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

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.