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.

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.

Protein Photodamaging Activity and Photocytotoxic Effect of an Axial-Connecting Phosphorus(V)porphyrin Trimer.

An axial-connecting trimer of the porphyrin phosphorus(V) complex was synthesized to evaluate the relaxation process of the photoexcited state and the photosensitizer activity. The photoexcitation energy was localized on the central unit of the phosphorus(V)porphyrin trimer. The photoexcited state of the central unit was relaxed through a process similar to that of the monomer phosphorus(V)porphyrin. The excited state of this axially connected type of phosphorus(V)porphyrin trimer was not deactivated through intramolecular electron transfer. The singlet oxygen generation quantum yield of the trimer was almost the same as that of the monomer. The phosphorus(V)porphyrin, trimer, and monomer bound to human serum albumin and oxidized the tryptophan residue via singlet oxygen generation and electron transfer during visible light irradiation. The photocytotoxicity of these phosphorus(V)porphyrins on two cell lines was examined. The monomer induced photocytotoxicity; however, the trimer did not show cytotoxicity with or without photoirradiation. In summary, the photoexcited state of the trimer was almost the same as that of the monomer, and these phosphorus(V)porphyrins demonstrated a similar protein-photodamaging activity. The difference in association between the photosensitizer molecules and cells is the key factor of phototoxicity by these phosphorus(V)porphyrins.

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.

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).

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.

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.

Enthalpy-Entropy Compensation Effect for Triplet Pair Dissociation of Intramolecular Singlet Fission in Phenylene Spacer-Bridged Hexacene Dimers.

Hexacene (Hc) is highly promising for singlet fission (SF). However, the number of SFs in Hc is extremely limited. As far as Hc dimers in solution are concerned, there is no report on the observation of the dissociation process from a correlated triplet pair (TT) to an individual one. The emphasis in this study is on the first observation of the quantitative TT generation together with the orientation-dependent photophysical discussions for TT dissociation using para- and meta-phenyl-bridged Hc dimers. Moreover, the activation enthalpies of Hc dimers in TT dissociation are smaller than those of pentacene (Pc) dimers, whereas the relative entropic contributions for Gibbs free energy of activation are much larger than the enthalpic ones in both Hc and Pc dimers. This implies that the vibrational motions are responsible for the intramolecular conformation changes associated with the TT dissociation. Consequently, "enthalpy-entropy compensation" has a large impact on the rate constants and quantum yields.

Orientations and water dynamics of photoinduced secondary charge-separated states for magnetoreception by cryptochrome.

In the biological magnetic compass, blue-light photoreceptor protein of cryptochrome is thought to conduct the sensing of the Earth's magnetic field by photoinduced sequential long-range charge-separation (CS) through a cascade of tryptophan residues, WA(H), WB(H) and WC(H). Mechanism of generating the weak-field sensitive radical pair (RP) is poorly understood because geometries, electronic couplings and their modulations by molecular motion have not been investigated in the secondary CS states generated prior to the terminal RP states. In this study, water dynamics control of the electronic coupling is revealed to be a key concept for sensing the direction of weak magnetic field. Geometry and exchange coupling (singlet-triplet energy gap: 2J) of photoinduced secondary CS states composed of flavin adenine dinucleotide radical anion (FAD-•) and radical cation WB(H)+• in the cryptochrome DASH from Xenopus laevis were clarified by time-resolved electron paramagnetic resonance. We found a time-dependent energetic disorder in 2J and was interpreted by a trap CS state capturing one reorientated water molecule at 120 K. Enhanced electron-tunneling by water-libration was revealed for the terminal charge-separation event at elevated temperature. This highlights importance of optimizing the electronic coupling for regulation of the anisotropic RP yield on the possible magnetic compass senses.

Geometries and Terahertz Motions Driving Quintet Multiexcitons and Ultimate Triplet-Triplet Dissociations via the Intramolecular Singlet Fissions.

