Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Since 1970s, magnetic field effects (MFEs) on photogenerated radical pairs have been the centre of focus in the field of spin chemistry. The MFE attributes to quantum mechanical interconversion between the singlet and triplet radical pair states and subsequent spin-selective recombination reactions. In this New View article, the author picks up two hot topics studied during the last two decades, which are (i) so-called low field effect (LFE) and (ii) 2J-resonance MFE on fixed distance donor-acceptor linked molecules. In both of the topics, quantum mechanical explanations are given referring to recent reports, and some novel calculations have been carried out for bridging theoretical and experimental data for long-lived radical pairs. For the first topic, time domain calculations of coherent state mixing have been carried out for elucidation of hyperfine (HF) structure dependence of the LFE. For the second topic, Monte Carlo simulations of the torsional motion of polyaromatic linker unit have been carried out for the demonstration of fast decoherence in such rigid molecules. From these considerations, future possibilities of MFE studies on photo-functional materials and biomolecules have been indicated.[GRAPHICS].

DOI： 10.1080/00268976.2019.1643510

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

For realization of low-cost organic photon-energy conversion, the supramolecular approach has been a focus of attention as a counter approach to precise synthesis of covalently linked donor (D)-acceptor (A) molecules. Here we report photogeneration of a long-lived (similar to 3 mu s) intermolecular charge-separated (CS) state of metal porphyries (D) and an alkyl viologen (A) at an interface of a vesicle membrane formed by self-assembly of nonionic surfactant and cholesterol molecules. The yield of escaped free radicals is negligibly low as in the case of CS states in covalently linked D-A systems. Furthermore, the transient concentration of the CS state dramatically increases by -100% upon application of a magnetic field of 250 mT at room temperature. The simulation of the spin dynamics of the CS state indicates that fast (similar to 10(7) s(-1)) spin-selective recombination and slow (10(5)-10(6) s(-1)) dissociation-re-encounter dynamics are the key processes for the long CS-state lifetime and the gigantic magnetic field effect. It has turned out that such dynamics are sharply dependent on temperature and alkyl chain length of the viologen. The present results would lead to the development of future materials for light energy conversion, drug delivery, and microscopic bioprobes.

DOI： 10.1021/acs.jpcb.8b08389

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Dynamics of the photogeneratedcharge-separated (CS) state is studied for. a newly synthesized molecular triad, in which the donor (D) dimethoiytriphenylamine; 1,3-bis(2-pyridylimino)isoindolate platinum (BPIPt), and the acceptor (A). naphthaldiimide are linked with a triethynylbenzene unit (BPIPt-DA). Photoexcitation of BPIPt gives rise to generation, of a long lived (similar to 4 mu s) CS Tstate BPIPt-D(+)A(-), Of which the lifetime is con, Siderably increased by applied magnetic field of 270 mT.: The positive magnetic field,effect (MFE) is in contrast to the negative MFE for the reference DA molecule, which indicates successful switching of the initial spin, state of the CS state from singlet to triplet Simulations of the MFE and time resolved electron paramagnetic resonance show that spin-selective charge recombination and spin relaxation Are. Unaffected by attachment of BPIPt. The minimum impact of heavy atom substitution on the. electronic and magnetic properties-hae-been.. realized by the small electronic coupling mediated by the rigid meta-triethynylbenzene.

DOI： 10.1021/acs.jpclett.6b02887

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Magnetic field effect is a powerful tool to study dynamics and kinetics of radical pairs (RPs), which are one of the most important intermediates for organic photon-energy conversion reactions. However, quantitative discussion regarding the relationship between the modulation of interelectron interactions and spin dynamics at low magnetic fields (<10 mT) is still an open question. We have studied the spin dynamics of a long-lived RP in a micelle by newly developed Monte Carlo simulation, in which fluctuations of the exchange and magnetic dipolar interactions by in-cage diffusion are directly introduced to the time-domain spin dynamics calculation. State-dependent relaxation/dephasing times of a few to a few tens of nanoseconds are obtained by simulations without hyperfine interactions (HFIs) as a function of the mutual diffusion constant (similar to 10(-6) cm(2)/s). Simulations with the HFIs exhibit incoherent singlet-triplet (S-T) mixings resulting from interplay between the HFIs and the fluctuating spin-spin interactions. The experimentally observed incoherent S-T mixing of similar to 20 ns at 3 mT for a singlet-born RP in a sodium dodecyl sulfate micelle is reproduced by the simulation with reasonable diffusion coefficients. The computational method developed here contributes to quantitative detection of molecular motion that governs the recombination efficiency of RPs.

