Singlet fission, in which a singlet exciton is converted to two triplet excitons, is a process that could be beneficial in photovoltaic applications. A full understanding of the dynamics of singlet fission in molecular systems requires detailed knowledge of the relevant potential energy surfaces and their (conical) intersections. However, obtaining such information is a nontrivial task, particularly for molecular aggregates. Here we investigate singlet fission in rubrene crystals using transient absorption spectroscopy and state-of-the-art quantum chemical calculations. We observe a coherent and ultrafast singlet-fission channel as well as the well-known and conventional thermally assisted incoherent channel. This coherent channel is accessible because the conical intersection for singlet fission on the excited-state potential energy surface is located very close to the equilibrium position of the ground-state potential energy surface and also because of the excitation of an intermolecular symmetry-breaking mode, which activates the electronic coupling necessary for singlet fission.

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Octadecanethiolate-protected gold (Au:SC18) clusters were prepared by the reaction of C18SH and Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) (PVP). Four samples were fractionated by recycling size exclusion chromatography of the as-prepared Au:SC18 clusters, and their core sizes were determined to be 8, 11, 21, and 26 kDa by using laser desorption ionization mass spectrometry. Unexpectedly, the sequence of these core sizes is different from that (8, 14, 22, and 29 kDa) obtained by conventional reduction of Au(I)−SC18 polymers, which is governed by kinetic factors. The present finding shows that the Au:SR (R = organic group) clusters with a high tolerance to thiol etching can be systematically synthesized by first populating precursory Au clusters in a PVP matrix with subsequent thiolation of the preformed Au clusters.

Bismuth vanadate (BiVO4) is an effective visible-light-driven photocatalyst for oxygen evolution from water. To understand the mechanism of photocatalytic oxidation of water, it is important to detect and characterize holes at the surfaces of powdered catalysts. Here, we report the transient absorption of BiVO4 in a wide time range from subpicosecond to 200 μs upon the excitation across the band gap with 400 nm femtosecond pulses. The effect of electron scavenger (Fe3+) on transient absorption decays indicates that the transitions at λ < 700 nm are mainly contributed by holes at the surfaces. While the transient absorption at λ > 700 nm rises almost instantaneously, the absorption λ < 700 nm has a slower rise component of τ ∼ 15 ps due to filling of surface traps with holes. Moreover, the rise component is modulated with strongly oscillating signals caused by coherent excitation of an external phonon mode between Bi3+ and VO4 3–. Thus, the transitions at λ < 700 nm associated with surface-trapped holes are strongly coupled to the external phonon mode. This study demonstrates that the time-domain spectroscopy is useful for characterizing the vibrational structure specific to the surface charge trap sites of powdered photocatalysts.

We used STM to observe visible light photo-oxidation reactions of formic acid on the ordered lattice-work structure of a TiO(2)(001) surface for the first time. The nanostructured surface makes the band gap significantly smaller than 3.0 eV only at the surface layer, and the surface state of the crystal enables a visible light response.

Heterogeneous photocatalysis is vital in solving energy and environmental issues that this society is confronted with. Although photocatalysts are often operated in the presence of water, it has not been yet clarified how the interaction with water itself affects charge dynamics in photocatalysts. Using water-coverage-controlled steady and transient infrared absorption spectroscopy and large-model (∼800 atoms) ab initio calculations, we clarify that water enhances hole trapping at the surface of TiO2 nanospheres but not of well-faceted nanoparticles. This water-assisted effect unique to the nanospheres originates from water adsorption as a ligand at a low-coordinated Ti–OH site or through robust hydrogen bonding directly to the terminal OH at the highly curved nanosphere surface. Thus, the interaction with water at the surface of nanospheres can promote photocatalytic reactions of both oxidation and reduction by elongating photogenerated carrier lifetimes. This morphology-dependent water-assisted effect provides a novel and rational basis for designing and engineering nanophotocatalyst morphology to improve photocatalytic performances.

The excited-state dynamics of molecular aggregates are governed by their potential energy landscape that can hardly be controlled artificially. However, it is possible to alter the excited state dynamics by a strong coupling between light and molecules (polariton formation) because it can decouple the electronic and vibrational degrees of freedom. Here, we demonstrate this polaron decoupling effect on the photochemical dynamics in singlet fission (SF) of amorphous rubrene thin films embedded in optical microcavities. The vibronic feature of polariton states in this system is characterized through the analysis of steady state absorption spectra by using the Holstein-Tavis-Cummings model. On the basis of this analysis, we show with time-resolved spectroscopy that the SF rate following a resonant excitation of the lowest energy polariton state is indeed modulated when the cavity photon energy is changed. A numerical simulation by using Fermi’s golden rule formula with the vibronic polariton feature successfully accounts for the observed modulation of the SF rate, indicating that the polaron decoupling plays a decisive role in the nonadiabatic dynamics.

The oxidation of methanol on a Pt(111)–(2×2)O surface has been investigated by infrared reflection absorption spectroscopy and temperature-programed desorption. Methanol is dehydrogenated to produce methoxy species in the annealing temperature range from 130 to 170 K. Above 170 K, the reaction proceeds differently, depending on methanol coverage. At the saturation coverage, methanol adsorbates partly desorb molecularly and partly react with precovered oxygen atoms to produce CO, H2, and H2O. No detectable formaldehyde or formate is formed. In contrast, at submonolayer coverages, methoxy species is dehydrogenated to yield formaldehyde at ∼180 K and further oxidized to formate at ∼200 K. Formate is decomposed by 300 K. Defect sites such as steps are not relevant to the formation of the intermediates. When CO is coadsorbed on the surface, it destabilizes the reaction intermediates. The destabilization by coadsorbed CO makes the reaction intermediates short lived so as not to be detectable at high initial coverages of methanol.