Porphyrin-anthraquinone dyads: Synthesis, spectroscopy and photochemistry

Free-base (H₂L²), copper(II) (CuL²) and zinc(II) (ZnL²) derivatives of a porphyrin-anthraquinone conjugate with an azomethine group separating the two photoactive subunits have been synthesized and characterized by mass (FAB), IR, UV-visible, ¹H NMR and ESR spectroscopic techniques and also by cyclic and differential pulse voltammetric methods. Analysis of the data reveals that the spectral and electrochemical properties of the individual chromophoric entities are retained and that there is no specific nπ-π interaction between the porphyrin and anthraquinone subunits. H₂L² and ZnL² are shown to exhibit substantial quenching (88-97%) of the porphyrin fluorescence compared to their corresponding monomeric analogues. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both derivatives. The fluorescence decays of porphyrin-anthraquinone conjugates are fit to 2/3 exponentials and indicate that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results are in good agreement with steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants are calculated, indicating k_ET values in the range of 1·1 × 10⁹ to 9·9 × 10¹⁰ s^-1 that are dependent upon the solvent.