DOI： 10.1021/acs.jpca.5b02183

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Construction of photogenerated long-lived charge-separated states is crucial for light-energy conversion using organic molecules. For realization of cheap and easy-to-make long-distance electron transfer (ET) systems, we have developed a supramolecular donor(D)chromophore(C)acceptor(A) triad utilizing a micellar interface. Alkyl viologen (A(2+)) is adsorbed on the hydrophilic interface of Triton X-100 micelle, which bears D units in the hydrophobic core. Excited triplet state of a hydrophobic flavin C entrapped in the supercage gives rise to primary ET from D, which is followed by the secondary ET from C to A(2+) to give the long-lived (>10 mu s) charge-separated state with negligible yield of escaped C. Analysis of magnetic field effect reveals that diffusion of C from the core to the hydrophilic interface leads to long-distance ET with a low charge recombination yield of similar to 20%. This novel concept of dynamic charge transporter has important implications for development of photon-energy conversion systems in solution phase.

DOI： 10.1021/jz502495u

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Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Spin selectivity in long-lived charge separation at the micellar interface is studied using the magnetic field effect (MFE). An amphiphilic viologen is complexed with a nonionic surfactant to form a supramolecular acceptor cage, of which the size is controlled by the acceptor concentration, as confirmed by dynamic light scattering measurement. Photoinduced electron transfer (ET) from a guest polyaromatic molecule to the viologen moiety is observed spin-dependently with time-resolved fluorescence (trFL) and transient absorption (TA). A negative MFE on the radical yield is successfully observed, which indicates generation of singlet-born long-lived radical pair that is realized by supramolecular control of the donor-acceptor (D-A) distances. The dominance of the singlet-precursor MFE is sensitive to the acceptor concentration, which presumably affects the D-A distance as well as the cage size. However, theoretical analysis of the MFE gives large recombination rates of ca. 10(8) s(-1), which indicate the contribution of spin-allowed recombination of the pseudocontact radical pair generated by still active in-cage diffusion. Dependence of the viologen concentration and alkyl chain length on the recombination and escape dynamics is discussed in terms of precursor spin states and the microenvironments in the cage.

DOI： 10.1021/jp401725f

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We have studied spin-dependent charge transfer dynamics in wirelike, donor-bridge-acceptor (D-B-A)molecules comprising a phenothiazine (PTZ) donor, an oligo(2,7-fluorene) (FL(n)) bridge, and a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, PTZ-FL(3)-PDI (1) and PTZ-FL(4)-PDI (2), dissolved in the magnetic field-aligned nematic phase of 4-cyano-4'-n-pentylbiphenyl (5CB) at 295 K. Time-resolved EPR spectroscopy using both continuous wave and pulsed microwaves shows that the photogenerated radical pairs (RPs), PTZ(+center dot)-FL(3)-PDI(-center dot) and PTZ(+center dot)-FL(4)-PDI(-center dot), recombine much faster from the singlet RP manifold than the triplet RP manifold. When a strong resonant microwave pi pulse is applied following RP photogeneration in 1 and 2, the RP lifetimes increase about 50-fold as indicated by electron spin-echo detection. This result shows that the RP lifetime can be greatly extended by rapidly switching off fast triplet RP recombination.

