Photoinduced Dynamics of Bis-dipyrrinato-palladium(II) and Porphodimethenato-palladium(II) Complexes: Governing Near Infrared Phosphorescence by Structural Restriction.

Although superficially similar, the bis-dipyrrinato-palladium(II) complex 1 and the bridged porphodimethenato-palladium(II) complex 2 possess dramatically different structures in the ground state (proved by X-ray structure analysis) and in the singlet and triplet excited states (calculated by density functional theory methods). While complex 2 is rather rigid, complex 1 undergoes a major structural reorganization in the excited state to yield a disphenoidal (seesaw) triplet state. The dynamics of the excited states were probed by transient absorption spectroscopy with femtosecond and nanosecond time resolution and with fluorescence upconversion and yield intersystem crossing rate constants of ca. (13-16 ps). The observation of significant near infrared phosphorescence in complex 2 but the absence of any emission in complex 1 in fluid solution could be rationalized by the structural reorganization of 1 which results in a nonemissive triplet metal centered state.