Andrew H. Comstock

Materials with exceptionally low thermal conductivity are desirable for thermal insulation and waste heat recovery. While foams and aerogels boast ultralow thermal conductivities akin to air, lack of mechanical stiffness in these soft materials necessitates a paradigm shift in materials design that can offer thermal insulation and mechanical rigidity simultaneously. Here, we show that spun-cast layered hybrid organic-inorganic perovskite thin films, azobenzene ethyl ammonium lead iodides, exhibit a record-low thermal conductivity, down to ∼0.04 watts per meter per kelvin at room temperature, while maintaining mechanical rigidity with an elastic modulus of 7.7 gigapascals that surpasses that of most plastics, foams, and aerogels. This unusual combination of ultralow thermal conductivity and high mechanical rigidity is attributed to the specially engineered organic cations in the layered structure. Our finding highlights the potential of molecular engineering in hybrid layered structures to push the extreme of thermal insulation in dense, rigid solids.

The combination of semiconducting properties and synthetically tunable chirality in chiral metal halide semiconductors (CMHSs) offers a compelling platform for room temperature control of electronic spin properties, leveraging effects such as chirality-induced spin selectivity (CISS). We report CISS-induced magnetoresistance (CISS-MR) exceeding 100% for spin valves operating at room temperature. The high CISS-MR is attributed to an interfacial tunneling barrier, modulated by chirality and spin, producing current dissymmetry factors ( g _c ) that surpass the limit imposed by the Julliere model, which is governed by the intrinsic spin polarization of the adjacent ferromagnetic (FM) contact. The CISS-MR exhibits a dependence on the CMHS composition, revealing structure-property relationships between CISS and structural chirality. The observed exceptionally large tunneling MR response differs from a subtle anisotropic MR arising from the proximity effect at the FM/CMHS interface in the absence of a tunneling barrier. Our study provides insights into charge-to-spin interconversion in chiral semiconductors, offering design principles to control and enhance the CISS response for functional platforms.

Doping is a cornerstone strategy for enhancing charge transport in semiconducting polymers, important for their application in, for example, semi‐transparent electrode materials, thermoelectric devices, and antistatic coatings. Both chemical and electrochemical doping have, for this purpose, been the focus of extensive research resulting in considerable progress. However, the interactions between neutral excitons and doping‐induced charges to form multi‐particle states are largely unexplored in soft organic semiconductors, and their signatures remain poorly understood. Here, we demonstrate that coupling between excitons and polarons in doped polymers can lead to bound states such as trions (i.e., quasiparticles of an electron and two holes delocalized across three chromophores for p ‐doping), or bound exciton‐hole pairs. Combining spectroscopic evidence with theoretical insights, we hypothesize that polymer architecture, dopant chemistry, and charge delocalization govern the formation and stability of these multi‐particle states. More broadly, our findings reveal that trions and bound exciton‐hole pairs—that is, three‐body entities—are a key species in organic semiconductors that could open new pathways toward optoelectronic functionalities beyond conventional doping, including enhanced charge transport and quantum‐coherent excitations.

Conjugated organic molecules with open‐shell diradical character ( y ) possess two weakly paired electron spins interacting across their constituent π ‐systems. These materials provide fundamental insight into the nature of electron pairing, enabling the utilization of the spin degree of freedom within emerging technologies. However, materials systems that synergistically offer high modularity, tunable y , high chemical stability, and interrelated (opto)electronic functionalities remain limited. Here, we report the facile synthesis of donor‐acceptor‐donor diradicaloids comprised of a central electron‐deficient 6,7,8,9‐tetrachloro‐[1,2,5]thiadiazolo[3,4‐ b ]phenazine acceptor flanked by electron‐rich thiophene‐based donors. Nuclear magnetic resonance and electron paramagnetic resonance spectroscopies, and theoretical investigations that account for the multiconfigurational nature of these species, connect a narrowing of the singlet‐triplet splitting (∆ E _ST ), extension of π ‐conjugation, and electronic correlations with the evolution of diradical character. These data demonstrate that the differences in structural, electronic, spin, magnetic, physicochemical, and transport properties of the materials can be modulated, while the inherent multireference nature of the electronic structure can be predicted using optimally tuned long‐range corrected Mixed‐Reference Spin‐Flip Time‐Dependent Density Functional Theory. These insights enable facile access to a broader range of open‐shell materials and facilitate the manipulation of important properties such as electronic structure, topology, exchange, and interrelated optoelectronic and transport functionalities.

Chirality-induced spin selectivity (CISS) phenomena arise from an interplay among structural chirality, electron spin orientation, and charge current. Steady-state observations such as magnetoresistance offer little insight into the timescales that govern the spin-charge interconversion and often conflate interfacial and bulk phenomena. By contrast, inverse CISS involves the conversion of spin to a charge current. Using terahertz (THz) emission spectroscopy, we directly measured an ultrafast charge current due to inverse CISS with picosecond time resolution. Polarity and polarization analysis of the THz emission map the induced charge current direction upon spin injection. We found that a charge current is generated along the spin orientation that changes direction with stereochemical configuration. These observations directly demonstrate the inherent coupling between spin and charge currents in chiral systems, offering key insights into their fundamental dynamics.

Device-level implementation of soft materials for energy conversion and thermal management demands a comprehensive understanding of their thermal conductivity and elastic modulus to mitigate thermo-mechanical challenges and ensure long-term stability. Thermal conductivity and elastic modulus are usually positively correlated in soft materials, such as amorphous macromolecules, which poses a challenge to discover materials that are either soft and thermally conductive or hard and thermally insulative. Here, we show anomalous correlations of thermal conductivity and elastic modulus in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIP) by engineering the molecular interactions between organic cations. By replacing conventional alkyl-alkyl and aryl-aryl type organic interactions with mixed alkyl-aryl interactions, we observe an enhancement in elastic modulus with a reduction in thermal conductivity. This anomalous dependence provides a route to engineer thermal conductivity and elastic modulus independently and a guideline to search for better thermal management materials. Further, introducing chirality into the organic cation induces a molecular packing that leads to the same thermal conductivity and elastic modulus regardless of the composition across all half-chiral 2D HOIPs. This finding provides substantial leeway for further investigations in chiral 2D HOIPs to tune optoelectronic properties without compromising thermal and mechanical stability.

The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.

Coupling of spin and charge currents to structural chirality in non-magnetic materials, known as chirality-induced spin selectivity, is promising for application in spintronic devices at room temperature. Although the chirality-induced spin selectivity effect has been identified in various chiral materials, its Onsager reciprocal process, the inverse chirality-induced spin selectivity effect, remains unexplored. Here we report the observation of the inverse chirality-induced spin selectivity effect in chiral assemblies of π-conjugated polymers. Using spin-pumping techniques, the inverse chirality-induced spin selectivity effect enables quantification of the magnitude of the longitudinal spin-to-charge conversion driven by chirality-induced spin selectivity in different chiral polymers. By widely tuning conductivities and supramolecular chiral structures via a printing method, we found a very long spin relaxation time of up to several nanoseconds parallel to the chiral axis. Our demonstration of the inverse chirality-induced spin selectivity effect suggests possibilities for elucidating the puzzling interplay between spin and chirality, and opens a route for spintronic applications using printable chiral assemblies.