On the Nanoscopic Environment a Neutral Fluorophore Experiences in Room Temperature Ionic Liquids

The nanoscopic environment experienced by a fluorophore while being dissolved in a complex nanoscopic structure of room temperature ionic liquid (RTIL) is not fully known. Here we have explored resonance energy transfer (RET) in order to have an insight in this direction in a noninvasive manner. We have varied the alkyl chain length of the cation and size of the anion in order to probe the nanoscopic environment within an RTIL. Other factors like different excitation wavelengths and different concentrations of the probe has been varied in order to understand the nature of the nanoscopic environment more accurately. Förster formulation has been shown to explain fluorescence dynamical parameters related to the RET phenomenon with reasonable accuracy. The rise time (varying from 3.00 to 4.00 ns) or rate constant of energy transfer has been shown to be independent of the excitation wavelength and the concentration of the acceptor. However, the magnitude of rise time varies with the nature of the cation (alkyl chain length) and size of the anion. By employing Förster formulation, we have obtained the donor–acceptor distance inside nanostructural RTIL media. The magnitude of donor–acceptor distance (varying from 30.35 to 36.48 Å) and hence the size of the nanostructural cage have been shown to be dependent on the size of the alkyl chain length of the cation as well as the size of the anion of the RTILs. Quite expectedly the size of the nanoscopic cages does not depend on the excitation wavelength or concentration of the acceptor. After detailed experiments and rigorous analysis, we have put forward a model that can successfully explain the nanoscopic environment that a fluorophore experiences in an RTIL.