) state is under debate, and from both experimental and theoretical calculations [39], the following widely accepted dynamical behavior has been proposed to understand the excited-state dynamics of DCM dye. The potential energy surface (PES) of the LE state (S1) for twisting motion of the central C=C bond (which bridges N,N-dimethylamino group with pyran group) is calculated to be very small (0.2 eV), and the barrier height is insensitive to the polarity of solvent. However, the shape of excited-state PESs of for the twisting motion of the CN single bond of the N,N-dimethylamino group of DCM is strongly influenced by the polarity of the solvent [39]. Moreover, in a polar media, the energy of the S1/LE state increases, whereas the energy of the S2/CT state decreases by twisting the CN single bond of the dimethylamino group and leads to a nonadiabatic curve crossing between the two states. Therefore, the formation of an emissive TICT state along the amino group twisting coordinate is more favored with increasing the polarity of the solvent. Trans and cis isomerization is dominated in polar solvents because of the increased the energy barrier in the TICT state along the torsional coordinate of the C=C double bond when the TICT state is formed at the perpendicular geometry

where the energy of the S1/LE state is higher than that of the S2/CT state.

A *chemosensor* can be any organic or inorganic complex molecule that is used for sensing of an analyte to produce a detectable change or a signal [40–43]. Similarly, Cambridge defined the chemical sensor as a 'miniaturized device that can deliver

**3. DCM derivatives as optical sensors**

**12**

• Chemodosimeter approach

These approaches only differ in the arrangement of two units (receptor and signaling) with respect to each other. In the 'binding site-signalling subunit' approach, two parts are linked through a covalent bond. The interaction of the analyte/guest with the binding site induces changes in the electronic properties of the signaling subunit that results sensing of the target anion. The displacement approach is based on the formation of molecular assemblies of binding site-signaling subunit, which in coordination of a certain anion with the binding site results in the release of the signaling subunit into the solution with a concomitant change in their optical properties. In the chemodosimeter approach, a chemical reaction results in an optical signal when a specific anion approaches the receptor. Depending on the type of signals that are produced upon the recognition event, chemosensors are classified into two categories: optical sensors and electronic sensors. While the former sensors change optical signals, the latter change electrochemical properties. Based on the type of optical signal, the optical sensors further can be classified into two categories.

*Chromogenic chemosensors* change the color upon the recognition event (binding of analyte/guest into the receptor subunit) and thus show variation in absorption of signaling unit. Since the color of parent solution is changing after recognition, these are also known as colorimetric sensors.

*Fluorogenic chemosensors* change the fluorescence of the signaling unit upon the recognition event. These are also called fluorosensors.

It has been demonstrated that the colorimetric sensors are simple and low-cost and offer both qualitative and quantitative information without any need of sophisticated spectroscopic instrumentation, and most often the colorimetric response can be visualized with the naked eye. On the other hand, the fluorescence measurement is a bit expensive but relatively more sensitive and versatile and offers microto nanomolar estimation of guest species. A wide variety of optical chemosensors have been reported for the cation, anion, and neutral molecules. Based on the nature of analyte being detected, irrespective of the photophysical phenomenon the receptors follows, the chemosensors may be broadly classified into three categories: cations sensors, anions sensors, neutral sensors.

The ICT mechanism has been exploited quite extensively in ion sensing and molecular switching applications [45, 46]. A fluorosensor is generally designed to have two units: a signaling unit typically a fluorophore and a receptor (recognition unit) which are covalently connected with a π-spacer for rendering the recognition event to the fluorophore that ultimately changes fluorescence signal. A group of fluorogenic sensors which has either weak fluorescence or no fluorescence (off state) by nature and that becomes fluorescent (on state) upon the receptor recognizes the analyte/guest molecule, and this type of fluorogenic sensors are called as off–on sensors. Similarly, on–off sensors can also be designed, where a sensor initially exhibits fluorescence (on state) and after the recognition event, the sensor becomes nonfluorescent/weakly fluorescent (off state). A schematic representation of off–on fluorogenic sensors is shown in **Figure 4**.

As discussed in the previous section, the DCM molecule and its derivatives are having unique advantages in terms of their photophysical properties such as red light emission, high quantum yield, and highly tunable fluorescence that is sensitive not only by solvent polarity but also structure modification. Unlike visible light fluorogenic sensors, red and NIR fluorogenic sensors (600–950 nm) have received considerable interest due to minimum fluorescence background, less light scattering, and less photodamage and are having certain advantages in bioimaging applications of live cells. Therefore, in recent years, there is a consistent growth of the colorimetric and fluorogenic sensors based on DCM and its analogues (**Figure 5**) for sensing cations, anions, and neutral species, which are summarized below.
