Chem Sci Trans., 2013, 2(3), 749-760 Chemical Science Transactions DOI:10.7598/cst2013.470 ISSN/E-ISSN: 2278-3458/2278-3318 Photoinduced Electron Transfer Reaction of 2-Thiobarbituric Acid and Methylene Blue: Mechanism and Kinetics MEDHA D. DAVE * and U. C. PANDE Department of Chemistry, Gujarat University, Navrangpura, Ahmedabad-380 009, Gujarat, India medhaddave@yahoo.com Received 6 December 2012 / Accepted 19 December 2012 Abstract: Barbiturates are known to have hypnotic properties and are used as active moiety on central nervous system. Depends up on the experiments conditions and functional group present on the drugs, make the drugs labile to photodecomposition. The photosensitized reaction of 2-thiobarbituric acid (2TBA) with methylene blue (MB) in alkaline medium using visible light has been studied. 2TBA exists in protonated and deprotonated form in equilibrium in alkaline medium. Photosensitized electron transfer from the deprotonated 2TBA to excited state MB takes place and the energy is transferred to 2TBA molecule which undergoes chemical reaction. The interconversion of protonated form to deprotonated form during the reaction is observed. The rate of the reaction has been calculated and the effect of pH, concentration of the sensitizer, the light intensity on the rate of the reaction has been studied. The quantum efficiency of the photo chemical reaction is calculated using potassium ferrioxalate actinometer and the effect of the concentration of the substrate on the quantum efficiency is calculated. The reaction mechanism and the excited states involved have been suggested. Keywords: 2-Thiobarbituric Acid, Photosensitized reaction, Electron transfer reaction, Singlet - singlet energy transfer, Exciplex formation Introduction Photochemistry and photo stability of drugs presents the basic elements of science and serves as an excellent introduction to this emerging field of photochemistry. The photochemistry of drugs does not differ from that of the other molecules, since Pasteur 1 in 1846, scientists have been aware that many drugs are photo reactive, but until recently research in this area had been somewhat limited. However, until recently the matter has received only limited attention, also because it is generally feasible to protect the drug preparation from light through an appropriate package. As a result, the available knowledge is quit sparse. All pharmacopoeias mention that some drugs have to be protected from light, but this cautionary note is accompanied only by qualitative and incomplete information. However, since the introduction of acutely sensitive analytical methods, the realization of the need to identify the photochemical properties of a potential drug as early in its development as possible and the increased attention to the phototoxic effect of drugs, more details are becoming available. RESEARCH ARTICLE