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, Cs + 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 crPc and Zn II crPc the excited-state dynamics are those of the π-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 π-system, a deactivation event with a 22 ns lifetime was attributed to the decay of the 4 T 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 2 T precursor to the 4 T state or to the population of a CT state situated between 2 T and 2 S 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) Pcs. 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 Δ g ).