Synthesis, characterization, and optical-limiting properties of axially substituted gallium(III) naphthalocyanines

The soluble axially substituted naphthalocyanine monomers, tBu(4)NcGaCl (1) and tBu(4)-NcGa(p-TMP) (p-TMP = p-trifluoromethylphenyl) (2) and the mu-oxo-bridged naphthalo-cyanine dimer [tBu(4)NcGa](2)O (3) are synthesized under mild conditions. In comparison to the corresponding gallium phthalocyanine compounds, tBu(4)PcGaCl, tBu(4)PcGa(p-TMP), and [tBu(4)PcGa](2)O, all gallium naphthalocyanines 1-3 are almost transparent in the yellow and red region of the UV/vis spectrum, suggesting that they are suitable for eye protection in a practical optical-limiting device. These compounds optically limit at sufficiently high intensity via a reverse saturable absorption mechanism into the excited T-T absorption band at 532-nm excitation. The substitution of the axially p-TMP ligand onto the central gallium atom results in a defocusing of the beam relative to the other gallium naphthalocyanine compounds. This added defocusing of the beam is important and desirable in practical optical limiters as it helps to spatially disperse the pulse, further reducing the energy density of incident laser pulses.