Improving the Visible Light Photocatalytic Aerobic Oxidation of Sulfides into Sulfoxides on Dye‐Sensitized TiO₂

Abstract

Visible light‐driven interfacial redox reactions during dye‐sensitized TiO2 are essential to meet the energy and environmental challenges in the form of dye‐sensitized solar cells and sensitized degradation of organic pollutants. Inspired by these prior successes, it is conceivable that the underlying principles of these processes can guide us to construct selective chemical transformations in which the control of selectivity is in higher demand. We attempt to improve the selectivity for sulfoxides during the visible light‐driven selective aerobic oxidation of sulfides in the merger of dye‐sensitized TiO2 photocatalysis with TEMPO [(2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl] catalysis. Thus, different factors like the TEMPO and its derivatives, initial O2 pressures, initial sulfide concentrations, solvents, light‐emitting diodes (LED) and TiO2 were examined to identify the critical factor for selectivity improvement. It was found that an easily synthesized anatase TiO2, sensitized by 0.33 mol % of alizarin red S, was significant in improving the selectivity for sulfoxides at high conversions of sulfides with the aid of 2 mol % of TEMPO as a redox mediator under blue LED irradiation. This work suggests that the engineering of the TiO2 materials can be a viable solution in improving the selectivity of visible light‐driven chemical transformations by dye‐sensitized TiO2 photocatalysis.

Benzodithiophenedione‐Based Conjugated Microporous Polymer Catalysts for Aerobic Oxidation Reactions Driven by Visible‐Light

“…The oxidation of thioanisole could occur in a variety of common solvent (acetonitrile, EtOH), which is differ from the other catalyst that only exhibit functions in several specific solvents. [17] As listed in Table 1, The oxidation of thioanisole catalyzed by CMP-BDD achieved a remarkable conversion (99 %) and excellent selectivity for methyl phenyl sulfoxide (99 %) in EtOH at room temperature, which are superior than most of the catalyst such as Ru(bpy) 3 Cl 2 , [12] Rose Bengal, [18] C-CMP [9] and CMPBDP [12] reported before. Control experiment conducted in darkness (without illumination), or without adding photocatalysts, or without O 2 led to only trace conversions, demonstrating the (Table S2, entries 13-15).…”

Photosensitive Hybrid Nanostructured Materials: The Big Challenges for Sunlight Capture

“…ARS was found to be more efficient than EY, thionine, and Flavin in benzyl alcohol oxidation [51], and than alizarin blue S and nuclear fast red in benzylamine oxidation [50]. The ARS-TiO 2 charge transfer pathway was proposed to be a direct injection rather than the indirect one typical of dyes [49]. The oxidation of sulfides was also performed on anthraquinones-sensitized TiO 2 (alizarin precursors), evidencing the key role of the chelation of Ti ions through the diol group for this family of compounds [52].…”

“…Alizarin (1,2-dihydroxy anthraquinone), a red dye naturally found in plant roots, is another common photosensitizer along with its derivatives, such as alizarin red S (ARS), containing a sulfonate group. Recent papers by Lang, Wang, Hao, and co-workers have dealt with TiO 2 /ARS systems for the selective aerobic oxidation of organic sulfides to sulfoxides [48,49], amines to imines [50], and alcohols to aldehydes or ketones [51] under visible light irradiation. Most of these reactions were carried out, taking advantage of TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) as a redox mediator, allowing to achieve excellent yields and selectivities and to improve dye stability ( Figure 4).…”

“…Indeed, in some cases, both pathways have a similar probability to be active, even if one can be kinetically favored. In addition, although dyes are commonly associated with indirect CT, some visible light-absorbing molecules are predicted to prefer a direct electron injection [49,69,100]. Direct photosensitization should result in a more efficient electron transfer to the conduction band; on the other hand, the back electron transfer (recombination) might also be favored [97].…”

“…Proposed mechanism for the visible-light-induced selective oxidation of sulfide into sulfoxide with air on alizarin red S (ARS)-sensitized TiO 2 with TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) as a redox mediator. Adapted and reprinted with permission from[49], Copyright (2018) Wiley.…”