**2.4 Ultra-fast spectroscopic studies of DCM**

As described in previous sections, since steady-state absorption and fluorescence studies were not conclusive about the nature of emitting state, one would always ask whether fluorescence emission is from direct charge-transfer state (CT) or relaxed CT state which is originated from locally excited state as shown in **Figure 3**. To answer this question, it is not necessary to have ultra-fast spectroscopy data; in fact simple steady-state fluorescence data would be sufficient to explain the nature of the emitting state. Suppose if it is encountered that the transition dipole moments for absorption (CT ← S0) and emission (CT → S0) are the same, one can conclude that the DCM photophysics are involved in two states (ground and CT states) [27]. Further, in such

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

smaller fraction of charge transfer than that of DMABN.

electron donor-acceptor (EDA) molecules.

**2.3 Fluorescence lifetimes**

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

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

Fluorescence lifetimes of DCM were measured in six different solvents for the first time, and it is found that the fluorescence times (τ) depend upon the polarity of the solvent [8]. Later on, wavelength dependent fluorescence decay profiles of DCM in protic-polar solvent (ethanol) and other solvents were measured, and it is found that all the decays profiles are fitting with single exponential function despite the strong overlap between the two fluorescence bands [9]. Moreover, these studies clearly reveal that the fluorescence lifetime value of DCM in a given solvent is independent of the fluorescence wavelength at which the measurement was made. In order to obtain more information about the nature of the emitting states of DCM in polar solvents, the fluorescence spectra of DCM in DMSO were recorded at various times after excitation. From typical time-resolved emission spectral data, it was observed that both short- and long-wavelength fluorescence bands appear within the 0.75 ns after excitation. Further, their relative intensities change with time until a time-independent intensity ratio is reached, at about 2.25 ns. Wavelengthdependent time-resolved fluorescence measurements also suggest that DCM exhibits dual fluorescence in polar solvents which is assigned to the two well-separated different emitting states. Based on the steady-state and time-resolved fluorescence data, Hsing-Kang and co-workers suggested two different intramolecular charge

**8**

**Figure 3.**

*Potential energy curves against generalized coordinates which include intramolecular and solvent modes for (A) direct vertical excitation to CT-state (B) population of LE state followed by LE → CT transition from ground state.*

a case, the solvatochromism of absorption and emission should be consistent with ground- and excited-state dipole moments and their difference. On the other hand, if fluorescence anisotropy of DCM is substantially smaller than 0.4, then it is possible that the fluorescence emission could be from a different state than that of populated by photoexcitation, perhaps it is direct indicative of a three-state system (ground, LE, and CT states). Therefore, explicit evidence of such a three-state system can only be obtained by time-resolved spectroscopy through the direct observation of the LE → CT transition. However, because of the interference of both population transfer and relaxation (solvent, vibration) in spectral dynamics, the interpretation of the transient spectra can sometimes be sensitive and may to lead confusion. Easter et al. have investigated ultra-fast dynamics of DCM for the first time and observed temporal evolution of its stimulated emission in methanol and ethylene glycol at several wavelengths using sub-picosecond pump-probe spectroscopy [28]. The observed temporal changes of the fluorescence intensity measured during the first 100 ps after excitation were assigned to the dynamic Stokes shift of the fluorescence emission from the CT state following its direct optical excitation. Time-resolved transient absorption spectroscopic studies of DCM solutions in weakly polar and polar were carried out by Martin and co-workers, and corresponding data exhibits an isosbestic point in the net gain spectra within a few picoseconds after excitation which suggest rapid evolution of an emissive intermediate state from the initial excited S1 state [29]. Solvatochromic behavior of the gain spectral position and its time-resolved redshift in slowly relaxing solvents support the CT character of the emissive intermediate state. Further, the overall intramolecular CT process is observed to take place within 30 ps in all solvents, and solvent relaxation time appears as an important parameter in the observed kinetics. Moreover, it was also found that the time constants associated with these changes depend upon the solvent polarity and vary from 2 ps (in acetonitrile) to 8 ps (in methanol). All these dynamics of DCM were interpreted to a transition that occurs from optically populated LE state to the CT state. However, there was no evidence of the twisted nature of this CT state which was suggested earlier [26].

Population relaxation within the fluorescent state was selectively monitored by Glasbeek and co-workers using femtosecond fluorescent up-conversion technique with a time-resolution of ~150 fs which does not permit to probe any influence of the dynamics within the electronic ground state [30]. It has been shown that intramolecular charge separation is taking more than 300 fs after the pulsed excitation. Following the pulsed excitation of the molecule, the integrated intensity of the spontaneous fluorescence decreased to approximately 50% of its initial value within few picoseconds. Moreover, it was observed that a significant portion of the charge

