Fluorescence of phthalocyanines: Emission from an upper excited state

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Abstract
The weak (φf < 103) fluorescence at around 430 nm of the S2 upper excited singlet state of metal-free phthalocyanine and metallophthalocyanines is presented. Polarization measurements indicate that the emission is short-lived (< 800 ps) contrary to the intense (φf > 0.3) normal emission at around 700 nm originating from S1 and having a lifetime in the 4.1 to 10.6 ns range, depending on the solvent. The short wavelength emitting S2 excited state has been populated by a two photon absorption process using the excitation light at 695 nm of a pulsed ruby laser. This process is shown not to involve the triplet state but the following stepwise two photon absorption process: