Pt–Au Triangular Nanoprisms with Strong Dipole Plasmon Resonance for Hydrogen Generation Studied by Single-Particle Spectroscopy

Three anisotropic Pt-covered, Pt-edged, and Pt-tipped Au triangular nanoprisms (TNPs) were prepared by controlling the overgrowth of Pt as photocatalysts for H₂ generation. With strong electric field and more interface for the hot electrons transfer, the H₂ generation rate of Pt-edged Au TNPs was 3 and 5 times higher than those of Pt-tipped and Pt-covered Au TNPs. Single-particle photoluminescence (PL) spectra and finite-difference-time-domain (FDTD) simulations demonstrated that dipole surface plasmon resonance (DSPR) of Au TNPs enhanced the hot electrons transfer from Au to Pt leading to H₂ generation. SPR bands of Au TNPs depending on the size play an important role on the photocatalytic activity of Pt-edged Au TNPs

Photoaccelerated Hole Transfer in Oligothiophene Assemblies

A new series of mesitylene-linked oligothiophenes (<i>n</i>T, <i>n</i> is the number of thiophene units), including 2T-M, 3T-M, 4T-M, 4T-M-2T, and 4T-M-3T, was prepared to investigate the intramolecular hole transfer (HT) from the excited radical cation for the first time. The results of spectroscopic and theoretical studies indicated that mesitylene acts as a spacer minimizing the perturbation to the thiophene π-conjugation and increasing the stability of <i>n</i>T radical cations (<i>n</i>T^•+). Femtosecond laser flash photolysis was applied to the FeCl₃-oxidized 4T^•+-M, 4T^•+-M-2T, and 4T^•+-M-3T. Upon 670 nm laser excitation, the transient absorption spectra of 4T^•+-M showed the existence of two species as the D₁ and D₀^hot states. The intramolecular HT processes from excited 4T^•+ with the time constants of 1.6 and 0.8 ps were observed upon excitation of 4T^•+-M-2T and 4T^•+-M-3T, respectively. This is the first capture of such ultrafast processes with the subsequent back HT from the ground-state 2T^•+ or 3T^•+ in <i>n</i>T assemblies. The current findings indicated an accelerated migration of photocarriers (polarons) in thiophene-based p-type semiconductor materials upon irradiation and provided a fresh viewpoint to understand the successive HT in polythiophenes for various organic molecular devices

Superstructure of TiO₂ Crystalline Nanoparticles Yields Effective Conduction Pathways for Photogenerated Charges

Materials with intricate nanostructures display fascinating properties, which have inspired extensive research on the synthesis of materials with controlled structures. In this study, we investigated the properties of superstructures of TiO₂ to understand the inter-relationship between structural ordering and photocatalytic performance. The nanoplate anatase TiO₂ mesocrystals were chosen as the typical investigation objects, which were newly synthesized by a topotactic structural transformation. The TiO₂ mesocrystals displayed the superstructure of crystallographically ordered alignment of anatase TiO₂ nanocrystals with high surface area and large high-energy surface {001} planes exposed. The photoconductive atomic force microscopy and time-resolved diffuse reflectance spectroscopy were utilized to determine the charge transport properties of TiO₂ mesocrystals, and their features were highlighted by a comparison with reference TiO₂ samples, for example, anatase TiO₂ nanocrystals with similar surface area and single crystal structure. Consequently, it was found for the first time that such a superstructure of TiO₂ could largely enhance charge separation and had remarkably long-lived charges, thereby exhibiting greatly increased photoconductivity and photocatalytic activity

Single-Particle Study of Pt-Modified Au Nanorods for Plasmon-Enhanced Hydrogen Generation in Visible to Near-Infrared Region

Pt-modified Au nanorods (NRs) synthesized by anisotropic overgrowth were used for producing H₂ under visible and near-infrared light irradiation. The Pt-tipped sample exhibited much higher activity compared with fully covered samples. Using single-particle spectroscopies combined with transmission electron microscopy, we observed obvious quenching phenomena for photoluminescence and light scattering from individual Pt-tipped NRs. The analysis of energy relaxation of plasmon-generated hot electrons indicates the electron transfer from the excited Au to Pt

