Electron spin exchange in linked phenothiazine-viologen charge transfer complexes incorporated in "through-ring" (rotaxane) α-cyclodextrins

Abstract A series of covalently bound phenothiazine (PHZ) donor and methylviologen (V) acceptor compounds with polymethylene chain spacers (C₈, C₁₀, C₁₂) were incorporated in a "through-ring" (rotaxane) fashion to α-cyclodextrin (α-CD) hosts such that the alkyl chains were fully extended, with the donor and acceptor on opposite sides of the α-CD cylinder. Photoexcitation of the PHZ unit induces electron transfer from the PHZ first excited triplet state to the V moiety, forming a biradicaloid charge-separated state. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy at the X-band and Q-band microwave frequencies was used to investigate the spin exchange interaction, J, in these biradicaloids. Simulation of the spectra using a "static" model for spin-correlated radical pairs allows extraction of the J values, which are negative in sign and have absolute values range from 2 to 1000 Gauss. Comparison of the PHZ_nV (n = 8, 10, 12) spectra to those obtained using phenyl ether spacers indicates that π-bonds may assist the electronic coupling. The results are discussed in terms of through-bond vs through-space electronic coupling mechanisms. Photoexcitation of covalently bound phenothiazine donors with methylviologen acceptors separated by C₈, C₁₀ and C₁₂ alkane chain spacers incorporated in a "through-ring" (rotaxane) fashion to α-cyclodextrin hosts leads to biradicaloid charge-separated states that are studied by time-resolved electron paramagnetic resonance spectroscopy at the X-band and Q-band microwave frequencies. Computer simulation of the spectra using a "static" model for spin-correlated radical pairs allows extraction of the spin exchange interactions, which range from -2 to -1000 Gauss. The results are discussed in terms of through-bond vs through-space electronic coupling mechanisms as a function of donor-acceptor distance.