Tomoaki Yago

Nanoscale Heterogeneous Structure of Ionic Liquid as Revealed by Magnetic Field Effects

Large magnetic field effects (MFEs) observed for photoinduced hydrogen abstraction reaction between benzophenone and thiophenol in an ionic liquid (N,N,N,-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide, TMPA TFSA) are analyzed by using the stochastic Liouville equation for the first time. The sphere cage model can well reproduce the observed MFEs and the nanoscale heterogeneous structure with a cage radius of 1.8 +/- 0.3 nm, and an effective viscosity in the cage of 1-2 cP is found to be formed in TMPA TFSA.

Anomalous magnetic field effects on photochemical reactions in ionic liquid under ultrahigh fields of up to 28 T.

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions between benzophenone and thiophenol in an ionic liquid, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), were studied by a nanosecond laser flash photolysis technique under ultrahigh fields of up to 28 T. Extremely large and anomalous stepwise MFEs were observed for the first time. The escape yield of benzophenone ketyl radical decreased with increasing magnetic field strength (B) at 0 T<B<or=2 T. The decrease was almost saturated at 2 T<B<or=10 T. At much higher fields (10 T<B<or=28 T), the yield decreased again with increasing B, producing a 25% decrease at 28 T.

Magnetic field effect on a radical pair reaction as a probe of microviscosity.

The magnetic field effects (MFEs), caused by the Delta g mechanism, on the photoinduced hydrogen abstraction reaction of benzopheneone with thiophenol were investigated in alcoholic solutions of varying viscosities (eta = 0.55 to 59.2 cP) by a nanosecond laser flash photolysis technique. The escape yield of benzophenone ketyl radicals ( Y) gradually decreased with increasing magnetic field strength ( B) from 0 to 1.6 T. The relative yield observed at 1.6 T, R(1.6 T) = Y(1.6 T)/ Y(0 T), decreased with increasing eta in the range of 0.55 cP < or = eta < or = 5 cP, and then increased with increasing eta in the range of 5 cP < eta < or = 55.3 cP. When eta was higher than 55.3 cP, the R(1.6 T) value became 1.0, and MFEs were completely quenched. The observed eta dependence of the MFEs was analyzed by the stochastic Liouville equation (SLE), in which the effects of spin-orbit coupling by a heavy atom such as sulfur were taken into account. The observed MFEs were reproduced fairly well by the SLE analysis. The diffusion coefficients of the radicals obtained by the SLE were about three times smaller than those expected from the macroscopic solvent viscosities. One can probe the microviscosity in the vicinity of the radical pairs by observing MFEs on the present photochemical reaction system.

Quantum Oscillations and Polarization of Nuclear Spins in Photoexcited Triplet States

The unique physical properties of photoexcited triplet states have been explored in numerous spectroscopic studies employing electron paramagnetic resonance (EPR). So far, however, no quantum interference effects were found in these systems in the presence of a magnetic field. In this study, we report the successful EPR detection of nuclear quantum oscillations in an organic triplet state subject to an external magnetic field. The observed quantum coherences can be rationalized using an analytical theory. Analysis suggests that the nuclear spins are actively involved in the intersystem crossing process. The novel mechanism also acts as a source of oscillatory nuclear spin polarization that gives rise to large signal enhancement in nuclear magnetic resonance (NMR). This opens new perspectives for the analysis of chemically induced dynamic nuclear polarization in mechanistic studies of photoactive proteins.

Photocyclization Reactions of Diarylethenes via the Excited Triplet State.

Cyclization reactions of three diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), 1,2-bis(2-hexyl-3-benzothienyl)perfluorocyclopentene (BTHex), and 1,2-bis(2-isopropyl-3-benzothienyl)perfluorocyclopentene (BTiPr), via their excited triplet states were studied by means of steady-state and nanosecond transient absorption spectroscopy. The excited triplet states of BT, BTHex, and BTiPr were generated by energy transfer from the photoexcited triplet states of sensitizers such as xanthone, phenanthrene, and pyrene. The single-step quantum yields of the cyclization reactions from the excited triplet states of BT, BTHex, and BTiPr were determined to be 0.34, 0.53, and 0.65, respectively. The triplet energies of these three BTs were estimated to be 190-200 kJ mol(-1).

