Photophysics, rotational dynamics and fluorescence lifetime imaging study of coumarin dyes in deep eutectic solvent

In this work, the rotational dynamics and photophysics of three structurally similar, coumarins: Coumarin 6, Coumarin 7 and Coumarin 30 in the deep eutectic solvent (DES) have been studied by using steady-state and time-resolved fluorescence emission spectroscopy, with the variation of temperature. In our system, with the increase in temperature, the fluorescence lifetime and average rotational relaxation time decreases in a regular manner for three coumarin dyes. From the temperature dependent study, we have estimated the activation energy of non-radiative decays in DES. It has been observed that below 298 K the anisotropy decay can't be fitted due to the hindered motion. However, the above 298 K anisotropy decays are fitted by stretched exponential functions. The rotational dynamics have been analyzed by Stokes-Einstein-Debye (SED) hydrodynamic model of rotational diffusion theory. We have observed significant deviation of rotational dynamics deviation from hydrodynamic theory. We have also reported absolute fluorescence quantum yield of three coumarin dyes in DES for the first time in the literature. Additionally, we have carried out fluorescence lifetime imaging study to understand the distribution of fluorescence lifetime of coumarin molecules in DES. (C) 2019 Elsevier B.V. All rights reserved.