Photophysics and nonlinear optical properties of tetra- and octabrominated silicon naphthalocyanines

The effect of the number of bromide substituents on the photophysical and nonlinear optical properties of the tetrabrominated naphthalocyanine Br-4(tBu(2)PhO)(4)NcSi[OSi(Hex)(3)](2) (1) and the octabrominated naphthalocyanine Br(8)NcSi[OSi(Hex)(3)](2) (2) has been investigated through various spectroscopic techniques. Absorption and emission of I and 2 have been studied at room temperature and 77 K to determine the spectral properties of the ground and the excited states and the lifetimes and quantum yields of formation of the excited states. There is a moderate increase of the quantum yield of the triplet excited-state formation ((Phi(T) = 0.10 vs 0.13) and a decrease of the triplet excited-state lifetime (tau(T) approximate to zt 70 vs 50 mu s) from 1 to 2. These can be attributed to the stronger heavy atom effect produced by the larger number of peripheral bromide substituents in 2 considering that an excited state with a triplet manifold is involved in the excitation dynamics of both complexes I and 2. The quantum yields of the singlet oxygen formation (Phi(Delta))) upon irradiation of 1 and 2 at 355 nm were also evaluated, and a value of Phi(Delta)(1) = Phi(Delta)(2) = 0.16 was obtained. In addition to that, octabrominated complex 2 displays a larger decrease of nonlinear optical transmission for nanosecond pulses at 532 nm with respect to the tetrabrominated complex 1. The nanosecond Z-scan experiments reveal that 1 and 2 exhibit both a reverse saturable absorption and a nonlinear refraction at 532 nm. However, both the sign and the magnitude of the nonlinear refraction change from 1 to 2. For picosecond Z-scan in the visible spectral region, these two complexes exhibit only reverse saturable absorption, and the excited-state absorption cross-section increases at longer wavelengths.