Saturation of Absorption and Fluorescence in Solutions of Phthalocyanines

Abstract

Lifetimes of the excited states of phthalocyanines are studied by the irradiation of solutions with a Q-switched ruby laser. By detailed measurements of the saturation of fluorescence and absorption with increasing-pulse laser power, the radiative and the nonradiative lifetimes of S1 as well as the time constants for the S1 → T1 and T1 → S0 transitions are determined. The transition between the ground state and the first excited singlet state in the metal-free as well as the metallophthalocyanine saturates at laser pulse flux power densities of 0.1 to 0.15 MW/cm2. A complete transition of the molecular system from a ground state, S0, into a triplet state, T1, is observed in the case of the metal-free phthalocyanine H2Pc. The triplet state has a relaxation time to the ground state of 1.0 μsec at 300°K and 150 μsec at 77°K. The absence of fluorescence in VOPc and CuPc implies that a third state also is involved in the saturation of the absorption of the metallophthalocyanines. In these solutions, the saturation of absorption recovers with a relaxation time of less than 20 nsec. All the data obtained at room temperature is consistent with a three energy-level model. The lifetime of the first excited singlet state, S1, is determined as ∼2.0 nsec for all Pc's. The radiative lifetime of S1 and the S1 → T1 intersystem crossing time for H2Pc are estimated from the measurements to be about 6 and 10 nsec, respectively.