Abstract: Susceptible tumors are treated by first sensitizing them to incident electromagnetic radiation by of a compound of the formula: ##STR1## where M is a diamagnetic metal ion or a diamagnetic metal compound or silicon or a compound of silicon or is 2H; one H being bonded to each of the two nitrogen atoms depicted as being bonded to M at positions 29 and 31;R.sub.1 to R.sub.25 are the same or different and are independently selected from C.sub.1 -C.sub.20, alkyl, C.sub.2 -C.sub.20 alkenyl, X--O--Y, ##STR2## X.sup.2 COOX.sup.1, x.sup.2 COONR.sup.1 R.sup.11 and H; wherein X and X.sub.2 are independently selected from a chemical bond, --(CH.sub.2).sub.n -- where n is an integer from 1 to 20 and --(CH.sub.2).sub.a CH.dbd.CH(CH.sub.2).sub.b where a and b are independently selected from integers 0-20 and a+b totals from 0 to 20;X.sup.1 is independently selected from C.sub.1-20 alkyl, C.sub.2-20 alkenyl and H;R.sup.1 and R.sup.11 are independently selected from H, C.sub.1-20 alkyl, C.sub.