Macrocyclic Parallel Dimer Showing Quantum Coherence of Quintet Multiexcitons at Room Temperature.

Singlet fission (SF) is a promising approach in quantum information science because it can generate spin-entangled quintet triplet pairs by photoexcitation independent of temperature. However, it is still challenging to rationally achieve quantum coherence at room temperature, which requires precise control of the orientation and dynamics of triplet pairs. Here we show that the quantum coherence of quintet multiexcitons can be achieved at room temperature by arranging two pentacene chromophores in parallel and in close proximity within a macrocycle. By making dynamic covalent Schiff-base bonds between aldehyde-modified pentacene derivatives, macrocyclic parallel dimer-1 (MPD-1) can be selectively synthesized in a high yield. MPD-1 exhibits fast subpicosecond SF in polystyrene film and generates spin-polarized quintet multiexcitons. Furthermore, the coherence time T2 of the MPD-1 quintet is as long as 648 ns, even at room temperature. This macrocyclic parallel dimer strategy opens up new possibilities for future quantum applications using molecular multilevel qubits.

Boosting charge separation in organic photovoltaics: unveiling dipole moment variations in excited non-fullerene acceptor layers.

The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has reached more than 19% due to the rapid development of non-fullerene acceptors (NFAs). To compete with the PCEs (26%) of commercialized silicon-based inorganic photovoltaics, the drawback of OPVs should be minimized. This drawback is the intrinsic large loss of open-circuit voltage; however, a general approach to this issue remains elusive. Here, we report a discovery regarding highly efficient NFAs, specifically ITIC. We found that charge-transfer (CT) and charge dissociation (CD) can occur even in a neat ITIC film without the donor layer. This is surprising, as these processes were previously believed to take place exclusively at donor/acceptor heterojunctions. Femtosecond time-resolved visible to mid-infrared measurements revealed that in the neat ITIC layers, the intermolecular CT immediately proceeds after photoirradiation (<0.1 ps) to form weakly-bound excitons with a binding energy of 0.3 eV, which are further dissociated into free electrons and holes with a time-constant of 56 ps. Theoretical calculations indicate that stacking faults in ITIC (i.e., V-type molecular stacking) induce instantaneous intermolecular CT and CD in the neat ITIC layer. In contrast, J-type stacking does not support such CT and CD. This previously unknown pathway is triggered by the larger dipole moment change on the excited state generated at the lower symmetric V-type molecular stacking of ITIC. This is in sharp contrast with the need of sufficient energy offset for CT and CD at the donor-acceptor heterojunction, leading to the significant voltage loss in conventional OPVs. These results demonstrate that the rational molecular design of NFAs can increase the local dipole moment change on the excited state within the NFA layer. This finding paves the way for a groundbreaking route toward the commercialization of OPVs.

Nonfullerene Acceptors Bearing Spiro-Substituted Bithiophene Units in Organic Solar Cells: Tuning the Frontier Molecular Orbital Distribution to Reduce Exciton Binding Energy.

The development of nonfullerene acceptors (NFAs), represented by ITIC, has contributed to improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Although tuning the electronic structures to reduce the exciton binding energy (Eb) is considered to promote photocharge generation, a rational molecular design for NFAs has not been established. In this study, we designed and developed two ITIC-based NFAs bearing spiro-substituted bithiophene or biphenyl units (named SpiroT-DCI and SpiroF-DCI) to tune the frontier molecular orbital (FMO) distribution of NFAs. While the highest occupied molecular orbitals (HOMOs) of SpiroF-DCI and ITIC are delocalized in the main π-conjugated framework, the HOMO of SpiroT-DCI is distributed on the bithiophene unit. Reflecting this difference, SpiroT-DCI exhibits a smaller Eb than either SpiroF-DCI or ITIC, and exhibits greater external quantum efficiency in single-component OSCs. Furthermore, SpiroT-DCI shows improved PCEs for bulk-heterojunction OSCs with a donor of PBDB-T, compared with that of either SpiroT-DCI or ITIC. Time-resolved spectroscopy measurements show that the photo-induced intermolecular charge separation is effective even in pristine SpiroT-DCI films. This study highlights the introduction of spiro-substituted bithiophene units that are effective in tuning the FMOs of ITIC, which is desirable for reducing the Eb and improving the PCE in OSCs.

Visible-light-induced direct C-H alkylation of polycyclic aromatic hydrocarbons with alkylsulfones.

Polycyclic aromatic hydrocarbons (PAHs) are fragments of graphene that have attracted considerable attention as a new class of carbon-based materials. The functionalization of edge positions in PAHs is important to enable the modulation of physical and chemical properties essential for various applications. However, straightforward methods that combine functional group tolerance and regioselectivity remain sought after. Here we report a photochemical approach for the direct alkylation of carbon-hydrogen bonds in PAHs that takes place in a regiospecific manner, an outcome that has never been achieved in related thermal reactions. A reaction mechanism involving a single electron transfer process from photo-excited PAHs to sulfones, and a rationale for the origin of regioselectivity are proposed on the basis of spectroscopic analyses and theoretical calculations.

