**2.2 Steady-state fluorescence**

Fluorescence emission behavior of DCM laser dye in a variety of solvents has been measured [11, 16]. DCM dye molecule exhibits a single structured fluorescence band in non-polar aprotic solvents. For example, the fluorescence maxima of DCM in isooctane and cyclohexane are found to be at 533 nm and 530 nm, respectively (Stoke shift of ~80 nm), which shifts its maxima (to 566 nm) upon increasing polarity of the solvent (1,4-dioxane). On the other hand, in dipolar aprotic and protic solvents, the fluorescence maxima shift towards the red region of the visible light, undergo a little change in shape of the band, and are accompanied by a new fluorescence band with its maximum above 610 nm. Similarly, the DCM emits at 635 and 626 nm in DMSO and DMF, respectively, that gives rise to ~150 nm Stokes shift. Furthermore, from the systematic fluorescence study, it was observed that the short-wavelength fluorescence intensities depend upon solvent polarity and that the intensity of the longerwavelength band enhanced monotonically with increasing polarity of the solvent. The structured fluorescence emission band in non-polar solvent is attributed to the Franck-Condon or locally excited (LE) state where the DCM molecular structure/ configuration is almost same as the ground-state configuration. The dynamic Stokes shift of the fluorescence emission maxima in polar solvents indicates that the nature of the emitting state is changing to a highly polar state and the solvation of DCM molecules further stabilizing the emitting state. From Stokes shift values obtained in different solvents and by using Lippert-Mataga theory, the excited-state dipole moment (μe) was estimated to be 26.3 D [16], which further supports the high dipolar nature of DCM emitting state. A large change in dipole moment (~20 D) from ground state to the excited state resulted in a large Stokes shift (~150 nm) from non-polar solvents to the polar solvents. The estimated μe and large change in dipole moment upon photoexcitation also explain why Stokes shift is more than the solvatochromic shift. Since μe is very high, it is likely that the DCM molecule mostly exists in the planar confirmation in charge-transfer (CT) state which will be relatively more stabilized by polar solvents rather than non-polar solvents. Further, it was observed that both the spectral shifts are correlating with Lippert-Mataga solvent parameter, ∆*f*.

In order to understand the nature of the emitting state, titration experiments were carried out in which aliquots of pure ethanol solvent are added gradually to DCM and dioxane solution [9]. It was observed that the original fluorescence band in pure dioxane is redshifted upon gradual addition of ethanol to the DCM-dioxane solution and concurrently produces initially a longer-wavelength fluorescence band with a maximum at 610 nm which reduces its intensity beyond certain ethanol concentration (10<sup>−</sup><sup>4</sup> M) and emerged to a new fluorescence band with a maximum at 630 nm. However, further increase of ethanol concentration beyond this limit did not shift the position of the longer-wavelength fluorescence maximum but increases intensity of fluorescence band despite the fact that there is significant increase in the polarity of the binary solvent mixture. Fluorescence quantum yield (ϕf) of DCM highly depends upon the polarity of the solvent. For example, in n-hexane solvent, DCM quantum yield is calculated to be 0.05, and in polar DMSO solvent, quantum yield is estimated to be 0.81 [15]. Therefore, the quantum yield of DCM in non-polar solvents are less and in polar solvents high (**Table 1**). The observed high fluorescence quantum yields in polar solvents can be understood in terms of the CT character of the DCM dye. From the initial steady-state fluorescence studies, it was proposed that the DCM dye molecule emits a single fluorescence, and a three-state model was proposed in order to explain the fluorescence

**7**

**Figure 2.**

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

spectral behavior, and all the solvents and the emitting state would be either LE state. Therefore, solvatochromic behavior of DCM was attributed to the change in their dipole moment of the ground state and excited state where fluorescence spectral shift increases due to an increased dipole moment upon excitation and to

observed that the cis-isomer fluorescence quenches to give the trans-DCM. Based on the steady-state absorption and fluorescence studies in a variety of solvents, a mechanism has been proposed to understand photophysical properties (**Figure 2**) [9]. The DCM dye may be thought of an ionic merocyanine-like electron donoracceptor (EDA) dye molecule in which an electron-donating N,Ndimethylaniline moiety is covalently connected with a conjugated π-electron spacer and an electron-accepting dicyanomethylene moiety. Electronic excitation of DCM molecules leads to the formation of locally excited (LE) state immediately after photoexcitation. So, the fluorescence emission of DCM in non-polar solvents predominantly occurs from LE state, formed via π-π\* transitions, and has an electronic configuration similar to that of the ground state, which is evident from the vibronic fluorescence emission band. However, in polar solvents, excited DCM molecules emitted from ICT state, which are characterized by a planar molecular conformation, are formed immediately after photoexcitation under the influence of the electric polarization of the surrounding solvent molecules, and it is argued that the shortwavelength fluorescence primarily originated from ICT state. This also explains why

