Crown ether substituted monomeric and cofacial dimeric metallophthalocyanines. 1. Photophysical studies of the free base, zinc(II), and copper(II) variants

Metallophthalocyanines have been prepared with 18-crown-6 residues at the four peripheral benzo sites (McrPc). Metal centers employed have been H-2 (free base), Zn(II), and Cu(II). In ethanol solution containing potassium acetate, such species incorporate K+ cations;into the crowns, one K+ per crown, and are monomeric in nature. When cesium acetate is present, Csf cations complex with a pair of crown residues, resulting in cofacial dimer species (McrPcD) in which one Cs+ ion links two ethers in a kind of sandwich arrangement. This dimerization results in spectral shifts to the blue. Photophysical examinations of these monomeric and dimeric entities have been carried out. For H(2)crPe and Zn(II)crPc the excited-state dynamics are those of the pi-macrocycle, and dimer formation caused no major changes except for increasing the rate constants of the excited-state deactivation. Such increases are anticipated owing to the proximity of the lower exciton state and the ground state. For Cu(II)crPc where now a d(9) metal is present within the ct-system, a deactivation event with a 22 ns lifetime was attributed to the decay of the T-4 State of the complex. The corresponding state of the cofacial dimer had a lifetime of 5.7 ns. Ultrafast experiments with ca. 500 fs resolution provided evidence of earlier processes in the Cu(II) system. Thus, in the monomer, a 2.9 ps lifetime event preceded the quartet-state decay. This may be attributable either to the decay of the T-2 precursor to the T-4 State or to the population of a CT state situated between T-2 and S-2(0) in energy. Similar early-time behavior was noted for the Cu(II) dimer. Triplet-state properties are reported for the monomeric and dimeric free base and Zn(II) Pea. Notably, the bimolecular rate constants for O-2 quenching were lower for the dimers compared to the monomers. This can be understood if the dimerization yields a triplet state with an energy lower than that of singlet oxygen, O-2((1)Delta(g)).