Solvent dependent photophysical study of stable and medicinally active diketone modified pyrazole derivatives of curcumin: A spectroscopic study

Curcumin pyrazole derivatives have already been extensively studied as potent drugs in various biological fields but the information about their fundamental physical parameters is still somewhat limited. Therefore, the study of the photophysical behaviour of pyrazole derivatives of curcumin in various solvents is a very interesting and an important research area that necessarily needs to be explored in order to understand its biological efficacy. Herein, the photophysics of stable and pharmaceutically active curcumin pyrazole derivative (CPD) and curcumin phenyl pyrazole derivative (CPPD) has been studied in various solvent environments. For the investigation of photophysical properties, various spectroscopic techniques have been utilized. Steady state spectroscopic results have revealed that the excited state of both the derivatives is largely affected by changing the polarity of the medium. The molar extinction coefficient (e) for CPD and CPPD has also been determined in various solvents. From the measurement of Stokes shift values for wide range of solvents, it could be concluded that there is difference in the dipole moment of the excited state and the ground state of both the molecules. The Stokes shift values for both the derivatives were found to be correlated linearly with the normalized molar transition energy ( E T N ) of solvents with exceptions in case of polar protic solvents. This indicates that H- bonding might play a major role in the solution phase. The difference in the dipole moment of ground state and excited state of CPD and CPPD were calculated as around 5.23 D and 5.41 D. Further, using multiple regression analysis, the solute -solvent interactions were characterized as specific and non-specific. The low quantum yields obtained in polar protic solvents revealed that the positive solvatokinetic effect could take place in these solvents. The results of TCSPC study showed that the process of deactivation of the excited state of these derivatives is very slow in the presence of acetonitrile, DCM and non-polar solvents. A very fast decay of excited state of both the derivatives has been observed in the case of polar protic solvents. This could be due to the presence of strong H-bonding between the derivatives and these solvents which could result in the deactivation of excited states via non-radiative pathway.