Diastereoisomers of l-proline-linked trityl-nitroxide biradicals: synthesis and effect of chiral configurations on exchange interactions

The exchange (J) interaction of organic biradicals is a crucial factor controlling their physiochemical properties and potential applications and can be modulated by changing the nature of the linker. In the present work, we for the first time demonstrate the effect of chiral configurations of radical parts on the J values of trityl-nitroxide (TN) biradicals. Four diastereoisomers (TNT(1), TNT(2), TNL(1) and TNL(2)) of TN biradicals were synthesized and purified by the conjugation of a racemic (R/S) nitroxide with the racemic (M/P) trityl radical vial-proline. The absolute configurations of these diastereoisomers were assigned by comparing experimental and calculated electronic circular dichroism (ECD) spectra as (M, S, S) for TNT(1), (P, S, S) for TNT(2), (M, S, R) for TNL(1) and (P, S, R) for TNL(2). Electron paramagnetic resonance (EPR) results showed that the configuration of the nitroxide part instead of the trityl part is dominant in controlling the exchange interactions and the order of the J values at room temperature is TNT(1) (252 G) > TNT(2) (127 G) ≫ TNL(2) (33 G) > TNL(1) (14 G). Moreover, the J values of TNL(1)/TNL(2) with the S configuration in the nitroxide part vary with temperature and the polarity of solvents due to their flexible linker, whereas the J values of TNT(1)/TNT(2) are almost insensitive to these two factors due to the rigidity of their linkers. The distinct exchange interactions between TNT(1,2) and TNL(1,2) in the frozen state led to strongly different high-field dynamic nuclear polarization (DNP) enhancements with ε = 7 for TNT(1,2) and 40 for TNL(1,2) under 800 MHz DNP conditions.