Photoresponsive J-Aggregation Behavior of a Novel Azobenzene−Phthalocyanine Dyad and Its Third-Order Optical Nonlinearity

The photoresponsive J-aggregation behaviors of a novel azobenzene-substituted zinc phthalocyanine (azo-ZnPc dyad) were studied by UV/vis, fluorescence, and 1H NMR spectroscopy. Upon illumination with 365 nm UV light, the trans−cis isomerization of azobenzene can efficiently reduce the steric hindrance around the peripheral oxygen atom of azo-ZnPc, shortening the possible distance between two phthalocyanine molecules and, consequently, greatly improving the tendency of J-aggregation of azo-ZnPc dyad. The third-order optical nonlinearities of the photoresponsive J-aggregates (before and after illumination) were measured by a Z-scan technique at 532 nm with a pulse duration of 25 ps. The Z-scan spectra revealed that all the samples possessed large positive nonlinear refraction and positive nonlinear absorption, exhibiting a self-focusing effect and reverse saturable absorption, respectively. The second molecular hyperpolarizabilities of the dyad in two conditions were measured to be 3.87 × 10−30 and 4.82 × 10−30 esu, respectively. All the results suggest that the azo-ZnPc dyad has potential in the field of nonlinear optics applications.