**2.1 Absorption**

Electronic absorption spectrum of DCM in polar medium, dimethylsulphoxide (DMSO) was reported for the first time by Hammond in 1979 [7]. Later on, the DCM dye absorption spectra in different medium were studied in various contexts, and it is observed that the electronic absorption behavior is quite similar to many charge transfer (CT) dyes [7–16]. The DCM dye has very broad absorption, typically in between 200 and 600 nm, and the nature of the absorption strongly depends upon polarity of the medium (**Figure 1** and **Table 1**) [11, 12, 15, 16]. The DCM exhibits two absorption bands where the longer-wavelength band is found to be more intense than the shorter-wavelength band. In non-polar solvents, the shape of the longerwavelength band is found to be more structured (vibronic structure) like any other CT dye molecules, and in polar solvents the structured nature disappears [17]. For example, the electronic absorption spectrum of DCM in cyclohexane shows two bands: structured longer-wavelength band with a maximum at 451 nm and shoulder at 340 nm [11]. However, when the same DCM dye is present in highly polar solvent like DMSO, a structureless longer-wavelength band is observed with maxima at 482 nm with a shoulder at 350 nm. Interestingly, the electronic absorption maximum of DCM undergoes a redshift upon increasing with the polarity of the medium which is commonly known as solvatochromic shift of the absorption. The solvatochromic behavior is more prominent in polar aprotic solvents than that of polar protic solvents with that of non-polar solvents. For example, the absorption maxima of DCM in cyclohexane and DMSO in the solvatochromic shift is found to be ~30 nm, which is relatively less (~20 nm) when compared to cyclohexane and ethanol absorption maxima. From the Lippert-Mataga theory [18–20], ground-state dipole moment (μa) of the DCM was estimated to be 5.6 D which suggests that the DCM is a dipolar molecule [11]. It is well-known in the literature that an electronic state of a dipolar molecule is more stabilized in polar solvents rather than in less polar or non-polar solvents. So, the observed solvatochromic behavior in different solvents

**5**

*fluorescence quantum yield.*

*Photophysical parameters DCM [7–16].*

**Table 1.**

**Figure 1.**

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

*Molecular structure of DCM (left). Right side, experimental (a) fluorescence and (b) absorption spectra of DCM dissolved in hexane (continuous lines), CHCl3 (dashed lines), and DMSO (dotted lines) and (c) fluorescence and (d) calculated absorption spectra. Reproduced with permission from ACS [16].*

**(nm)**

 n-Hexane 451 530 79 0.05 0.015 Cyclohexane 454 533 79 — — 1,4-Dioxane 456 566 100 — — Toluene 461 567 106 0.08 0.022 Chloroform 471 565 94 0.35 — Tetrahydrofuran — — — 0.49 1.24 Dichloromethane 468 587 109 — — Acetonitrile 463 617 154 0.45 1.95 DMF 475 626 151 — — DMSO 481 644 163 0.80 2.25 Methanol 466 623 157 0.3 1.36 EtOH 470 614 144 — — n-Propanol 472 614 142 0.57 2.10 PMMA 453 550 97 0.76 2.00 λa*, absorbance maxima (nm);* λf*, fluorescence emission maxima;* λ<sup>a</sup> − λf*, Stokes shift; τ, fluorescence lifetime; ϕf,* 

*λa − λf* **(nm) ϕ<sup>f</sup> τ (ns)**

**Sl. no Solvent** *λa* **(nm)** *λ<sup>f</sup>*

is attributed to the extent of dipole–dipole interactions in the respective solvents. Dipole–dipole interactions are prominent in polar solvents and aromatic solvents, and corresponding energy state will be relatively more stabilized; thereby a redshift of the absorption maxima is quite obvious. Similarly, the structured longerwavelength absorption band, observed in non-polar solvents, is primarily due to the vibronic coupling where the vibrational energy levels are well separated and thus their vibrational transitions become prominent. The vibronic coupling is even more prominent at the low-temperature (77 K) experiments and can be attributed to the absence of dipole–dipole interactions [15]. Molar absorption coefficients of DCM in

