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The most definitive detection of superoxide anion O 2 -, which is of considerable importance in living systems, 1 has been made by the ESR spectrum which gives a characteristic two-line highly anisotropic signal due to the large spin-orbit coupling of oxygen in frozen media containing aprotic solvents or on various solid oxide surfaces at 77 K. 2-4 However, the disadvantage of the ESR detection of O 2 -is the significant line broadening resulting from fast relaxation because of the quenching of the orbital angular momentum by fluctuating solvent interactions or orbital degeneracy which has precluded the detection in fluid solution particularly at higher temperatures. 5,6 Thus, no ESR spectra of O 2 -have so far been reported in fluid solution at room temperature. 7 We report herein that the isotropic ESR spectra of O 2 -can be obtained successfully in solution at temperatures much higher than the melting point of aprotic solvents even at 60°C when O 2 -forms a 1:1 complex with scandium ion. The superhyperfine splitting due to the scandium nucleus and the hyperfine coupling of oxygen enriched in 17 O provide valuable information about the structure of the O 2 --Sc 3+ complex in solution.The dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA) 2 ], which can act as a unique two-electron donor, 8 is used as an electron source to reduce O 2 to O 2 -photochemically. As shown in Scheme 1, the photoinduced electron transfer from the singlet excited state of (BNA) 2 to O 2 gives (BNA) 2 •+ and O 2 -, followed by a fast cleavage of the C-C bond of the dimer to produce N-benzylnicotinamide radical (BNA • ) and BNA + . 8,9 The subsequent second electron transfer from BNA • to O 2 occurs rapidly, judging from the oxidation potential of BNA • (E 0 ox vs SCE ) -1.08 V) 10 lower than the reduction potential of O 2 (E 0 red ) -0.8 V). 11 Thus, once photoinduced electron transfer from (BNA) 2 to O 2 occurs, 2 equiv of O 2 -molecules are produced. 12 When an oxygen-saturated propionitrile solution containing (BNA) 2 (1.7 × 10 -3 M) is irradiated with a high-pressure mercury lamp, O 2 -formed photochemically is detected by the ESR spectrum in frozen propionitrile at 77 K which shows a wellknown signal with g | ) 2.072 and g ⊥ ) 2.009. 5 When the temperature is raised above the frozen temperature (e.g., -60°C), no ESR signal can be detected due to the line-broadening resulting from the fast relaxation, although the signal appears clearly again by lowering the temperature to 77 K.Addition of scandium triflate (Sc(OTf) 3 ) and 3 equiv of hexamethylphosphoric triamide (HMPA) ligand to the (BNA) 2 -O 2 system, however, results in the appearance of the sharp isotropic ESR signal under irradiation of the light in fluid solution at 25°C as shown in Figure 1, where oxygen is enriched in 17 O to provide valuable information about the inequivalency of oxygen nuclei in the O 2 -complex. 13,14 The clear eight-line isotropic spectrum at the center is ascribed to the superhyperfine coupling of O 2 -with the 7 / 2 nuclear spin of the scandium nucle...

High-performance GaInN-based solar cells with high open-circuit voltage, high short-circuit current density, and good fill factor have been obtained using a combination of two different GaInN superlattice structures. The GaInN barrier thicknesses (3 and 0.6 nm) in both superlattice structures were optimized, resulting in a thick GaInN-based active layer with a low pit density in the device. The conversion efficiency is approximately 2.5% under a solar simulator of air mass 1.5G and an irradiation intensity of 155 mW/cm 2 .

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