Hiroaki Yonemura

The effect of silver nanoplate (AgPL) on the singlet exciton fission (SF) of rubrene (Rub) was verified by examining the magnetic field effects (MFEs) on the SF of Rub through measuring magnetic field dependence of fluorescence intensity of Rub in two thick Rub polymer-composite films in the absence and the presence of AgPL. The fluorescence intensity of Rub in the Rub-AgPL polymer-composite film was larger than that in the Rub polymer-composite film. The enhancement (1.4 times) must be caused by strong electric fields due to localized surface plasmon resonance (LSPR) of the embedded AgPL near the Rub in polymer matrix. The magnitude of the MFEs in the Rub-AgPL polymer-composite film was almost same as those in the Rub polymer-composite film. These results strongly indicate that the efficiency of SF was enhanced by the LSPR due to the embedded AgPL into the polymer matrix in the Rub-AgPL polymer-composite film.

Effects of silver nanoplate (AgPL) and magnetic field on photon upconversion based on sensitized triplet–triplet annihilation (PUC-TTA) were examined in the system of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA), when AgPLs were incorporated into polymer thin films containing PtOEP and DPA. In the presence of AgPL, the PUC emission intensity increased as compared with that in the absence of AgPL. The result is most likely attributable to the localized surface plasmon resonance of AgPL. On the other hand, in the absence of AgPL, the PUC emission intensity decreased in the presence of magnetic field, while in the presence of AgPL, the PUC emission intensity increased drastically in the presence of magnetic field. The synergetic effect of AgPL and magnetic field on the PUC emission intensity in PUC-TTA was observed for the first time.

The fluorescence intensity from rubrene (Rub) in Rub-silver nanoparticle (AgNP) composite film was larger than that in Rub film. The enhancement is most likely attributable to large electric fields due to localized surface plasmon resonance of AgNP. The magnitude of the magnetic field effects in the Rub-AgNP composite film was almost same as those in the Rub film. These results indicate that the triplet excited states of Rub due to the singlet exciton fission in the presence of AgNP are generated much more than those in the absence of AgNP.

Effect of gold nanoparticle (AuNP) on photon upconversion based on sensitized triplet-triplet annihilation (PUC-TTA) were examined in the system of platinum(II) octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA), when AuNPs were incorporated into polymer thin films containing PtOEP and DPA. The enhancements of the upconversion fluorescence from DPA and the phosphorescence of PtOEP were observed in the presence of AuNP. The enhancements were varied with AuNP loading and the maximum of enhancements was obtained at the AuNP loading (3.0%). The enhancements of PUC-TTA are most likely attributable to large electric fields due to localized surface plasmon resonance of AuNP.

The magnetic orientation of single-walled carbon nanotubes (SWNTs) or the SWNT composites wrapped with polymer using poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) as the conducting polymer were examined. The formation of SWNT/MEHPPV composites was confirmed by examining absorption and fluorescence spectra. The N,N-dimethylformamide solution of SWNT/MEHPPV composites or the aqueous solution of the shortened SWNTs was introduced dropwise onto a mica or glass plate. The magnetic processing of the composites or the SWNTs was carried out using a superconducting magnet with a horizontal direction (8 T). The AFM images indicated that the SWNT/MEHPPV composites or the SWNTs were oriented randomly without magnetic processing, while with magnetic processing (8 T), they were oriented with the tube axis of the composites or the SWNTs parallel to the magnetic field. In polarized absorption spectra of SWNT/MEHPPV composites on glass plates without magnetic processing, the absorbance due to semiconducting SWNT in the near-IR region in horizontal polarized light was almost the same as that in vertical polarized light. In contrast, with magnetic processing (8 T), the absorbance due to semiconducting SWNT in the horizontal polarization direction against the direction of magnetic field was stronger than that in the vertical polarization direction. Similar results were obtained from the polarized absorption spectra for the shortened SWNTs. These results of polarized absorption spectra also support the magnetic orientation of the SWNT/MEHPPV composites or the SWNTs. On the basis of a comparison of the composites and the SWNTs alone, the magnetic orientation of SWNT/MEHPPV composites is most likely ascribable to the anisotropy in susceptibilities of SWNTs.

Magnetic field effects (MFEs) and time-resolved EPR on photogenerated biradical from intramolecular electron-transfer reactions in zinc-tetraphenylporphyrin (ZnP)–C₆₀ linked compounds (ZnP(4)C₆₀ and ZnP(8)C₆₀) with flexible four and eight methylene groups have been investigated in benzonitrile. At low temperature (288 K), the decay rate constant of the biradical decreased steeply in low magnetic fields (60 and ZnP(8)C₆₀. At high temperature (323 K), in contrast, the decay rate constant decreased slightly in low magnetic fields (60 anion radical due to the spherical π-system. The mechanism is also supported by the time-resolved EPR spectra.

Magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes modified with C60 nanoclusters were examined, intended for the development of nanodevices where photofunctions were controllable with magnetic field. Mixed clusters of a C60 derivative (C60N+) containing a positive charge and methylphenothiazine (MePH) were prepared by dissolving them in the THF–H2O mixed solvent. Diameters of the clusters were estimated to be ca. 100 nm from dynamic light scattering and AFM measurements. Photofunctional electrodes with the nanoclusters of C60N+ and MePH were fabricated on the electrodes modified with a self-assembled monolayer of HS(CH2)2SO3−Na+ on a gold substrate. The photocurrent action spectrum of the cluster-modified electrode strongly indicates that the photocurrents are ascribed to the photoexcitation of the nanoclusters of C60N+. Furthermore, the photocurrents increased with increasing an external magnetic field, typically 3% at 0.5 T. The profile of magnetic field dependence on the photocurrents is in good agreement with the results on the dynamics of radical pair in the nanoclusters of C60N+ and MePH in the THF–H2O mixed solvent. This is suggested that the magnetic field effects are ascribable to the contribution of triplet radical pair that are generated by photoinduced intermolecular electron-transfer from MePH to triplet excited state of nanocluster of C60N+, and are explained in terms of spin-lattice relaxation mechanism in radical pair mechanism.