The intramolecular [4+4] photodimerization of anthracene chromophores in covalent assemblies can be harnessed to create negative photochromic systems. This paper reports the characterization of the photophysical and photochemical properties of a new class of asymmetric phenyl-linked bis(anthracene) photochromes and compares their behavior with that of a previously studied symmetric ethylene-linked analog. Steady-state and femtosecond time-resolved spectroscopic experiments show that both types of bis(anthracenes) support a neutral bright state along with a lower-energy charge resonance state. After photoexcitation, both states relax on sub-10 ps timescales, but with significantly different photodimerization quantum yields: 0.83 using 532 nm excitation of the charge-resonance state versus 0.46 for 400 nm excitation of the neutral bright state. The phenyl-linked bis(anthracene) derivatives exhibit superior thermal stability and reversibility due to a lower activation energy for dimer dissociation (94 kJ mol-1versus 110 kJ mol-1). Quantum chemical calculations reveal the structure of the neutral and charge-resonance excited states and can rationalize the higher photodimerization quantum yield of the latter. The phenyl linker enforces close alignment of the anthracene moieties, shifting the charge resonance state to lower energy and extending the wavelength range of the photochrome while also raising the energy of the photodimer ground state to enhance the backward reaction rate. The phenyl-linked bis(anthracenes) provide a promising system to harness the [4+4] photodimerization reaction with high quantum yield, room temperature reversibility, and cyclability.

Plasmonic Au nanoparticle photoelectrodes were fabricated and characterized with ultrafast transient absorption (TA) and photomodulated cyclic voltammetry (CV) to determine whether the plasmonic photoenhancement mechanism for electrooxidation of ethanol is driven by hot electron transfer or by photothermal heating. The different timescales of the ultrafast TA and CV experiments enable the characterization of the photocatalytic event from start to finish. From TA experiments, visible-light photoexcitation of plasmonic Au nanoparticles is followed by rapid picosceconds excited-state relaxation, whereas photomodulated CV experiments exhibited slow photophysics, with the ethanol electrooxidation photocurrent growing in and decaying over 100s of milliseconds. These outcomes are only consistent with a photothermal heating mechanism for ethanol electrooxidation. The slow timing of the photocurrent decay after illumination has ceased is incompatible with a photoinduced electron-transfer mechanism given the rapidity of excited-state relaxation and carrier recombination. Systematic analysis of the photomodulated CV further corroborates the dominance of the photothermal mechanism in that it shows the photocurrent response quantitatively follows the temperature response in photothermal heating experiments, with no contribution from photoinduced electron transfer being detected whatsoever. Beyond elucidating the photoenhancement mechanism for ethanol electrooxidation, the methodology presented here provides a logical framework for the photomechanistic analysis of other reactions and systems driven by light absorption of plasmonic nanoparticles.

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al2O3 seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm2 under simulated sunlight (AM1.5G, 100 mW/cm2) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.

Energy transfer was explored in a series of poly(phenylene ethynylene) (PPE) polymers with increasing lengths that were end-capped with thiophene-benzothiadiazole (TBT) groups to form fully conjuga...

Interface dynamics of nanocrystalline NiO thin films sensitized with two ruthenium polypyridyl chromophores have been investigated to examine the influence that excited-state dipole orientation and the position of the bipyridine radical formed in the charge-separated state have on interfacial electron transfer yields. In ultrafast transient absorption experiments, the charge separated state is observed on the nanosecond timescale for the trifluoromethyl-substituted chromophore, [Ru(flpy)(2)(dcb)](2+) (flpy = 4,4'-bis(trifluoromethyl)-2,2'-bipyridine, dcb = 4,4'-dicarboxy-2,2'-bipyridine), but not for [Ru(bpy)(2)(dcb)](2+) (bpy = 2,2'-bipyridine). Differences are attributed to the positioning of the bipyridine radical formed in the charge separated state; for [Ru(flpy)(2)(dcb)](2+), the electron is localized on the flpy ligand distal to the surface, whereas for [Ru(bpy)(2)(dcb)](2+), the electron is localized on the dcb ligand, proximal to the NiO surface. Enhanced photovoltaic performance is observed for dye-sensitized solar cell devices prepared with [Ru(flpy)(2)(dcb)](2+), demonstrating that enhanced charge separation can be correlated with device efficiency.

Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor-π-acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO2, and catalyst binding through phosphonate linkage to the TiO2 layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO2 or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. The absolute magnitudes of photocurrent changes with the core material, TiO2 spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The results of transient absorption measurements and DFT calculations show that electron injection by the photoexcited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye's molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. Although chromophore decomposition limits the efficiency of the DSPEC devices toward O2 production, the flexibility of the strategy presented here offers a new approach to photoanode design.

The hole-injection and recombination photophysics for NiO sensitized with RuP ([RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron-hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway.

Ultrafast energy and electron transfer (EnT and ET, respectively) are characterized in a light-harvesting assembly based on a π-conjugated polymer (poly(fluorene)) functionalized with broadly absorbing pendant organic isoindigo (iI) chromophores using a combination of femtosecond transient absorption spectroscopy and large-scale computer simulation. Photoexcitation of the π-conjugated polymer leads to near-unity quenching of the excitation through a combination of EnT and ET to the iI pendants. The excited pendants formed by EnT rapidly relax within 30 ps, whereas recombination of the charge-separated state formed following ET occurs within 1200 ps. A computer model of the excited-state processes is developed by combining all-atom molecular dynamics simulations, which provides a molecular-level view of the assembly structure, with a kinetic model that accounts for the multiple excited-state quenching pathways. Direct comparison of the simulations with experimental data reveals that the underlying structure has a dramatic effect on the partitioning between EnT and ET in the polymer assembly, where the distance and orientation of the pendants in relation to the backbone serve to direct the dominant quenching pathway.

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru(III/)* couple, E(+/)*, in 0.1 M perchloric acid (HClO4(aq)) to be tuned from -0.69 to -1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350-800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3-30 and 30-500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to E(+/)*. The correlation between the exponentially diminishing density of TiO2 sub-band acceptor levels and injection rate is well described using Marcus-Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the (3)MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the (3)MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials.

Sensitized SrTiO3 films were evaluated as potential photoanodes for dye-sensitized photoelectrosynthesis cells (DSPECs). The SrTiO3 films were grown via pulsed laser deposition (PLD) on a transparent conducting oxide (fluorine doped tin oxide, FTO) substrate, annealed, and then loaded with zinc(II) 5,10,15-tris(mesityl)-20-[(dihydroxyphosphoryl)phenyl] porphyrin (MPZnP). When paired with a platinum wire counter electrode and an Ag/AgCl reference electrode these sensitized films exhibited photocurrent densities on the order of 350 nA/cm(2) under 0 V applied bias conditions versus a normal hydrogen electrode (NHE) and 75 mW/cm(2) illumination at a wavelength of 445 nm. The conditions of the post-deposition annealing step namely, a high-temperature reducing atmosphere-proved to be the most important growth parameters for increasing photocurrent in these electrodes.

Singlet fission, in which an initially excited singlet state spontaneously splits into a pair of triplet excitons, is a process that can potentially boost the efficiency of solar energy conversion. The separate electronic bands in organic semiconductors make them especially useful for dividing a high-energy singlet exciton into a pair of lower-energy triplet excitons. Recent experiments illustrate the role of spin coherence in fission, while kinetic models are used to describe how triplet and singlet states interact on longer time scales. Despite insights gained from recent experiments, the detailed structure and dynamics of the electronic states involved in the initial step of singlet fission remain active areas of investigation. On longer time scales, finding ways to efficiently harvest the triplet excitons will be an important challenge for making devices based on this phenomenon. A full understanding of singlet fission requires consideration of a sequence of photophysical events (decoherence, relaxation, and diffusion) occurring on different time scales.

The dynamics of singlet fission (SF) are studied in monoclinic and orthorhombic crystals of 1,6-diphenyl-1,3,5-hexatriene. Picosecond time-resolved fluorescence measurements and the presence of a strong magnetic field effect indicate that up to 90% of the initially excited singlets undergo SF in both forms. The initial SF and subsequent triplet pair dissociation rates are found to be more rapid in the monoclinic crystal by factors of 1.5 and 3.5, respectively. These results provide clear evidence that molecular organization affects the rates of triplet pair formation and separation, both important parameters for determining the ultimate utility of a SF material.

