Photoinduced Electron Transfer in 9‐Substituted 10‐Methylacridinium Ions

Abstract

A series of 9-substituted 10-methylacridinium ions (Acr -R) in which an electron-donor moiety (R) is directly linked with an electron-acceptor moiety (Acr ) at the 9-position was synthesized, and the photodynamics was fully investigated to determine the rate constants of photoinduced electron transfer (ET) and back electron transfer. The driving forces of photoinduced electron transfer and back electron transfer were determined by means of electrochemical and photophysical measurements. The dependence of the ET rate constants on driving force was well analyzed in the light of the Marcus theory of ET. The quantum yields of formation of the triplet ET states vary significantly, depending on the interaction between the donor (R) and acceptor (Acr ) moieties. Among the Acr -R examined, the 9-mesityl-10-methylacridinium ion (Acr -Mes) exhibits the best performance in terms of the lifetime of the triplet ET state and the quantum yield. Photoexcitation of Acr -Mes results in formation of the triplet ET state [ (Acr -Mes )], which has a long lifetime, a high energy (2.37 eV), and a high quantum yield (>75 %) in acetonitrile. The triplet ET state exhibits both the oxidizing and reducing activity of the Mes and Acr moieties, respectively.

Immobilization of Molecular Catalysts for Enhanced Redox Catalysis

“…‐Mes .+ molecule to the Mes .+ moiety in another Acr . ‐Mes .+ molecules . Acr + ‐Mes has merited increasing attention as an excellent organic photocatalyst for various organic transformations via a wide range of electron‐transfer reactions, because the Mes .+ moiety in Acr .…”

“…Photoexcitation of Acr + ‐Mes@AlMCM‐41 dispersed in MeCN results in intramolecular electron transfer from the Mes moiety in Acr + ‐Mes to the singlet excited state (SExS) of Acr + moiety in Acr + ‐Mes to produce the singlet ET state, which undergoes intersystem crossing to the triplet ET state, 3 (Acr . ‐Mes .+ ) . Because the Mes .+ moiety in 3 (Acr .…”

“…[112] However,a tr oom temperature in solution, the ET state decays via diffusion-limited intermolecularb ack electron transfer from the AcrC moiety in one AcrC-MesC + molecule to the MesC + moiety in anotherA crC-MesC + molecules. [112,114] Acr + -Mes has merited increasing attention as an excellent organic photocatalyst for variouso rganic transformationsv ia a wide range of electron-transfer reactions, because the MesC + moiety in AcrC-MesC + has high oxidizing ability (E red = 2.06 Vv s. SCE) whereas the AcrC moiety in AcrC-MesC + has high reducing ability (E ox = À0.57 Vv s. SCE). When Acr + -Mes was immobilized by cation exchange in nanosized tubularm esoporous silica-alumina (tAlMCM-41) to form an anocomposite (Acr + -Mes@tAlMCM-41), the bimolecular decay of the ET state (i.e.,A crC-MesC + )w as prevented completely to afford the long lifetimeo f2sd ue to intramolecular back electron transfer from the AcrC moiety to the MesC + moiety in the ET state (i.e., AcrC-MesC + )i nM eCN at 298 Ka sd epicted in Figure 13 (part D), [140] where the ESR spectrum of AcrC-MesC + (part A) is the superposition of AcrC-Mes (part B) and MesC + (part C).…”

“…[140] Photoexcitation of Acr + -Mes@AlMCM-41 dispersed in MeCN results in intramolecular electron transfer from the Mes moiety in Acr + -Mes to the singlet exciteds tate (SExS) of Acr + moietyi nA cr + -Mes to produce the singlet ET state, which undergoes intersystem crossingt ot he triplet ET state, 3 (AcrC-MesC + ). [114] Because the MesC + moiety in 3 (AcrC-MesC + )h as the strong oxidizing ability (E red vs. SCE = 2.06 V), electron transfer from p-xylene (E ox vs. SCE = 1.93 V) to the MesC + moiety in AcrC-MesC + occurs efficiently to produce p-xylene radical cation that undergoes deprotonation to produce p-methylbenzylr adical. [140] The carbon centered p-methylbenzyl radical reacts rapidly with O 2 to give p-methylbenzylperoxyl radical.…”

Catalytic Asymmetric Allylation of Aldehydes with Alkenes Mediated by Organophotoredox and Chiral Chromium Hybrid Catalysis

“…Extension of the chemistry to facilitate addition of these nucleophiles to polar moieties, such as carbonyl groups, has been hitherto unexplored with the exception of one recent report by the Glorius group. 7,15 Based on this hypothesis, we commenced optimization of the reaction conditions using benzaldehyde (2a) and cyclohexene (1a: 20 equiv) as model substrates, and a combination of 5 mol % CrCl2 and 2.5 mol % acridinium photoredox catalysts (2,6-Xyl-Acr + •ClO4 -; 10), 16 under 430 nm visible light irradiation at room temperature ( Table 1). As expected, the desired reaction did not proceed at all in the absence of chromium complex (entry 1).…”