Red light-driven electron sacrificial agents-free photoreduction of inert aryl halides via triplet-triplet annihilation

The study addresses the challenge of activating strong carbon–halogen bonds in inert aryl bromides and chlorides using photoredox catalysis. Conventional approaches rely on high-energy UV or blue light (<500 nm), which suffer from shallow penetration, side reactions, and scalability issues. Utilizing long-wavelength red light is desirable for deeper penetration and simpler scaling, but single red photons lack sufficient energy for energy-demanding reductions. Prior multiphoton Z-scheme strategies require sacrificial electron donors and have not been demonstrated under red light. The authors hypothesize that a two-component triplet–triplet annihilation (TTA) system can harness two red photons to generate a high-energy singlet state of perylene derivatives that can reduce aryl halides without sacrificial electron donors, enabling efficient and scalable red-light-driven photoreductions.

Previous work established blue-light TTA-based photocatalytic reductions of aryl halides but still required sacrificial agents. Near-infrared TTA upconversion (UC) systems have coupled to visible-light photocatalysts mainly via inefficient emission–reabsorption pathways, limiting their use for demanding reductions. Z-scheme multiphoton strategies form radical anions that absorb a second photon but rely on excess toxic sacrificial donors and have not operated with long-wavelength red light. Engineering approaches (e.g., micro-flow reactors) improve light–matter contact for blue/UV systems but are complex and not easily scalable. These limitations motivate a direct, sacrificial-agent-free TTA strategy that operates efficiently under red light with better penetration and simpler implementation.

• Photocatalytic system: Two-component TTA system comprising a red-absorbing photosensitizer, PdTPBP (meso-tetraphenyltetrabenzoporphine Pd complex), and perylene-based annihilators/photocatalysts (Py0–Py4). The perylene derivatives were designed with phenyl substituents and 2'-methyl steric hindrance (Py3, Py4) to restrict phenyl rotation and suppress triplet nonradiative decay.
• Photophysical and electrochemical characterization: Determined oxidation potential of perylene (0.90 V vs SCE) and excited-state reducing power (Ered* ≈ −1.88 V vs SCE), sufficient for reducing substrates like 4-bromoacetophenone (Ered = −1.84 V vs SCE). UV–vis, fluorescence spectroscopy, quantum yields, lifetimes, Stern–Volmer quenching (kSV), bimolecular quenching (kq), triplet–triplet energy transfer efficiency (φTET), normalized TTA efficiency (ηTTA), upconversion quantum yield (ηUC; ΦUC = 2×ηUC), excitation threshold intensity (Ith), and delayed fluorescence lifetimes (τDF) were measured.
• TTA-UC measurements: PdTPBP at 10 μM; optimized Py concentrations (Py0–Py4: 500–1000 μM) in argon-saturated toluene; excitation at ~653–656 nm; power-dependent UC to determine Ith and ηUC.
• Theoretical calculations: DFT (B3LYP/6-31G(d)) and TD-DFT to optimize ground-state geometries, compute vertical excitation energies, frontier orbitals, triplet spin densities, and assess effects of steric hindrance on S1/T1 properties and localization.
• Photocatalytic reactions: Model substrate 4-bromoacetophenone; coupling with heteroarenes (e.g., N-methylpyrrole), arenes, and triethyl phosphite. Conditions: For blue light (455 nm): photocatalyst Py (0.02 equiv), base K2CO3 (3 equiv), DMSO (2 mL), heteroarene (2 equiv), argon purge and freeze–pump–thaw, 455 nm LED ~2 cm, 400 rpm stirring; workup by water quench, DCM extraction, silica gel chromatography. For red light TTA: Py (5 mM), PdTPBP (25 μM), K2CO3 (3 equiv), aryl bromide/chloride (1 equiv), solvent (2 mL), heteroarene (2 equiv), argon removal of O2, 656 nm LED ~2 cm, same workup. Light intensity typically 60 mW/cm2.
• Mechanistic studies: Rehm–Weller analysis of ΔGet indicates reduction occurs from the TTA-generated S1 state of Py, not T1. Quenching studies show 4-bromoacetophenone quenches TTA-UC (S1 of Py) with KSV = 5.0 M−1, kq = 1.04×10^9 M−1 s−1, while PdTPBP phosphorescence unaffected, and TTA-UC lifetime unchanged upon substrate addition, supporting S1-mediated electron transfer. Calculated ET activation barrier ΔG‡ ≈ 3.52 kJ/mol. Control experiments excluded N-methylpyrrole as sacrificial donor (1:1 reagent still gives 67% yield). Scaling tests compared 2 mL vs 20 mL under identical spot size and intensity for blue vs red illumination.

