Synthesis and excited state dynamics of μ-oxo group IV metal phthalocyanine oligomers:: Trimers and tetramers

Two mu-oxo silicon phthalocyanines have been synthesized and subjected to photophysical examination. The results have been compared to the monomeric and dimeric systems published previously. The Q-bands in the ground state absorption spectra undergo a blue shift with respect to the monomer due to an excitonic interaction of the transition dipoles of the coplanar silicon phthalocyanine units having central symmetry along the Si-O-Si axis, with no significant molecular orbital overlap. This excitonic interaction generates a new set of excited states. The Q-band shift (with respect to the monomer) is greater as the number of phthalocyanine rings is increased consistent with the molecular exciton model of a linear polymer. On suprananosecond time scales, photoexcitation generated optical absorptions of triplet states that decayed with intrinsic lifetimes in the tens of microseconds region. These were quenched in the presence of O-2 with bimolecular rate constants for the trimer and tetramer of near 2 x 10(6) M-1 s(-1), indicating that the T-1 states of the trimer and tetramer lie significantly below the energy of the (1)Delta(g) state of oxygen (22.5 kcal mol(-1)). Estimates of the energy of T, generated values of approximately 17 kcal mol(-1) for both compounds, some 2 kcal mol(-1) lower than that calculated for the dimer. Photon absorption results in the formation of the highest lying exciton state, which deactivates to the lowest exciton state, which subsequently undergoes intersystem crossing to the triplet manifold. The ultrafast pump-probe results demonstrated that part of the lowest exciton state population was complete within the instrument rise time, whereas other populations required several picoseconds to develop, probably on account of the requirement for torsional/vibrational relaxation of excited state populations produced from ground state molecules having lower symmetry than D-4h.