Tetrathiafulvalene-Fused Heterabuckybowl: Protonation-Induced Electron Transfer and Self-Sensitized Photooxidation

The electron donor tetrathiafulvalene (D-1) was fused onto the electron-rich heterabuckybowl trichalcogenasumanene (D-2) through an electron-deficient pyrazine unit (A) to give 1 c, 1 d, 2 c, and 2 d, featuring the D-1-A-D-2 structure. Both D-1 and D-2 play a pivotal role in intramolecular charge-transfer (ICT) transitions, consequently 1 c, 2 d, 2 c, and 2 d show a broad ICT band at 450-720 nm in steady state. They exhibit two charge-separated transient states, CS1 and CS2, that appear in sequence. CS1 has a short lifetime (542 fs), and the D-1 moiety on CS1 is in the radical cation state with an absorption maximum (lambda(max)) at 889 nm. CS1 then converts into CS2 (lambda(max), 1105 nm) through an ICT between D-1(.+) and D-2, affording D-1((1-delta).+) and D-2(delta.+). Compounds 1 c, 1 d, 2 c, and 2 d show protonation-induced intramolecular electron transfer that leads to absorption at lambda=700-1300 nm. Owing to the existence of an electron-rich C=C bond on the D-1 moiety and in situ generation of O-1(2) by the pyrazine-fused D-2 moiety, compounds 1 c, 1 d, 2 c, and 2 d display self-sensitized photooxidation in 50 s.