Hydrogen Bonding Effects on the Reorganization Energy for Photoinduced Charge Separation Reaction between Porphyrin and Quinone Studied by Nanosecond Laser Flash Photolysis

Alcohol concentration dependences of photoinduced charge separation (CS) reaction of zinc tetraphenyl-porphyrin (ZnTPP) and duroquinone (DQ) were investigated in benzonitrile by a nanosecond laser flash photolysis technique. The photoinduced CS reaction was accelerated by the addition of alcohols, whereas the addition of acetonitrile caused little effect on the CS reactions. The simple theory was developed to calculate an increase in reorganization energies induced by the hydrogen bonding interactions between DQ and alcohols using the chemical equilibrium constants for the hydrogen bonding complexes through the concerted pathway and the stepwise one. The experimental results were analyzed by using the Marcus equation where we took into account the hydrogen bonding effects on the reorganization energy and the reaction free energy for the CS reaction. The observed alcohol concentration dependence of the CS reaction rates was well explained by the formation of the hydrogen bonding complexes through the concerted pathway, demonstrating the increase in the reorganization energy by the hydrogen bonding interactions.

Time-resolved EPR study on reorganization energies for charge recombination reactions in the systems involving hydrogen bonding

Abstract Hydrogen bonding effects on reorganization energies ( λ ) in the photoinduced electron donor–acceptor system of duroquinone (DQ)–1,2,4-trimethoxybenzene (TMB) in benzonitrile have been investigated by time-resolved EPR and cyclic voltammetry measurements. The solvent reorganization energies ( λ S ) determined for the radical ion pair systems involving the hydrogen-bonded complexes are larger by ∼0.2 eV than those calculated from the Marcus continuum dielectric model. The present results suggest that dissociation of the DQ anion–alcohol hydrogen-bonded complex results in the new component of λ S for the charge recombination reaction in polar solvents.

Magnetic Field Effects on Hydrogen Abstraction of Thiobenzophenone as a Probe of Microviscosity

Hydrogen abstraction reactions of thiobenzophenone with thiophenol in solutions of varying viscosities (η=0.29-42.0 cP) were studied by a nanosecond laser flash photolysis under magnetic fields of 0-15.5 T. In alcoholic solutions, the escaped radical yield (Y) of thiobenzophenone ketyl radical showed appreciable magnetic field effects (MFEs). The observed MFEs can be interpreted with the Δg mechanism through the triplet radical pair. The relative escaped radical yield (R(1.7T)=Y(1.7T)/Y(0T)) decreased with increasing η at 0<η≤3.33 cP, but then the yield increased with increasing η at 3.33 cP<η≤22.2 cP. At much higher viscosity 22.2 cP<η≤42 cP, R(1.7T) values become 1.0 within experimental errors. Such quenching of MFE was explained by the spin-orbit coupling recombination of close radical pairs associated with high viscosity. The MFEs on the present reaction is extremely sensitive to the solvent viscosity in the vicinity of the radical pairs. Using this probe reaction, microviscosities of sodium dodecyl sulfate (SDS) and Brij35 micellar solutions were estimated.

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (reff) effectively contribute to the CIDEP generated from diffusive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into account the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperfine-dependent RPM polarization (PRPM) phase is varied with the driving force (−ΔGCR) for the charge-recombination (CR) process and that the boundary −ΔGCR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distance,reff=1.2 nm, between the ion radicals. For the first time, thereff is well described by the exponent parameter (β) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate thereff and thePRPM in homogeneous liquid polar solvents.

Structure and dynamics of triplet-exciton pairs generated from singlet fission studied via magnetic field effects

Singlet fission is the conversion of a singlet exciton to a pair of triplet excitons followed by a diffusion process to form two free triplet excitons. The quantum yield of singlet fission per photon can exceed 100%. Singlet fission is thus an attractive way to enhance solar-cell performance. However, singlet fission events are not well characterized. In particular, the structure and diffusion pathways of triplet-exciton pairs, which strongly affect the efficiency of the singlet fission event, are unclear. Here we study the magnetic field effects (MFEs) on the singlet fission of diphenylhexatriene (DPH) and fluorinated DPHs crystals. Their fluorescence intensities show clear MFEs and the shape of the MFE curve depends on the crystal structure. Analysis of MFEs with the stochastic Liouville equation reproduces the MFE curve well. This use of MFEs allows one to determine the structure and diffusion pathways of triplet-exciton pairs, and to predict the efficiency of singlet fission events.Singlet fission events could be exploited to improve solar cell performance, but currently their characterization is challenging. Here, the authors exploit magnetic field effects at low magnetic field strengths to determine the structure and diffusion pathways of triplet-exciton pairs and to predict the efficiency of singlet fission events.