A new 2,6-bis(5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazolin-6-yl)-4-methylphen ol (1) serves as a highly selective and sensitive fluorescent probe for Zn2+ in a HEPES buffer (50 mM, DMSO:water = 1:9 (v/v), pH = 7.2) at 25 degrees C. The increase in fluorescence in the presence of Zn2+ is accounted for by the formation of dinuclear Zn2+ complex [Zn-2(C35H25N6O)(OH)(NO3)(2)(H2O)] (2), characterized by X-ray crystallography. The fluorescence quantum yield of the chemosensor 1 is only 0.019, and it increases more than 12-fold (0.237) in the presence of 2 equiv of the zinc ion. Interestingly, the introduction of other metal ions causes the fluorescence intensity to be either unchanged or weakened. By incubation of cultured living cells (A375 and HT-29) with the chemosensor 1, intracellular Zn2+ concentrations could be monitored through selective fluorescence chemosensing.