DOI： 10.1021/ja110789q

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Photoinitiated charge separation and thermal charge recombination (CR) in a covalent donor-bridge-acceptor (D-B-A) system consisting of a perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptor, 2,7-oligofluorene bridge (FLn), and phenothiazine donor (PTZ) (PTZ-FLn-PDI) have been shown to transition from superexchange to charge hopping mechanisms as the D-A distance increases. In work presented here, the spin-selective multiple CR pathways in PTZ-FLn-PDI are studied by a detailed analysis of the magnetic field effect (MFE) on the radical ion pair (RP) lifetime and triplet yield. A kinetic analysis of the MFE gives the spin-selective CR rates and the RP singlet-triplet (S-T) relaxation rates for n = 2-4. When n = 2 and 3, where the S-T splitting (2J) of the RP is large, slow S-T relaxation results in a kinetic bottleneck slowing the observed total CR rate at zero magnetic field. These results show that spin state mixing is an important controlling factor for CR reaction rates in these systems. The CR rate constant for the triplet RP (k(CRT)) obtained by MFE analysis is about 10 times faster than the corresponding rate for the singlet RP (k(CRS)) when n = 2-4, indicating that k(CRT) occurs near the maximum of the Marcus rate vs free energy dependence, whereas k(CRS) is deep in the inverted region. The distance dependence of both k(CRS) and k(CRT) is explained by the crossover from superexchange (n = 1 and 2) to distant independent thermal hopping (n = 3 and 4). A possible mechanism of the S-T relaxation is proposed based on S-T dephasing, which may be induced by fluctuations of 2J resulting from bridge torsional dynamics.

DOI： 10.1021/jp103441n

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Photoreaction of N,N,N`,N`-tetramethyl-1,4-phenylenediamine (TMPD) in an aerosol OT (AOT) reversed micelle (RM) is studied by time-resolved EPR (tr-EPR) and the transient absorption detected magnetic field effect (MFE) Tr-EPR and transient absorption spectra indicate electron transfer from a highly excited triplet state of TMPD to the AOT headgroup regardless of W = [H(2)O]/[AOT] values from 0 to 40 Noticeable MFEs on the yield of TMPD cation radical (TMPD(+)) are observed at W > 0 and maximized at W similar to 10 The dynamics of TMPD(+) in the bound water region of the RM has been precisely analyzed by theoretical analysis of time-resolved magnetically affected reaction yield (MARY) spectra The simulation of the MARY spectra indicates that two kinds of radical pairs exist, both of which are composed of an AOT alkyl radical and TMPD(+) One system has TMPD(+) strongly bound to the anionic interface, where the radical pair shows very slow relaxation and recombination Another system has TMPD(+) diffusing in the bound water, which shows a smaller diffusion coefficient than that in bulk water by 1 order of magnitude In the larger water pool (W > 15), the spin correlated radical pair of the hydrated electron and TMPD(+) generated by photoionization is observed by tr-EPR The ionization reaction is followed by electron attachment to the AOT headgroup and generation of the sulfite radical However these radical pairs are not thought to contribute significantly to the observed MFEs Spin multiplicities of the precursor state and recombination products have been discussed from the different sign of J values for the radical pairs at larger W

DOI： 10.1021/jp102300z

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4: 9,10-bis(dicarboximide) (PDI), PTZ-FL(n)-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI ((3)*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. (3)*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T(0) mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n >= 3 is incoherent thermal hopping, which results in wire-like charge transfer.

DOI： 10.1021/jp101523n

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The singlet-triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off-on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S-T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet-triplet and triplet-triplet dephasing rate constants are determined independently to be 2 x 10(8) and 0.2 x 10(8) s(-1), respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair.

DOI： 10.1021/jp077505x

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond Pulsed magnetic field effect oil transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron -electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.

DOI： 10.1021/jp056488d

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A large magnetic field effect (MFE) up to 13% of the yield of the free radical formation is found in the photoinduced electron-transfer reaction from the tryptophan residue of the hen egg-white lysozyme (HEWL) to the flavin mononucleotide, (FMN). The magnetic field effect is special in the water solution of FMN, and that is reduced to 5-7% by the addition of sodium chloride to the solution and in the system of riboflavin. The MFE up to 13% coincides with the appearance of the fast quenching process of the excited singlet state. The large MFE is attributed to association by Coulomb interaction between the ionic phosphate group of FMN and the cationic surface of the protein molecule.

