No abstract

Photophysical properties of a natural plant alkaloid, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), which comprises both proton donating and accepting sites, have been studied in different solvents using steady state and time-resolved fluorescence techniques primarily to understand the origin of dual fluorescence that this molecule exhibits in some specific alcoholic solvents. Ground and excited state calculations based on density functional theory have also been carried out to help interpretation of the experimental data. It is shown that the long-wavelength emission of the molecule is dependent on the hydrogen bond donating ability of the solvent, and in methanol, this emission band arises solely from an excited state reaction. However, in ethylene glycol, both ground and excited state reactions contribute to the long wavelength emission. The time-resolved fluorescence data of the system in methanol and ethylene glycol indicates the presence of two different hydrogen bonded species of ellipticine of which only one participates in the excited state reaction. The rate constant of the excited state reaction in these solvents is estimated to be around 4.2-8.0 × 10(8) s(-1). It appears that the present results are better understood in terms of solvent-mediated excited state intramolecular proton transfer reaction from the pyrrole nitrogen to the pyridine nitrogen leading to the formation of the tautomeric form of the molecule rather than excited state proton transfer from the solvents leading to the formation of the protonated form of ellipticine.

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.