The invention relates to a compound of formula (M2L4 where: M = a palladium or platinum metal ion; L = X = -O-, -S-, -NH-, or (-CH2-)n, where n = 1, 2 or 3; R2, R4, R5, R6 are each independently selected from -H, -F, -Cl, -Br, -I, -methyl, -ethyl, -propyl, -O-methyl, -OH, -CH2OH, and - CH2CH2OH, R7,...
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