DOI： 10.1021/jp034068n

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jacs.3c01076

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpcc.3c02128

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/jacs.3c01264

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The time-of-flight method is a fundamental approach for characterizing the transport properties of semiconductors. Recently, the transient photocurrent and optical absorption kinetics have been simultaneously measured for thin films; pulsed-light excitation of thin films should give rise to non-negligible in-depth carrier injection. Yet, the effects of in-depth carrier injection on the transient currents and optical absorption have not yet been elucidated theoretically. Here, by considering the in-depth carrier injection in simulations, we found a 1/ t^{1− α/2} initial time ( t) dependence rather than the conventional 1/ t^{1− α} dependence under a weak external electric field, where α < 1 is the index of dispersive diffusion. The asymptotic transient currents are not influenced by the initial in-depth carrier injection and follow the conventional 1/ t^{1+ α} time dependence. We also present the relation between the field-dependent mobility coefficient and the diffusion coefficient when the transport is dispersive. The field dependence of the transport coefficients influences the transit time in the photocurrent kinetics dividing two power-law decay regimes. The classical Scher–Montroll theory predicts that a₁ + a₂ = 2 when the initial photocurrent decay is given by [Formula: see text] and the asymptotic photocurrent decay is given by [Formula: see text]. The results shed light on the interpretation of the power-law exponent of [Formula: see text] when a₁ + a₂ ≠ 2.

DOI： 10.1063/5.0143683

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Layered hybrid organic-inorganic halide perovskites have emerged as durable and tunable candidates for optoelectronic materials due to their outstanding properties. Here, we report the temperature-induced structural transition and photophysical properties of the layered perovskite (MA)(2)PbI2-xBrx(SCN)(2) (MA = CH3NH3; 0 <= x <= 1.2). Synchrotron X-ray diffraction measurements demonstrate that there is a first-order structural phase transition with an increase of the interlayer distance in (MA)(2)PbI2(SCN)(2) (x = 0) at 315 K. We attribute this transition to order-disorder transformations of the molecular ions, MA(+) and SCN-. The observed transition temperature decreases with Br substitution (from 320 K to 270 K), suggesting a weakening of bonding interaction between the molecular ions. A decrease in the decomposition temperature was also observed by Br substitution, indicating that the strength of interlayer bonding is coupled with the thermal stability of the layered perovskite. Photocurrent transient measurements for (MA)(2)PbI2(SCN)(2) reveal a dramatic decrease of the fast decay component upon heating, indicating that the structural transition decreases the carriers in the crystalline domain by random distortion of the perovskite layer derived from disordering of molecular ions. This study provides insight into the structure and photophysical properties of the thiocyanate layered perovskite family.

DOI： 10.1039/d2ce00733a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CHEMICAL SOC JAPAN  

The synthesis, excited state dynamics, and substituent effect on the 1O2-generation efficiency of several copper(II) complexes of 5,15-diazaporphyrin (CuDAPs) bearing peripheral substituents are reported. 10,20-Diaryl-CuDAPs were prepared via a new method consisting of metal-templated cyclization and N-dealkylation as key steps. The effect of the β-substituents on the optical and redox properties of CuDAP was stronger than that of the meso-aryl groups, which can be explained by considering the characteristics of the HOMO and LUMO of the DAP ring. The excited state dynamics and 1O2-generation efficiency of CuDAPs were studied using time-resolved spectroscopic techniques; the electronic effect of the meso-aryl groups on the triplet lifetime and 1O2-generation quantum yield was relatively weak, whereas that of the β-(1-pyrazolyl) groups was appreciable. The analysis of the temperature dependence of phosphorescence spectra of 10,20-bis(2,4,6-trimethylphenyl)-CuDAP allowed the energy gap between the excited trip-quartet and trip-doublet states to be determined. The relatively long triplet lifetimes of CuDAPs show their potential for future application as photosensitizers.

