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

Abstract

The electron donor tetrathiafulvalene (D₁ ) was fused onto the electron-rich heterabuckybowl trichalcogenasumanene (D₂ ) through an electron-deficient pyrazine unit (A) to give 1 c, 1 d, 2 c, and 2 d, featuring the D₁ -A-D₂ structure. Both D₁ and D₂ 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 D₁ moiety on CS¹ is in the radical cation state with an absorption maximum (λ_max ) at 889 nm. CS¹ then converts into CS² (λ_max , 1105 nm) through an ICT between D₁ ^.+ and D₂ , affording D₁ ^(1-δ).+ and D₂ ^δ.+ . Compounds 1 c, 1 d, 2 c, and 2 d show protonation-induced intramolecular electron transfer that leads to absorption at λ=700-1300 nm. Owing to the existence of an electron-rich C=C bond on the D₁ moiety and in situ generation of ¹ O₂ by the pyrazine-fused D₂ moiety, compounds 1 c, 1 d, 2 c, and 2 d display self-sensitized photooxidation in 50 s.

Keywords: conjugation; donor-acceptor systems; electron transfer; heterabuckybowls; photochemistry.