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.

Room-temperature quantum coherence of entangled multiexcitons in a metal-organic framework.

Singlet fission can generate an exchange-coupled quintet triplet pair state 5TT, which could lead to the realization of quantum computing and quantum sensing using entangled multiple qubits even at room temperature. However, the observation of the quantum coherence of 5TT has been limited to cryogenic temperatures, and the fundamental question is what kind of material design will enable its room-temperature quantum coherence. Here, we show that the quantum coherence of singlet fission-derived 5TT in a chromophore-integrated metal-organic framework can be over hundred nanoseconds at room temperature. The suppressed motion of the chromophores in ordered domains within the metal-organic framework leads to the enough fluctuation of the exchange interaction necessary for 5TT generation but, at the same time, does not cause severe 5TT decoherence. Furthermore, the phase and amplitude of quantum beating depend on the molecular motion, opening the way to room-temperature molecular quantum computing based on multiple quantum gate control.

Radical qubits photo-generated in acene-based metal-organic frameworks.

A series of metal-organic frameworks (MOFs) assembled with diazatetracene (DAT)-based linkers were synthesized and characterized. Despite different chromophore orientations and spacings, photoinduced persistent radicals were generated in all the MOFs, and their spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) were found to be relatively long even at room temperature. The generality of long T1 and T2 values of photogenerated radicals in the chromophore-assembled MOFs provides a new platform towards quantum sensing applications.

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.

Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density.

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1-4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g-1 that exceeds previous records for well-defined molecular STFs.

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.

Singlet fission as a polarized spin generator for dynamic nuclear polarization.

Singlet fission (SF), converting a singlet excited state into a spin-correlated triplet-pair state, is an effective way to generate a spin quintet state in organic materials. Although its application to photovoltaics as an exciton multiplier has been extensively studied, the use of its unique spin degree of freedom has been largely unexplored. Here, we demonstrate that the spin polarization of the quintet multiexcitons generated by SF improves the sensitivity of magnetic resonance of water molecules through dynamic nuclear polarization (DNP). We form supramolecular assemblies of a few pentacene chromophores and use SF-born quintet spins to achieve DNP of water-glycerol, the most basic biological matrix, as evidenced by the dependence of nuclear polarization enhancement on magnetic field and microwave power. Our demonstration opens a use of SF as a polarized spin generator in bio-quantum technology.

Dynamic Electron Polarization Lasting More Than 10 µs by Hybridizing Porphyrin and TEMPO with Flexible Linkers.

Dynamic electron polarization (DEP), induced by quenching of photoexcited species by stable radicals, can hyperpolarize electron spins in solution at room temperature. Recently, development of technologies based on electron spin polarization such as dynamic nuclear polarization (DNP) has been progressing, where it is important to design molecules that achieve long-lasting DEP in addition to high DEP. Hybridization by linking dyes and radicals is a promising approach for efficient DEP, but strong interactions between neighboring dyes and radicals often result in the rapid decay of DEP. In this study, we introduce a flexible linker into the hybrid system of porphyrin and TEMPO to achieve both efficient DEP and long-lasting DEP. The structural flexibility of the linker switches the interaction between the radical and the triplet, which promotes the DEP process by bringing the radical and the triplet into close proximity, while avoiding abrupt relaxation due to strong interactions. As a result, the new hybridized system exhibits a larger DEP than the unlinked system, while at the same time achieving a DEP lasting more than 10 µs.

Recyclable optical bioplastics platform for solid state red light harvesting via triplet-triplet annihilation photon upconversion.

Sustainable photonics applications of solid-state triplet-triplet annihilation photon upconversion (TTA-UC) are limited by a small UC spectral window, low UC efficiency in air, and non-recyclability of polymeric materials used. In a step to overcome these issues, we have developed new recyclable TTA-UC bioplastics by encapsulating TTA-UC chromophores liquid inside the semicrystalline gelatin films showing broad-spectrum upconversion (red/far-red to blue) with high UC efficiency in air. For this, we synthesized a new anionic annihilator, sodium-TIPS-anthracene-2-sulfonate (TIPS-AnS), that combined with red/far-red sensitizers (PdTPBP/Os(m-peptpy)2(TFSI)2), a liquid surfactant Triton X-100 reduced (TXr) and protein gelatin (G) formed red/far-red to blue TTA-UC bioplastic films just by air drying of their aqueous solutions. The G-TXr-TIPS-AnS-PdTPBP film showed record red to blue (633 to 478 nm) TTA-UC quantum yield of 8.5% in air. The high UC quantum yield has been obtained due to the fluidity of dispersed TXr containing chromophores and oxygen blockage by gelatin fibers that allowed efficient diffusion of triplet excited chromophores. Further, the G-TXr-TIPS-AnS-Os(m-peptpy)2(TFSI)2 bioplastic film displayed far-red to blue (700-730 nm to 478 nm) TTA-UC, demonstrating broad-spectrum photon harvesting. Finally, we demonstrated the recycling of G-TXr-TIPS-AnS-PdTPBP bioplastics by developing a downstream approach that gives new directions for designing future recyclable photonics bioplastic materials.

Far-red triplet sensitized Z-to-E photoswitching of azobenzene in bioplastics.

We report the first example of direct far-red triplet sensitized molecular photoswitching in a condensed phase wherein a liquid azobenzene derivative (Azo1) co-assembled within a liquid surfactant-protein film undergoes triplet sensitized Z-to-E photoswitching upon far-red/red light excitation in air. The role of triplet sensitization in photoswitching has been confirmed by quenching of sensitizer phosphorescence by Z-Azo1 and temperature-dependent photoswitching experiments. Herein, we demonstrate new biosustainable fabrication designs to address key challenges in solid-state photoswitching, effectively mitigating chromophore aggregation and requirement of high energy excitations by dispersing the photoswitch in the trapped liquid inside the solid framework and by shifting the action spectrum from blue-green light (450-560 nm) to the far-red/red light (740/640 nm) region.

