Delayed fluorescence from inverted singlet and triplet excited states.

Hund's multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration1. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund's rule and has a negative singlet-triplet energy gap of -11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 µs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers.

Chirality-Induced Magnetoresistance Due to Thermally Driven Spin Polarization.

Chirality-induced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from current-induced spin polarization in molecules. The current-induced spin polarization is widely recognized as a fundamental principle of chiral-induced spin selectivity (CISS). In this study, we investigate chirality-induced current-in-plane magnetoresistance (CIP-MR) in a chiral molecule/ferromagnetic metal bilayer at room temperature. In contrast to CPP-MR, CIP-MR observed in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests that thermally driven spontaneous spin polarization in chiral molecules is the key to the observed MR. The novel MR is consistent with recent CISS-related studies, that is, chiral molecules in contact with a metallic surface possess a finite spin polarization.

Solvent-free autocatalytic supramolecular polymerization.

Solvent-free chemical manufacturing is one of the awaited technologies for addressing an emergent issue of environmental pollution. Here, we report solvent-free autocatalytic supramolecular polymerization (SF-ASP), which provides an inhibition-free template-assisted catalytic organic transformation that takes great advantage of the fact that the product (template) undergoes a termination-free nucleation-elongation assembly (living supramolecular polymerization) under solvent-free conditions. SF-ASP allows for reductive cyclotetramerization of hydrogen-bonding phthalonitriles into the corresponding phthalocyanines in exceptionally high yields (>80%). SF-ASP requires the growing polymer to form hexagonally packed crystalline fibres, which possibly preorganize the phthalonitriles at their cross-sectional edges for their efficient transformation. With metal oleates, SF-ASP produces single-crystalline fibres of metallophthalocyanines again in exceptionally high yields, which grow in both directions without terminal coupling until the phthalonitrile precursors are completely consumed. By taking advantage of this living nature of polymerization, multistep SF-ASP without/with metal oleates allows for the precision synthesis of multi-block supramolecular copolymers.

A Design Principle for Polar Assemblies with C3 -Sym Bowl-Shaped π-Conjugated Molecules.

Polar materials attract wide research interest due to their unique properties, such as ferroelectricity and the bulk photovoltaic effect (BPVE), which are not accessible with nonpolar materials. However, in general, rationally designing polar materials is difficult because nonpolar materials are more favorable in terms of dipole-dipole interactions. Here, we report a rational strategy to form polar assemblies with bowl-shaped π-conjugated molecules and a molecular design principle for this strategy. We synthesized and thoroughly characterized 12 single crystals with the help of various theoretical calculations. Furthermore, we demonstrated that it can be possible to predict whether polar assemblies become more favorable or not by estimating their lattice energies. We believe that this study contributes to the development of organic polar materials and their related studies.

Distinct Pathways in "Thermally Bisignate Supramolecular Polymerization": Spectroscopic and Computational Studies.

In general, supramolecular polymers are thermally labile in solution and easily depolymerized upon heating. This dynamic nature is beneficial in many aspects but limits certain applications. Recently, we developed "thermally bisignate supramolecular polymerization", through which a polymer is formed upon heating as well as cooling in a hydrocarbon solvent containing a small amount of alcohol. Here, we present a detailed mechanistic picture for this polymerization based on both spectroscopic and computational studies. For this particular type of polymerization, we mainly employed a copper porphyrin derivative ((S)PORCu) as a monomer with eight hydrogen-bonding (H-bonding) amide units in its chiral side chains. Because of a strong multivalent interaction, the resulting supramolecular polymer displayed an extraordinarily high thermal stability in a hydrocarbon medium such as methylcyclohexane (MCH)/chloroform (CHCl3) (98/2 v/v; denoted as MCH*). However, when a small volume (<2.0 vol %) of ethanol (EtOH) was added to this solution at ambient temperatures as a H-bond scavenger, the supramolecular polymer dissociated into its monomers. Here, it should be noted that, both upon cooling (clustering of EtOH) and heating (lower-critical-solution-temperature behavior, LCST), the monomer was liberated from the H-bond scavenger and underwent supramolecular polymerization. In this Article, we conducted detailed spectroscopic studies, analyzed the results using theoretical models, and eventually succeeded in supporting the pathways explaining why the monomer deactivated by the H-bond scavenger turns active upon both heating and cooling. We also investigated the thermally bisignate nature of the supramolecular polymerization of other monomers such as triphenylamine ((S)TPA) and pyrene ((S)Py) derivatives together with free-base ((R)POR2H) and zinc porphyrin ((S)PORZn) derivatives and rationalized the large potential for this multicomponent supramolecular polymerization.

Thermally bisignate supramolecular polymerization.

One of the enticing characteristics of supramolecular polymers is their thermodynamic reversibility, which is attractive, in particular, for stimuli-responsive applications. These polymers usually disassemble upon heating, but here we report a supramolecular polymerization that occurs upon heating as well as cooling. This behaviour arises from the use of a metalloporphyrin-based tailored monomer bearing eight amide-containing side chains, which assembles into a highly thermostable one-dimensional polymer through π-stacking and multivalent hydrogen-bonding interactions, and a scavenger, 1-hexanol, in a dodecane-based solvent. At around 50 °C, the scavenger locks the monomer into a non-polymerizable form through competing hydrogen bonding. On cooling, the scavenger preferentially self-aggregates, unlocking the monomer for polymerization. Heating also results in unlocking the monomer for polymerization, by disrupting the dipole and hydrogen-bonding interactions with the scavenger. Analogous to 'upper and lower critical solution temperature phenomena' for covalently bonded polymers, such a thermally bisignate feature may lead to supramolecular polymers with tailored complex thermoresponsive properties.