Light-induced electron spin polarization in rigidly linked, strongly coupled triplet–doublet spin pairs

Perturbation theory is used to predict the light-induced spin polarization associated with the quartet state of rigid, strongly coupled triplet–doublet spin pairs. It is shown that spin–orbit coupling mixes the trip–doublet and trip–quartet states and that if the mixing determines the rate of ISC, it leads to net and multiplet spin polarization of the quartet state. Analytical expressions for the contributions to the spin polarization are derived using first-order perturbation theory. Using these expressions, the sign, magnetic field dependence and orientation dependence of the spin polarization are discussed in terms of the exchange coupling, zero-field splitting and spin–orbit coupling within the system.