Hans L. J. Bäckström

Bio: Hans L. J. Bäckström is an academic researcher. The author has contributed to research in topics: Quenching (fluorescence) & Phosphorescence. The author has an hindex of 6, co-authored 9 publications receiving 370 citations.

Influence of Energy Transfer by the Exchange Mechanism on Donor Luminescence

Abstract: The decay of donor luminescence in a rigid solution when modified by electronic energy transfer by the exchange mechanism is treated theoretically. The rate constant for the elementary process of energy transfer is taken to be of the Dexter form, const exp(−2R/L), where R is the donor—acceptor distance and L is a positive constant. Calculations are made of the yield and decay time of the donor luminescence as functions of the acceptor concentration. The resulting relationship among the above quantities enables one to analyze experimental data in a quantitative manner, and thereby to obtain information about an intermolecular exchange interaction. As an example of such an analysis, Ermolaev's data on triplet—triplet transfer between some aromatic molecules are compared with our results, and very good agreement is found with a choice of the single parameter L.

Mechanistic insight into the photoredox catalysis of anti-markovnikov alkene hydrofunctionalization reactions.

Abstract: We describe our efforts to understand the key mechanistic aspects of the previously reported alkene hydrofunctionalization reactions using 9-mesityl-10-methylacridinium (Mes-Acr+) as a photoredox catalyst. Importantly, we are able to detect alkene cation radical intermediates, and confirm that phenylthiyl radical is capable of oxidizing the persistent acridinyl radical in a fast process that unites the catalytic activity of the photoredox and hydrogen atom transfer (HAT) manifolds. Additionally, we present evidence that diphenyl disulfide ((PhS)2) operates on a common catalytic cycle with thiophenol (PhSH) by way of photolytic cleaveage of the disulfide bond. Transition structure analysis of the HAT step using DFT reveals that the activation barrier for H atom donation from PhSH is significantly lower than 2-phenylmalononitrile (PMN) due to structural reorganization. In the early stages of the reaction, Mes-Acr+ is observed to engage in off-cycle adduct formation, presumably as buildup of PhS− becomes sig...

Role of Singlet Excited States of Molecular Oxygen in the Quenching of Organic Triplet States

Abstract: The quenching of triplet‐state molecules by molecular oxygen is examined theoretically. Two different quenching processes are considered: namely, (i) quenching by transfer of electronic excitation energy from the triplet state of the donor molecule to oxygen, resulting in a ground‐state donor molecule and an electronically excited singlet‐state oxygen molecule; and (ii) quenching by enhancement of intersystem crossing in the donor molecule, resulting in a vibrationally excited donor molecule in its ground electronic state and a ground‐state molecule. The relative importance of these two quenching processes is determined almost entirely by Franck—Condon factors, rather than by electronic factors. From our analysis we conclude that quenching by transfer of electronic excitation energy from the triplet‐state molecule to oxygen is usually 100–1000 times faster than quenching by enhanced intersystem crossing.Exchange interactions appear to be less important in promoting the radiationless transitions than do in...