Efficient Photoredox Cycles to Control Perylenediimide Self-Assembly

Abstract

Photoreduction of perylenediimide (PDI) derivatives has been widely studied for use in photocatalysis, hydrogen evolution, photo-responsive gels, and organic semiconductors. Upon light irradiation, the radical anion (PDI⋅^- ) can readily be obtained, whereas further reduction to the dianion (PDI^2- ) is rare. Here we show that full 2-electron photoreduction can be achieved using UVC light: 1) in anaerobic conditions by 'direct photoreduction' of PDI aggregates, or 2) by 'indirect photoreduction' in aerobic conditions due to acetone ketyl radicals. The latter strategy is also efficient for other dyes, such as naphthalenediimide (NDI) and methylviologen (MV²⁺ ). Efficient photoreduction on the minute time-scale using simple LED light in aerobic conditions is attractive for use in dissipative light-driven systems and materials.

Keywords: ketyl radical; perylenediimide; photoreduction; redox; self-assembly.

Figure 1

Photoreduction under anaerobic conditions followed by aerobic oxidation. A) The molecular structure of monomeric PDI‐1 (pink), single reduced PDI‐1⋅ ⁻ that photoreduces and disassembles to PDI‐1⋅ ⁻ and upon further illumination to PDI‐1^{2 −}. Exposure to air oxidizes PDI‐1^{2 −} to PDI‐1, followed by re‐assembly into PDI‐1_agg . B) UV‐vis spectra of the PDI‐1_agg (33 μM+3000 eq. TEOA, pink curve) in deoxygenated sodium bicarbonate‐carbonate buffer. Upon UVC LED (275 nm, 1×8200 mW/m²) illumination it first forms PDI‐1⋅ ⁻ (blue curve) and then PDI‐1^{2 −} (purple dashed curve). C) Maximum reduced species (1 e⁻ or 2 e⁻ reduction) versus aggregation constant α. D) Maximum reduced species in different solvents; from low to high dielectric constant: toluene, isopropanol, acetone, ethanol, DMF, DMSO, buffer. E) Confocal micrographs (overlay of transmitted light and fluorescence with pseudo‐colour red) of a degassed 33 μM PDI‐1_agg solution+3000 eq. of TEOA. After 3 minutes irradiation at the central green circle (355 nm laser) the PDI‐1_agg assemblies are reduced and disassemble in the region indicated with the asterisk (see Supporting Video S1).

Figure 2

Acetone catalysed ‘indirect photoreduction’ under aerobic conditions (33 μM PDI‐1_agg in buffer, 9 % v/v acetone, 3000 eq. TEOA). A) UV‐Vis spectra upon UVC illumination. The inset shows absorbances of PDI‐1_agg , PDI‐1⋅ ⁻, and PDI‐1^{2 −} 526, 725, and 543 nm, respectively. B) 10 photoredox cycles of PDI‐1 in buffer (2.25 mL, 50 μM, 3000 eq. of TEOA) under constant stirring to introduce O₂. Shaded salmon coloured areas shown when the UVC light is on. C) Acetone‐catalyzed photoreduction mechanism (see main text).

Figure 3

UV‐Vis spectra of PDIA, NDI‐PA, and MV²⁺ before and after irradiation with UVC LED light (open cuvette, 9 % v/v acetone in carbonate buffer pH 10.8, 3000 eq. TEOA). A) PDIA (46.7 μM) shows fully reduced PDIA^{2 −} after 43 s of UVC irradiation. B) NDI‐PA (100 μM) after 34 s. C) MV²⁺ (134 μM) after 73 s.