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[EN] Imatinib is a synthetic tyrosinase inhibitor that is employed for the treatment of some kinds of human cancer. This drug has a low phototoxicity towards DNA, but its pyridylpyrimidine (1) fragment by itself exhibits significant phototoxicitiy. The intrinsic mechanism that leads to the enhanced photosafety of Imatinib is not yet known. Here, the properties of the excited state and interchromophoric interactions of Imatinib have been explored by using ultrafast laser flash photolysis and agarose electrophoresis studies. An intramolecular charge separation was directly observed for the irradiated Imatinib, which accounts for the relaxation of its excited state. An anionic form of pyridylpyrimidine (1) was deduced from the results of time-resolved resonance Raman spectra and by quenching experimental studies on compound 1 and diaminotoluene. In contrast, compound 1 efficiently transformed into triplet excited states with a long lifetime, which explained the phototoxicity associated with this fragment. This work provides insight into how to design drugs with lower phototoxicitiy or improved photostability by using interchromophoric interactions., This work was supported by Natural Science Foundation of China (21773151) and Shantou University Initial Funding (NTF16010). The Support from the Hong Kong Research Grants Council grants GRF 17307916, AoE/P-03/08, SEG HKU/07, The University of Hong Kong Development Fund 2013-2014 project "New Ultrafast Spectroscopy Experiments for Shared Facilities", the Spanish Government (CTQ2015-70164-P and BES-2013-066566) are also acknowledged.