Fission of entangled spins: An electronic structure perspective
Electronic structure aspects of singlet fission process are discussed. Correlated many-electron adiabatic wave functions of the bright singlet states and dark multi-exciton states of tetracene and pentacene dimers are analyzed in terms of their character (excitonic, charge-resonance, multi-exciton). At short interfragment separation (3.5-4 A), both multi-excitonic and singly-excited singlet states have noticeable charge-resonance contributions, which fall off quickly at longer distances. Non-adiabatic couplings between the states are discussed. The limitations of diabatic framework in the context of singlet fission are analyzed. We propose using the norm of one-particle transition density matrix, ||gamma||, as a proxy for non-adiabatic coupling. Using the Cauchy-Schwarz inequality, we show that any one-electron interstate property is symbatic with the respective ||gamma||. The analysis of ||gamma|| and state characters at selected configurations reveals that the couplings between the multi-exciton and singly-excited states depend strongly on the weights of charge-resonance configurations in excitonic and multi-exciton states. To characterize energetics relevant to triplets separation step, we consider multi-exciton binding energy (Eb) defined as the difference between the quintet and singlet multi-exciton states. The effect of fragment orientation on the couplings and multi-exciton binding energy is analyzed. Contrarily to previous studies, we observe significant state interactions at parallel cofacial (and not slip-stacked) orientations.