Radiative pumping in a strongly coupled microcavity filled with a neat molecular film showing excimer emission.

Strong light-matter interactions have attracted much attention as a means to control the physical/chemical properties of organic semiconducting materials with light-matter hybrids called polaritons. To unveil the processes under strong coupling, studies on the dynamics of polaritons are of particular importance. While highly condensed molecular materials with large dipole density are ideal to achieve strong coupling, the emission properties of such films often become a mixture of monomeric and excimeric components, making the role of excimers unclear. Here, we use amorphous neat films of a new bis(phenylethynyl anthracene) derivative showing only excimer emission and investigate the excited-state dynamics of a series of strongly coupled microcavities, with each cavity being characterised by a different exciton-photon detuning. A time-resolved photoluminescence study shows that the excimer radiatively pumps the lower polariton in the relaxation process and the decay profile reflects the density of states. The delayed emission derived from triplet-triplet annihilation is not sensitive to the cavity environment, possibly due to the rapid excimer formation. Our results highlight the importance of controlling intermolecular interactions towards rational design of organic exciton-polariton devices, whose performance depends on efficient polariton relaxation pathways.

Spin-Polarized Radicals with Extremely Long Spin-Lattice Relaxation Time at Room Temperature in a Metal-Organic Framework.

The generation of spin polarization is key in quantum information science and dynamic nuclear polarization. Polarized electron spins with long spin-lattice relaxation times (T1) at room temperature are important for these applications but have been difficult to achieve. We report the realization of spin-polarized radicals with extremely long T1 at room temperature in a metal-organic framework (MOF) in which azaacene chromophores are densely integrated. Persistent radicals are generated in the MOF by charge separation after photoexcitation. Spin polarization of a triplet generated by photoexcitation is successfully transferred to the persistent radicals. Pulse electron spin resonance measurements reveal that the T1 of the polarized radical in the MOF is as long as 214 µs with a relatively long spin-spin relaxation time T2 of the radicals of up to 0.98 µs at room temperature. The achievement of extremely long spin polarization in MOFs with nanopores accessible to guest molecules will be an important cornerstone for future highly sensitive quantum sensing and efficient dynamic nuclear polarization.

Noncovalent Modification of Cycloparaphenylene by Catenane Formation Using an Active Metal Template Strategy.

The active metal template (AMT) strategy is a powerful tool for the formation of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes, allowing the synthesis of a variety of MIMs, including π-conjugated and multicomponent macrocycles. Cycloparaphenylene (CPP) is an emerging molecule characterized by its cyclic π-conjugated structure and unique properties. Therefore, diverse modifications of CPPs are necessary for its wide application. However, most CPP modifications require early stage functionalization and the direct modification of CPPs is very limited. Herein, we report the synthesis of a catenane consisting of [9]CPP and a 2,2'-bipyridine macrocycle as a new CPP analogue that contains a reliable synthetic scaffold enabling diverse and concise post-modification. Following the AMT strategy, the [9]CPP-bipyridine catenane was successfully synthesized through Ni-mediated aryl-aryl coupling. Catalytic C-H borylation/cross-coupling and metal complexation of the bipyridine macrocycle moiety, an effective post-functionalization method, were also demonstrated with the [9]CPP-bipyridine catenane. Single-crystal X-ray structural analysis revealed that the [9]CPP-bipyridine catenane forms a tridentated complex with an Ag ion inside the CPP ring. This interaction significantly enhances the phosphorescence lifetime through improved intermolecular interactions.

Proton Hyperpolarization Relay from Nanocrystals to Liquid Water.

Dynamic nuclear polarization (DNP) using transient electron spin polarization generated by photoexcitation can improve nuclear magnetic resonance (NMR) sensitivity far beyond the thermal equilibrium limit for analysis in life science and drug discovery. However, DNP of liquid water at room temperature remains an important challenge. Here, we propose a new method called hyperpolarization relay, in which the nonequilibrium polarization of electron spins is transferred to proton spins in the nanocrystals and then to proton spins in bulk water. Molecular nanocrystals doped with a polarizing agent that generates a highly polarized photoexcited triplet are synthesized by a reprecipitation method while controlling the size of the nanocrystals. The triplet-DNP sequence of repeated laser and microwave irradiation enhances the NMR signal of bulk water as well as nanocrystals. The smaller size of the nanocrystals increases the efficiency of polarization transfer from the nanocrystals to water due to the increased surface area. A series of control experiments and simulations based on Solomon equations confirmed the hyperpolarization relay mechanism.

Osmium Complex-Chromophore Conjugates with Both Singlet-to-Triplet Absorption and Long Triplet Lifetime through Tuning of the Heavy-Atom Effect.

Os(II) complexes showing singlet-to-triplet absorption are of growing interest as a new class of triplet sensitizers that circumvent energy loss during intersystem crossing, and they enable effective utilization of input photon energy in various applications, such as photoredox catalysis, photodynamic therapy, and photon upconversion. However, triplet excited-state lifetimes of Os(II) complexes are often too short (τ < 1 µs) to transfer their energy to neighboring molecules. While the covalent conjugation of chromophores has been known to extend the net excited-state lifetimes through an intramolecular triplet energy transfer (IMET), heavy-atom effects of the central metals on the attached chromophore units have rarely been discussed. Here, we investigate the relationship between the spin-density contribution of the heavy metals and the net triplet excited-state lifetimes for a series of Os(II) and Ru(II) bis(terpyridine) complexes modified with perylene units. Phosphorescence lifetimes of these compounds strongly depend on the lifetimes of the perylenyl group-localized excited states that are shortened by the heavy-atom effect. The degree of heavy-atom effect can be largely circumvented by introducing meta-phenylene bridges, where the perylene unit retains its intrinsic long excited-state lifetime. The thermal activation to the short-lived excited states is suppressed, thanks to sufficient but still small energy losses during the IMET process. Involvement of the metal center was also confirmed by the prolonged lifetime by replacing Os(II) with Ru(II) that possesses a smaller spin-orbit coupling constant. These results indicate the importance of ligand structures that give a minimum heavy-atom effect as well as the sufficient energy gap among the excited states and fast IMET for elongating the triplet excited-state lifetime without sacrificing the excitation energy.

Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal-Organic Framework.

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced-fit-type structural transformation of a crystalline yet flexible metal-organic framework (MOF). In contrast to the usual hosts, 1 H spin-lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5-fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host-guest interactions. Combined with triplet-DNP and cross-polarization (CP), this system realizes an enhanced 19 F NMR signal of guest target molecules.

A Novel Thermocell System Using Proton Solvation Entropy.

The entropy change associated with proton-coupled electron transfer (PCET) reactions significantly enhance the Seebeck coefficient (Se ) of thermocells. A redox pair of [Ru(Hx im)6 ]2+/3+ (Him=imidazole, x=0≈1) releases three protons in their one-electron redox reactions in thermocells, which gave a remarkably high Se of -3.7 mV K-1 as confirmed by temperature-dependent square wave voltammetry. The value of Se is proportional to the redox reaction entropy (ΔSrc ), which increased with the number of dissociating protons. This result demonstrates the utility of PCET reaction toward efficient thermoelectric conversion.

Design Guidelines to Elongate Spin-Lattice Relaxation Times of Porphyrins with Large Triplet Electron Polarization.

The spin-polarized triplet state generated by light irradiation has potential for applications such as triplet dynamic nuclear polarization (triplet-DNP). Recently, we have reported free-base porphyrins as versatile and biocompatible polarizing agents for triplet-DNP. However, the electron polarization of free-base porphyrins is not very high, and the dilemma is that the high polarization of metalloporphyrins is accompanied by a too short spin-lattice relaxation time to be used for triplet-DNP. We report here that the introduction of electron-withdrawing fluorine groups into Zn porphyrins enables a long enough spin-lattice relaxation time (>1 µs) while maintaining a high polarization (Px:Py:Pz = 0:0:1.0) at room temperature. Interestingly, the spin-lattice relaxation time of Zn porphyrin becomes much longer by introducing fluorine substituents, whereas the spin-lattice relaxation time of free-base porphyrin becomes shorter by the fluorine substitution. Theoretical calculations suggest that this is because the introduction of the electron-withdrawing fluorine substituents reduces the spin density on Zn atoms and weakens the spin-orbit interaction.

Discovery of Key TIPS-Naphthalene for Efficient Visible-to-UV Photon Upconversion under Sunlight and Room Light*.

While many studies have been done on triplet-triplet annihilation-based photon upconversion (TTA-UC) to produce visible light with high efficiency, the efficient TTA-UC from visible to UV light, despite its importance for a variety of solar and indoor applications, remains a challenging task. Here, we report the highest visible-to-UV TTA-UC efficiency of 20.5 % based on the discovery of an excellent UV emitter, 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Nph). TIPS-Nph is an acceptor with desirable features of high fluorescence quantum yield and high singlet generation efficiency by TTA. TIPS-Nph has a low enough triplet energy level to be sensitized by Ir(C6)2 (acac), a superior donor that does not quench UV emission. The combination of TIPS-Nph and Ir(C6)2 (acac) realizes the efficient UV light production even with weak light sources such as an AM 1.5 solar simulator and room LEDs.

Light-Triggered, Non-Centrosymmetric Self-Assembly of Aqueous Arylazopyrazoles at the Air-Water Interface and Switching of Second-Harmonic Generation.

Trans-p-methoxy arylazopyrazole spontaneously forms non-centrosymmetric polar crystals, which reversibly undergo liquefaction upon photoisomerization to the cis-isomer. This liquid cis-isomer has a large electric dipole moment and is highly soluble in water (solubility up to ≈58 mM), which is remarkably higher than that of the trans-isomer (690 µM). Vis-light illumination of the aqueous cis-isomer generates macroscopically oriented, non-centrosymmetric crystals at the air-water interface. Polar crystals are also formed in sandwich glass cells (spacing, 20 µm) upon photo-induced crystallization of the liquid cis-isomer. The trans-crystals thus formed showed second harmonic generation (SHG) whose intensity is switched on/off in response to the photo-induced phase transition.

Thermocells Driven by Phase Transition of Hydrogel Nanoparticles.

Thermoelectric conversion of low temperature, delocalized, and abundant thermal sources is crucial for the development of the Internet of Things (IoT) and/or a carbon-free society. Thermocells are of great interest in thermoelectric conversion of low-temperature heat due to the low cost and flexibility of components. However, significant improvement of the conversion efficiency is required for the practical use of the cells. Here, we report thermo-electrochemical cells driven by volume phase transition (VPT) of hydrogel nanoparticles (NPs). Entropically driven VPT of poly(N-isopropylacrylamide) NPs containing carboxylic acids and amines generates a pH gradient of up to 0.049 and -0.053 pH K-1, respectively, around physiological temperature. The pH gradient triggers the proton-coupled electron transfer (PCET) reactions of quinhydrone on the electrodes, resulting in the highly efficient thermoelectric conversion with a Seebeck coefficient (Se) of -6.7 and +6.1 mV K-1. Thermocells driven by phase transition of hydrogels provide a nontoxic, flexible, and inexpensive charger that harvests carbon-free energy from abundant energy sources such as solar, body and waste heat.

Enhanced Electric Polarization and Polar Switching of Dipolar Aromatic Liquids Confined in Supramolecular Gel Networks.

Dipolar aromatic liquids confined in the interstitial domains of chiral organogels show significantly enhanced electric polarization, as compared with those of pure liquids alone or organogels formed with nonpolar liquids. Intriguingly, nitrobenzene gels showed a supramolecular polar switching phenomenon; i.e., hysteresis in the polarization (P)-electric field (E) curves was observed for the gel above the melting point of the solvent. This indicates that the nitrobenzene molecules confined in the chiral nanofibrous gel networks exert macroscopic polarization whose direction is inversed depending on the direction of the external electric field. The anomalously enhanced electric polarization and polar switching phenomenon of supramolecular gels in varied solvents are scrutinized by the positive-up-negative-down (PUND) measurements, and the interactions between the gel nanofibers and the polar solvent molecules play crucial roles for the emergence of the polar switching phenomenon. This work presents for the first time that dipolar liquid molecules filling the interstitial space of supramolecular gels exhibit a significant confinement effect. It provides a new perspective to design electric-field-responsive soft materials based on the functional liquid domains confined in their porous networks.

Stimuli-Responsive Molecular Photon Upconversion.

The addition of stimuli-responsiveness to anti-Stokes emission provides a unique platform for biosensing and chemosensing. Particularly, stimuli-responsive photon upconversion based on triplet-triplet annihilation (TTA-UC) is promising due to its occurrence at low excitation intensity with high efficiency. This Minireview summarizes the recent developments of TTA-UC switching by external stimuli such as temperature, oxygen, chemicals, light, electric field, and mechanical force. For the systematic understanding of the underlying general mechanisms, the switching mechanisms are categorized into four types: 1) aggregation-induced UC; 2) assembly-induced air-stable UC; 3) diffusion-controlled UC; and 4) energy-transfer-controlled UC. The development of stimuli-responsive smart TTA-UC systems would enable sensing with unprecedented sensitivity and selectivity, and expand the scope of TTA-UC photochemistry by combination with supramolecular chemistry, materials chemistry, mechanochemistry, and biochemistry.

Transcription of Chirality from Metal-Organic Framework to Polythiophene.

Here, we report an unprecedented chirality transfer from a metal-organic framework (MOF) to a polymer. In this work, unsubstituted polythiophene (PTh) was prepared in the nanochannels of a chiral MOF. Circular dichroism spectroscopy revealed that nanoconfinement of the polymer chains could endow optically inactive PTh with a chiral nature. The thickness of polymer chain assemblies could be controlled by tuning the loading amount of PTh, which resulted in a drastic change in the chiroptical properties. Note that PTh liberated from the host still exhibited chirality even without the chiral support. Remarkably, the recovered PTh presented high thermal stability of chirality up to 250 °C. Our findings show that the encapsulation of the polymer chains in chiral MOFs is a simple and effective methodology not only to express the chirality of polymers but also to elucidate the inter- and intrachain chirality in polymer assemblies.

Supramolecular Crowding Can Avoid Oxygen Quenching of Photon Upconversion in Water.

A common challenge in chemistry that deals with photoexcited states is to avoid oxygen quenching. This is crucial for hot research fields such as photon upconversion (UC), in which oxygen-sensitive triplet excited states play pivotal roles. However, methods to avoid oxygen quenching in aqueous media are far more limited despite eagerly anticipated catalytic and biological applications. This work introduces a simple strategy to achieve air-stable triplet-triplet annihilation (TTA)-based UC in water, namely, supramolecular crowding. Amphiphilic cationic acceptor molecules and anions with long alkyl chains co-assemble in water in which hydrophobic donor molecules are molecularly dispersed. Despite the common notion that oxygen molecules diffuse readily across hydrophobic domains in water, more than 80 % of the TTA-UC emission of the obtained hydrophobic co-assemblies is maintained in air-saturated water. This work demonstrates the new promising potential of supramolecular chemistry for photophysical and photochemical functions with oxygen-sensitive species.

Synthesis of Chiral Labtb and Visualization of Its Enantiomeric Excess by Induced Circular Dichroism Imaging.

Crystalline particles of a microporous, robust, and chiral metal-organic framework (MOF) were synthesized and their enantiomer excess (ee) was visualized for each microparticle by CD imaging. Labtb, a thermally and chemically robust MOF, was employed in this study because it shows a chiral space group. Although Labtb has been obtained as a racemic conglomerate, enantioselective synthesis of Labtb was achieved via a chiral precursor complex consisting of lanthanum and homochiral phenylalanine. Methyl orange (MO) was introduced into the micropores of chiral Labtb, which showed a strong induced CD signal for the absorption band of MO chromophores. High ee of the chiral Labtb was revealed by microscopic CD observation at the particle-level. This result provides a facile way to obtain a robust MOF that has chiral nanospace.

Hierarchical Hybrid Metal-Organic Frameworks: Tuning the Visible/Near-Infrared Optical Properties by a Combination of Porphyrin and Its Isomer Units.

Hybrid metal-organic frameworks (MOFs) with core/shell-like hierarchical structure comprised of zirconium metal and porphyrin (e.g., TPP) and its isomer, N-confused porphyrin (NCP), were synthesized through a seed-mediated reaction. The hierarchical structures of hybrid MOFs were characterized by the microscopic image analyses (e.g., scanning electron microscope (SEM), energy dispersive X-ray (EDX) spectrometry, and confocal laser scanning microscope (CLSM)). Taking advantage of the intrinsic light-harvesting properties of the porphyrin dye and the N-confused isomer, changing the core/shell layer structures of hybrid MOFs allows for tuning of the visible-to-near-infrared (NIR) absorption/emission characters, excited-state energy migrations, and photosensitization capabilities. The Förster energy transfer event occurring in the bulk MOF samples by photoexcitation enabled us to control the photoinduced singlet oxygen generation through the comprehensive light-harvesting ability of these hybrid porphyrinic MOFs. Therefore, implementation of a precisely designed porphyrin "substitute" into the MOF-based materials indeed provides a new mimic of the photosynthetic pigment system and should be potentially applicable for solar-light-driven devices.

Triplet dynamic nuclear polarization of nanocrystals dispersed in water at room temperature.

While dynamic nuclear polarization using photo-excited triplet electrons (triplet-DNP) can improve the sensitivity of nuclear magnetic resonance at room temperature, it has not been carried out in water. Here, we report the first example of triplet-DNP in water by downsizing the conventional bulk crystals to nanocrystals.

Absolute Method to Certify Quantum Yields of Photon Upconversion via Triplet-Triplet Annihilation.

For the consistent development of the field of photon upconversion via triplet-triplet annihilation (TTA-UC), it is pivotal to know the true quantum yield of TTA-UC emission. Although TTA-UC quantum yields have been determined by common relative measurements using quantum yield standards, there is still a discrepancy between the reported values even for the benchmark sensitizer-emitter pair of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA). Here, to resolve this situation, we show a method to obtain the absolute quantum yield of TTA-UC photoluminescence. The difficulty in obtaining absolute TTA-UC quantum yield by the integrating sphere measurement is to accurately calibrate the contribution of reabsorbed upconverted emission by triplet sensitizers. The reabsorption correction is successfully carried out by comparing sensitizer phosphorescence with and without the integrating sphere. An absolute TTA-UC quantum yield of the PtOEP-DPA pair is obtained as 36%, which shows a good agreement with the relative TTA-UC quantum yield. An absolute TTA-UC quantum yield of another red-to-blue TTA-UC pair, platinum(II) meso-tetraphenyltetrabenzoporphyrin (PtTPBP) and 2,5,8,11-tetra-tert-butylperylene (TTBP), is obtained as 27%. These absolute TTA-UC quantum yields can be used as certified values to check the measurement setup and sample condition for determining relative TTA-UC quantum yields in each laboratory.

Near-Infrared Optogenetic Genome Engineering Based on Photon-Upconversion Hydrogels.

Photon upconversion (UC) from near-infrared (NIR) light to visible light has enabled optogenetic manipulations in deep tissues. However, materials for NIR optogenetics have been limited to inorganic UC nanoparticles. Herein, NIR-light-triggered optogenetics using biocompatible, organic TTA-UC hydrogels is reported. To achieve triplet sensitization even in highly viscous hydrogel matrices, a NIR-absorbing complex is covalently linked with energy-pooling acceptor chromophores, which significantly elongates the donor triplet lifetime. The donor and acceptor are solubilized in hydrogels formed from biocompatible Pluronic F127 micelles, and heat treatment endows the excited triplets in the hydrogel with remarkable oxygen tolerance. Combined with photoactivatable Cre recombinase technology, NIR-light stimulation successfully performs genome engineering resulting in the formation of dendritic-spine-like structures of hippocampal neurons.