Excited-state dynamics of porphyrin-naphthalenediimide-porphyrin triads

The excited-state dynamics of two triads consisting of a naphthalenediimide (cNDI) substituted at the core by two zinc (ZnP) or free-base tetraphenylporphyrins (FbP) was investigated by ultrafast fluorescence and transient absorption spectroscopy. The electronic absorption spectra of the triads are almost the composites of those of the constituents, pointing to a weak electronic coupling and to a localization of the excitation energy on one of the porphyrins. In cyclohexane, the excited-state dynamics of the triads are essentially the same as those of the individual porphyrins, with the exception of the Soret emission of the ZnP triad, whose lifetime exhibits a more than 10 fold shortening compared to ZnP. A similarly ultrafast fluorescence decay was measured in tetrahydrofuran and benzonitrile. In these two solvents, charge separation from the excited porphyrin to the cNDI was found to take place with similar to 1 ps and similar to 25 ps time constants in the ZnP and FbP triads, respectively. The build up of the charge-separated state population in the ZnP triad is independent of the excitation wavelength, indicating that charge separation takes place from the lowest singlet excited state. Charge recombination occurs with a time constant of around 8 ps in both triads, i.e. it is slower than charge separation in the ZnP triad but faster in the FbP triad. These differences are rationalized in terms of the driving forces for charge separation and recombination.