Importance of vibronic effects has been highlighted for the singlet-fission (SF) that converts one high-energy singlet exciton into doubled triplet excitons, as strongly coupled multiexcitons. However, molecular mechanisms of spin conversion processes and ultimate decouplings in the multiexcitons are poorly understood. We have analyzed geometries and exchange couplings (singlet-quintet energy gaps: 6J) of the photoinduced multiexcitons in the pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m-(Pc)2] by simulations of the time-resolved electron paramagnetic resonance spectra. We clarified that terahertz molecular conformation dynamics play roles on the spin conversion from the singlet strongly coupled multiexcitons 1(TT) to the quintet multiexcitons 5(TT) and on the intramolecular decouplings in the 6J to form spin correlated triplet pairs (T+T). The strongly coupled 5(TT) multiexcitons are revealed to possess entirely planar conformations stabilized by mutually delocalized spin distributions, while the intramolecular decoupled spin-correlated triplet pairs generated at 1 µs are also stabilized by distorted conformations resulting in two separately localized biradical characters.

Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin.

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S(•2-)-tryptophan214 radical cation (W214(•+)) separated by 1.2 nm. The ligand of AQ1S(-) has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218(+)) residue bound to AQ1S(•2-) mediates the long-range electron-transfer. The energetics of triad states of AQ1S(•2-)-R218(+)-W214(•+) and AQ1S(-)-R218(•)-W214(•+) have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.

Structure and dynamics of photogenerated triplet radical ion pairs in DNA hairpin conjugates with anthraquinone end caps.

A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, (1*)Aq undergoes rapid intersystem crossing to yield (3*)Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-•)Gn(+•)). All (3)(Aq(-•)Gn(+•)) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the (3*)Aq precursor. The TREPR spectra of the (3)(Aq(-•)Gn(+•)) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of (3)(Aq(-•)Gn(+•)) makes it possible to determine that the π systems of Aq(-•) and G(+•) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-•)Gn(+•)) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.

Protein-ligand structure and electronic coupling of photoinduced charge-separated state: 9,10-anthraquinone-1-sulfonate bound to human serum albumin.

To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved electron paramagnetic resonance spectra of photoinduced charge-separated (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to determine both the geometries and the exchange couplings of the CS states of AQ1S(2-•)-histidine-242 radical cation (H242(+•)) and AQ1S(2-•)-tryptophan-214 radical cation (W214(+•)). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied molecular orbitals of AQ1S(2-•) and H242(+•), the electronic coupling (5.4 cm(-1)) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge separation.

Electron spin polarization transfer to the charge-separated state from locally excited triplet configuration: theory and its application to characterization of geometry and electronic coupling in the electron donor-acceptor system.

We present a theoretical model of analysis of the time-resolved electron paramagnetic resonance (TREPR) spectrum of the charge-separated (CS) state generated by the photoinduced electron transfer (ET) reaction via the locally excited triplet state in an electron donor-acceptor (D-A) system with a fixed molecular orientation. We show, by the stochastic-Liouville equation, that chemically induced dynamic electron polarization (CIDEP) of the triplet mechanism is explained by lack of transfer of quantum coherence terms in the primary triplet spin state, resulting in net emissive or absorptive electron spin polarization (ESP) which is dependent on anisotropy of the singlet-triplet intersystem crossing in the precursor excited state. This disappearance of the coherence is clearly shown to occur when the photoinduced ET rate is smaller than the angular frequency of the Zeeman splitting: the transferred coherence terms are averaged to be zero due to effective quantum oscillations during the time that the chemical reaction proceeds. The above theory has been applied to elucidate the molecular geometries and spin-spin exchange interactions (2J) of the CS states for both folded and extended conformers by computer simulations of TREPR spectra of the zinc porphyrin-fullerene dyad (ZnP-C(60)) bridged by diphenyldisilane. On the extended conformation, the electronic coupling is estimated from the 2J value. It has been revealed that the coupling term is smaller than the reported electronic interactions of the porphyrin-C(60) systems bridged by diphenylamide spacers. The difference in the electronic couplings has been explained by the difference in the LUMO levels of the bridge moieties that mediate the superexchange coupling for the long-range ET reaction.