Competing mechanisms of radical-enhanced intersystem crossing in rigid chromophore–radical conjugates

Enhanced intersystem crossing (ISC) in rigid chromophore-radical conjugates is often discussed in terms of exchange asymmetry, although the physical basis of this effect remains insufficiently resolved. In this work, three possible pathways are examined within a common three-spin framework: intersystem crossing driven by intrinsic chromophore spin-orbit coupling, direct sing-doublet to trip-doublet conversion induced by exchange asymmetry (ΔJ mechanism), and a spin-orbit borrowing (SOB) mechanism involving a transient charge-transfer state. The comparison shows that the ΔJ mechanism can become less efficient relative to the SOB pathway when the charge-transfer detuning is favorable. Literature-based estimates of coupling strengths, energy detuning, Franck-Condon factors, and excited-state lifetimes are used to identify the conditions under which each mechanism may become important. The results provide a new basis for interpreting radical-enhanced ISC in rigid chromophore-radical systems and for assessing the relative roles of the underlying mechanisms.