Novel unsymmetrically substituted push–pull phthalocyanines for second-order nonlinear optics Original Research Article

The γ(3ω; ω, ω, ω) and γ(2ω; ω, ω, 0) hyperpolarizabilities of some novel unsymmetrically substituted phthalocyanines (Pcs) have been measured at λ=1.34 μm and λ=1.9 μm, respectively. The EFISH values are positive as expected from a strong orientational contribution. By using a two-level model including the ground and the Q-excited levels, the off-resonant dipolar βv(0) component of the β tensor has been evaluated. Values are quite significant and reach 46.7×10−30 esu for the free molecule with an extended conjugated path to the NO2 acceptor group. The molecules containing a transition metal yield similar values that increase with the sequence Co, Ni, Cu in correlation with the magnitude of the ground state dipole moment. THG susceptibilities are negative and much larger than those measured for similar symmetric compounds. This has been related to a dominant asymmetric contribution associated to the Q-band which is resonant at the second-harmonic frequency.