Photoexcited State Properties of Silicon Phthalocyanine Monomer, Dimer, and Trimer

Properties of the excited states of silicon phthalocyanine monomer and its coplanar-stacked dimer and trimer have been investigated. Absorption bands of the oligomers showed blue-shift due to exciton coupling. Weak fluorescence bands of the dimer and trimer appeared in the near-IR region. The lifetimes of the singlet excited states of the dimer and trimer were estimated to be 196 and 53 ps, respectively, from the absorption-time profiles of the singlet excited states. The T–T absorption bands showed decays according to the intermolecular T–T annihilation processes. The rate constants for the intermolecular T–T annihilation processes were one-order smaller than the diffusion-limiting rates of the solvents, indicating some steric hindrance by the substituted group at the axial position. The quantum yields for the intersystem crossing processes of the dimer and trimer were quite small compared to that of the monomer. These findings indicate the extensive contribution of the non-radiative processes in the excited states of the dimer and trimer. The energy transfer rate constants between of the triplet excited phthalocyanines and O2 decreased in the order of monomer > dimer > trimer. The small rate constants of the dimer and trimers can be attributed to the lower triplet energy levels of the dimer and trimer compared to that of the monomer.