Effect of solvent on the photophysical properties of isoxazole derivative of curcumin: A combined spectroscopic and theoretical study

The present work aims to decipher the photophysics of a β-diketo modified curcumin analog named isoxazole derivative of curcumin (IOC). IOC itself happens to be a potential drug molecule possessing numerous biological applications such as; anti-cancer, anti-malarial, anti-Alzheimer’s, anti-Parkinson’s etc. Herein, the photophysical properties of IOC have been explored in various sets of solvents. In order to investigate, steady state and time resolved spectroscopy have been utilized as a tool. To elucidate the experimental observations at molecular level, electronic structure calculations with density functional theory (DFT) as well as classical molecular dynamics (MD) simulations in explicit solvents have been deployed. The DFT has established that the most stable ground state electronic structure of the IOC molecule is the all trans- planar conformation where the conjugated pi-electrons are delocalized over the entire molecule. The MD simulations have provided an insight into the specific and non-specific interactions with the solvent molecules. Steady state spectroscopic techniques have shown that both the absorption and emission maxima experienced a traditional red shift upon increase in the solvent polarity. The absorption maxima for IOC got red shifted from 332 nm in hexane to 344 nm in DMSO. However, the red shift in the emission maxima is more pronounced than that of absorption maxima starting from 356 nm in hexane to 431 nm in water. The value of molar extinction coefficient has also been determined for IOC in different solvents from the concentration dependent absorption spectra. The time correlated single photon counting (TCSPC) data has revealed that the excited state of IOC molecule could relax via three probable pathways. Also, the overall process of decay of the excited state is comparatively slower in case of non-polar solvents. On the other hand, the fluorescence lifetime has been observed to be unusually low in case of dimethyl sulfoxide (DMSO) as compared to its other fellow polar aprotic solvents.