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 PHZn V (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.

Inverse photochromism in viologen–tetraarylborate ion-pair complexes: optical write/microwave erase switching in polymer matrices

“…The electron spin-spin exchange interaction is exponentially distance dependent and directly reflects the donor-acceptor electronic coupling, specially into host-guest supramolecular systems where p-p stacking interaction may be observed. 20,28,[32][33][34][35][36] In fact, the presence of microwave radiation increases the yield of the viologen (a) species. This in turn increases the recombination of radicals to form excited PSCcomplex species, which may be deactivated by radiative (fluorescence) and non-radiative processes (thermal deactivation) to return to the ground state.…”

Spin Polarization Transfer from a Photogenerated Radical Ion Pair to a Stable Radical Controlled by Charge Recombination

“…The RP is generated in the singlet (S) state, which then decays through recombination to the singlet ground state and intersystem crossing to the three triplet RP states (T –1 , T 0 , T +1 ) in the high magnetic field limit. In the TREPR experiments, B ≫ 2 J , such that spin mixing primarily involves the S and T 0 states. , As shown in Figure b, this results in population of the two states Ψ A and Ψ B that result from S-T 0 mixing in the RP. Spin relaxation in the RP or in the equilibrated 3 *NDI can lead to population of the T ±1 levels. , Because the two mixed states Ψ A and Ψ B have some singlet character, these populations recombine quickly to the singlet ground state.…”

“…In the TREPR experiments, B ≫ 2J, such that spin mixing primarily involves the S and T 0 states. 5,51 As shown in Figure 6b, this results in population of the two states Ψ A and Ψ B that result from S-T 0 mixing in the RP. Spin relaxation in the RP or in the equilibrated 3 *NDI can lead to population of the T ±1 levels.…”

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor‐Acceptor Systems

“…The study show that the J is exponentially depended on the distance between the radical anion and the cations. The threading of the electron donor‐acceptor dyads with α‐CD to prepare rotaxane may provide a way to separate the through‐bond and the through‐space components of the electronic coupling mechanisms in spin exchange, electron transfer or energy transfer processes [104] …”