**11**

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

separation trajectory (~30%) is controlled by the solvation process on a picosecond time scale. Therefore, it is inferred that LE and CT states of photoexcited DCM strongly coupled adiabatically in the inverted region where a large extent of the charge separation process occurs on a picosecond time scale controlled by the excited state solvation process. However, subsequent high-resolution (<100 fs) fluorescence up-conversion studies of the DCM dye molecule in methanol and chloroform reveal that there is no change of the integrated spectral intensity during the first 25 ps after vertical excitation for the LE → CT transition [31]. Besides, for all times only one fluorescent excited state was noticeable, and the observed dynamic Stokes shift is attributed to solvent relaxation. Mean position of the time-resolved fluorescence spectrum of DCM in methanol shifts towards the red side with bi-exponential (175 fs and 3.2 ps) behavior, while in chloroform the spectral position remains practically unchanged for all times. The collected time-resolved data concluded that DCM has a

single emitting state, which is directly populating upon photoexcitation.

separation (≥70%) is completed within 300 fs of the pulsed excitation.

and the CN stretching region, ~2208 cm<sup>−</sup><sup>1</sup>

state absorption bands in the fingerprint region (1495 cm<sup>−</sup><sup>1</sup>

1520, and 1590 cm<sup>−</sup><sup>1</sup>

Later, time-resolved visible pump and infrared (IR) probe transient absorption measurements of the DCM and its isotopomer DCM-*d*6 were studied by Fleming and co-workers to probe the ultra-fast charge-transfer state formation in polar solvents: dimethylsulphoxide (DMSO) and acetonitrile (MeCN) [34]. Transient infrared absorption bands at both a fingerprint region between 1440 and 1620 cm<sup>−</sup><sup>1</sup>

is assigned to the LE state of DCM, while the higher-frequency absorptions (1495,

frequency upshift and/or changes in band shape on a few ps time scale (∼1–2 ps) which was attributed to the formation of the excited state and charge-transfer state via twisting and pyramidalization of the C–N(Me)2 group and associated changes in C–C bonding character throughout the molecule. That is the fast rise in the CT bands was assigned to the rapid evolution of the LE state into the CT state.

Excited state non-radiative relaxation dynamics of DCM in hexane have been investigated using femtosecond fluorescence up-conversion technique at three

) are assigned to the CT state. The results reveal that excited-

, were probed. The IR band at 1440 cm<sup>−</sup><sup>1</sup>

) are exhibiting a

A binodal dynamic Stokes shift was observed with time constants, one is about 100 fs, and another is of few picoseconds, respectively, when DCM is present in highly polar solvent media (methanol, ethylene glycol, ethyl acetate, and acetonitrile) [32]. The initial fast component is attributed to the free streaming motions of the solvent molecules and the second slow time component to the rotational diffusion motions of the solvent molecules. However, from the rapid sub-picosecond rise of the integrated emission intensity, it was suggested that the excited state electron transfer is preferentially taking place within about 100 fs from a higher-lying less emissive state to a lower-lying more emissive CT state. That is, the charge separation process in DCM is completed within about 100 fs. The LE and CT states are pictured as strongly coupled in the inverted region which is already reported earlier by Gustavsson et al. [31], and the gradual charge separation is treated as diffusional motion on the resulting barrierless potential. On the other hand, transient absorption spectra of DCM dye in methanol were measured using pump-supercontinuum probe technique with 40 fs time resolution and also revealed two components [33]. Initially (before 70 fs), a prominent spectral structure is observed which is primarily due to resonance Raman processes. At longer times (>70 fs), the spectrum undergoes a significant redshift, and shape of the band changes with a well-defined isosbestic point, and these observations are quite similar to earlier study done by Martin and co-workers [29]. The early transient component has been assigned to the locally excited state of DCM. Further, it was found that LE → CT transition is much faster than that suggested by Martin et al. and concluded that a substantial fraction of the intramolecular charge

*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*

separation trajectory (~30%) is controlled by the solvation process on a picosecond time scale. Therefore, it is inferred that LE and CT states of photoexcited DCM strongly coupled adiabatically in the inverted region where a large extent of the charge separation process occurs on a picosecond time scale controlled by the excited state solvation process. However, subsequent high-resolution (<100 fs) fluorescence up-conversion studies of the DCM dye molecule in methanol and chloroform reveal that there is no change of the integrated spectral intensity during the first 25 ps after vertical excitation for the LE → CT transition [31]. Besides, for all times only one fluorescent excited state was noticeable, and the observed dynamic Stokes shift is attributed to solvent relaxation. Mean position of the time-resolved fluorescence spectrum of DCM in methanol shifts towards the red side with bi-exponential (175 fs and 3.2 ps) behavior, while in chloroform the spectral position remains practically unchanged for all times. The collected time-resolved data concluded that DCM has a single emitting state, which is directly populating upon photoexcitation.

