Photoinduced electron transfer in zinc naphthalocyanine-naphthalenediimide supramolecular dyads

Photoinduced electron transfer was studied in self-assembled donor-acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI-py:ZnNc (1) and NDI(CH₂)₂-py:ZnNc (2) self-assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10⁴ and 2.2×10⁴ M^-1, respectively, from the steady-state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine-appended NDI to ZnNc in toluene. The energy-level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc ( ¹ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge-separation of 1 and 2 were found to be 2.2×10¹⁰ and 4.4×10⁹ s ^-1, respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge-separated states were found to be 8.50×10⁸ s ^-1 (1.2 ns) for 1 and 1.90×10⁸ s^-1 (5.3 ns) for 2. These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self-assembled NDI-py:ZnNc and NDI(CH₂)₂-py:ZnNc dyads are useful as photosynthetic models. Long live the dyads: Efficient electron transfer and relatively long-lived charge-separated states are achieved of light-harvesting zinc naphthalocyanine:naphthalenediimide supramolecular dyads utilizing the femtosecond transient absorption technique in toluene.