Push−Pull Phthalocyanines: A Hammett Correlation between the Cubic Hyperpolarizability and the Donor−Acceptor Character of the Substituents

The nonlinear optical (NLO) properties at the molecular level of a family of unsymmetrically substituted metal-free phthalocyanines ( 3 − 9 ) have been studied for the first time. NLO characterization has been performed by electric field-induced second harmonic (EFISH) generation and third harmonic generation (THG) experiments in solution at 1.064 and 1.34 mm, respectively. The NLO response has been investigated for different types of substituents. Although no experimental evidence of a quadratic hyperpolarizability (β) contribution to γ EFISH has been obtained, a significant influence of intramolecular charge transfer (ICT) on the cubic nonlinear response has been evidenced and correlated to the relative strengths of donor and acceptor substituents. Thus, γ THG shows a clear absolute minimum for low Hammett parameters values of the substituents and becomes maximal for high values, i.e., for compounds having strong donor or acceptor groups such as alkoxy (3) or nitro (8) substituents. Such an enhancement effect of the γ THG values due to the high dipolar moment associated with strong donor or acceptor substituents follows the trend previously observed in some porphyrin and phthalocyanine systems.