Abstract
Imiquimod, commonly formulated as AldaraTM, is one of the most widely used drugs in clinic to treat several skin diseases such as superficial basal cell carcinoma (BCC), viral warts, etc. As this kind of important skin drugs tends to be exposed to light the characterization of their photophysical properties would help to further scrutinize the molecule's biological activity. Herein, for the first time we characterize the photophysical and acid-base properties of Imiquimod in aqueous solutions using different spectroscopic techniques and quantum chemical calculations. Steady-state spectroscopy revealed that the drug molecule can exist in three different forms depending upon pH of the medium. The excited-state lifetimes of these three distinct forms of IMQ were well characterized by using time-resolved spectroscopic technique. Quantum chemical calculations confirm the molecular structures of the three forms as cationic, neutral and anionic forms of IMQ that are observed in different pH conditions. A unique excited state phenomenon is observed at higher pH, where the neutral form of the drug is being converted to anionic form exclusively in the excited state via excited state proton transfer mechanism.
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