A novel conjugated phthalocyanine pentamer was studied for optical limiting performance. Its fluence-dependent transmittance in different solvents was measured using 10 ns, 532 mn laser pulses in a collimated optical setup. Compared with in chloroform, phthalocyanine pentamer dissolved in chloroform/methyl iodide mixed solvent shows better optical limiting performance at the same linear transmittance. Explanation based on the triple-state absorption enhanced by the increase of the intersystem crossing rate through external heavy atom effect was proposed.