Synthesis and Excited State Dynamics of μ-Oxo Group IV Metal Phthalocyanine Dimers: A Laser Photoexcitation Study

The synthesis of two metal phthalocyanine monomers, GePc[OSi( n -C 6 H 13 ) 3 ] 2 and SnPc[OSi( n -C 6 H 13 ) 3 ] 2, and two μ-oxo-bridged dimers, ( n -C 6 H 13 ) 3 SiOSiPcOGePcOSi( n -C 6 H 13 ) 3 and ( n -C 6 H 13 ) 3 SiOSiPcOSnPcOH, are described. The ground-state absorption spectra and excited-state dynamics of these compounds together with those of ( n -C 6 H 13 ) 3 SiOSiPcOSiPcOSi( n -C 6 H 13 ) 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 fluorescence in the visible, the dimers show only a weak emission in the near-IR. These observations are characteristic for the presence of exciton interactions in all three dimers, as had been reported earlier for the Si−O−Si dimer. Subnanosecond laser flash photolysis experiments on all five compounds yielded triplet−triplet absorption spectra, triplet lifetimes, triplet quantum yields, and bimolecular rate constants for quenching of the triplet states by O 2 . The triplet quantum yields and lifetime for the monomers and the dimers were fairly similar. The oxygen quenching rate constants indicate a diffusion-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. Singlet oxygen quantum yields were measured. These are close to the triplet yields for the monomers, but markedly less for the dimers. These results were interpreted as resulting from reversible energy transfer in the dimers in competition with quenching to the ground-state surface. Reversible energy transfer with molecular oxygen occurs because the dimer triplet energies are significantly lower than those of the monomers, probably because of charge resonance interactions between the closely lying π-planes. The reversible 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. It has been determined that the rate constant for intersystem crossing between the dimer lower exciton state and the triplet state was near 10 -10 s, varying somewhat with central metal. Experiments at high time resolution indicated that the lower exciton state is formed initially in a torsionally excited state, the cooling of which has a lifetime of about 10 ps.