Hydrogen treatment of TiO2 has been demonstrated to significantly alter its optical properties, including substantially enhanced visible light absorption that has important implications for various applications. The chemical nature of the bandgap states responsible for the increased visible absorption is not yet well understood. This work reports a detailed study of the structural, optical, electronic, and ultrafast properties of hydrogen-treated TiO2 (H:TiO2) nanowires (NWs) using a combination of experimental techniques including high-resolution transmission electron microscopy (HRTEM), electron spin resonance spectroscopy (ESR), time-resolved fluorescence (TRF), and femto-second transient absorption (TA) spectroscopy in order to explain the origin of the strong visible absorption. The combined TEM, ESR, TRF, and TA data suggest that the presence of a localized mid-bandgap oxygen vacancy (V-O) occupied by a lone electron in an antibonding orbital situated at a surface site is likely responsible for the visible absorption of the material. The data further indicate that while untreated TiO2 NWs are fluorescent, the hydrogen treatment leads to quenching of the fluorescence and highly efficient charge carrier recombination from the V-O state following excitation with visible light. With UV excitation, however, the charge carrier recombination of the H:TiO2 NWs exhibits a larger component of a slow decay compared to that of untreated TiO2, which is correlated with enhanced photoelectrochernical performance. Both the treated and untreated samples exhibit a fast decay that dominates the TA signals, which is likely caused by a high density of surface trap states. A simple model is proposed to explain all the key optical and dynamic features observed. The results have provided deeper insight into the chemical nature and photophysical properties of bandgap states in chemically modified TiO2 nanomaterials.

The spectroscopic and photocatalytic properties of a series of Au@TiO(2) core-shell nanostructures are characterized. The crystallinity of the TiO(2) shells was varied by changing the etching and calcination conditions. Measurements of the photoluminescence, transient absorption, and H(2) production rate permit us to look for correlations between the spectroscopic and catalytic behaviors. We found that there is a strong effect of crystallinity on the H(2) production rate and also the stretched exponential lifetime of the photoluminescence created by short-wavelength (266 and 300 nm) photoexcitation. As the TiO(2) crystallinity is increased, the photoluminescence lifetime increases from 22 to 140 ps in a 1 ns detection window, while the H(2) production rate increases by a factor of ~4. There is no discernible effect of crystallinity on the photoluminescence dynamics excited at 350 or 430 nm, or on the electronic dynamics measured by femtosecond transient absorption after excitation at 300 nm. We hypothesize that high-energy photons create reactive and emissive charge-separated states in parallel, and that both species are subject to similar electron-hole recombination processes that depend on sample crystallinity. Based on our observations, it can be concluded that the photoluminescence dynamics may be used to evaluate the potential performance of this class of photocatalysts.

The properties and applications of materials are significantly controlled by their physical characteristics, such as size, shape, and structural state. Many processes are governed by interface reactions by which the surface energy and reactivity depend on the spatial configuration, coordination, and structural state of surface atoms and molecules. For crystals, this dependence is directly related to the expression of specific crystallographic faces, which exhibit different surface structures and atomic configurations. These differences explain why some applications, such as molecular adsorption and desorption, gas sensing, drug molecule delivery and release, and heterogeneous catalysis are highly sensitive to the surface atomic structures. Recent progress in the engineering of crystal morphology has included the synthesis of polyhedral silver nanocrystals by the polyol method, the epitaxially seeded growth of highly faceted Pt-Pd nanocrystals, and the controlled overgrowth of Pd-Au core–shell structures enclosed by {111} facets. Apart from these metallic nanocrystals, binary or ternary compounds with preferentially developed facets have also been reported. The facet effect is an important factor for heterogeneous photocatalysts, because surface atom arrangement and coordination intrinsically determine the adsorption of reactant molecules, surface transfer between photoexcited electrons and reactant molecules, and desorption of product molecules. This phenomenon has been well studied in TiO2 photocatalysts. TiO2 is one of the most extensively studied photocatalysts owing to its abundance, nontoxicity, and stability. However, for practical applications, pure TiO2 is not a good candidate because it is only active under ultraviolet (UV) irradiation owing to the band gap of 3.2 eV for the anatase phase. Therefore, band-gap engineering is required to use TiO2 as a water-splitting catalyst under visible-light irradiation. Reduced TiO2 (TiO2 x), containing Ti or O vacancies, has been reported to show visible-light absorption. Various strategies have been applied to synthesize reduced TiO2, such as heating under vacuum [8] or reducing gas, laser irradiation, and high-energy particle bombardment (electrons or Ar ions). A big challenge for the application of reduced TiO2 is that the surface oxygen defects are highly unstable in air owing to the susceptibility of Ti toward oxidation by O2. [13] Recently, we reported a facile one-step combustion method to synthesize partially reduced TiO2. [14] The presence of Ti in the sample extends the photoresponse of TiO2 from the UV to the visible light region, which leads to high visible-light photocatalytic activity for the generation of hydrogen gas from water. However, in the rapid and harsh combustion process, there is very limited control over the crystallization process, which results in the irregularly shaped products. Herein we report the development of a simple solution method to grow non-stoichiometric rutile TiO2 crystals with desired facets. The incorporation of Ti, which extends the light absorption from the UV into the visible range, along with the development of facets with high reactivity, results in a material exhibiting greatly enhanced photocatalytic H2 production activity relative to the combustion product we reported before. Powder X-ray diffraction analysis (Figure 1a) shows that the sample of as-produced TiO2 (sample S1) has rutile structure. All of the diffraction peaks can be assigned to

