Synthesis of Trityl-based Spin Systems and Investigation of Spin-Spin Interactions for Quantum Information Science

Abstract

Quantum information science and molecular magnetism rely on precise control and understanding of spin-spin interactions in multi-spin systems. The exchange coupling constant J, which governs the magnetic interaction between unpaired electron spins, stands as a central parameter determining the ground-state spin multiplicity and the energy gap between spin states. Establishing quantitative relationships between molecular structure and exchange coupling remains a fundamental challenge requiring integration of synthetic chemistry, spectroscopy, and quantum chemical calculations. Tetrathiatriarylmethyl (trityl) radicals have emerged as adequate building blocks for investigating exchange-coupled systems due to their narrow EPR linewidth, long phase memory times, and outstanding chemical stability. The carbon-based scaffold with negligible hyperfine coupling to magnetic nuclei minimizes decoherence, while modular functionalization via palladium-catalyzed cross-coupling reactions allows for systematic structural variation with large degree of π-conjugation. These properties position trityl radicals as ideal platforms for elucidating structure-property relationships in molecular spin systems. <br/> This cumulative dissertation establishes quantitative correlations between molecular architecture and magnetic properties through systematic synthesis and detailed electron paramagnetic resonance spectroscopic investigation of trityl-based biradicals and radical-chromophore systems. The goal is to elucidate how structural parameters determine the magnitude, and sign of exchange coupling constants. <br/> Palladium-catalyzed <em>Suzuki-Miyaura</em> cross-coupling reactions allowed modular preparation of perylene diimide (PDI)-trityl dyads and trityl-PDI-trityl triads with systematic variation in linker length, chromophore substitution pattern, and number of radical centers. Multi-frequency continuous-wave and pulsed EPR measurements at cryogenic and ambient temperatures allowed for the determination of <em>g</em>-tensor components, ¹³C hyperfine coupling constants, and spin relaxation times. The PDI-trityl radicals demonstrate microsecond-scale phase memory times at room temperature, establishing their suitability for ambient condition quantum applications. Double quantum coherence measurements on trityl-PDI-trityl biradicals indicated weak antiferromagnetic exchange coupling combined with dipolar coupling, placing these systems in the intermediate coupling regime. Transient EPR investigations of photoexcited PDI-trityl dyads confirmed photogenerated quartet state formation with substantially improved coherence times compared to PDI-nitroxide analogues and demonstrated room-temperature quartet polarization arising from anisotropic molecular motion. <br/> Investigations of a Cu(II)porphyrin-trityl system provided ground-state structural analogues for photoexcited chromophore-trityl systems. Multi-frequency EPR spectroscopy combined with broken-symmetry density functional theory calculations showed ferromagnetic exchange coupling with substantial distribution width arising from thermal population of conformers with varying phenyl bridge dihedral angles. The near-perpendicular orientation of the phenyl linker disrupts π-conjugation between porphyrin and trityl, favoring ferromagnetic exchange through orthogonal magnetic orbitals, which is an unusual finding for <em>para</em>-substituted phenyl-bridged biradicals that typically yield antiferromagnetic coupling. Systematic investigation of conjugated trityl-phenylene-trityl biradicals with varying bridge lengths showed exponential decay of exchange coupling, demonstrating efficient spin conductivity through oligo(<em>p</em>-phenylene) scaffolds. Comparison of <em>meta</em>-, and <em>para</em>-phenylene connectivity showed the same sign of <em>J</em>, although with different magnitudes. The magnitude of <em>J</em> for <em>meta</em>-phenylene connectivity showed strong dependence with the environment due to conformational flexibility modulating the magnitude of <em>J</em>. <br/> These results establish important design principles for coupled trityl systems and provide experimental data for understanding spin-spin interactions in molecular architectures relevant to quantum information science and molecular magnetism. The modular synthesis developed herein allows systematic variation of structural parameters including bridge length, topology, and dihedral angles, enabling quantitative correlation of molecular structure with magnetic properties essential for rational optimization of, e.g., molecular spin qubits and qudits.