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 Ψ_R=ψ₁+λψ₂+μψ₃ and Ψ_P=ψ₄+μψ₂+λψ₃. Use of the latter wave functions leads to the establishment of the matrix element T_RP= 〈Ψ_R‖H−E₁‖Ψ_P〉≊T₁₄−(T₁₂T₂₄+T ₁₃T₃₄)/A, with T_ij=〈ψ_i‖H−E₁‖ψ_j〉 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.

A detailed theory for electronic aspects of electronic excitation (energy) transfer (EET) for sandwich dimers was derived in paper II of this series [J. Chem. Phys. 101, 10 521 (1994)]. In II, the electronic transfer matrix element for EET was evaluated, then simplified to various levels of approximation. The results of ab initio molecular orbital calculations on an ethene sandwich dimer are reported here in order to test and quantify the theory of II. The calculations were undertaken using a STO‐6G basis set and localized molecular orbitals, with separations of 4, 5, and 6 Å between the molecules. It is demonstrated that the important electronic factors contributing to EET are the Coulombic interaction (for direct singlet–singlet transfer) and, for both singlet–singlet and triplet–triplet EET, orbital overlap‐dependent interactions. The dominant orbital overlap‐dependent terms arise from through‐configuration interaction, which involves successive one‐electron transfers mediated via bridging ionic configurations, first presented in II. The results confirm that the Dexter‐type exchange interaction is small in comparison.

The theory established in J. Chem. Phys. 101, 10521 (1994), for electronic factors which promote interchromophore electronic energy transfer, exciton interactions and which provide the stabilization of excimers, is extended; first so as to include the possible contribution of doubly excited configurations. It is ascertained that there is a resultant effect upon the (interchromophore orbital overlap-dependent) through-configuration interaction, and a significant correction to the simple expression obtained previously for the Coulombic interaction. These CI effects are admitted to the general theory of the previous work and the cases of nonidentical, identical, and orthogonal donor and acceptor are discussed. Second, a description of superexchange effects is admitted to the theory. Two possible formalisms are developed and discussed.

The origin of electronic energy transfer (EET) between two chromophores (D and A) is explored further for several molecular situations that may be encountered in experiment—namely, nonoverlapping active‐space orbitals of the D and A chromophores, forbidden electronic excitations for both chromophores, and an allowed and a forbidden electronic excitation for the D and A chromophores, respectively. The theory is illustrated via the results of calculations of the EET matrix elements for model systems with both four–eight active‐space electrons and all of the electrons included explicitly. In each case, it is found that any overlap contribution to these matrix elements is associated much more with charge‐transfer and penetration terms rather than it is with the Dexter exchange integral. The calculated magnitude of the latter integral is always smaller than that of the Coulomb integral.