Organic Photoredox Catalysts Exhibiting Long Excited-State Lifetimes

Abstract

Organic photoredox catalysts with a long excited-state lifetime have emerged as promising alternatives to transition-metal-complex photocatalysts. This Account explains the effectiveness of using long-lifetime photoredox catalysts for organic transformations, focusing on the structures and photophysics that enable long excited-state lifetimes. The electrochemical potentials of the reported organic, long-lifetime photocatalysts are compiled and compared with those of the representative Ir(III)- and Ru(II)-based catalysts. This Account closes by providing recent demonstrations of the synthetic utility of the organic catalysts.

Unleashing the potentiality of metals: synergistic catalysis with light and electricity

“…23 Luminescent Ru( ii ) and Ir( iii ) polypyridyl complexes exhibit long-lived excited states with redox activity. 24 In recent years, efforts have been devoted to enhancing the excited-state lifetimes of iron complexes to achieve more efficient photo-reactivity. 1 d ,25 However, increasing the excited-state lifetime is not the sole approach to realizing useful photo-reactivity of iron complexes.…”

Multielectron Reduction of Esters by a Diazabenzacenaphthenium Photoredox Catalyst

“…During the past decade, photoredox catalysis has emerged as a rapidly growing field in chemistry . Within this field, a wide variety of radical-based organic molecular transformations involving a photocatalytic single-electron-transfer (SET) process have been developed .…”

“…During the past decade, photoredox catalysis has emerged as a rapidly growing field in chemistry . Within this field, a wide variety of radical-based organic molecular transformations involving a photocatalytic single-electron-transfer (SET) process have been developed . Compared with photoinduced radical reactions, the development of photocatalytic molecular transformations via ionic (anion/cation) species generated through a multielectron-transfer process has received scant attention .…”

“…Cyclic voltammetry of N- BAP in MeCN solution revealed that its ground-state reduction ( E 1/2 red ) and oxidation potentials ( E 1/2 ox ) are −1.89 V and +1.08 V (vs SCE), respectively (Figure B and C). Redox potentials of the singlet excited state ( E ox S 1 , E red S 1 ) and the triplet excited state ( E ox T 1 , E red T 1 ) of N -BAP were calculated from the ground-state redox potentials and the excitation energy. , N- BAP exhibited high reducing and oxidizing abilities in its ground and singlet excited states. Considering the reduction potential of aromatic esters (<−1.72 V vs SCE), we surmise that the ground and excited singlet states of N- BAP play key roles in the ester reduction, rather than its triplet state…”

“…When the reaction of 1a with N- BAP was performed using N , N -diisopropylethylamine ( i -Pr 2 NEt) or triethanolamine [N­(CH 2 CH 2 OH) 3 ] instead of ammonium oxalate, 1a was completely consumed, and 2-pyridinemethanol ( 2a ) was formed, but in <3% yield (entries 5 and 6) . Tertiary amine reductants such as i -Pr 2 NEt or N­(CH 2 CH 2 OH) 3 are known to be oxidized to iminium cations in the reduction step of the photocatalysis. , Consequently, the low yield of 2a would be due to interruption of the four-electron-transfer process by the oxidized byproducts from the amines. In contrast, oxalate is known to serve as a “traceless reductant”.…”