Magnesium phthalocyanine (MgPc) was covalently attached by four imidazole units to form a novel photosensitizer (PS). The photophysical processes within the dyad PS were explored by steady state and time-resolved fluorescence as well as laser flash photolysis. Although the imidazole units caused a 50% decrease in fluorescence quantum yield and a remarkable shortening of fluorescence lifetime of the MgPc moiety, the triplet yield (Phi(T)) is higher and the triplet lifetime becomes longer. The transient absorption bands for MgPc center dot- were observed, indicating the occurrence of intramolecular photoinduced electron transfer (PET) from imidazole subunits to the lowest excited singlet state (S-1) of the MgPc moiety. The kinetic and thermodynamic analysis also supports the involvement of PET in S-1 deactivation. The quantum efficiency of photosensitized oxidation of diphenylisobenzofuran (DPBF) by the PS is 0.52 This value is much higher than Phi(T) (026), since DPBF is photo oxidized not only by singlet oxygen (type II reaction, 54%) but also by superoxide anion radical (type I reaction, 46%). The result suggests that the mechanism of photosensitized oxidation could be changed upon the conjugation of a PS to biological molecules, so that the importance of type I reaction is enhanced