End-to-End Bent Perylene Bisimide Cyclophanes by Double Sulfur Extrusion.

Bending inherently planar π-cores consisting of only six-membered rings has traditionally been challenging because a powerful transformation is required to compensate for the significant strain energy associated with bending. Herein, we demonstrate that sulfur extrusion can achieve substantial molecular bending of a perylene structure to form a substructure of a Vögtle belt, a proposed yet hitherto elusive carbon nanotube fragment. Bent perylene bisimide (PBI) derivatives were synthesized through a double-sulfur-extrusion reaction from the corresponding sulfur-containing V-shaped precursors with an internal alkyl tether. The effect of bending the inherently planar PBI core, which is a recent topic of interest for the design of advanced organic electronic and optoelectronic materials, was investigated systematically. Increasing the curvature leads to a red shift in the absorption and emission spectra, while the fluorescence quantum yields remain high. This stands in contrast with the nonemissive features of previously reported nonplanar PBI derivatives based on conjugative tethers. Detailed photophysical measurements indicated that the increasing curvature with shorter alkyl tethers (i) slightly facilitates intersystem crossing and (ii) significantly suppresses the internal conversion in the excited state of the present bent PBI derivatives. The latter characteristics originate from the restricted dynamic motion associated with the charge-transfer (CT) character between the core chromophores and the N-aryl units.

Efficient Spin Interconversion by Molecular Conformation Dynamics of a Triplet Pair for Photon Up-Conversion in an Amorphous Solid.

Solid-state materials with improved light-to-energy conversions in organic photovoltaics and in optoelectronics are expected to be developed by realizing efficient triplet-triplet annihilation (TTA) by manipulating the spin conversion processes to the singlet state. In this study, we elucidate the spin conversion mechanism for delayed fluorescence by TTA from a microscopic view of the molecular conformations. We examine the time evolution of the electron spin polarization of the triplet-pair state (TT state) in an amorphous solid-state system exhibiting highly efficient up-conversion emission by using time-resolved electron paramagnetic resonance. We clarified that the spin-state population of the singlet TT increased through the spin interconversion from triplet and quintet TT states during exciton diffusion with random orientation dynamics between the two triplets for the modulation of the exchange interaction, achieving a high quantum yield of up-conversion emission. This understanding provides us with a guide for the development of efficient light-to-energy conversion devices utilizing TTA.

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.

The Effect of Torsional Motion on Multiexciton Formation through Intramolecular Singlet Fission in Ferrocene-Bridged Pentacene Dimers.

A series of ferrocene(Fc)-bridged pentacene(Pc)-dimers [Fc-Ph(2,n)-(Pc)2 : n=number of phenylene spacers] were synthesized to examine the tortional motion effect of Fc-terminated phenylene linkers on strongly coupled quintet multiexciton (5 TT) formation through intramolecular singlet fission (ISF). Fc-Ph(2,4)-(Pc)2 has a relatively small electronic coupling and large conformational flexibility according to spectroscopic and theoretical analyses. Fc-Ph(2,4)-(Pc)2 exhibits a high-yield 5 TT together with quantitative singlet TT (1 TT) generation through ISF. This demonstrates a much more efficient ISF than those of other less flexible Pc dimers. The activation entropy in 1 TT spin conversion of Fc-Ph(2,4)-(Pc)2 is larger than those of the other systems due to the larger conformational flexibility associated with the torsional motion of the linkers. The torsional motion of linkers in 1 TT is attributable to weakened metal-ligand bonding in the Fc due to hybridization of the hole level of Pc to Fc in 1 TT unpaired orbitals.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure.

Invited for the cover of this issue are the groups of Kazuteru Usui and Satoru Karasawa at Showa Pharmaceutical University and Yasuhiro Kobori of Kobe University. The image depicts chirality control of helical compounds through cycles of photocleavage and recombination under sunlight with a "Jack and the Beanstalk" motif. Read the full text of the article at 10.1002/chem.202302413.

Biomimetic design of an α-ketoacylphosphonium-based light-activated oxygenation auxiliary.

The biomimetic design of a transition metal complex based on the iron(iv)-oxo porphyrin π-cation radical species in cytochrome P450 enzymes has been studied extensively. Herein, we translate the functions of this iron(iv)-oxo porphyrin π-cation radical species to an α-ketoacyl phosphonium species comprised of non-metal atoms and utilize it as a light-activated oxygenation auxiliary for ortho-selective oxygenation of anilines. Visible light irradiation converts the α-ketoacyl phosphonium species to the excited state, which acts as a transiently generated oxidant. The intramolecular nature of the process ensures high regioselectivity and chemoselectivity. The auxiliary is easily removable. A one-pot protocol is also described.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure.