DOI： 10.1246/bcsj.20220002

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/anie.202106490

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/anie.202106490

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.orgel.2019.105383

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Metal complexes of 3,7,13,17-tetrakis(di(4-carboxyphenyl)amino)-5,15-diazaporphyrin (MDAP-COOH; M=Pd, Cu) and their ethyl ester precursors (MDAP-COOEt; M=Pd, Cu) have been synthesized for use as near-infrared (NIR)-light-responsive photosensitizers. Under irradiation with visible or NIR light, PdDAP-COOEt in toluene generated singlet oxygen (O-1(2)) with an excellent quantum yield (phi(Delta)=0.99), whereas CuDAP-COOEt exhibited a lower efficiency (phi(Delta)=0.21). The water-soluble Pd-II complex PdDAP-COOH also behaved as a photosensitizer (phi(Delta)=0.20) in a micellar solution. The photophysical properties of these dyes were measured by transient absorption techniques. It was found that the efficiency of the energy transfer from the triplet state of MDAP-COOR (R=Et, H) to the ground state of dioxygen was highly dependent on the peripheral substituents, the central metal, and the solvent. Furthermore, the phototoxicity of PdDAP-COOH toward HeLa cells under irradiation of NIR light (720 nm) was evaluated. As expected, PdDAP-COOH exhibited good photodynamic activity, and control experiments confirmed that O-1(2) was generated as the active oxygen species.

DOI： 10.1002/cplu.201900087

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/cplu.201900087

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Copyright © 2018 American Chemical Society. A visible light promoted process for desulfonylation of N-sulfonylamides and -amines has been developed, in which 1,3-dimethyl-2-hydroxynaphthylbenzimidazoline (HONap-BIH) serves as a light absorbing, electron and hydrogen atom donor, and a household white light-emitting diode serves as a light source. The process transforms various N-sulfonylamide and -amine substrates to desulfonylated products in moderate to excellent yields. The observation that the fluorescence of 1-methyl-2-naphthoxy anion is efficiently quenched by the substrates suggests that the mechanism for the photoinduced desulfonylation reaction begins with photoexcitation of the naphthoxide chromophore in HONap-BIH, which generates an excited species via intramolecular proton transfer between the HONap and BIH moieties. This process triggers single electron transfer to the substrate, which promotes loss of the sulfonyl group to form the free amide or amine. The results of studies employing radical probe substrates as well as DFT calculations suggest that selective nitrogen-sulfur bond cleavage of the substrate radical anion generates either a pair of an amide or amine anio

DOI： 10.1021/acs.joc.8b01536

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PubMed

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

The magnetoconductance (MC) effect was investigated for two types of organic solar cells with single junction (SJ) and bulk junction (BJ) of poly(3-hexylthiophene) (P3HT) as donor (D) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) as acceptor (A). Three components with different half-field-at-half-maximums (B-1/2) of 4 +/- 1, 20 +/- 15 and >400 mT, hereafter referred to MCS,M,B in a sequence, were observed in the magnetic field dependence of the MC effects measured under dark and light conditions. The magnitude of the MCS,M,B components is sensitive to not only the junction structure of the cell but also the presence or absence of incident light. The bias voltage (17) dependence of the MC effect in the dark for the SJ-cell is maximized around the turn-on voltage (V-ON) of the dark current, where a flat band condition of the active layer is achieved. The B-1/2 for the MCM component of the SJ-cell increases with V beyond V-ON. In light, the BJ-cell exhibits the MC effect, whereas no effect is detected for the SJ-cell. The MCS,M components for the BJ-cell in light increase with the incident light power. The transient MCS,M components for the BJ-cell measured using a nanosecond pulse laser increases with the delay time after the flash. By integrating these phenomena and the phase of the MC effect, it is concluded that all of the MC components arise from the magnetic field effect on the spin conversion of nongeminate electron (e)-hole (h) pairs with spin-dependent charge recombinations at the D/A-interface. The B-1/2 values for MCS,M,B are, respectively, understood by the spin conversion due to the hyperfine interaction, the spin relaxation, and the g-factor difference for e (PCBM-) and h (P3HT(+)). Kinetic simulations of the MCS,M components for the BJ-cell observed at the short-circuit condition in light yield an efficiency of ca. 40% for the nongeminate recombination, which is accompanied by the generation of triplet excitons as well as relaxation to a ground singlet state. The loss mechanism of moderate triplet recombination suggests an important possibility to improve the power conversion efficiency by harvesting of the triplet excitons.

