PHOTOCHEMISTRY
Categories of PCs
There is always a quest to conduct photochemical reactions in the visible region for milder reaction conditions , eco-friendliness , and reliability . Interestingly , the issue is addressed through the usage of PCs , their propensity to harness light energy , getting excited to reactive species ( Figure 2 ) and manoeuvering electron mobility by the redox potential of reaction media to construct new chemical bonds via heat transfer , isomerisation , degradation , photosensitisation , reduction and oxidation . Organic PCs – cyanoarenes , benzophenones , quinones , pyryliums , etc . - are well covered in review published by Nicewicz et al . in 2016 . 1 There were sporadic reports documented earlier in metal PCs , but this concept was
Figure 2 – SET cycle & organometallic catalysts revolutionised with key publications from MacMillan et al ., Yoon et al . and Stephenson et al . 2-4 . Most of the commonly employed visible photocatalysts are polypyridyl complexes of ruthenium and iridium ( Figure 2 ). These complexes provide a unique proposition , with a lower energy of ligand-centered π * orbital in comparison to the metal-centred eg orbital . This has benefitted both organic and semiconductor photocatalysis .
Scope of chemical conversions
The most promising influence of visible photoredox catalysis is on exceptional bond formations like C ( sp3 ) -C ( sp3 ), C ( sp3 ) -C ( sp2 ) couplings and so on , that are not amenable under conventional conditions , and its ability to operate in redox neutral conditions . Various organic transformations , including oxidation , reductions , cyclisations , dehalogenations , cycloadditions , C-H arylations , trifluoromethylation and decarboxylative couplings , have been investigated to establish its versatility and diversity . New advances in PCs with ‘ dual catalysis ’ or metallaphotoredox catalysis by pioneering contribution of MacMillan , Doyle and others in cross-coupling reactions have been reported . 5 These successful transformations have made PCs a preferred tool for late-stage functionalisation on advanced drug-like molecules for building focused libraries and guiding SPR modifications to address ADMEtox properties .
Photocatalysis platforms
Despite the tremendous advances in this field , the chemical industry is still conservative in assimilating this technology for scale-up and in industrial settings . However , advanced photoreactors ( parallel photoscreening and narrow wavelength platforms ) and process intensification methods ( i . e . photoflow reactors ) are creating new prospects for PC technology acquisition at commercial scales to make it amenable for process efficiency and safety ( Figure 3 ). Another key issue is the availability of precious rare-earth metals like ruthenium and iridium , which are costly and limited in the Earth ’ s crust , and could be subject to supply chain disruption in the future . Nevertheless , the systematic development of organic PCs with computer-aided design is paving the way for greater resilience and effectiveness in PCs .
Ru ( bpy ) 32 + Ir ( dp )( bpy ) 23 +
Application in chemical biology
Photo-catalysis has expanded its horizon in chemical biology applications as well , for instance :
• Photo-uncaging of a microtubuletargeted rigidin analogue in hypoxia cancer cells using green light 6
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