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy.

Increasing levels of CO2 in the atmosphere is a problem that must be urgently resolved if the rise in current global temperatures is to be slowed. Chemically reducing CO2 into compounds that are useful as energy sources and carbon-based materials could be helpful in this regard. However, for the CO2 reduction reaction (CO2RR) to be operational on a global scale, the catalyst system must: use only renewable energy, be built from abundantly available elements and not require high-energy reactants. Although light is an attractive renewable energy source, most existing CO2RR methods use electricity and many of the catalysts used are based on rare heavy metals. Here we present a transition-metal-free catalyst system that uses an organohydride catalyst based on benzimidazoline for the CO2RR that can be regenerated using a carbazole photosensitizer and visible light. The system is capable of producing formate with a turnover number exceeding 8,000 and generates no other reduced products (such as H2 and CO).

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State.

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H2BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C61-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

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.

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers.

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)n : n=2, 4, 6]. Then, a comparison of (Tc)n and Tc-hetero-oligomer [TcF3 -(Tc)4 -TcF3 ] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF3 units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: ΦTrT =176 %).

Diphenyldihydropentalenediones: Wide Singlet-Triplet Energy Gap Compounds Possessing the Planarly Fixed Diene Subunit.

2,2,5,5-Tetramethyl-3,6-diphenyl-2,5-dihydropentalene-1,4-dione (PD-H) and its dimethoxy (PD-OCH3) and bis(trifluoromethyl) derivatives (PD-CF3) were developed as a new class of compounds possessing a wide excited singlet-triplet energy gap. The PD derivatives would also have a high energy level of the triplet-excited state (E T) due to the planarity of the fused-diene subunit. The results of photophysical studies revealed that the energy level of the singlet-excited state (E S) and E T of PD-H are 2.88 and 1.43 eV, respectively. These values indicate that PD-H has the energy relationship, E S > 2E T, required for it to be a singlet fission (SF) material. Moreover, the introduction of electron-donating or -withdrawing groups on the benzene rings in PD-H enables fine-tuning of E S and E T. The results of transient absorption spectroscopic studies show that PD-H, PD-OCH3, and PD-CF3 in CH2Cl2 have respective T1 lifetimes of 71, 118, and 107 µs, which are long enough to utilize its triplet exciton in other optoelectronic systems. These findings suggest that the PDs are potential candidates for SF materials with high E T levels.

Photochromism of colloidal ZnO nanocrystal powders under ambient conditions.

Zinc oxide (ZnO) nanocrystals (NCs) exhibit photochromic reactions under specific conditions upon ultraviolet light irradiation. Since the color is originated from the excited electrons at the conduction band of ZnO NCs, the photoinduced absorption is observed only in the solution with hole acceptors under inert conditions. ZnO is earth-abundant and less toxic than many other substances, and has been widely used in various industrial fields. If the photochromic reaction of ZnO can be observed consistently under ambient conditions, the material may pave the way for large-scale photochromic applications such as in pigments, windows, and building materials in addition to conventional photochromic applications. In this study, we synthesize hydrophilic ZnO NCs and observe the solid-state photochromic reactions in the visible to mid-infrared regions even in humid-air conditions. We reveal that the coloration of powders of ZnO NCs under ambient conditions originates mainly from two factors: (1) charge separation induced by hole trapping by water molecules adsorbed on the surface of NCs, and (2) deceleration of the reactions involving the electrons in the conduction band of ZnO NCs with molecular oxygen and the adsorbed water molecules.

Caging and photo-triggered uncaging of singlet oxygen by excited state engineering of electron donor-acceptor-linked molecular sensors.

Singlet oxygen (1O2), one of the most sought-after species in oxidative chemical reactions and photodynamic cancer therapy, is activated and neutralized in the atmosphere and living cells. It is essential to see "when" and "where" 1O2 is produced and delivered to understand and utilize it. There is an increasing demand for molecular sensor tools to capture, store, and supply 1O2, controlled by light and engineered singlet and triplet states, indicating the 1O2-capturing-releasing state. Here, we demonstrate the outstanding potential of an aminocoumarin-methylanthracene-based electron donor-acceptor molecule (1). Spectroscopic measurements confirm the formation of an endoperoxide (1-O2) which is not strongly fluorescent and remarkably different from previously reported 1O2 sensor molecules. Moreover, the photoexcitation on the dye in 1-O2 triggers fluorescence enhancement by the oxidative rearrangement and a competing 1O2 release. The unique ability of 1 will pave the way for the spatially and temporally controlled utilization of 1O2 in various areas such as chemical reactions and phototherapies.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System.

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.