DOI： 10.1021/acsomega.8b01746

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DOI： 10.20665/kakyoshi.66.7_338

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society  

Benzimidazolium naphthoxide (-ONap-BI+) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of -ONap-BI+ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of -ONap-BI+ was proposed.

DOI： 10.1021/acs.joc.8b00282

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The effect of the solvent viscosity dependence of time-resolved magnetoluminescence (ML) on the delayed fluorescence of 9,10-diphenylanthracene (DPA) sensitized by platinum octaethylporphyrin has clarified the structure and dynamics of the triplet triplet pair (TT), i.e., the transition state of triplet fusion. Phase inversion of the ML effect with time provides evidence for the recycle dynamics of the excited triplet state for DPA in triplet fusion. The electron spin-relaxation by random molecular rotation causes intersystem crossing among the different spin states of the triplet-triplet pair and allows the (TT)-T-3,5 to engage in triplet fusion. Therefore, slow-down of the molecular diffusion by an increase in the solvent viscosity can enhance the triplet fusion yield. However, the reduction of the ML effect observed in quite high viscosity solvents suggests that the substantially slow rotational motion decreases the triplet fusion yield due to steric factors in electron exchange from the triplet-triplet pair.

DOI： 10.1021/acs.jpcb.5b11208

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Photoinduced electron transfer (PET) reactions promoted by 2-aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) were investigated. Excited states of Ar-DMBIH, formed by irradiation using light above 360 initiate PET reductions of various organic substrates, including transformations of epoxy ketones to aldols, free radical rearrangements such as the Dowd-Beckwith ring-expansion and 5-exo hexenyl cyclization, deprotection of N-sulfonyl-indols, and allylation of acyl formates. In these processes, Ar-DMBIH possessing 1-naphthyl, 2-naphthyl, 1-pyrenyl and 9-anthryl substituents formally act as two electron and one proton donors while the hydroxynaphthyl substituted derivative serves as a two electron and two proton donor. On the basis of the results of absorption spectroscopy studies, cyclic voltammetry and DFT calculation, a mechanistic sequence for these reduction reactions is proposed that involves initial photoexcitation of the aryl chromophore of the Ar-DMBIH followed by single electron transfer (SET) to the organic substrate to generate the radical cation of benzimidazoline and the radical anion of the substrate. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.tet.2015.06.071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We demonstrate the magnetoconductance (MC) effect originated from depressing the spin mixing in encounter pairs under the external magnetic field provides quantitative information about the singlet fission, the charge recombination, and the trap-related dynamics with triplet exciton in a bilayer device of pentacene (Pen) and fullerene (C-60). Three MC effects in low-, moderate-, and high-fields were detected in the bilayer device at room temperature. Kinetic analysis of the low-field MC effect showed that the charge recombination yield at the Pen|C-60 interface is similar to 1%. Quantum mechanical simulations for dynamics of spin-carrying pairs following the conservation rule of spin angular momentum in recombination showed that the moderate- and high-field MC effects are caused by, respectively, the trap-related dynamics with triplet exciton and the singlet fission with a maximum yield of 52% in the layers. The quantitative information obtained by investigating the MC effect will contribute to the development of high-efficiency organic solar cells devices.

DOI： 10.1021/jp508799j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A series of donor-chromophore-acceptor-stable radical (D-C-A-R-center dot) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D+center dot-C-A(-center dot) spin-correlated radical pair (RP) to the stable radical R-center dot. Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2J(DA)) of D+center dot-C-A(-center dot). Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R-center dot molecules described herein, D is 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C is 4-aminonaphthalene-1,8-dicarboximide, and A is naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R-center dot) (1B-6B). Photoexcitation of C with 416-nm laser pulses results in two-step charge separation to yield D+center dot-C-A(-center dot)-(R-center dot). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous-wave (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin-echo detection) was used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R-center dot radical. The TREPR spectra show that vertical bar 2J(DA)vertical bar for D+center dot-C-A(-)center dot decreases in the order MeOAn(+center dot) > DioxAn(+center dot) > BDXAn(+center dot) as a result of their spin density distributions, whereas the spin-spin dipolar interaction (d(DA)) remains nearly constant. Given this systematic variation in vertical bar 2J(DA)vertical bar, electron spin-echo-detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R-center dot depends on vertical bar 2J(DA)vertical bar.

