Organic Photocatalyst Utilizing Triplet Excited States for Highly Efficient Visible-Light-Driven Polymerizations

Abstract

ConspectusUltraviolet (UV) light has traditionally been used to drive photochemical organic transformations, mainly due to the limited visible-light absorption of most organic molecules. However, the high energy associated with UV light often causes undesirable side reactions. In the late 2000s, MacMillan, Yoon, and Stephenson pioneered the use of visible light in conjunction with photocatalysts (PCs) to initiate organic transformations. This innovative approach overcame the limitations of UV light by utilizing visible-light-absorbing PCs in their photoexcited states for electron or energy transfer, generating reactive radical species and promoting the photoreactions. Furthermore, while the photocatalysis has predominantly relied on transition-metal complexes, concerns over the potential toxicity, cost, and sustainability of these metals have driven the development of organic PCs. These organic PCs eliminate the need for metal removal, offer structural diversity, and enable tuning of their properties, thus paving the way for the creation of a tailored library of PCs.In recent decades, significant advancements have been made in the development of novel organic PCs with diverse scaffolds, with a notable example being the work of Zhang et al. in 2016. They demonstrated that cyanoarene analogues, originally developed by Adachi et al. for thermally activated delayed fluorescence (TADF) in organic light-emitting diodes, could function effectively as PCs. Building on these insights, we developed a PC design platform featuring TADF compounds with twisted donor-acceptor structures, which paved the way for new PC discoveries. We showcased these PCs' ability (i) to generate long-lived lowest triplet excited (T₁) states and (ii) to tune redox potentials by independently modifying donor and acceptor moieties. Through this platform, we discovered PCs with varying redox potentials and the capability to effectively populate T₁ states, establishing structure-property relationships within our PC library and creating PC selection criteria for targeted reactions. Specifically, we tailored PCs for highly efficient reversible-deactivation radical polymerizations, including organocatalyzed atom transfer radical polymerization, photoinduced electron/energy transfer reversible addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization with dual photoredox/copper catalysis as well as rapid free radical polymerizations. These advancements have also facilitated the development of functionalized, visible-light-cured adhesives for advanced display technologies. Furthermore, we investigated the origins of the exceptional catalytic performance of these PCs through comprehensive mechanistic studies, including electrochemical and photophysical measurements, quantum chemical calculations, and kinetics simulations. Specifically, we studied the formation and degradation of key PC intermediates in photocatalytic dehalogenations of alkyl and aryl halides. Our findings revealed a distinctive photodegradation pattern in the cyanoarene-based PCs, which significantly impact their catalytic efficiency in the reaction. Additionally, this discovery led us to introduce a concept of beneficial PC degradation for the first time.Over the past decades, organic photocatalysis based on the T₁ state has become central to polymerization and organic synthesis. Utilizing our PC design platform, we have developed novel PCs and catalytic systems that enhance the overall efficiency of various organic transformations. In this overview of our contributions to visible-light-driven organic photocatalysis, we highlight the role of the T₁ state in broadening applications through mechanistic analysis, enabling more sustainable transformations.

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(a) Schematic energy and Jablonski diagrams of strongly twisted donor–acceptor compounds. Calculated HOMO and LUMO topologies of 4DP-IPN are shown (top). D, donor moiety; A, acceptor moiety; LE, locally excited; CT, charge-transfer; IC, internal conversion; Flour., fluorescence; ISC, intersystem crossing; Phos., phosphorescence; ΔE _ST, energy gap between singlet and triplet states; δ, first-order mixing coefficient between singlet and triplet states; ¹ Ψ, singlet wave functions; ³ Ψ, triplet wave functions; H _SO, spin–orbit Hamiltonian; Φ _ISC, the quantum yield of ISC. (b) PC design principles for tuning the light absorption and redox potentials of ground- and excited-state correlated with electron affinity (EA) and ionization potential (IP). (c) Stability control of PC intermediates in the PC cycle, with exemplified electrostatic potential maps of 4DP-IPN^•+ and 4DP-IPN^•– (top). (d) General photoredox catalysis mechanism. (e) Relative populations of S₁ and T₁ states of 4DP-IPN, compared with those of the representative PCs, based on kinetic simulations.

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Schematic representation of the strongly twisted donor–acceptor PC design platform.

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(a) Previously reported PC library for O-ATRP. PCs reported to utilize the T₁ state are highlighted in the box. (b) Proposed mechanism of O-ATRP and key challenges to be addressed.

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PC design strategies for addressing challenges in O-ATRP using 4DP-IPN and 2DHPZ-DPS.

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(a) Aqueous PET-RAFT polymerization with high oxygen tolerance. (b) Proposed mechanism of PET-RAFT polymerization accelerated by O₂. (c) PC design strategies for 3DP-MSDP-IPN to enhance water-solubility. (d) Aqueous PET-RAFT polymerization using 3DP-MSDP-IPN under aqueous and aerobic conditions for PPCs.

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(a) Proposed mechanisms of ATRP with photoredox/copper dual catalysis. (b) PC design strategies for 4DCDP-IPN to enhance its regeneration ability. (c) Oxygen-tolerant large-scale ATRP with photoredox/copper dual catalysis using 4DCDP-IPN at 50 ppb without any degassing process. Images of the reaction batch and isolated products are also provided.

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(a) Competitive interaction between PC degradation and the ET process. (b) Monitored PC degradation during the reductive dehalogenation. (c) Highly efficient photoredox reductive dehalogenation using 4DP-IPN. (d) Comparison of reaction kinetics with 4DP-IPN and 4DP-Me-BN in the reductive dehalogenation. (e) Mechanism of solvated electron process by Ceroni et al.

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(a) Proposed mechanism of XAT-induced dehalogenation leveraging beneficial PC degradation. (b) PC design strategies for tailoring 3DP-DCDP-IPN as the precatalyst to accelerate PC degradation. (c) Comparison of reaction kinetics with 4DP-IPN, 4DP-Me-BN, 3DP-DCDP-IPN, and 3DP-DCDP-Me-BN in the XAT-induced dehalogenation.

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(a) Proposed mechanism of PIS using 4Cz-IPN and amine-based co-initiator. (b) Synthesis of UV-blocking OCAs under ambient conditions using 4Cz-IPN and amines, incorporating UVA as a functionalizing agent. (c) Synthesis of UV-debondable OCAs under ambient conditions using 4Cz-IPN and amine-based co-initiator, incorporating a benzophenone-based acrylate as a functionalizing monomer.

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Inhibition of the PC cycle of 4Cz-IPN in the presence of UVA and advanced PIS design strategies to overcome this inhibition, including the use of 4DP-IPN as the PC and incorporation of ionic co-initiators.

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(a) Proposed mechanism of PIS using 4DP-IPN and ionic co-initiators. (b) Rapid and efficient PIS for synthesizing UV-blocking OCA using 4DP-IPN and ionic co-initiators.