*Schematic diagram of the dynamic behavior of low-lying singlet states of DCM.*

As can be understood from the molecular structure (**Figure 1**), the DCM dye can present in either cis-confirmation or trans-configuration because of π-spacer. So, the photophysics of cis- and trans-isomerization of DCM were studied by Drake and co-workers [10]. The DCM solutions were analyzed by high-pressure liquid chromatography (HPLC) and nuclear magnetic resonance (NMR), and they found that in the freshly prepared solutions, the DCM exists in trans-configuration (in dark). However, DCM solution when exposed to ambient light, trans-DCM converts in to cis-DCM whose ratio depends on the solvent. From HPLC study, absolute absorption cross sections for both isomers were measured for the first time. The fluorescence quantum yield of trans-isomer is found to be more than that of the cis-isomer because of the less non-radiative rate of the trans-DCM. Temperature dependence of the fluorescence emission spectra of both isomers in methanol, dimethylsulphoxide (DMSO), and lipid bilayers was studied [14]. These results suggest that the fluorescence spectral behavior of the two isomers is almost overlapping while their fluorescence decay times are found to be distinct. Furthermore, cis-DCM fluorescence was measured for the first time in DMSO solvent along with the trans-DCM, and it is

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

the interaction of this dipole with the polar solvent cage.

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

spectral behavior, and all the solvents and the emitting state would be either LE state. Therefore, solvatochromic behavior of DCM was attributed to the change in their dipole moment of the ground state and excited state where fluorescence spectral shift increases due to an increased dipole moment upon excitation and to the interaction of this dipole with the polar solvent cage.

As can be understood from the molecular structure (**Figure 1**), the DCM dye can present in either cis-confirmation or trans-configuration because of π-spacer. So, the photophysics of cis- and trans-isomerization of DCM were studied by Drake and co-workers [10]. The DCM solutions were analyzed by high-pressure liquid chromatography (HPLC) and nuclear magnetic resonance (NMR), and they found that in the freshly prepared solutions, the DCM exists in trans-configuration (in dark). However, DCM solution when exposed to ambient light, trans-DCM converts in to cis-DCM whose ratio depends on the solvent. From HPLC study, absolute absorption cross sections for both isomers were measured for the first time. The fluorescence quantum yield of trans-isomer is found to be more than that of the cis-isomer because of the less non-radiative rate of the trans-DCM. Temperature dependence of the fluorescence emission spectra of both isomers in methanol, dimethylsulphoxide (DMSO), and lipid bilayers was studied [14]. These results suggest that the fluorescence spectral behavior of the two isomers is almost overlapping while their fluorescence decay times are found to be distinct. Furthermore, cis-DCM fluorescence was measured for the first time in DMSO solvent along with the trans-DCM, and it is observed that the cis-isomer fluorescence quenches to give the trans-DCM.

Based on the steady-state absorption and fluorescence studies in a variety of solvents, a mechanism has been proposed to understand photophysical properties (**Figure 2**) [9]. The DCM dye may be thought of an ionic merocyanine-like electron donoracceptor (EDA) dye molecule in which an electron-donating N,Ndimethylaniline moiety is covalently connected with a conjugated π-electron spacer and an electron-accepting dicyanomethylene moiety. Electronic excitation of DCM molecules leads to the formation of locally excited (LE) state immediately after photoexcitation. So, the fluorescence emission of DCM in non-polar solvents predominantly occurs from LE state, formed via π-π\* transitions, and has an electronic configuration similar to that of the ground state, which is evident from the vibronic fluorescence emission band. However, in polar solvents, excited DCM molecules emitted from ICT state, which are characterized by a planar molecular conformation, are formed immediately after photoexcitation under the influence of the electric polarization of the surrounding solvent molecules, and it is argued that the shortwavelength fluorescence primarily originated from ICT state. This also explains why

