Strong Modulation of the Exchange Interaction in a Spin-Polarized, Aryl Ether-Linked 1,14-Biradical

Abstract

The time-resolved electron paramagnetic resonance spectrum of a 1,14-bis(alkyl)biradical linked via the para position of an aryl ether moiety has been studied. Previously the spectrum obtained at 50°C could not be simulated using spin-correlated radical pair (SCRP) theory. It is successfully simulated here by adding modulation of the exchange interaction (J) to the fitting routine as a T 2 relaxation mechanism. Inclusion of this line-broadening effect allowed an accurate value of the exchange interaction to be determined for this temperature. Comparison of this J value with that obtained in previous work for the meta isomer of the same biradical showed that the dominant mechanism of J coupling is through-solvent rather than through-bond. The magnitude of the J modulation matrix elements in the two isomers are discussed as are their temperature dependencies.In a recent paper 1 we presented the experimental and simulated time-resolved electron paramagnetic resonance (TREPR) spectra of biradical 1a (shown in Scheme 1) obtained in n-octane solution at two temperatures. The purpose of that work was to attempt to differentiate between two coupling mechanisms for the spin exchange interaction J: through-bond (TB) and throughsolvent (TS). It has been shown in another of our papers 2 that the TB and TS mechanisms have opposite temperature dependencies for flexible alkane chain biradicals. While the TS coupling becomes greater at higher temperatures, the TB mechanism dominates at the lower ones. This means that a temperature dependence of the TREPR spectrum provides an excellent way to test the dominant mechanism of J coupling in these structures. As demonstrated in additional published work from our laboratory, construction of biradical precursors with double bonds 3,4 or conjugated π-systems 1 can change both mechanisms simultaneously. Figure 1A shows the X-band TREPR spectrum of biradical 1a at 50°C, obtained by laser flash photolysis of precursor ketone 1. All experimental details regarding the collection of this data are given in ref 1. To get a better idea of which electronic coupling mechanism is dominant, our previous experiments were also performed on the meta isomer of 1a, and the temperature dependencies of the TREPR spectra were also studied for both isomers. 1 At higher temperatures (105°C ), good quality simulations of spectra from 1a were obtained using SCRP theory in its most basic form. By comparison to the meta isomer, it could be concluded that the dominant mechanism was TS at 105°C. However, at lower temperatures, a very poor quality fit was obtained, as shown in Figure 1B. It is more accurate to state that we were unable to simulate the spectrum of 1a at 50°C. At that temperature, however, we could stimulate the spectrum from the meta isomer very well.In this paper we demonstrate that the inclusion of J modulation (caused by conformational motion) as a relaxation mechanism into the SCRP simulation routine allows us to resolve this situation and reach excellent agreement between the experimental and sim...

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