Fluorescence Response of 4-(N,N′-Dimethylamino)benzonitrile in Room Temperature Ionic Liquids: Observation of Photobleaching under Mild Excitation Condition and Multiphoton Confocal Microscopic Study of the Fluorescence Recovery Dynamics

Abstract

The fluorescence behavior of 4-(N,N'-dimethylamino) benzonitrile has been studied in room temperature ionic liquids (ILs) as a function of temperature, excitation wavelength, and exposure time. Dual emission from the locally excited (LE) and intramolecular charge transfer (ICT) states of the molecule has been observed and the relative intensities of the two emission bands and the peak position of the ICT emission are found consistent with the viscosity and polarity of the ILs. Temperature dependence study reveals a blue shift of the ICT emission peak with lowering of temperature indicating that under this condition the emission occurs from incompletely solvated state of the molecule. The observed excitation wavelength dependence of the emission behavior has been attributed to the microheterogeneity of the media. Exposure of the solution to the exciting radiation under very mild condition is found to influence the relative intensities of the two emission bands; an enhancement of the LE emission accompanied by a slight decrease of the ICT emission is observed. The emission intensities, however, return almost to their original values when the exposed solution is kept in the dark. The observation has been attributed to photoreaction of the exposed molecules and the recovery to replenishment of phototransformed molecules by the surrounding unexposed molecules. Fluorescence recovery after photobleaching has been studied by multiphoton confocal fluorescence microscopic technique to obtain insight into the recovery dynamics. The diffusion coefficient estimated from this study is found to be lower than that predicted by the Stokes-Einstein equation by a factor of nearly 7 indicating the microheterogeneous nature of the ILs.

Excitation wavelength dependence of photo-induced intramolecular proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavone in various liquids

“…On the other hand, in less polar solvents such as triacetin, which is most viscous conventional molecular liquids in this study (16.31 mPa·s [24]), decrease of the tautomer form fluorescence intensity was found from 420 nm to 470 nm. A significant decrease of the tautomer form fluorescence intensity was observed with increasing excitation wavelength from 380nm to 470nm in [P 6,6,6,14 ][NTf 2 ] (Figure 1(c)).…”

“…In triacetin the decrease was apparent (from 16.5 to 7.38 from 380 nm to 470 nm). The decrease in [P 6,6,6,14 ][NTf 2 ] (460mPa·s [17]) was remarkable, where the ratio at 400 nm excitation (6.02) was rather close to the value in non-polar solvent, while the ratio at 470 nm excitation (1.82) was similar to that of in polar liquids such as ACN. The excitation wavelength in other ionic liquids ([BMIm][PF 6 ] (360mPa·s [17]) and [BMIm][NTf 2 ](58mPa·s [17])) were also apparent, although the values were similar to those in polar conventional molecular liquids.…”

“…Using the different color of the excitation wavelength, we selectively excite the solute molecule which is differently solvated. The result in [P 6,6,6,14 ][NTf 2 ] is the most illustrative in this sense, since this ionic liquid has a very long alkyl chain and it has been suggested that polar and nonpolar domains are suggested to exist [1,4]. The excitation wavelength dependence is understandable as follow: the solute molecule in polar domain is selectively excited in red-side excitation since the ratio of tautomer to the normal is close to the value excited in polar solvent.…”

“…Maroncelli et al have shown that time constants associated with isomerization reactions of the solute (julolidine Malononitrile) are dependent on the excitation wavelength [11]. Samanta et al also found that the charge transfer reaction yield of 4-(N,N'-dimethylamino)benzonitrile is dependent on the excitation wavelength [14]. Very recently, Sahu et al have investigated the effect of local inhomogeneity to a reaction rate using new spectroscopic method [15].…”

Medium decoupling of dynamics at temperatures ∼100 K above glass-transition temperature: A case study with (acetamide + lithium bromide/nitrate) melts

“…123 At lower temperatures where heterogeneities are longer-lived than τ r , molecular aspects get prominence and a departure from the SED description ensues. Results obtained from experimental studies of reactive and non-reactive dynamics in ionic liquids, 52,124,125 and those from very recent measurements after isothermal introduction of ∼2 nm structural inhomogeneity in an otherwise homogeneous liquid 126 suggest that both the static and dynamic heterogeneities could contribute to the observed decoupling between solute rotation and viscosity in these melts. In the absence of LiNO 3 (f LiBr = 1.0) the extent of decoupling becomes even stronger as one finds p = 0.50 at this composition (see Fig.…”