Furan-Indole-Chromenone-Based Organic Photocatalyst for α-Arylation of Enol Acetate and Free Radical Polymerization Under LED Irradiation

Abstract

In this study we report on the efficiency of a furane-indole-chromenone-based organic derivative (FIC) as a photocatalyst in the α-arylation of enol acetate upon LED irradiation at 405 nm, and as a photoinitiator/photocatalyst in the free radical polymerization of an acrylate group in the presence of bis-(4-tert-butylphenyl)iodonium hexafluorophosphate (Iod) as an additive, or in the presence of both Iod and ethyl-4-(dimethyl amino) benzoate (EDB) under LED irradiation at 365 nm. The photochemical properties of this new light-sensitive compound are described, and the wide redox window (3.27 eV) and the high excited-state potentials FIC*/FIC^●- (+2.64 V vs. SCE) and FIC^●+/FIC* (-2.41 V vs. SCE) offered by this photocatalyst are revealed. The chemical mechanisms that govern the radical chemistry are discussed by means of different techniques, including fluorescence-quenching experiments, UV-visible absorption and fluorescence spectroscopy, and cyclic voltammetry analysis.

Keywords: alkene arylation; free radical polymerization; organic photocatalyst; photoinduced electron transfer; photoredox catalysis; redox potential.

Scheme 1

Organic photocatalysts (PCs) with high excited-state reduction and/or oxidation potentials E*_red (PC*/PC^●−) > +2 V and/or E*_ox (PC^●+/PC*) < −2 V vs. SCE. n/a: Data not available. ^s: data for singlet excited state. ^t: data for triplet excited state. Reported excited-state redox potentials E* V vs. SCE, see ref. [6].

Scheme 2

α-Arylation of enol acetate and free radical polymerization of acrylate group mediated by FIC.

Scheme 3

Two-step synthesis of FIC.

Scheme 4

Arylation of enol acetate 6 with diazonium salt 5a.

Scheme 5

Arylation of enol acetate 6 with diazonium salts 5a–e.

Scheme 6

Mechanistic hypothesis for the formation of 7.

Figure 1

Photopolymerization profiles of TA (acrylate function conversion vs. irradiation time) upon exposure to a LED (λ = 365 nm) in the presence of FIC/Iod (0.5%/1% w/w) and FIC/Iod/EDB (0.5%/1%/1% w/w/w): (A) in laminate (thickness = 25 μm) and (B) in air (thickness = 2.3 mm). The irradiation starts at t = 10 s.

Figure 1

Photopolymerization profiles of TA (acrylate function conversion vs. irradiation time) upon exposure to a LED (λ = 365 nm) in the presence of FIC/Iod (0.5%/1% w/w) and FIC/Iod/EDB (0.5%/1%/1% w/w/w): (A) in laminate (thickness = 25 μm) and (B) in air (thickness = 2.3 mm). The irradiation starts at t = 10 s.

Figure 2

(A) UV-visible absorption and emission spectra of FIC in acetonitrile. (B) Cyclic voltammetry of FIC using tetrabutylammonium hexafluorophosphate dissolved in acetonitrile as the electrolyte.

Scheme 7

Mechanistic hypothesis for the FRP initiation.

Figure 3

Fluorescence quenching study of FIC: (A) with Iod, and (C) with EDB in acetonitrile. Associated Stern–Volmer plot of FIC: (B) with Iod, and (D) with EDB.