DOI： 10.1021/jp4045012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A covalent, fixed-distance donor-bridge- acceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T+1) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of (3*) D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of (3*) D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T-0 eigenstate.

DOI： 10.1021/ja1094815

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3)*PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1)*PDI -&gt; TEMPO(+)*-PDI(-)* occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3)*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3*PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3)*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3)*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism. which requires that the radical and attached triplet are in the weak exchange regime.

DOI： 10.1021/jp909212c

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Language：English   Publisher：AMER CHEMICAL SOC  

A series of donor-bridge-acceptor (D-B-A) triads have been synthesized in which the donor, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An), and the acceptor, naphthalene-1,8:4,5-bis(dicarboximide) (NI), are linked by p-oligophenylene (Ph-n) bridging units (n = 1-5). Photoexcitation of DMJ-An produces DMJ(+center dot)-An(-center dot) quantitatively, so that An(-center dot) acts as a high potential electron donor, which rapidly transfers an electron to NI yielding a long-lived spin-coherent radical ion pair (DMJ(+center dot)-An-Ph-n-NI-center dot). The charge transfer properties of 1-5 have been studied using transient absorption spectroscopy, magnetic field effects (MFEs) on radical pair and triplet yields, and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The charge separation (CS) and recombination (CR) reactions exhibit exponential distance dependencies with damping coefficients of beta = 0.35 angstrom(-1) and 0.34 angstrom(-1), respectively. Based on these data, a change in mechanism from superexchange to hopping was not observed for either process in this system. However, the CR reaction is spin-selective and produces the singlet ground state and both (3)*An and (3)*NI. A kinetic analysis of the MFE data shows that superexchange dominates both pathways with beta = 0.48 angstrom(-1) for the singlet CR pathway and beta = 0.35 angstrom(-1) for the triplet CR pathway. MFEs and TREPR experiments were used to measure the spin-spin exchange interaction, 2J, which is directly related to the electronic coupling matrix element for CR, V-CR(2). The magnitude of 2J also shows an exponential distance dependence with a damping coefficient alpha = 0.36 angstrom(-1), which agrees with the P values obtained from the distance dependence for triplet CR. These results were analyzed in terms of the bridge molecular orbitals that participate in the charge transport mechanism.

DOI： 10.1021/ja907625k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The spin mixing process of a radical pair in a micelle in a low magnetic field is investigated by transient absorption detected MARY spectra. In a system comprising the radical pair generated by photoreaction of 2-methyl-1,4-naphthoquinone in sodium dodecylsulfate micelle, two techniques were applied: time resolved MARY spectra and pulsed MARY spectra. In order to analyse the results, a practical calculation method that lends itself to the simulation of experimental results has been developed. In the calculation, a semi-classical model was used for the evaluation of the hyperfine mechanism (HFM) in the single-site Liouville equation in which one can take spin relaxation and dephasing phenomena into account. The interrelation between the HFM and relaxation mechanism (RM) was discussed. A new insight into the HFM induced spin mixing process, which is assisted by fast spin dephasing phenomena, is proposed. This mechanism is important for the MARY spectral shape and connects the HFM and RM smoothly.

DOI： 10.1080/14767050600588106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

xxThe spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)-diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field affect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE Studied by AD-SEMF is shorter than the lifetime 0 of the recombination kinetics of the radical pair. These results indicate that the LIFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.

DOI： 10.1021/jp053989q

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Presentation type：Symposium, workshop panel (nominated)  

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Language：English   Presentation type：Oral presentation (invited, special)  

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