A binodal dynamic Stokes shift was observed with time constants, one is about 100 fs, and another is of few picoseconds, respectively, when DCM is present in highly polar solvent media (methanol, ethylene glycol, ethyl acetate, and acetonitrile) [32]. The initial fast component is attributed to the free streaming motions of the solvent molecules and the second slow time component to the rotational diffusion motions of the solvent molecules. However, from the rapid sub-picosecond rise of the integrated emission intensity, it was suggested that the excited state electron transfer is preferentially taking place within about 100 fs from a higher-lying less emissive state to a lower-lying more emissive CT state. That is, the charge separation process in DCM is completed within about 100 fs. The LE and CT states are pictured as strongly coupled in the inverted region which is already reported earlier by Gustavsson et al. [31], and the gradual charge separation is treated as diffusional motion on the resulting barrierless potential. On the other hand, transient absorption spectra of DCM dye in methanol were measured using pump-supercontinuum probe technique with 40 fs time resolution and also revealed two components [33]. Initially (before 70 fs), a prominent spectral structure is observed which is primarily due to resonance Raman processes. At longer times (>70 fs), the spectrum undergoes a significant redshift, and shape of the band changes with a well-defined isosbestic point, and these observations are quite similar to earlier study done by Martin and co-workers [29]. The early transient component has been assigned to the locally excited state of DCM. Further, it was found that LE → CT transition is much faster than that suggested by Martin et al. and concluded that a substantial fraction of the intramolecular charge separation (≥70%) is completed within 300 fs of the pulsed excitation.

Later, time-resolved visible pump and infrared (IR) probe transient absorption measurements of the DCM and its isotopomer DCM-*d*6 were studied by Fleming and co-workers to probe the ultra-fast charge-transfer state formation in polar solvents: dimethylsulphoxide (DMSO) and acetonitrile (MeCN) [34]. Transient infrared absorption bands at both a fingerprint region between 1440 and 1620 cm<sup>−</sup><sup>1</sup> and the CN stretching region, ~2208 cm<sup>−</sup><sup>1</sup> , were probed. The IR band at 1440 cm<sup>−</sup><sup>1</sup> is assigned to the LE state of DCM, while the higher-frequency absorptions (1495, 1520, and 1590 cm<sup>−</sup><sup>1</sup> ) are assigned to the CT state. The results reveal that excitedstate absorption bands in the fingerprint region (1495 cm<sup>−</sup><sup>1</sup> ) are exhibiting a frequency upshift and/or changes in band shape on a few ps time scale (∼1–2 ps) which was attributed to the formation of the excited state and charge-transfer state via twisting and pyramidalization of the C–N(Me)2 group and associated changes in C–C bonding character throughout the molecule. That is the fast rise in the CT bands was assigned to the rapid evolution of the LE state into the CT state.

Excited state non-radiative relaxation dynamics of DCM in hexane have been investigated using femtosecond fluorescence up-conversion technique at three

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

a case, the solvatochromism of absorption and emission should be consistent with ground- and excited-state dipole moments and their difference. On the other hand, if fluorescence anisotropy of DCM is substantially smaller than 0.4, then it is possible that the fluorescence emission could be from a different state than that of populated by photoexcitation, perhaps it is direct indicative of a three-state system (ground, LE, and CT states). Therefore, explicit evidence of such a three-state system can only be obtained by time-resolved spectroscopy through the direct observation of the LE → CT transition. However, because of the interference of both population transfer and relaxation (solvent, vibration) in spectral dynamics, the interpretation of the transient spectra can sometimes be sensitive and may to lead confusion. Easter et al. have investigated ultra-fast dynamics of DCM for the first time and observed temporal evolution of its stimulated emission in methanol and ethylene glycol at several wavelengths using sub-picosecond pump-probe spectroscopy [28]. The observed temporal changes of the fluorescence intensity measured during the first 100 ps after excitation were assigned to the dynamic Stokes shift of the fluorescence emission from the CT state following its direct optical excitation. Time-resolved transient absorption spectroscopic studies of DCM solutions in weakly polar and polar were carried out by Martin and co-workers, and corresponding data exhibits an isosbestic point in the net gain spectra within a few picoseconds after excitation which suggest rapid evolution of an emissive intermediate state from the initial excited S1 state [29]. Solvatochromic behavior of the gain spectral position and its time-resolved redshift in slowly relaxing solvents support the CT character of the emissive intermediate state. Further, the overall intramolecular CT process is observed to take place within 30 ps in all solvents, and solvent relaxation time appears as an important parameter in the observed kinetics. Moreover, it was also found that the time constants associated with these changes depend upon the solvent polarity and vary from 2 ps (in acetonitrile) to 8 ps (in methanol). All these dynamics of DCM were interpreted to a transition that occurs from optically populated LE state to the CT state. However, there was no evidence of the twisted nature of this CT state which was suggested earlier [26]. Population relaxation within the fluorescent state was selectively monitored by Glasbeek and co-workers using femtosecond fluorescent up-conversion technique with a time-resolution of ~150 fs which does not permit to probe any influence of the dynamics within the electronic ground state [30]. It has been shown that intramolecular charge separation is taking more than 300 fs after the pulsed excitation. Following the pulsed excitation of the molecule, the integrated intensity of the spontaneous fluorescence decreased to approximately 50% of its initial value within few picoseconds. Moreover, it was observed that a significant portion of the charge

*Potential energy curves against generalized coordinates which include intramolecular and solvent modes for (A) direct vertical excitation to CT-state (B) population of LE state followed by LE → CT transition from* 

**10**

**Figure 3.**

*ground state.*

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 dynamics of DCM in non-polar solvent.
