Striking Similarities in the Fluorescence Behavior between Carbon Dots and Ionic Liquids: Toward Understanding the Fluorescence Behavior of Carbon Dots

Despite several studies, convincing explanation for the fluorescence of carbon dots (CDs) and its excitation wavelength dependence behavior has not yet emerged. It may be noted that direct structure–property correlation can be misleading based on solely transmission electron microscopy micrographs as it does not fully reveal the possibility of heterogeneous nature of the samples in a sense that it does not fully reveal the possibility of having both carbonaceous nanoparticles as well as small organic molecular-based systems. The present work is undertaken specifically to address this issue. A detailed spectroscopic investigation comprising steady-state absorption, emission, time-resolved fluorescence, and fluorescence correlation spectroscopy (FCS) studies has been carried out on CDs, synthesized from two different sources. Similar investigations have also been carried out on the systems such as aromatic and aliphatic ionic liquids (ILs), which are known to be fluorescent in their neat conditions. Interestingly, the fluorescence behavior of CDs is observed to be very similar to that of neat ILs. Recent studies by Kim and co-workers have categorically demonstrated that fluorescence from neat ILs can originate from associated structures of ILs. In the present work, the excitation wavelength-dependent fluorescence measurements, emission wavelength-dependent radiative recombination and FCS studies on CDs and other systems including ILs have established that the presence of energetically different associated structures (in the ground state) in CD solution is primarily responsible for the fluorescence behavior of CDs. Dynamic light scattering measurements and dilution studies through FCS have also provided evidence in favor of associated structures in CD solution. Excitation wavelength-dependent fluorescence behavior of CDs can also be explained on the basis of energetically different associated structures that are formed in CD solution during its synthesis. Essentially the present investigations have revealed that carbon dots are not inherently fluorescent, rather fluorescence from CD solution arises due to the presence of associated/networked structures similar to what has been observed in systems such as neat ILs.