Energetics of Photoinduced Electron Transfer in the Indole‐O₂ Cluster in Gas Phase: Possible Consequences for Photoexcited Tryptophan in Solution

Abstract

A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole+.O2- clusters in gas phase a neutral indole+.O2- charge-separated exciplex state is found at 2.25 +/- 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 +/- 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states 1Lb and 1La and the lower even below the first triplet state T1 of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan.O2 in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan*-O2 complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O2 and propose electron transfer to be the relevant process.

Throwing light on dark states of α-oligothiophenes of chain lengths 2 to 6: radical anion photoelectron spectroscopy and excited-state theory

“…5,6 Since the ionization energies (IE) of a-oligothiophenes are low 7,8 such magnetic momentum (spin) and electronic energy transfer could happen through an intermolecular charge transfer (CT) state as proposed by some of us for other systems. 9,10 Note that even if, upon irradiation, only a small fraction of the molecules undergoes photochemistry, after some time the device such as for example an organic solar cell will be destroyed.…”

Microsolvation of similar-sized aromatic molecules: Photoelectron spectroscopy of bithiophene–, azulene–, and naphthalene–water anion clusters

“…Molecular EAs are of interest because they can be used for calibration of theoretical calculations, for estimation of the energies of charge transfer states in gas phase 1,2 or for determination of LUMO energies. 3 For example in donorbridge-acceptor systems EAs and LUMO energies of acceptor and bridge are important to decide whether electron transfer is a superexchange or charge hopping mechanism.…”

“…Due to experimental and theoretical reasons this is only possible in mass-selected clusters with a relatively small number of solvent molecules. By spectroscopy one finds for example that in O 2 Ϫ ͑H 2 O͒ 4 the sites where the solvent molecules attach are influenced by a directed orbital interaction. 6 Clearly even more rigid anion-to-solvent binding is expected for strong hydrogen bonds and clearly a classical concept must fail.…”

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

“…49 In a previous work we already investigated the CT state of O 2 Áindole using PD-PES. 50 However, the results have been difficult to understand for a broad audience since the photoabsorption cross-section of O 2 is very small and as a result predominantly the CCT state was observed. In the interpretation of these former results we concentrated on a situation in a polar solvent and discussed charge solvation in the CCT sate and formation of free O 2…”