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.

Near-infrared vapochromism in lipid-packaged mixed-valence coordination polymers.

Reversible vapochromism in the NIR region is achieved for a mixed-valence platinum complex with lipid counterions, from which exclusion of crystallization water by organic vapor alters the lipid molecular orientation, which amplifies the information to changes in the 1D coordination structure and the electronic state.

Design Guidelines for Rigid Epoxy Resins with High Photon Upconversion Efficiency: Critical Role of Emitter Concentration.

For the practical application of triplet-triplet annihilation-based photon upconversion (TTA-UC), the development of rigid, transparent, air-stable, and moldable materials with a high TTA-UC efficiency remains a challenging issue. In addition to the noncovalent introduction of ionic liquid emitters into the epoxy network, we covalently introduce emitters with polymerization sites to increase the emitter concentration to 35.6 wt %. A TTA-UC quantum yield ΦUC of 5.7% (theoretical maximum: 50%) or a TTA-UC efficiency ηUC of 11.4% (theoretical maximum: 100%) is achieved, which is the highest value ever achieved for a rigid polymer material. More importantly, the high emitter concentration speeds up the triplet diffusion and suppresses the back energy transfer from the emitter to sensitizer so that the sensitized emitter triplet can be effectively utilized for TTA. The generality of our finding is also confirmed for epoxy resins of similar emitter unit concentrations without the ionic liquid. This work provides important design guidelines for achieving highly efficient TTA-UC in rigid solid materials, which has been very difficult to achieve in the past. Furthermore, the solid-state TTA-UC exhibits high air stability, reflecting the high oxygen barrier performance of epoxy resins. The high moldability of epoxy resins allows the construction of upconversion materials with complex geometries at nano- to macroscopic scales.

Nanoencapsulated Phase-Change Materials: Versatile and Air-Tolerant Platforms for Triplet-Triplet Annihilation Upconversion.

Efficient and long-term stable triplet-triplet annihilation upconversion (TTA-UC) can be achieved by effectively protecting the excited organic triplet ensembles from photoinduced oxygen quenching, and discovery of a new material platform that promotes TTA-UC in ambient conditions is of paramount importance for practical applications. In this study, we present the first demonstration of an organic nonparaffin phase-change material (PCM) as an air-tolerant medium for TTA-UC with a unique solid-liquid phase transition in response to temperature variation. For the proposed concept, 2,4-hexadien-1-ol is used and extensively characterized with several key features, including good solvation capacity, mild melting point (30.5 °C), and exclusive antioxidant property, enabling a high-efficiency, low-threshold, and photostable TTA-UC system without energy-intensive degassing processes. In-depth characterization reveals that the triplet diffusion among the transient species, i.e., 3sensitizer* and 3acceptor*, is efficient and well protected from oxygen quenching in both aerated liquid- and solid-phase 2,4-hexadien-1-ol. We also propose a new strategy for the nanoencapsulation of PCM by employing hollow mesoporous silica nanoparticles as vehicles. This scheme is applicable to both aqueous- and solid-phase TTA-UC systems as well as suitable for various applications, such as thermal energy storage and smart drug delivery.

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.

Green-to-UV photon upconversion enabled by new perovskite nanocrystal-transmitter-emitter combination.

The first example of triplet-triplet annihilation-based photon upconversion (TTA-UC) from green light to ultraviolet (UV) light sensitized by lead halide perovskite nanocrystals is demonstrated. The combination of a new transmitter that extracts triplet energy from perovskite and a UV emitter with a low triplet energy level lengthens the excitation wavelength of perovskite-sensitized upconverted UV emission.

Liquid-Based Multijunction Molecular Solar Thermal Energy Collection Device.

Photoswitchable molecules-based solar thermal energy storage system (MOST) can potentially be a route to store solar energy for future use. Herein, the use of a multijunction MOST device that combines various photoswitches with different onsets of absorption to push the efficiency limit on solar energy collection and storage is explored. With a parametric model calculation, it is shown that the efficiency limit of MOST concept can be improved from 13.0% to 18.2% with a double-junction system and to 20.5% with a triple-junction system containing ideal, red-shifted MOST candidates. As a proof-of-concept, the use of a three-layered MOST device is experimentally demonstrated. The device uses different photoswitches including a norbornadiene derivative, a dihydroazulene derivative, and an azobenzene derivative in liquid state with different MOSTproperties, to increase the energy capture and storage behavior. This conceptional device introduces a new way of thinking and designing optimal molecular candidates for MOST, as much improvement can be made by tailoring molecules to efficiently store solar energy at specific wavelengths.

High seebeck coefficient in middle-temperature thermocell with deep eutectic solvent.

Deep eutectic solvent (DES) was applied to the solvent of thermocell and high Seebeck coefficient (Se) of the thermocell was achieved at high-temperatures operation. The Se of a redox couple of ferricyanide and ferrocyanide ([Fe(CN)6]3-/4-) reaches - 1.67 mV/K in a DES consisting of ethylene glycol and choline chloride. Spectroscopic analysis reveals that this is due to the strong interactions between the redox couple and the DES. Furthermore, the cell can operate over a wide temperature range of 135-165 °C. This result is a desired feature for waste-heat recovery applications.

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.