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.