Theoretical analysis of the electron paramagnetic resonance spectra of the moderately coupled chromophore-radical conjugate

A theoretical study of transient electron paramagnetic resonance (TREPR) spectra of rigidly linked chromophore-paramagnetic species, in which the magnitude of the exchange coupling J between the two species is noticeable but smaller than the zero field splitting (ZFS), is presented. The TREPR spectrum of these conjugates is too complex to be characterized by two ZFS parameters (D and E). The spectral characteristics derived from this study provide practical tools for differentiating intersystem crossing (ISC) mechanisms and estimating exchange interaction strengths. This has the potential to aid in identifying the D/J-dominant coupling regime. When the chromophore's D-parameter is strong compared to other spin-spin interaction terms, the spectral properties can be classified into three regions: the central region around the resonance fields Bres near the spectrum's center B0 (Bres≈B0), the mid regions (Bres≈B0±D/2), and the outer regions (Bres≈B0±D). Analytical expressions are obtained for magnetic fields aligned along one of the chromophore's principal axes. Sharp peaks near the center of the spectrum split by ∼4 J arise when radical-enhanced ISC is efficient. This makes them a unique marker that distinguishes this mechanism from the triplet mechanism of ISC.