Guest Seminar - Design of Purely Organic Photocatalysts for Visible-light Driven Polymerizations 

The development of new chemical reactions using photoredox catalysts (PCs) is one of the most important topics in the field of organic and polymer synthesis since photoredox-mediated reactions are “green” and “mild” alternatives to thermal processes, and structurally unusual organic scaffolds can be built under extremely mild conditions using these catalysts. A variety of photoredox-mediated reactions have been actively developed in which organometallic complexes, including Ru(II) and Ir(III), have been mostly employed as PCs. However, concerns about toxicity, cost, precious-metal sustainability, and trace-metal contamination have limited their widespread use, especially in biomedical and electronic applications. In addition, the rather narrow redox-potential ranges and limited structural diversities of these transition-metal complexes limit their scope.
Due to their broad structural diversity and benign environmental profiles, purely organic PCs provide attractive alternatives to transition-metal-based catalysts. However, the full potential of organic PCs has not been realized, as the criteria for their design are not well established. Therefore, the development of new transformations using organic PCs is limited to a few organic dyes commonly used for imaging purposes and whose parameters are not straightforwardly tuned through structural alternation.
In this talk, we propose a general strategy for the design of purely organic PCs.[1] Strongly twisted donor–acceptor structures are introduced as a general scaffold design for these PCs. The charge transfer (CT) characteristics of the lowest excited states of the designed PCs greatly promote the generation of the lowest triplet excited states (T1) and allow systematic control of the essential catalyst parameters over broad ranges, thereby facilitating efficient catalytic performance. Through combined computational and experimental studies, we provide a flow chart that facilitates the computationally directed rational design of organic PCs for the development of new photoredox-mediated reactions. Based on this strategy, highly efficient organic PCs were discovered that addressed important issues associated with photoredox-mediated atom transfer radical polymerization (ATRP),[1-4] reversible addition-fragmentation chain transfer (RAFT),[5-7] and free radical polymerization.[8,9]