**2.3 Fluorescence lifetimes**

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

**9**

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

transfer (ICT) emitting states for DCM which are in dynamic equilibrium with each other, where a short-wavelength emission was assigned to a planar conformation

Fluorescence decay measurements of cis- and trans-isomers of DCM were carried out in six solvents using PRA photon counting system [10]. The fluorescence decays of DCM are fitting with mono-exponential despite the presence of two isomers. On the contrary, quite different results were observed when lifetime measurements are carried out using picosecond time-correlated single photon counting technique [23]. The fluorescent decay profiles in methanol, acetonitrile, and chloroform are fitting in bi-exponential. A short component (~25–48 ps) is having longer lifetime in methanol and acetonitrile solvents than that in chloroform. On the other hand, long component has a lifetime (τ) of 1.38 ns in methanol and chloroform solvents and τ ~1.94 ns in acetonitrile. Furthermore, fluorescence decay is fitting with single exponential function in DMSO with a lifetime ~2.25 ns. Thus, it is proved that solvent plays an important role in the non-radiative decay processes of the DCM in excited state which ultimately changes the fluorescence lifetimes. Bi-exponential nature of DCM clearly suggests the presence of two fluorescent species, and similarly, single exponential decay fitting in DMSO indicates single fluorescence species. The long-lived species are predominant in methanol, and acetonitrile solvent attributed to a trans-isomer which is produced while synthesizing DCM. From the relative weight component ratio (*a2/a1 + a2*) analysis at a fixed excitation wavelength, it was observed that the relative contribution of the cis-DCM increases in the order methanol, acetonitrile, and chloroform which is inconsistent with the cis-DCM percentages obtained by Drake et al. [10] Further, a short component is attributed to cis-DCM with fluorescence lifetime in picoseconds and low fluorescence efficiency. The cis-DCM is having steric hindrance that inhibits planarity and rigidity of the molecule and thereby favors electronic to vibrational energy conversion. The decay behavior in highly polar DMSO solvent medium is attributed to relaxed fluorescence state from LE state, and it was pointed that TICT model is not necessary to describe single exponential decay [24]. Based on steady-state and time-resolved fluorescence studies, photoisomerization mechanism was suggested as follows: excitation of the trans-DCM followed by a nonadiabatic curve crossing process in which a surface crossing leads directly to the photoisomer. Another possible scheme would involve production of an intermediate, twisted internal charge transfer (TICT) state from the excited trans-configuration followed by partitioning to the cis and trans ground state which is similar to the Rullière model [25]. Solvatochromic absorption and emission behavior and the fact that the molecule possesses well-separated donor (amino) and acceptor (cyano) groups are consistent with the well-known charge-transfer properties. Therefore, it is likely that the geometrical configuration is skewed and intermediate between the cis and trans excited states. On the other hand, non-exponential fluorescence decay of DCM was observed at low temperature (5 and −35°C) in dibutyl

and a longer-wavelength emission to a twisted (TICT) conformation.

ether, and the main fluorescent state was attributed to a TICT state [26].

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

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

*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*

transfer (ICT) emitting states for DCM which are in dynamic equilibrium with each other, where a short-wavelength emission was assigned to a planar conformation and a longer-wavelength emission to a twisted (TICT) conformation.

Fluorescence decay measurements of cis- and trans-isomers of DCM were carried out in six solvents using PRA photon counting system [10]. The fluorescence decays of DCM are fitting with mono-exponential despite the presence of two isomers. On the contrary, quite different results were observed when lifetime measurements are carried out using picosecond time-correlated single photon counting technique [23]. The fluorescent decay profiles in methanol, acetonitrile, and chloroform are fitting in bi-exponential. A short component (~25–48 ps) is having longer lifetime in methanol and acetonitrile solvents than that in chloroform. On the other hand, long component has a lifetime (τ) of 1.38 ns in methanol and chloroform solvents and τ ~1.94 ns in acetonitrile. Furthermore, fluorescence decay is fitting with single exponential function in DMSO with a lifetime ~2.25 ns. Thus, it is proved that solvent plays an important role in the non-radiative decay processes of the DCM in excited state which ultimately changes the fluorescence lifetimes. Bi-exponential nature of DCM clearly suggests the presence of two fluorescent species, and similarly, single exponential decay fitting in DMSO indicates single fluorescence species. The long-lived species are predominant in methanol, and acetonitrile solvent attributed to a trans-isomer which is produced while synthesizing DCM. From the relative weight component ratio (*a2/a1 + a2*) analysis at a fixed excitation wavelength, it was observed that the relative contribution of the cis-DCM increases in the order methanol, acetonitrile, and chloroform which is inconsistent with the cis-DCM percentages obtained by Drake et al. [10] Further, a short component is attributed to cis-DCM with fluorescence lifetime in picoseconds and low fluorescence efficiency. The cis-DCM is having steric hindrance that inhibits planarity and rigidity of the molecule and thereby favors electronic to vibrational energy conversion. The decay behavior in highly polar DMSO solvent medium is attributed to relaxed fluorescence state from LE state, and it was pointed that TICT model is not necessary to describe single exponential decay [24]. Based on steady-state and time-resolved fluorescence studies, photoisomerization mechanism was suggested as follows: excitation of the trans-DCM followed by a nonadiabatic curve crossing process in which a surface crossing leads directly to the photoisomer. Another possible scheme would involve production of an intermediate, twisted internal charge transfer (TICT) state from the excited trans-configuration followed by partitioning to the cis and trans ground state which is similar to the Rullière model [25]. Solvatochromic absorption and emission behavior and the fact that the molecule possesses well-separated donor (amino) and acceptor (cyano) groups are consistent with the well-known charge-transfer properties. Therefore, it is likely that the geometrical configuration is skewed and intermediate between the cis and trans excited states. On the other hand, non-exponential fluorescence decay of DCM was observed at low temperature (5 and −35°C) in dibutyl ether, and the main fluorescent state was attributed to a TICT state [26].
