Saturation of Absorption and Fluorescence in Solutions of Phthalocyanines

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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 S 1 as well as the time constants for the S 1 → T 1 and T 1 → S 0 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,S 0, into a triplet state, T 1, 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, S 1, is determined as ∼2.0 nsec for all Pc's. The radiative lifetime of S 1 and the S 1 → T 1 intersystem crossing time for H2Pc are estimated from the measurements to be about 6 and 10 nsec, respectively.

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