Rate expressions for excitation transfer. II. Electronic considerations of direct and through–configuration exciton resonance interactions

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Abstract
The electronic interactions which promote singlet–singlet and triplet–triplet electronic excitation (energy) transfer (EET) are investigated in detail. Donor and acceptor locally excited configurations, ψ1(A*B) and ψ4(AB*), respectively, are each allowed to mix with bridging ionic configurations, ψ2(A+B−) and ψ3(A−B+) to form the new donor and acceptor wave functions ΨR=ψ1+λψ2+μψ3 and ΨP=ψ4+μψ2+λψ3. Use of the latter wave functions leads to the establishment of the matrix element TRP= 〈ΨR‖H−E1‖ΨP〉≊T14−(T12T24+T 13T34)/A, with Tij=〈ψi‖H−E1‖ψj〉 and A=E2−E1, as the exciton resonance interaction term for EET. Introduction of the Mulliken approximation shows that the ‘‘direct’’ exciton resonance interaction term (T14) contributes primarily a Coulombic interaction, for singlet–singlet EET, while the ‘‘through–configuration’’ exciton resonance interaction term [−(T12T24+T13T34)/A] replaces the Dexter exchange integral (which is a component of H14) as the primary source of short‐range orbital overlap‐dependent EET. The origins of ‘‘Dexter‐type’’ energy transfer are thus shown to be quite different from that originally outlined.