octakis( mercaptopropylisobutyl-POSS) functional group was achieved. By the reaction of [ 1-(3-mercapto) propyl- 3,5,7,9,11,13,15- isobutylpentacyclo[ 126.96.36.199(3,9).1(5,15).1(7,13)] octasiloxane 1 with 4,5- dichloro- 1,2-dicyanobenzene 2 in THF as the solvent in the presence of K2CO3 as the base, the phthalonitrile derivative 3 was synthesized. Compound 3 reacted with CoCl2 center dot 6H(2)O in ethylene glycol to furnish a novel cobalt( II) phthalocyanine 4. The tetramerization of 3 with urea and CuCl in the absence of solvent gave the novel Cu( II) phthalocyanine 5; while with Zn(OAc) 2 center dot 2H(2)O in dry DMF gave the novel zinc( II) phthalocyanine 6. The structures of the target compounds were confirmed by elemental analysis, UV/VIS, IR, MALDI-TOF MS and H-1 NMR spectra. Nonlinear absorptions of MPcs 4-6 in chloroform solution were investigated by using Z-scan measurement technique with 4 ns pulses at 532 nm wavelength. While CuPc 5 showed very high nonlinear absorption, MPcs 4 and 6 did not show considerable nonlinear absorption. Investigations of optical limiting properties of 5 revealed that this material is a very good candidate for optical limiting applications.