• Perylene acts as a metal-free photocatalyst enabling sacrificial-agent-free photoreduction of aryl halides under blue light; control experiments without perylene, base, or light gave no conversion.
• Under red light (656 nm, 60 mW/cm2), neither perylene nor PdTPBP alone produced product, but together (TTA system) gave substantial yields; e.g., with perylene (Py0) + PdTPBP: 36.9% isolated yield for coupling of 4-bromoacetophenone with N-methylpyrrole.
• Sterically hindered perylene derivatives (Py3, Py4) suppress triplet nonradiative decay, increasing TTA efficiency and upconversion performance: ηUC up to 23% (ΦUC = 45.6%) for PdTPBP/Py4, 3.2× perylene (Py0). Threshold intensities Ith < 0.1 W/cm2 (e.g., 82.5 mW/cm2 for Py4).
• Photocatalysis yields under 656 nm (PdTPBP + Py derivatives): Py0 36.9%, Py1 53.7%, Py2 59.5%, Py3 68.0%, Py4 79.4% (TURNOVER NUMBERS up to 39.7). Direct blue-light (455 nm) photoreduction with Py4 gave 70.3%, lower than red-light TTA with PdTPBP/Py4 under identical intensity.
• Overall photocatalytic quantum yield Φoverall for PdTPBP/Py4 was 1.9% (with [PdTPBP]=10 μM, [Py]=500 μM, [substrate]=50 mM); approximately 20% of TTA-generated 1Py used in photoreduction across TTA pairs.
• Mechanism: Only S1 of perylene derivatives is thermodynamically competent for ET to 4-bromoacetophenone; quenching constants for substrate on PdTPBP/Py4 TTA-UC: KSV = 5.0 M−1, kq = 1.04×10^9 M−1 s−1; PdTPBP phosphorescence unquenched by substrate; TTA-UC lifetime unaffected by substrate, implicating 1Py in ET; computed ET barrier ΔG‡ ≈ 3.52 kJ/mol.
• Substrate scope: High yields (>60%) for couplings of 4-bromoacetophenone with pyrrole derivatives, indole, 1,3,5-trimethoxybenzene, and for Arbuzov phosphonylation with triethyl phosphite. Electron-deficient aryl bromides (e.g., bromoacetophenone, bromobenzaldehyde, bromobenzonitrile) gave high yields; electron-deficient heteroaryl bromide also successful (62.7%). Aryl chlorides gave modest yields (e.g., 2-chlorobenzonitrile 14.0%; 4-chlorophenone 17.2%).
• Scalability and penetration: For 4-bromoacetophenone + triethyl phosphite, blue-light Py4 system yield dropped from 83% (2 mL) to 39% (20 mL), while red-light TTA system maintained high yield (84% to 66%) at identical intensity and spot size, evidencing superior red-light penetration and reduced photobleaching.

The work demonstrates that combining a red-absorbing sensitizer (PdTPBP) with perylene-based annihilators enables sacrificial-agent-free photoreduction of inert aryl halides using low-power red light via TTA. The research question—how to activate strong aryl halide bonds under red light without sacrificial donors—is addressed by generating high-energy 1Py through sensitized TTA, which directly participates in single-electron transfer to aryl halides. Steric hindrance in perylene derivatives (Py3, Py4) restricts phenyl rotation, reduces triplet nonradiative decay, extends triplet lifetimes, raises ηTTA and ηUC, and consequently increases the population of reactive 1Py. This leads to significantly higher catalytic yields and turnover numbers under red light compared to both the original perylene (Py0) and direct blue-light excitation. Mechanistic data (thermodynamics, quenching kinetics, unaffected PdTPBP phosphorescence, unchanged TTA-UC lifetime upon substrate addition) corroborate an S1-mediated electron transfer pathway. Practically, the red-light system mitigates substrate/intermediate side-absorption and photocatalyst bleaching, affording superior performance in larger volumes, aligning with green chemistry and industrial scalability.

This study introduces a red light-driven, sacrificial electron donor-free method for photoreducing inert aryl halides by leveraging a sensitized TTA mechanism. Perylene acts as a metal-free photocatalyst; coupling it with PdTPBP enables efficient red-light (656 nm) activation. Engineering perylene derivatives with steric hindrance (Py3, Py4) suppresses triplet nonradiative decay, boosting TTA upconversion efficiency (to 23%) and elevating catalytic yields (e.g., 79.4% for 4-bromoacetophenone + N-methylpyrrole), surpassing analogous blue-light conditions. The method effectively activates electron-deficient aryl bromides and demonstrates notable scalability due to superior red-light penetration. Future work could broaden substrate scope (especially aryl chlorides and less-activated aryl halides), optimize catalyst structures and sensitizer–annihilator pairing, and integrate the approach into larger-scale continuous processes.

• Substrate scope favors strongly electron-deficient aryl bromides; electron-neutral or electron-rich aryl bromides are not reported to give high yields.
• Aryl chlorides show only low to moderate yields (e.g., 14–17%).
• Reactions require inert atmosphere and specific solvent/base conditions (e.g., DMSO, K2CO3) and defined concentrations for optimal TTA efficiency.
• Although the photocatalyst (perylene) is metal-free, the sensitizer PdTPBP contains palladium, which may be a consideration for certain green chemistry or cost constraints.
• The method relies on precise control of light intensity and sensitizer/annihilator concentrations to maintain high ηUC and catalytic performance.