The photoresponsive J-aggregation behaviors of a novel azobenzene-substituted zinc phthalocyanine (azo-ZnPc dyad) were studied by UV/vis, fluorescence, and (1)H 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 x 10(-30) and 4.82 x 10(-30) esu, respectively. All the results suggest that the azo-ZnPc dyad has potential in the field of nonlinear optics applications.
The absorption, fluorescence, and transient absorption spectra of Tetra(alpha-amino) zinc phthalocyanine, ZnPc(alpha-NH2)4, have been measured in polar solvents and compared with that of ZnPc(alpha-R)4 (R = H, NO2, OCH(CH3)2). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(alpha-NH2)4 exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(alpha-NH2)4. NMR indicates that intramolecular H-bonding makes atoms in NH2 actually coplanar with other elements of ZnPc(alpha-NH2)4. The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.
A novel family of thiophene-decorated phthalocyanines (M-TPcs) was efficiently synthesized, during which the key intermediate of these compounds was purified through a chemical approach. In the optimized geometries of M-TPcs, the peripherally linked thiophene rings are tilted from the Pc core, which oppose aggregation considering the mutual steric hindrance. However, Zn-TPc formed J-aggregates in many solvents while Ni-TPc and Cu-TPc did not. Octaphenyl-substituted zinc Pc (Zn-PPc) showed similar J-aggregation behavior as Zn-TPc. This unusual J-aggregation was attributed to the conformational planarization of the corresponding molecules.
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