Photocatalysis

LED based NMR illumination device for mechanistic studies on photochemical reactions – Versatile and simple, yet surprisingly powerful

C. Feldmeier, H. Bartling, E. Riedle, R.M. Gschwind,

Jounral of Magnetic Resonance 2013, 232, 39-44

Web-Edition

An LED based illumination device for mechanistic studies on photochemical reactions by means of NMR spectroscopy is presented. The LEDs are directly switched by the NMR spectrometer with the help of a one-stage electronic circuit. This allows for continuous or alternatively pulsed operation of the LEDs. Continuous operation provides direct comparability with conditions in synthetic chemistry, in pulsed operation the short time light power can be enhanced ninefold. The LEDs are efficiently coupled to a 1000 μm core optical fiber guiding the light into the spectrometer by simply bringing it in close contact to the fiber. The tip of the fiber is roughened by sandblasting and thus emits light in a uniform and efficient way over the full length of the receiver coil. The combination of these techniques tremendously increases the amount of light brought into the NMR sample and makes LEDs an easy, versatile and handy light source for the in situ illumination of NMR samples allowing even for single millisecond time resolved Photo-CIDNP spectroscopy.

Aggregation Effects in Visible-Light Flavin Photocatalysts: Synthesis, Structure, and Catalytic Activity of 10-Arylflavins

J. Dadova, S. Kümmel, C. Feldmeier, J. Cibulkova, R. Pazout, J. Maixner, R.M. Gschwind, B. König, R. Cibulka

Chemistry A European Journal 2013, 19, 1066-1075

Web-Edition

A series of 10-arylflavins (10-phenyl-, 10-(2′,6′-dimethylphenyl)-, 10-(2′,6′-diethylphenyl)-, 10-(2′,6′-diisopropylphenyl)-, 10-(2′-tert-butylphenyl)-, and 10-(2′,6′-dimethylphenyl)-3-methylisoalloxazine (2 a–f)) was prepared as potentially nonaggregating flavin photocatalysts. The investigation of their structures in the crystalline phase combined with ¹H-DOSY NMR spectroscopic experiments in CD₃CN, CD₃CN/D₂O (1:1), and D₂O confirm the decreased ability of 10-arylflavins 2 to form aggregates relative to tetra-O-acetyl riboflavin (1). 10-Arylflavins 2 a–d do not interact by π–π interactions, which are restricted by the 10-phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3H⋅⋅⋅O hydrogen bonds were detected in their crystal structures. In the structure of 10-aryl-3-methylflavin (2 f) with a substituted N3 position, weak CH⋅⋅⋅O bonds and weak π–π interactions were found. 10-Arylflavins 2 were tested as photoredox catalysts for the aerial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (model reaction), thus showing higher efficiency relative to 1. The quantum yields of 4-methoxybenzyl alcohol oxidation reactions mediated by arylflavins 2 were higher by almost one order of magnitude relative to values in the presence of 1