Photoinduced Electron Transfer Reactions in Zeolite-Based Donor-Acceptor and Donor-Donor-Acceptor Diads and Triads

Electron transfer (ET) reactions in a series of donor-acceptor (D-A) molecules (2, 2ʹ-bipyridine)₂Ru(4-CH₃-2,ʹ-bipyridine-4ʹ)-(CH₂)_n-(4, 4ʹ-bipyridinium-CH₃)⁴⁺ (n = 2-5, 7, 8) exchanged onto the surface of large-pore zeolites (Y, L, and mordenite) were studied in suspensions by nanosecond flash photolysis/transient diffuse reflectance techniques. From solid state CP-MAS spectra of ¹³C-labeled compounds, it was established that the D-A molecules occupy surface sites in which the A end is occluded by the zeolite channels, while the size-excluded D end is exposed. The rate of forward ET reactions from photoexcited D to A decreases with increasing spacer length. The back ET reaction is approximately 10⁵ times slower for D—A diads on the zeolite surface than in solution. Slowing of the back ET rate and a maximum in charge separation quantum yields at n = 4, 5 are attributed to lateral charge transfer diffusion on the zeolite surface to form an intermolecular charge-separated state. Addition of size-or charge-excluded electron donors (Dʹ) to the suspension gives a Dʹ-D-A triad, in which the initial ET reaction can be between D and A, or Dʹ and D, depending on energetics and the spacer length n. In both cases a long-lived charge-separated state is formed between Dʹ and A.