of porphyrin-phthalocyanine heterodimers have been measured by degenerate four-wave mixing (DFWM) method.The third-order susceptibilies
X
(3)
of the heterodimers are more than that of free base porphyrin (H
2
TTP) and phthalocyanine (H
2
Pc)
the
X
(3)
values of porphyrin-phthalocyaine heterodimers first increase directly proportional to the chain length between porphyrin and phthalocyalocyanine (
X
n=3
(3)
>
X
n=2
(3)
).The
X
(3)
value reaches the largest when the chain length equal the number of the 3 methylene.Then the
X
(3)
) values of heterodimers decrease (
X
n=3
(3)
>
X
n=4
(3)
>
X
n=5
(3)
) even though the chain lengths increase further.These results are related to flexible chains which are covalently-linked porphyrin and phthalocyanine.We give a tentative explanation for these results.