heat a medium with a large refractive index response to heat will accentuate this thermal refractive effect the polysiloxane structure has a very large refractive index response to heat and by virtue of being bonded to the periphery of the phthalocyanine ring is well positioned to accentuate this effect the temperature dependence of the refractive index was measured for the polysiloxane phthalocyanine of example 5 via ellipsometry to be u2212 5 4 u00b1 1 u00d7 10 u2212 4 u00b0 c u2212 1 between 25 and 40 u00b0 c and an average dn dt of u2212 4 u00b1 1 u00d7 10 u2212 4 u00b0 c u2212 1 between 25 and 95 u00b0 c the latter value compares well with that found for linear polydimethylsiloxane liquids relative to other polymers polydimethylsiloxane has an exceptionally large dn dt this thermal refractive enhancement to the optical limiting is depicted in fig1 by comparing the optical limiting curve with that attributable to the only the nonlinear optical absorption fig3 shows the optical limiting measurements when carried to higher energies other favorable properties that the polysiloxane peripheral groups confer on the phthalocyanine materials are chemical inertness and moisture resistance the heavy metal ions particularly lead are labile to displacement from the phthalocyanine.