Effect of axial substitution on the optical limiting properties of indium phthalocyanines

Nonlinear absorption, refraction, and optical limiting by a series of manoa xially chloro- and aryl-substituted indium(III) phthalocyanines are describ ed, The absorption cross sections and temporal evolution of the low-lying e xcited states are also reported. A large nonlinear absorption that increase d with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be d ominated by a strong excited stair: absorption fi om an orientationally ave raged tripler state. We derive and experimentally confirm the relation betw een the molecular absorption cross sections and the fluence-dependent nonli near absorption coefficients. The effective nonlinear refraction on the nan osecond time scale was reduced because the electronic contribution to the n onlinear refractive index was of the opposite sign from the thermal contrib ution. An optical limiter using the new material, p-(trifluoromethyl)phenyl indium(III) tetra-tert-butylphthalocyanine [(t-Bu)(4)PcIn(p-TMP)], showed a much lower threshold for optical limiting and a much lower transmission at high fluences than previously reported indium phthalocyanine limiters. Thi s improved optical limiting was due both to the larger nonlinear absorption coefficient and to the design of the limiter device. The optical propertie s of the in phthalocyanine moiety were found to be surprisingly robust to s tructural changes in the axial position.