Modulating electron injection from an organic dye to a titania nanoparticle with a photochromic energy transfer acceptor

We have prepared titania nanoparticles with an organic dye sensitiser and diarylethene molecular switch attached to the surface. Spectroscopic investigations show that the dye sensitiser's electron injection efficiency is reduced when the diarylethene is switched from its colourless, ring-open isomer to its coloured, ring-closed isomer, due to the introduction of a competing energy transfer pathway

Spectroscopic study of the benchmark Mn+-H2 complex

We have recorded the rotationally resolved infrared spectrum of the weakly bound Mn+-H2 complex in the H-H stretch region (4022-4078 cm(-1)) by monitoring Mn+ photodissociation products. The band center of Mn+-H2, the H-H stretch transition, is shifted by -111.8 cm(-1) from the transition of the free H2 molecule. The spectroscopic data suggest that the Mn+-H2 complex consists of a slightly perturbed H2 molecule attached to the Mn+ ion in a T-shaped configuration with a vibrationally averaged intermolecular separation of 2.73 A. Together with the measured Mn+...H2 binding energy of 7.9 kJ/mol (Weis, P.; et al. J. Phys. Chem. A 1997, 101, 2809.), the spectroscopic parameters establish Mn+-H2 as the most thoroughly characterized transition-metal cation-dihydrogen complex and a benchmark for calibrating quantum chemical calculations on noncovalent systems involving open d-shell configurations. Such systems are of possible importance for hydrogen storage applications

Onset of carbon-carbon bonding in the Nb5Cy (y = 0-6) clusters: a threshold photo-ionisation and density functional theory study

We have used photo-ionisation efficiency spectroscopy to determine the ionisation potentials (IPs) of the niobium–carbide clusters, Nb5Cy (y = 0–6). Of these clusters Nb5C2 and Nb5C3 exhibit the lowest IPs. Complementary density functional theory calculations have been performed to locate the lowest energy isomers for each cluster. By comparing the experimental IPs with those calculated for candidate isomers, the structures of the Nb5Cy clusters observed in the experiment are inferred. For all these structures, the underlying Nb5 cluster has either a ‘prolate’ or ‘oblate’ trigonal bipyramid geometry. Both Nb5C5 and Nb5C6 are shown to contain carbon–carbon bonding in the form of one and two molecular C2 units, respectively

Photochrome-doped organic films for photonic key-pad locks and multi-state fluorescence

The spectroscopic properties of poly(methyl methacrylate) polymer films doped with two kinds of photochromic molecular switches are investigated. A green-fluorescent sulfonyl diarylethene (P1) is combined with either a non-fluorescent diarylethene (P2) or red-fluorescent spiropyran (P3). Photoswitching between the colorless and colored isomers (P1: o-BTFO4 ↔ c-BTFO4, P2: o-DTE ↔ c-DTE, P3: SP ↔ MC) enables the P1+P2 and P1+P3 films to be cycled through three distinct states. From the initial state (o-BTFO4 + o-DTE/SP), irradiation with UV light generates the second state (c-BTFO4 + c-DTE/MC), where c-BTFO4 → c-DTE/MC energy transfer is established. Irradiation with green light then generates the third state (c-BTFO4 + o-DTE/SP), where the energy transfer acceptor is no longer present. Finally, irradiation with blue light regenerates the initial state. For the P1+P2 film, only one state is fluorescent, with the irradiation inputs required to be entered in the correct order to access this state, acting as a keypad lock. For the P1+P3 film, the states emit either no fluorescence, red fluorescence, or green fluorescence, all using a common excitation wavelength. Additionally, once the fluorescence is activated with UV light, it undergoes a time-dependent color transition from red to green, due to the pairing of P-type and T-type photochromes. These multi-photochromic systems may be useful for security ink or imaging applications

Photoionization efficiency spectroscopy and density functional theory investigations of RhHo2On, (n=0-2) clusters

The experimental and theoretical adiabatic ionization energies (IEs) of the rhodium-holmium bimetallic clusters RhHo(2)O(n) (n=0-2) have been determined using photoionization efficiency spectroscopy and density functional theory (DFT) calculations. Both sets of data show the IE of RhHo(2)O to be significantly lower than the values for RhHo(2) and RhHo(2)O(2), which are found to be similar. This indicates that there are significant changes in electronic properties upon sequential addition of oxygen atoms to RhHo(2). The DFT investigations show that the lowest energy neutral structures are a C(2v) triangle for RhHo(2), a C(2v) planar structure for RhHo(2)O where the O atom is doubly bridged to the Ho-Ho bond, and a C(2v) nonplanar structure for RhHo(2)O(2), where the O(2) is dissociative and each O atom is doubly bridged to the Ho-Ho bond in the cluster above and below the RhHo(2) trimer plane. Good correlation between the experimental and computational IE data imply that the lowest energy neutral structures calculated are the most likely isomers ionized in the molecular beam. In particular, the theoretical adiabatic IE for the dissociative RhHo(2)O(2) structure is found to compare better with the experimentally determined value than the corresponding lowest energy O(2) associative structure.Alexander S. Gentleman, Matthew A. Addicoat, Viktoras Dryza, Jason R. Gascooke, Mark A. Buntine, and Gregory F. Meth

