Christopher J. Bardeen

Active facets on titanium(III)-doped TiO2: an effective strategy to improve the visible-light photocatalytic activity.

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

Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S(1) state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S(0)→S(1) transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T(1)→T(N) absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracenes photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.

High-power femtosecond chirped pulse excitation of molecules in solution

Abstract The effects of varying the chirp and intensity of an ultrashort pulse exciting the molecules LD690 and LDS750 in room temperature liquid solutions are investigated experimentally and theoretically. For low-power excitation, we find that the absorption and amount of excited state population are independent of chirp, while for high-power excitation we observe a strong chirp dependence. The high-power data results from an intra-pulse pump-dump process which is enhanced by negatively chirped pulses and prevented by positively chirped pulses.

Reversible photoinduced twisting of molecular crystal microribbons.

9-Anthracenecarboxylic acid, a molecule that undergoes a reversible [4 + 4] photodimerization, is prepared in the form of oriented crystalline microribbons. When exposed to spatially uniform light irradiation, these photoreactive ribbons rapidly twist. After the light is turned off, they relax back to their original shape over the course of minutes. This photoinduced motion can be repeated for multiple cycles. The final twist period and cross-sectional dimensions of individual microribbons are measured using a combination of atomic force and optical microscopies. Analysis of this data suggests that the reversible twisting involves the generation of interfacial strain within the ribbons between unreacted monomer and photoreacted dimer regions, with an interaction energy on the order of 3.4 kJ/mol. The demonstration of reversible twisting without the need for specialized irradiation conditions represents a new type of photoinduced motion in molecular crystals and may provide new modes of operation for photomechanical actuators.

Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K10[Nb2O2(H2O)2][SiNb12O40]·12H2O

The synthesis and photocatalytic properties of a heteropolyoxoniobate, K(10)[Nb(2)O(2)(H(2)O)(2)][SiNb(12)O(40)]·12H(2)O (1), are reported, revealing an important role of Zr(4+) additives in the crystallization. Compound 1 exhibits overall photocatalytic water splitting activity, and its photocatalytic activity is significantly higher than that of Na(10)[Nb(2)O(2)][SiNb(12)O(40)]·xH(2)O (2). Fluorescence lifetime measurements suggest that the enhanced photocatalytic activity of 1 likely results from a larger yield of longer-lived charge trapping states in 1 due to the coordination of one water molecule to the bridging Nb(5+), leading to highly unsymmetrical seven-coordinated Nb(5+) sites.

Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix

We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly ...

Phosphorescence from iridium complexes doped into polymer blends

Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, tris[9,9-dihexyl-2-(phenyl-4′-(-pyridin-2″-yl))fluorene] iridium (III) [Ir(DPPF)3], is investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy. A redshifted PL and slow fluorescence decay are due to the formation of an exciplex in PVK-PBD blends. A decrease in intensity in polymer blends observed at 425 nm with increasing concentrations of Ir(DPPF)3 and an evident rising feature observed in films with 1 wt % Ir(DPPF)3 in the range of 578 to 615 nm within a 200 ns timescale indicate that efficient Forster energy transfer from exciplex to Ir(DPPF)3 occurs. The electrophosphorescent light-emitting diodes fabricated with PVK-PBD doped with Ir(DPPF)3 have external quantum efficiency of 8% ph/el, luminous efficiency of 29 cd/A and brightness greater than 3500 cd/m2 at 1 wt % Ir(DPPF)3. The devices exhibited no electroluminescence (...

The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: Kinetic evidence for a dark intermediate state and implications for singlet fission

The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.

Self-absorption correction for solid-state photoluminescence quantum yields obtained from integrating sphere measurements

A new method is presented for analyzing the effects of self-absorption on photoluminescence integrating sphere quantum yield measurements. Both the observed quantum yield and luminescence spectrum are used to determine the self-absorption probability, taking into account both the initial emission and subsequent absorption and reemission processes. The analysis is experimentally validated using the model system of the laser dye perylene red dispersed in a polymer film. This approach represents an improvement over previous methods that tend to overestimate the true quantum yield, especially in cases with high sample absorbance or quantum yield values.

Hybrid Molecule-Nanocrystal Photon Upconversion Across the Visible and Near-Infrared.

The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.