Flexible Biradicals in Liquid and Supercritical Carbon Dioxide:  The Exchange Interaction, the Chain Dynamics, and a Comparison with Conventional Solvents

Abstract

X-band time-resolved electron paramagnetic resonance (TREPR) spectra of three flexible biradicals of varying chain length and structure were obtained in liquid and supercritical carbon dioxide (CO2) solutions and compared to conventional solvents. For C16 acyl-alkyl biradical 1a, an average spin exchange interaction between the radical centers, J(avg), was obtained by spectral simulation using a simple model for spin-correlated radical pairs (SCRPs) and a small amount of T2 relaxation from a previously established J modulation mechanism. A large solvent effect on J(avg) was observed for the first time, varying by almost 1 order of magnitude from CO2 (J(avg) = -120 +/- 10 MHz) to heavy mineral oil (-11 +/- 3 MHz) for 1a. For C15 bis(alkyl) biradical 1b, spectra obtained under supercritical conditions are only slightly different from those detected in liquid CO2 but differ from spectra taken in benzene. For C10 acyl-alkyl biradical 2a, more emissive spin polarization due to S-T- mixing is observed in CO2 than in benzene. These results are discussed in terms of solvent properties such as dielectric constant, viscosity, and specific interactions. Both chain dynamics and changes to the equilibrium distribution of end-to-end distances can alter J(avg) and the observed ratio of S-T0 to S-T- mixing; however, faster chain dynamics is concluded to be the most likely cause of the observed effects in these systems.

TEMPO Radical-Functionalized Supramolecular Coordination Complexes with Controllable Spin–Spin Interactions

“…Noncovalent spin–spin interactions (dipolar and exchange) are the result of the intrinsic properties of (poly)­radicals, and they are closely correlated with their structures, e.g., the molecular geometry and conjugation, structural rigidity, linker length between unpaired electrons, and so on. − The spin–spin interactions between unpaired electrons in organic radicals are important to their properties and applications. , For example, spin–spin interactions are the origin of magnetic interactions, which is a critical factor for the design of organic magnetic materials and for their magnetic properties. − The radicals and their spin–spin interactions also provide a unique platform and tool to investigate electron transfer dynamics. − Site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy have emerged as versatile approaches to study protein structures and interactions, mainly through the qualitative and quantitative analysis of electron spin–spin interactions between spin labels. − Recently, the spin–spin interactions of organic radicals have also garnered much attention in supramolecular chemistry . For instance, Kim et al reported the spin–spin interactions of radical cation dimers such as methyl viologen and tetrathiafulvalene cation radicals and their host–guest interactions with cucurbit[8]­uril. , Zhang and co-workers proposed a supramolecular approach to regulate the spin–spin interactions and reactivities of organic radicals. − In addition, spin–spin interactions have emerged as a new type of noncovalent driving force to construct sophisticated supramolecular architectures and advanced materials.…”

Electron Spin Exchange in Linked Phenothiazine–Viologen Charge Transfer Complexes Incorporated in “Through‐Ring” (Rotaxane) α‐Cyclodextrins

“…The three cases considered are: A) J ≪ a I , B) J ~ a I and C) J ≫ a I (this is shown explicitly for J ~ g β B 0 , the Zeeman energy, where B 0 is the applied magnetic field, g is the chemical shift of the unpaired electron, and β is the Bohr magneton, a constant). The transition probabilities follow directly from spin selection rules and the ratio of J to a I . For Fig.…”

Structural Factors Controlling the Spin–Spin Exchange Coupling: EPR Spectroscopic Studies of Highly Asymmetric Trityl–Nitroxide Biradicals

“…10–12 Several factors such as nature of the linkers between two spins, conformations, substituents and environment (e.g., temperature, solvent, etc) control the exchange coupling magnitude in biradicals. 27 Through conformational constraint into the co-planarity of two radical moieties with m-phenylene 10,28,29 or simple direct linkage, 30 stable trimethylenemethane (TMM)-type biradicals with large positive exchange interactions have been recently obtained which show great potential as building blocks for robust magnetic materials. In contrast, biradicals with rigid geometries holding two nitroxide moieties approximately orthogonal to one another have weak exchange coupling but show enhanced DNP properties.…”

“…The spin–spin coupling interaction can be through-bond and/or through-space, and its value varies by many orders of magnitude. − Several factors, such as the nature of the linker between the two spins, the conformation, substituents, and the environment (e.g., temperature, solvent, etc. ), control the magnitude of the exchange coupling in biradicals . Through conformational constraints to enforce coplanarity of the two radical moieties with m -phenylene ,, or simple direct linkage, stable trimethylenemethane (TMM)-type biradicals with large po­sitive exchange interactions that show great potential as building blocks for robust magnetic materials have recently been obtained.…”