A chlorophyll-a derivative, zinc methyl 3-diethylaminomethyl-pyropheophorbide-a, was tightly bound with N-fluorenylmethyloxycarbonyl-histidines in chloroform to form a 1: 1 complex through the coordination of the imidazolyl nitrogen atom of N-protected histidine to the central zinc atom of the semi-synthetic chlorophyll and hydrogen bonding ( or salt bridging) of the former carboxy group with the latter amino group.