# Viscosity-Controlled Magnetic Field Effects in Homogeneous Photoredox Catalysis Enabled by Ionic Liquids

## TL;DR

This study shows how ionic liquids can control magnetic field effects in chemical reactions by adjusting viscosity and influencing radical pair behavior.

## Contribution

The novel use of ionic liquids as a tunable solvent cage to enhance and study magnetic field effects in photoredox catalysis.

## Key findings

- Magnetic field effects increase with solvent viscosity up to 15% enhancement in ionic liquids.
- Ionic liquids reduce radical cage escape rates, aligning with spin evolution rates.
- Triplet charge recombination contributes moderately, limiting magnetic enhancement amplitude.

## Abstract

In conventional low-viscosity solvents, magnetic field effects (MFEs) in photoredox catalysis are often negligible because photogenerated radical ion pairs (RIPs) diffuse apart before significant spin evolution occurs. This study reports using ionic liquids (ILs) as a tunable homogeneous “solvent cage” to observe distinct low-field MFEs in the phenothiazine-mediated photoinduced reductive dechlorination of aryl chlorides. Experimental results demonstrate that MFEs increase significantly with bulk viscosity, reaching saturation at approximately 1000 Gs with a maximum enhancement of about 15%, consistent with the hyperfine coupling mechanism (HFCM). Femtosecond transient absorption spectroscopy (fs-TA) reveals that the ionic liquid environment effectively reduces the radical cage escape rate, matching it with the spin evolution rate. This allows the external magnetic field to intervene in the back electron transfer (BET) process. However, unlike strongly confined micellar systems, the contribution of the triplet charge recombination (TCR) pathway here is moderate, intrinsically limiting the magnetic enhancement amplitude. These findings establish that MFE magnitude is determined by both viscosity-controlled cage dynamics and the efficiency of the TCR channel, providing a mechanistic basis for designing spin-modulated homogeneous photoredox systems.