In this paper, we have investigated the ability to sensitize the
phototoxicity toward HeLa cells in vitro, of tetra-tert-butylphthalocyaninatosilicon (
SiPc) covalently linked to one or two
2,2,6,6-tetramethyl-1-piperidinyloxy (
TEMPO) radicals (R1c or R2c), which are shown as
photosensitizers efficiently producing
singlet oxygen (1Delta(g)). Addition of R1c or R2c encapsulated in
liposomes to cultures, followed by irradiation with a 680-nm
dye laser, resulted in a highly significant
phototoxicity toward HeLa cells, in contrast to negligible
phototoxicity observed with (dihydroxy)
SiPc (R0). EPR measurements indicate that R1c and R2c exist in some degree as
nitroxide radicals even in HeLa cells. Electronic absorption spectra indicate that the degree of aggregation increases in the order R2c<R1c<R0. Thus, the high
phototoxicity of R1c and R2c toward HeLa cells is reasonably interpreted by both the large
singlet oxygen yield and the inhibition of aggregation due to the bulky
TEMPO radicals. This increase in photodynamic effect on HeLa cells is an unusual and important example for increasing the photobiological reaction yields using paramagnetic radicals.