Indium phthalocyanines with different axial ligands: A study of the influence of the structure on the photophysics and optical limiting properties

The photophysical properties of four axially substituted indium phthalocyanines, namely, 2,(3)- tetra- tert-butyl-phthalocyaninato indium chloride (1), 2,(3)-tetra-[(3,5-di-tert-butyl)-phenyloxyl-phthalocyaninato indium bromide (2), 2,(3)-tetra-[(3,5-di-tert-butyl)-phenyloxyl-phthalocyaninato indium iodide (3), and 2,3-octa-[(2-hexyl)-ethyloxyl-phthalocyaninato indium trifluoroacetate (4), have been investigated, and their optical limiting properties with nanosecond light pulses were evaluated. All complexes behave as reverse saturable absorbers in the range of 400-625 nm due to a triplet-triplet excited-state transition. Excited-state absorption cross sections and triplet state lifetimes are not significantly affected by the nature of the axial ligand. On the other hand, remarkable differences in the variation of nonlinear transmittance are observed for 1-4 due to significantly different intersystem crossing rates. Heavier axial ligands in phthalocyanines 2 and 3 produce the largest variations of nonlinear transmission (heavy-atom effect). Complex 1 in polystyrene matrix shows reversible nonlinear absorption when incident fluence does not exceed 0.025 J cm(-1).