Synthesis and Optical Limiting Properties of Axially Bridged Phthalocyanines: [(tBu4PcGa)2O] and [(tBu4PcIn)2O]

Highly soluble [(tBu4PcM)2O] phthalocyanine dimers (M=GaIII (3), InIII (4)) were prepared by the reaction of [tBu4PcMCl] (M=GaIII (1), InIII (2)) with excess of concentrated H2SO4 at −20 °C. The MOM linkages in 3 and 4 are not stable against concentrated H2SO4 at room temperature, 6 n HCl at reflux, or during isolation under column chromatographic conditions (e.g. silica gel/toluene). The stability of 3 in solution is considerably higher than that of 4. The μ‐oxo‐bridged phthalocyanine dimers 3 and 4 have a more intense photoluminescence emission in the red region than the monomers 1 and 2. The gallium phthalocyanines 1 and 3 have fluorescence lifetimes of a few nanoseconds, while those of the indium phthalocyanines 2 and 4 last for only several hundred picoseconds. Comparison of the fluorescence lifetimes of monomers 1 and 3 with dimers 2 and 4, reveals that the dimers have longer lifetimes of the excited singlet states. The transient absorption spectrum is similar for all of the compounds, and the transient absorption band at about 520 nm, observed by nanosecond laser irradiation, can be assigned to the transition from the lowest triplet excited state to the upper triplet excited states (T–T absorption). The magnitude of the optical limiting exhibited by 1, 3, and 4 in toluene at 532 nm laser pulse irradiation is in the order: 3>4>1. The values of the imaginary third‐order nonlinear susceptibility Im{χ(3)} of the above compounds at 532 nm in toluene are also reported. These results demonstrate that these compounds are candidates for optical limiting applications. Highly soluble [(tBu4PcM)2O] phthalocyanine dimers (M = GaIII, InIII) were prepared, and an examination of the magnitude of the optical limiting in toluene at a laser pulse irradiation of 532 nm revealed the order shown in the figure.