Direct spectroscopy of contact charge transfer states: Possible consequences for tryptophan excited-state deactivation pathways by O2 and formation of reactive oxygen species

Abstract

This paper demonstrates that photodetachment photoelectron spectroscopy (PD-PES) of non-covalently bound heteromolecular anion clusters allows direct access to neutral contact charge transfer (CCT) states. The 2,2′-bithiophene·indole cluster is investigated as a model system in such a way that both chromophores have similar optical excitation cross-sections and hence the PD-PES shows the signature of both molecules. For the complexes O₂·stilbene, O₂·indole and O₂·N-methylindole the optical cross-sections of the conjugated chromophores are much larger than that of O₂. This forces most of the anion-to-neutral photoexcitation into the CCT state. The CCT states lie below S₁ for 2,2′-bithiophene·indole and O₂·stilbene or even below T₁ for O₂·indole and O₂·N-methylindole. Significant differences are found between the PD-PES of O₂ with indole and N-methylindole indicating that different collision sites may have different CCT state energies and as a result different ¹Δ_g oxygen formation efficiencies. We discuss the possible consequences of the energetics and the geometry changes for the excited-state deactivation of tryptophan via the CCT state O₂⁻·tryptophan⁺.