Catalytic Photooxidation of 4-Methoxybenzyl Alcohol with a Flavin-Zinc(II)-Cyclen Complex

Porphycene-mediated photooxidation of benzylamines by visible light

A variety of primary and secondary benzylic amines were oxidized efficiently to N-benzylidenebenzylamines and imines, respectively, using 2,7,12,17-tetrapropylporphycene (H(2)TPrPc) photocatalyst and blue light emitting diodes (LEDs). The photooxidation of 4-methoxybenzylamine in the presence of H(2)TPrPc and its tin(IV) complex Sn(TPrPc)Cl(2) was studied in detail in order to show that operating mechanisms can be different depending on the photosensitizer type. Two experiments involving solvent deuterium isotope effect and competitive quenching with DABCO provide evidence for the singlet oxygen mechanism as the major pathway in the H(2)TPrPc-catalyzed reaction and the predominance of the direct electron transfer from the photoexcited dye to the amine when the Sn(TPrPc)Cl(2) complex was used as a photocatalyst.

Synthesis of rigidified flavin-guanidinium ion conjugates and investigation of their photocatalytic properties

Flavin chromophores can mediate redox reactions upon irradiation by blue light. In an attempt to increase their catalytic efficacy, flavin derivatives bearing a guanidinium ion as oxoanion binding site were prepared. Chromophore and substrate binding site are linked by a rigid Kemp's acid structure. The molecular structure of the new flavins was confirmed by an X-ray structure analysis and their photocatalytic activity was investigated in benzyl ester cleavage, nitroarene reduction and a Diels-Alder reaction. The modified flavins photocatalyze the reactions, but the introduced substrate binding site does not enhance their performance.

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.

Signal amplification and transduction by photo-activated catalysis

A simple flavin-based catalytic system is able to transform light into chemical output with amplified response utilizing a Cu(I)-catalyzed cycloaddition reaction.