Three 2,6 dialkoxyanthracenes with varying ether sidechain lengths (methyl, n-propyl and n-hexyl) are synthesized and characterized. When compared to unsubstituted anthracene, the spectroscopic properties of the alkoxy-anthracenes are significantly different. The oscillator strength appears to be evenly distributed between the La and Lb states, rather than concentrated in the low-energy La state, and the transition dipole moments are rotated by ∼30° in the molecular frame. More importantly, all three derivatives undergo an intramolecular conformational change in the excited state that gives rise to a reshaping of the fluorescence spectrum on the nanosecond timescale. This process has an activation energy of 8±1kJ/mol. By ∼150K the fluorescence spectrum reflects only the high energy conformer, whose emission lineshape is similar to that of the unsubstituted anthracene. The temperature-dependent changes in the monomeric fluorescence spectra of these compounds will have to be taken into account in future studies of their solid-state spectroscopy.

Previous studies of solid-state tetracyanobenzene-based donor-acceptor complexes showed that these materials were highly susceptible to both laser and mechanical damage that complicated the analysis of their electron-transfer kinetics. In this paper, we characterize the optical properties of a pyrene/tetracyanoquinodimethane charge-transfer crystal that is much more robust than the tetracyanobenzene compounds. This donor-acceptor complex has a charge-transfer absorption that extends into the near-infrared, rendering the crystal black. We use time-resolved fluorescence and diffuse reflectance transient absorption to study its dynamics after photoexcitation. We show that the initially excited charge-transfer state undergoes a rapid, monoexponential decay with a lifetime of 290 ps at room temperature. There is no evidence for any long-lived intermediate or dark states; therefore, this decay is attributed to charge recombination back to the ground state. Fluorescence lifetime measurements demonstrate that this process becomes temperature-independent below 60 K, indicative of a thermally activated tunneling mechanism. The subnanosecond charge recombination makes this low-band-gap donor-acceptor material a poor candidate for generating long-lived electron-hole pairs.

Molecular crystals composed of 9-anthracene carboxylic acid (9AC) can undergo reversible light-induced mechanical motions driven by a [4 + 4] photodimerization reaction. This paper explores the structure, photo. physics, and photomechanical response of a family of anthracene carboxylic acid derivatives, with the goal of finding materials that have comparable or improved photomechanical properties. We find that methyl or phenyl substitution at the 10-position leads to a complete loss of photoreactivity due to changes in crystal packing. A series of halogen (F, Cl, Br) 10-substituted 9AC molecules all showed a similar stacked packing motif, but only the fluoro-substituted molecule;was photoreactive in the solid. Its photomechanical response was Similar to that of 9AC but with a much longer recovery time. Extending the carboxylic acid by adding a vinylene group at the 9-position resulted in crystals that showed good photoreactivity and a lack of fracture but no reversibility. Attempts to self consistently rationalize observed trends in terms of excited state lifetimes or steric effects were only partly successful. Balancing factors such as electronic relaxation, steric interactions, and crystal packing present a challenge for engineering photoactive solid-state materials based on molecular crystals.