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

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

#### **Figure 1.**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

**2. Photophysical properties of DCM**

**2.1 Absorption**

materials [1]. However, in subsequent years it was found that DCM exhibit high fluorescence quantum efficiency, large Stokes shift, and solvatochromic behavior. Further, the absorption spectrum of DCM dye has minimum overlap with its fluorescence spectrum which was utilized in lasing action, for developing red lasers, and organic light-emitting diode (OLED) materials [2, 3]. Because of their interesting photophysical and optoelectronic properties, several research groups actively involved in developing DCM analogues not only for OLED application but also for logic gates, lasers, bioimaging, sensors, photovoltaics, and NLO applications. Way back in 2004, Chen reviewed how red emitting DCM derivatives have evolved as dopants for OLED device applications [4]. Later in 2012, Tian has published one review article which describes not only OLED applications of DCM-type materials but also fluorescent sensors, logic gates, photovoltaic sensitizers, nonlinear optical materials, bioimaging dyes, etc. [5]. Considering simple synthetic procedures of DCM derivatives [6], many optical sensors were reported based on DCM derivatives for recognizing various guest analytes, and the number of publications is rapidly increasing day by day. However, on the other hand, a comprehensive summery of DCM photophysical behavior has not been reported till date. Moreover, to the best of our knowledge, there is no single report that describes optical sensing behavior of DCM and its derivatives. The book chapter describes both the fundamental photophysics of DCM and recent progress on DCM derivatives as optical sensors.

Electronic absorption spectrum of DCM in polar medium, dimethylsulphoxide (DMSO) was reported for the first time by Hammond in 1979 [7]. Later on, the DCM dye absorption spectra in different medium were studied in various contexts, and it is observed that the electronic absorption behavior is quite similar to many charge transfer (CT) dyes [7–16]. The DCM dye has very broad absorption, typically in between 200 and 600 nm, and the nature of the absorption strongly depends upon polarity of the medium (**Figure 1** and **Table 1**) [11, 12, 15, 16]. The DCM exhibits two absorption bands where the longer-wavelength band is found to be more intense than the shorter-wavelength band. In non-polar solvents, the shape of the longerwavelength band is found to be more structured (vibronic structure) like any other CT dye molecules, and in polar solvents the structured nature disappears [17]. For example, the electronic absorption spectrum of DCM in cyclohexane shows two bands: structured longer-wavelength band with a maximum at 451 nm and shoulder at 340 nm [11]. However, when the same DCM dye is present in highly polar solvent like DMSO, a structureless longer-wavelength band is observed with maxima at 482 nm with a shoulder at 350 nm. Interestingly, the electronic absorption maximum of DCM undergoes a redshift upon increasing with the polarity of the medium which is commonly known as solvatochromic shift of the absorption. The solvatochromic behavior is more prominent in polar aprotic solvents than that of polar protic solvents with that of non-polar solvents. For example, the absorption maxima of DCM in cyclohexane and DMSO in the solvatochromic shift is found to be ~30 nm, which is relatively less (~20 nm) when compared to cyclohexane and ethanol absorption maxima. From the Lippert-Mataga theory [18–20], ground-state dipole moment (μa) of the DCM was estimated to be 5.6 D which suggests that the DCM is a dipolar molecule [11]. It is well-known in the literature that an electronic state of a dipolar molecule is more stabilized in polar solvents rather than in less polar or non-polar solvents. So, the observed solvatochromic behavior in different solvents

**4**

*Molecular structure of DCM (left). Right side, experimental (a) fluorescence and (b) absorption spectra of DCM dissolved in hexane (continuous lines), CHCl3 (dashed lines), and DMSO (dotted lines) and (c) fluorescence and (d) calculated absorption spectra. Reproduced with permission from ACS [16].*


λa*, absorbance maxima (nm);* λf*, fluorescence emission maxima;* λ<sup>a</sup> − λf*, Stokes shift; τ, fluorescence lifetime; ϕf, fluorescence quantum yield.*

#### **Table 1.**

*Photophysical parameters DCM [7–16].*

is attributed to the extent of dipole–dipole interactions in the respective solvents. Dipole–dipole interactions are prominent in polar solvents and aromatic solvents, and corresponding energy state will be relatively more stabilized; thereby a redshift of the absorption maxima is quite obvious. Similarly, the structured longerwavelength absorption band, observed in non-polar solvents, is primarily due to the vibronic coupling where the vibrational energy levels are well separated and thus their vibrational transitions become prominent. The vibronic coupling is even more prominent at the low-temperature (77 K) experiments and can be attributed to the absence of dipole–dipole interactions [15]. Molar absorption coefficients of DCM in ethanol are estimated to be 4.2 × 104 mol<sup>−</sup><sup>1</sup> cm<sup>−</sup><sup>1</sup> at its absorption maxima (470 nm) and 1.2 × 104 mol<sup>−</sup><sup>1</sup> cm<sup>−</sup><sup>1</sup> at the shoulder (337 nm).