**Figure 2.** *Schematic diagram of the dynamic behavior of low-lying singlet states of DCM.*

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

mol<sup>−</sup><sup>1</sup>

Fluorescence emission behavior of DCM laser dye in a variety of solvents has been measured [11, 16]. DCM dye molecule exhibits a single structured fluorescence band in non-polar aprotic solvents. For example, the fluorescence maxima of DCM in isooctane and cyclohexane are found to be at 533 nm and 530 nm, respectively (Stoke shift of ~80 nm), which shifts its maxima (to 566 nm) upon increasing polarity of the solvent (1,4-dioxane). On the other hand, in dipolar aprotic and protic solvents, the fluorescence maxima shift towards the red region of the visible light, undergo a little change in shape of the band, and are accompanied by a new fluorescence band with its maximum above 610 nm. Similarly, the DCM emits at 635 and 626 nm in DMSO and DMF, respectively, that gives rise to ~150 nm Stokes shift. Furthermore, from the systematic fluorescence study, it was observed that the short-wavelength fluorescence intensities depend upon solvent polarity and that the intensity of the longerwavelength band enhanced monotonically with increasing polarity of the solvent. The structured fluorescence emission band in non-polar solvent is attributed to the Franck-Condon or locally excited (LE) state where the DCM molecular structure/ configuration is almost same as the ground-state configuration. The dynamic Stokes shift of the fluorescence emission maxima in polar solvents indicates that the nature of the emitting state is changing to a highly polar state and the solvation of DCM molecules further stabilizing the emitting state. From Stokes shift values obtained in different solvents and by using Lippert-Mataga theory, the excited-state dipole moment (μe) was estimated to be 26.3 D [16], which further supports the high dipolar nature of DCM emitting state. A large change in dipole moment (~20 D) from ground state to the excited state resulted in a large Stokes shift (~150 nm) from non-polar solvents to the polar solvents. The estimated μe and large change in dipole moment upon photoexcitation also explain why Stokes shift is more than the solvatochromic shift. Since μe is very high, it is likely that the DCM molecule mostly exists in the planar confirmation in charge-transfer (CT) state which will be relatively more stabilized by polar solvents rather than non-polar solvents. Further, it was observed that both the

at the shoulder (337 nm).

spectral shifts are correlating with Lippert-Mataga solvent parameter, ∆*f*.

In order to understand the nature of the emitting state, titration experiments were carried out in which aliquots of pure ethanol solvent are added gradually to DCM and dioxane solution [9]. It was observed that the original fluorescence band in pure dioxane is redshifted upon gradual addition of ethanol to the DCM-dioxane solution and concurrently produces initially a longer-wavelength fluorescence band with a maximum at 610 nm which reduces its intensity beyond certain ethanol

at 630 nm. However, further increase of ethanol concentration beyond this limit did not shift the position of the longer-wavelength fluorescence maximum but increases intensity of fluorescence band despite the fact that there is significant increase in the polarity of the binary solvent mixture. Fluorescence quantum yield (ϕf) of DCM highly depends upon the polarity of the solvent. For example, in n-hexane solvent, DCM quantum yield is calculated to be 0.05, and in polar DMSO solvent, quantum yield is estimated to be 0.81 [15]. Therefore, the quantum yield of DCM in non-polar solvents are less and in polar solvents high (**Table 1**). The observed high fluorescence quantum yields in polar solvents can be understood in terms of the CT character of the DCM dye. From the initial steady-state fluorescence studies, it was proposed that the DCM dye molecule emits a single fluorescence, and a three-state model was proposed in order to explain the fluorescence

M) and emerged to a new fluorescence band with a maximum

cm<sup>−</sup><sup>1</sup>

at its absorption maxima (470 nm)

ethanol are estimated to be 4.2 × 104

cm<sup>−</sup><sup>1</sup>

mol<sup>−</sup><sup>1</sup>

**2.2 Steady-state fluorescence**

and 1.2 × 104

**6**

concentration (10<sup>−</sup><sup>4</sup>

a gradual shift in the position of the fluorescence band is observed from a non-polar aprotic solvent to a polar solvent. Further, interpretation of the additional long-wavelength fluorescence was not that easy as expected; however, the preliminary fluorescence lifetime data suggest that it is generated from excited DCM in a new ICT state which is formed during the lifetime of the lowest excited singlet state and equilibrates with the ICT state emitting at 610 nm. It was suggested that the dual fluorescence originates from the excited DCM in the ICT state with a twisted conformation formed by internal rotation of the donor moiety with simultaneous ICT from this group to a suitable acceptor orbital. The new state is commonly known as twisted intramolecular charge transfer state (TICT) which was first reported by Grabowski and co-workers [21] to explain dual fluorescence of structurally different compounds such as p-cyano and p-(9-anthryl) derivatives of N,N-dimethylaniline in polar solvents [17, 22]. Typically, the TICT state is characterized by a perpendicular conformation of donor and acceptor moieties which is responsible for dual fluorescence of p-N,Ndimethylaminobenzonitrile (DMABN). However, unlike DMABN molecule, it should be noted that the difference between the short- and long-wavelength maxima of the dual fluorescence of DCM is somewhat smaller than that calculated for DMABN. This may be because the larger separation between the D and A moieties in DCM leads to a smaller fraction of charge transfer than that of DMABN.

Contrary to the above three-state model, a combined experimental and theoretical study revealed quite different results from the measured absorption and steadystate emission spectra of DCM dye upon its comparison with Nile red in a series of aprotic solvents with similar refractive index and different polarity [16]. Unlike many other studies reported earlier, the observed spectral behavior is interpreted to two-state electronic model accounting for the coupling to internal molecular vibrations and to an effective solvation coordinate. This study pointed out that change in band shapes upon varying solvent cannot be accounted as an evidence for two different emitting states and explained all the observed solvatochromic behavior of absorption and fluorescence spectra. Based on the consistency between experimental and calculated spectral data, a two-state model was suggested for understanding DCM photophysical properties which is generally also valid for most of the of the electron donor-acceptor (EDA) molecules.
