Tuning Deazaflavins Towards Highly Potent Reducing Photocatalysts Guided by Mechanistic Understanding - Enhancement of the Key Step by the Internal Heavy Atom Effect.

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo-induced electron transfer, in which their triplet state and semiquinone - the latter is formed from the former after electron transfer from a sacrificial electron donor - are key intermediates. Guided by mechanistic investigations aiming to increase intersystem crossing by the internal heavy atom effect and optimising the concentration conditions to avoid unproductive excited singlet reactions, we synthesised 5-aryldeazaflavins with Br or Cl substituents on different structural positions via a three-component reaction. Bromination of the deazaisoalloxazine core leads to almost 100 % triplet yield but causes photo-instability and enhances unproductive side reactions. Bromine on the 5-phenyl group in ortho position does not affect the photostability, increases the triplet yield, and allows its efficient usage in the photocatalytic dehalogenation of bromo- and chloroarenes with electron-donating methoxy and alkyl groups even under aerobic conditions. Reductive powers comparable to lithium are achieved.

Combining Flavin Photocatalysis and Organocatalysis: Metal-Free Aerobic Oxidation of Unactivated Benzylic Substrates.

We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential ( Eox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b ( E(2b*) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH.

Electronic excitations in Fischer-type Cr and W aminocarbene complexes: a combined ab initio and experimental study.

The influence of the substitution on the carbene ligand in the series of Fischer-type Cr and W aminocarbene complexes was studied experimentally by UV-vis spectroscopy and theoretically by comparative ab initio SA-CASSCF/MS-CASPT2 and TD-DFT methods. Both calculations interpreted the experimental UV-vis spectra and their variations caused by substitution effects well. TD-DFT analysis of individual transitions using electron density redistributions indicated that the variation of the absorption spectra due to substitution is accompanied by a change in the character of the low-lying excited states participating in the visible bands. Correlated MS-CASPT2 calculations confirmed the TD-DFT assignments of the lowest-lying transitions in the visible region almost quantitatively.