Masanobu Wakasa

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.

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.

Magnetic field effects on the reaction of a triplet-born radical pair consisting of two equivalent sulphur-centre radicals under ultrahigh fields of up to 28 T

A water-cooled pulsed magnet having a room temperature bore of diameter 20u2009mm was constructed. Magnetic field effects on a triplet sensitization reaction of p-aminophenyl disulfide (APDS) with xanthone (Xn) were studied in a sodium dodecylsulfate (SDS) micellar solution by a nanosecond laser flash photolysis technique under ultrahigh magnetic fields of up to 28u2009T. Upon irradiation of the SDS micellar solution containing APDS and Xn, triplet radical pairs of p-aminophenylthiyl radicals were generated. The yield of the escaped p-aminophenylthiyl radical was found to decrease by 6u2009±u20092% at 28u2009T compared with that at 0u2009T.

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.

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.