Ap. Pelliccioli et al., Synthesis and excited state dynamics of mu-oxo group IV metal phthalocyanine dimers: A laser photoexcitation study, J PHYS CH A, 105(10), 2001, pp. 1757-1766
The synthesis of two metal phthalocyanine monomers, GePc[OSi(n-C6H13)(3)](2
) and SnPc[OSi(n-C6H13)(3)](2), and two mu -oxo-bridged dimers, (n-C6H13)(3
)SiOSiPcOGePcOSi(n-C6H13)(3) and (n-C6H13)(3)SiOSiPcOSnPcOH, are described.
The ground-state absorption spectra and excited-state dynamics of these co
mpounds together with those of (n-C6H13)(3)SiOSiPcOSiPcOSi(n-C6H13)(3) have
been measured. The absorption spectra of the dimers are blue shifted with
respect to the monomers and whereas the latter exhibit a strong fluorescenc
e in the visible, the dimers show only a weak emission in the near-IR. Thes
e-observations are characteristic for the presence of exciton interactions
in all three dimers, as had been reported earlier for the Si-O-Si dimer. Su
bnanosecond laser flash photolysis experiments on all five compounds yielde
d triplet-triplet absorption spectra, triplet lifetimes, triplet quantum yi
elds, and bimolecular rate constants for quenching of the triplet states by
O-2. The triplet quantum yields and lifetime for the monomers and the dime
rs were fairly similar. The oxygen quenching rate constants indicate a diff
usion-controlled energy transfer process for the monomers; but in the case
of the dimers, these rate constants are up to 2 orders of magnitude less. S
inglet oxygen quantum yields were measured. These are close to the triplet
yields for the monomers, but markedly less for the dimers. These results we
re interpreted as resulting from reversible energy transfer in the dimers i
n competition with quenching to the ground-state surface. Reversible energy
transfer with molecular oxygen occurs because the dimer triplet energies a
re significantly lower than those of the monomers, probably because of char
ge resonance interactions between the closely lying pi -planes. The reversi
ble energy transfer kinetics allow estimation of the triplet energies which
are 1-2 kcal mol(-1) lower than the energy gap in oxygen (22.5 kcal mol(-1
)). Ultrafast pump-probe spectrometry measurements were used to investigate
the early dynamic events in the dimers. Tt has been determined that the ra
te constant for intersystem crossing between the dimer lower exciton state
and the triplet state was near 10(-10) s, varying somewhat with central met
al. Experiments at high time resolution indicated that the lower exciton st
ate is formed initially in a torsionally excited state, the cooling of whic
h has a lifetime of about 10 ps.