Phenothiazine--perylene diimide , . After photoexcitation of PDI, a hole is transferred from the PDI HOMO through the bridge to the PTZ HOMO to form the radical ion pair.
Plot of , the magnetic superexchange interaction between the spins of the radical pair vs donor-acceptor distance, , for the series of molecules in Fig. 1, , as measured by the magnetic field-effect experiment (Ref. 9) (black), and calculated to second order in and second order in (red), to second order in and third order in (green), to third order in and second order in (blue), to third order in and third order in (light blue). The best fit line, , for the experimental data yields and .
DFT (B3LYP, ) geometry-optimized structures of (a) and (b), representative donor-bridge and bridge-acceptor pairs, respectively. Torsional angles between phenyl units is in all cases, . The PTZ donor and PDI acceptor are twisted at angles of and , respectively, from the adjacent phenyl unit.
The HOMO of PTZ (a) and LUMO of PDI (b), obtained from extended Hückel calculations on DFT (B3LYP, ) optimized structures. A nodal plane exists in the LUMO of PDI at the point of bridge attachment, resulting in a lower effective coupling between bridge and acceptor compared to that between donor HOMO and bridge.
Relevant orbital energies and Coulomb repulsions for PTZ, PDI, and benzene corrected for solvent stabilization using Eq. (12). All values are in eV.
Local orbital couplings, , calculated with the extended Huckel method [Eqs. (15) and (16)] on density-functional theory (DFT) geometry-optimized structures. stands for donor(HOMO), for acceptor(LUMO), for the bridge(HOMO), etc., Chart 4.
Superexchange couplings, , calculated with second- and third-order perturbation expansions of the molecular orbitals [Eq. (13)] and the resulting values from Eqs. (6) and (10), and measured values (Ref. 9) for , , see Fig. 2. All values are in eV.
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