Improved Flavin-Based Catalytic Photooxidation of Alcohols through Intersystem Crossing Rate Enhancement

Abstract

The triplet excited-state formation efficiency in a flavin derivative was increased by the introduction of iodine into the molecular framework. The transient absorption measurements showed that the intersystem crossing rate was 1.1 × 10(10) s(-1), significantly faster than in the parent flavin compound. Furthermore, the photocatalytic efficiency of iodoflavin was evaluated using the oxidation of benzyl alcohol as a model reaction. The benzaldehyde product yields were higher when iodoflavin was used as a photocatalyst, showing that the increased triplet yield directly translates into improved photocatalysis. The iodoflavin catalyst also allowed the use of higher substrate concentrations (since the undesired electron transfer from singlet excited state was minimized), which is expected to improve the practical aspects of photocatalysis by flavins.

Molecular Editing of Flavins for Catalysis

“…In 2016, Glusac and co-workers reported C9-iodoflavin 29 which benefits from a drastically faster intersystem crossing (ISC) to the triplet state after photoexcitation (Figure 7A). 69 They demonstrated that this heavy atom effect made 29 a more active catalyst for the oxidation of benzyl alcohol to benzaldehyde when compared to the unsubstituted analogue. Similar observations were made by Zhao, Guo, and co-workers who reported the C7,C8-dibrominated flavin 30, again with the intention to benefit from the heavy atom effect (Figure 7B).…”

Tuning Riboflavin Derivatives For Photodynamic Inactivation of Pathogens

“…47 Heavy atom substitution has been shown to improve both the rate of ISC and the singlet oxygen quantum yield (ФΔ) in flavin derivatives used for synthetic photooxidation reactions. 48,49 Although the heavy-atom effect has been used to boost photodynamic efficacy for other PS dyes in PDI applications, it has not yet been explored for flavin derivatives. 50,51 To afford both methylated (F1-2) and brominated derivatives (F3-4), a Boc-protected ethylene amino component was first installed to the methylated or brominated arene core prior to cyclisation of the isoalloxazine ring system.…”

Nature is the Cure: Engineering Natural Redox Cofactors for Biomimetic and Bioinspired Catalysis

“…This dual catalysis involves a photooxidation cycle and a dark organocatalytic cycle, and uses a specific synthetic flavinium salt that exhibits a high photoexcited oxidation potential above +2.5 V (Figure ). The use of flavin derivatives as photocatalysts has led to the modification of the core scaffold with the goal of improving the photophysical properties such as rate of intersystem crossing and long‐lived charge separation (Figure ) …”

“…[49] This dual catalysis involves a photooxidation cycle and a dark organocatalytic cycle, and uses a specific synthetic flavinium salt that exhibits a high photoexcited oxidation potential above + 2.5 V (Figure 4). The use of flavin derivatives as photocatalysts has led to the modification of the core scaffold with the goal of improving the photophysical properties such as rate of intersystem crossing [50] and long-lived charge separation (Figure 4). [51] Beyond dual catalysis, supramolecular or nanoscale systems can be achieved through modification of the flavin core scaffold with dedicated subunits to combine flavin-based features with properties such as anion-binding with crown ethers, [52,53] and substrate-binding sites with cyclodextrin [54] and zinc(II)-cyclen.…”