Intramolecular Charge Resonance in Dimer Radical Anions of Di-, Tri-, Tetra-, and Pentaphenylalkanes

Intramolecular dimer radical anions of di-, tri-, tetra-, and pentaphenylalkanes were investigated on the basis of absorption spectral measurements during γ-radiolysis in 2-methyltetrahydrofuran (MTHF) glassy matrix at 77 K and theoretical calculations. The absorption spectrum of 1,1,2,2-tetraphenylethane (1,1,2,2-Ph₄E) radical anion showed two bands in the near-infrared (NIR) region (900–2600 nm). One band observed at shorter wavelength than 2000 nm is assigned to the intramolecular charge resonance (CR) band between two phenyl groups of the 1,1-diphenylmethyl chromophore (1,1-dimer radical anion). The intramolecular CR band of the 1,1-dimer radical anion was observed for various alkanes having 1,1-diphenylmethyl chromophore such as 1,1,1-triphenylmethane (1,1,1-Ph₃M), 1,1,1,1-tetraphenylmethane (1,1,1,1-Ph₄M), and so on. The other intramolecular CR band observed at longer wavelength than 2200 nm is assigned to intramolecular dimer radical anion between two phenyl groups of the 1,2-diphenylethyl chromophore (1,2-dimer radical anion). The intramolecular CR band of the 1,2-dimer radical anion was observed for various alkanes having a 1,2-diphenylethyl chromophore, such as 1,1,2-triphenylethane (1,1,2-Ph₃E), 1,1,2,2-Ph₄E, and 1,1,1,2,2-pentaphenylethane (1,1,1,2,2-Ph₅E) and so on. No dimer radical anion was observed for 1,<i>n</i>-diphenylalkanes (<i>n</i> > 2) without 1,1-diphenylmethyl chromophore. The relationship between the structure and negative charge delocalization over two phenyl groups connected by an sp³ carbon is discussed

HOMO Energy Gap Dependence of Hole-Transfer Kinetics in DNA

DNA consists of two type of base-pairs, G-C and A-T, in which the highest occupied molecular orbital (HOMO) localizes on the purine bases G and A. While the hole transfer through consecutive Gs or As occurs faster than 10⁹ s^–1, a significant drop in the hole transfer rate was observed for G-C and A-T mixed random sequences. In this study, by using various natural and artificial nucleobases having different HOMO levels, the effect of the HOMO-energy gap between bases (Δ_HOMO) on the hole-transfer kinetics in DNA was investigated. The results demonstrated that the hole transfer rate can be increased by decreasing the Δ_HOMO and can be finely tuned over 3 orders of magnitude by varying the Δ_HOMO

Mesolysis of Radical Anions of Tetra‑, Penta‑, and Hexaphenylethanes

A central carbon–carbon (C–C) σ bond dissociation of polyphenylethane radical anions (Ph_<i>n</i>E^•‑, <i>n</i> = 3–6), mesolysis, was investigated by the transient absorption measurement during pulse radiolysis of Ph_<i>n</i>E in 2-methyltetrahydrofuran. The charge resonance (CR) band of 1,1,2,2-tetraphenylethane radical anion (1,1,2,2-Ph₄E^•‑) was observed in the near-infrared region immediately after an electron pulse to be attributed to the intramolecular dimer radical anion. The CR band disappeared with simultaneous formation of two absorption bands at 330 and 460 nm corresponding to diphenylmethyl radical and diphenylmethyl anion, respectively, indicating the occurrence of the mesolysis in 1,1,2,2-Ph₄E^•‑. During pulse radiolysis of 1,1,1,2,2,2-hexaphenylethane (Ph₆E), an absorption band of triphenylmethyl radical was observed at 340 nm immediately after an electron pulse. It is suggested that one electron attachment to Ph₆E is followed by the subsequent rapid C–C σ bond dissociation. Formation of intramolecular dimer radical anions in Ph_<i>n</i>E^•‑ such as 1,1,2-triphenylethane (Ph₃E), 1,1,1,2-tetraphenylethane (1,1,1,2-Ph₄E), and 1,1,1,2,2-pentaphenylethane (Ph₅E) was also studied together with the subsequent mesolysis. The mesolysis of Ph_<i>n</i>E^•‑ is discussed in terms of charge delocalization in the intramolecular dimer radical anions and the central C–C σ bond as well as bond dissociation energy of the central C–C σ bond of Ph_<i>n</i>E^•‑

Plasmon-Enhanced Formic Acid Dehydrogenation Using Anisotropic Pd–Au Nanorods Studied at the Single-Particle Level

Plasmonic bimetal nanostructures can be used to drive the conventional catalytic reactions efficiently at low temperature with the utilization of solar energy. This work developed Pd-modified Au nanorods, which work as the light absorber and the catalytically active site simultaneously, and exhibit efficient plasmon-enhanced catalytic formic acid dehydrogenation even when below room temperature (5 °C). Plasmon-induced interface interaction and photoreaction dynamics of individual nanorods were investigated by single-particle photoluminescence measurement, and a complete quenching phenomenon at the LSPR region was observed for the first time. More importantly, the spatial distribution of the SPR-induced enhancement, analyzed by the finite difference time domain (FDTD) simulation, shows that only tip-coated Pd can be affected for the occurrence of plasmon resonance energy transfer. This finding provides a route to decrease the amount of Pd species by the selective deposition only at the field-enhanced sites

Photochemistry of Singlet Oxygen Sensor Green

To detect singlet oxygen (¹O₂), the commercially available fluorescent sensor named Singlet Oxygen Sensor Green (SOSG) has been the most widely used from material studies to medical applications, for example, photodynamic therapy. In light of the previous studies, SOSG is a dyad composed of fluorescein and anthracene moieties. In the present study, we carried out quantitative studies on photochemical dynamics of SOSG for the first time, such as the occurrence of intramolecular photoinduced electron transfer (PET), ¹O₂ generation, and two-photon ionization. It was revealed that these relaxation pathways strongly depend on the irradiation conditions. The visible-light excitation (ex. 532 nm) of SOSG induced intramolecular PET as a major deactivation process (<i>k</i>_PET = 9.7 × 10¹¹ s^–1), resulting in fluorescence quenching. In addition, intersystem crossing occurred as a minor deactivation process that gave rise to ¹O₂ generation via the bimolecular triplet–triplet energy transfer (<i>k</i>_q = 1.2 × 10⁹ M^–1 s^–1). Meanwhile, ultraviolet-light excitation (355 nm) of SOSG caused the two-photon ionization to give a SOSG cation (Φ_ion = 0.003 at 24 mJ cm^–2), leading to SOSG decomposition to the final products. Our results clearly demonstrate the problems of SOSG, such as photodecomposition and ¹O₂ generation. In fact, these are not special for SOSG but common drawbacks for most of the fluorescein-based sensors

Super-Resolution Mapping of Reactive Sites on Titania-Based Nanoparticles with Water-Soluble Fluorogenic Probes

Interfacial charge transfer at the heterogeneous surface of semiconductor nanoparticles is a fundamental process that is relevant to many important applications, such as photocatalysis, solar cells, and sensors. In this study, we developed new water-soluble fluorogenic probes for interfacial electron transfer reactions on semiconductor nanoparticles. The synthesized boron-dipyrromethene-based fluorescence dyes have one or two sulfonate groups, which confer solubility in aqueous media, and a dinitrophenyl group as a redox reaction site. These probes produce the corresponding fluorescent products <i>via</i> multiple interfacial electron transfer processes, allowing us to investigate the photoinduced redox reactions over individual pristine and Au-nanoparticle-deposited TiO₂ nanoparticles at the single-particle, single-molecule levels. The minimum probe concentration to detect single-product molecules on a single TiO₂ nanoparticle was found to be in the nanomolar range (<10 nM) in acidic solution. Furthermore, super-resolution mapping of the reaction sites revealed that visible-light-induced reduction reactions preferentially occurred on the TiO₂ surface within a distance of a few tens of nanometers around the deposited Au nanoparticles. This result was qualitatively interpreted on the basis of plasmon-induced electron and/or energy transfer mechanisms. Overall, this study provides a great deal of valuable information related to solar-energy-conversion processes that is impossible or difficult to obtain from ensemble-averaged experiments