Rational Synthesis and Characterization of Isatin-based Quinoidal Conjugated Molecules and Polymers for Organic Charge and Spin Transport

Abstract

Conjugated systems with quinoidal characters facilitate efficient charge transport through extended π-conjugation. Moreover, such molecular structures can stabilize open-shell diradicals via resonance, giving rise to unique intrinsic spin properties. Through structural engineering, this dissertation systematically describes the control of charge and spin transport in quinoid-based conjugated systems for organic devices. Chapter 1 establishes the paradigm shift from traditional aromatic conjugated systems to quinoidal conjugated systems, exploring the fundamental principles of how quinoidal character modulates molecular conformations and electronic states. Linking these fundamentals to device applications establishes the logical foundation for the research presented throughout this dissertation. Chapter 2 investigates the structural and property modulation of isoindigo-based conjugated systems via an asymmetric monochlorination strategy. By selectively introducing a single chlorine atom, this approach resolves the steric hindrance issues of conventional dichlorination, facilitating the development of promising acceptor units and semi-regioregular polymers. The newly synthesized monochlorinated isoindigo （1ClID） was copolymerized with centrosymmetric （2T） and axisymmetric （DTBT） donors, revealing that donor symmetry substantially influences backbone linearity and dipole moments. Notably, P1ClID-2T, which incorporates the centrosymmetric 2T unit, simultaneously demonstrated a highly linear backbone and a substantial dipole moment of 10.20 D. These properties contributed to highly ordered crystallinity with a distinct 5th-order lamellar diffraction peak. Moreover, under shear force, P1ClID-2T showed markedly improved molecular alignment, resulting in an outstanding electron mobility of 1.22 cm2 V-1 s-1—an over eight-fold improvement in organic field-effect transistor （OFET） performance. As the first report of monochlorinated isoindigo polymers, this study highlights the potential of asymmetric monochlorination for developing high-performance n-type semiconductors and provides invaluable insights into structure-property relationships. Beyond charge transport, Chapter 3 investigates the systematic modulation of open-shell polymers to enable efficient spin transport. A series of copolymers （Q-PQ-0, 0.2, and 0.5） were synthesized by precisely controlling the ratio of quinoid （Q） and pro-quinoid （PQ） open-shell moieties. The incorporation of PQ units enhanced spin-orbit coupling and stabilized the spin-parallel triplet state, leading to most superior spin properties in Q-PQ-0.5. However, the final magnetic and transport properties were predominantly governed by structural ordering rather than intrinsic spin density alone. Q-PQ-0.2 achieved the highest crystallinity via optimal structural balance; consequently, it demonstrated superior electron mobility （4.60×10-2 cm2 V-1 s-1） and robust long-range spin alignment, exhibiting a remanent magnetization （Mr） of 0.15 memu/g and a coercivity （Hc） of 89.29 Oe. In contrast, the disordered morphology of Q-PQ-0.5 hindered both charge transport and the maintenance of spin alignment, proving that synergistic control over crystallinity and spin properties is essential for high-performance organic spin materials. Based on these findings, current efforts are directed toward applying these polymers to spintronic devices to observe spin transport properties. This comprehensive research, ranging from rational molecular design to device-level practical validation, will significantly contribute to the advancement of high-performance organic spintronic technologies. Finally, Chapter 4 explores the chiral assembly of open-shell quinoids and the observation of chirality-induced spin selectivity （CISS） effect. A series of quinoid molecules with intrinsic spins （QFn） was developed and their chiral assemblies were established by blending with the chiral additive 1,1’-binaphthyl-2,2’-diamine （BN）. Systematic structural modulation via fluorine substitution primarily determined the interactions with BN, resulting in distinct chiroptical properties among the QFn. Thermal annealing promoted co-crystallization in the QF1+BN system, leading to a remarkable 30-fold chirality amplification with an absorption dissymmetry factor （gabs） of 1.23 × 10-2. In contrast, the QF2+BN system exhibited pronounced phase separation upon annealing, yielding only marginal chiral enhancement. Notably, the integration of QF1+BN co-crystals into magneto-field-effect transistors （m-FETs） enabled the clear observation of CISS. This system achieved a maximum spin polarization of 7.7%, indicating the highest degree of spin-dependent charge transport for device-level measurements among reported chiral open-shell systems. These findings provide foundational insights into the chiral assembly of open-shell molecules and the chirality–spin relationship, advancing the field of organic spintronics.