Crown ether substituted monomeric and cofacial dimeric metallophthalocyanines. 1. Photophysical studies of the free base, zinc(II), and copper(II) variants
Av. Nikolaitchik et al., Crown ether substituted monomeric and cofacial dimeric metallophthalocyanines. 1. Photophysical studies of the free base, zinc(II), and copper(II) variants, J PHYS CH A, 103(38), 1999, pp. 7587-7596
Metallophthalocyanines have been prepared with 18-crown-6 residues at the f
our peripheral benzo sites (McrPc). Metal centers employed have been H-2 (f
ree base), Zn(II), and Cu(II). In ethanol solution containing potassium ace
tate, 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 arrang
ement. This dimerization results in spectral shifts to the blue. Photophysi
cal 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 th
e pi-macrocycle, and dimer formation caused no major changes except for inc
reasing the rate constants of the excited-state deactivation. Such increase
s 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 cofa
cial 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. Thu
s, 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. Tr
iplet-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 o
f singlet oxygen, O-2((1)Delta(g)).