Frustrated Radical Pairs: From Fleeting Intermediates to Isolable Species

Herein we report the design of stable para-substituted triarylamine−DDQ radical ion pairs (RIPs). We investigated several combinations using a wide variety of spectroscopic methods such as IR, EPR and NMR, both in solution and in the solid state. We quantified the degree of single-electron transfer (SET) and found that both the substituents (MeO > tBu > Br) and the polarity of solvents (MeCN > DCM > toluene) can lead to a greater degree of SET.

We present the design and comprehensive investigation of stable para-substituted triarylamine–2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) radical ion pairs (RIPs) generated via single-electron transfer (SET). We quantified the degree of SET in both solution and solid phases, utilising a suite of spectroscopic techniques including IR, EPR, NMR, and single-crystal X-ray diffraction (SC–XRD). Our findings reveal that the extent of SET is significantly influenced by the nature of the substituents (MeO > ^tBu > Br) and the polarity of the solvent (MeCN > DCM > toluene). The radical ion pair [(pMeOPh)₃N]⋅⁺[DDQ]⋅⁻ was unambiguously identified using EPR and UV–vis spectroscopy, and its structure was confirmed by SC–XRD. Detailed analysis indicates an open-shell singlet ground state with a thermally accessible triplet state, as corroborated by EPR, magnetic susceptibility measurements, and DFT calculations. This study offers crucial insights into the mechanistic pathways of RIP formation and tuning both in solution and solid states, laying the groundwork for future exploration of their reactivity and potential applications.