The triplet mechanism of electron spin polarization in moderately coupled triplet-doublet rigid complexes as a source of the enhanced +1/2 ↔ -1/2 transitions.

The light-induced electron spin polarization generated in the excited quartet and doublet states of a system consisting of a chromophore with an attached radical is investigated theoretically. Excitation of the chromophore and subsequent relaxation leads to a coupled triplet-doublet spin system. In many such systems, the electronic coupling between the triplet and doublet spins is expected to be strong enough to split the spin system into so-called trip-doublet and trip-quartet states but sufficiently weak that it does not promote significant mixing between the sing-doublet and trip-doublet states. In such moderately coupled systems, the sing-doublet can relax to the trip-doublet and trip-quartet states by spin-orbit coupling mediated intersystem crossing within the chromophore. An analytical expression is derived for the intensity of the polarization generated by this mechanism for the m = +1/2 ↔ -1/2 electron paramagnetic resonance transitions of the trip-doublet and trip-quartet states. The expression shows that the intensity and sign of the polarization depend strongly on ratio j = 3J/ω between the triplet-doublet exchange interaction J and the Zeeman energy ω. The polarization becomes undefined when j = 1 and when j = 2 because level-anticrossings between the trip-doublet and trip-quartet sublevels occur. The sign of the polarization is also found to change above and below these values. Thus, for such moderately coupled systems, the sign of the polarization and its Zeeman energy dependence can be used to estimate the magnitude of the exchange coupling.