Threshold photo-ionisation and density functional theory studies of metal-carbide clusters.

Neutral gas-phase metal-carbide clusters are generated by laser ablation and are detected in the constructed time-of-flight mass-spectrometer by laser ionisation. Photo-ionisation efficiency (PIE) experiments are performed on the metal-carbide clusters to determine their ionisation potentials (IPs). Complimentary density functional theory (DFT) calculations are performed on the energetically favorable structural isomers of the metalcarbide clusters. Comparison between the calculated IPs of the isomers and the experimental IP allows the carrier of the observed ionisation onset for a metal-carbide cluster to be assigned. The niobium-carbide clusters Nb₃Cy (y = 0–4), Nb₄Cy (y = 0–6) and Nb₅Cy (y = 0–6) are examined by PIE experiments and DFT calculations. The IPs of the niobium-carbide clusters are found to be either left reasonably unchanged from the IPs of the bare metal clusters or moderately reduced. The clusters Nb₃C₂, Nb₄C₄, Nb₅C₂ and Nb₅C₃ display the largest IP reductions for their corresponding cluster series. The structures assigned to the IPs of the Nb₃Cy (y = 1–3) clusters are based on the carbon atoms attaching to the niobium faces and/or niobium-niobium edges of the triangular Nb₃ cluster. However, for Nb₃C₄ the ionisation onset is assigned to a low-lying isomer, which contains a molecular C₂ unit, rather than the lowest energy isomer, a niobium atom deficient 2×2×2 face-centred cubic (fcc) nanocrystal structure. The structures assigned to the IPs of the Nb₄Cy (y = 1–4) clusters are based on the carbon atoms attaching in turn to the niobium faces of the tetrahedral Nb₄ cluster, developing a 2×2×2 fcc nanocrystal structure for Nb₄C₄. For Nb₄C₃ two ionisation onsets are observed; one weak onset at low energy and another more intense onset at high energy. It is proposed that the two onsets are due to ionisation from both a metastable ³A₁ state and the ground ¹A₁ state of the lowest energy isomer. The ionisation onsets of Nb₄C₅ and Nb₄C₆ are also proposed to originate from metastable triplet states of the lowest energy isomers, with the transitions from the ground singlet states calculated to be greater than the highest achievable photon energy in the laboratory. The structures of Nb₄C₅ and Nb₄C₆ have one and two carbon atoms in a 2×2×2 fcc nanocrystal substituted with molecular C₂ units, respectively. The structures assigned to the IPs of the Nb₅Cy (y = 1–6) clusters are based on the underlying Nb₅ cluster being in either a “prolate” or “oblate” trigonal bipyramid geometry; the former has six niobium faces available for carbon addition, while the latter has two niobium butterfly motifs and two niobium faces available for carbon addition. Both the structures of Nb₅C₅ and Nb₅C₆ have the underlying Nb₅ cluster in the oblate trigonal bipyramid geometry and contain one and two molecular C₂ units, respectively. The tantalum-carbide clusters Ta₃Cy (y = 0–3), Ta₄Cy (y = 0–4) and Ta₅Cy (y = 0–6) are examined by PIE experiments and DFT calculations. The IPs of the tantalum-carbide clusters in each series show trends that are very similar to the corresponding iso-valent niobium-carbide cluster series, although the IP reductions upon carbon addition are smaller for the former. For the vast majority of tantalum-carbide clusters, the same structural isomer is assigned to the ionisation onset as that assigned for the corresponding niobium-carbide cluster. Bimetallic tantalum-zirconium-carbide clusters are generated using a constructed double ablation cluster source. The Ta₃ZrCy (y = 0–4) clusters are examined by PIE experiments and DFT calculations. The IP trend for the Ta₃ZrCy cluster series is reasonably similar to that of the Ta₄Cy cluster series, although the IP reductions upon carbon addition are greater for the former. The structures assigned to the IPs of the Ta₃ZrCy (y = 1–4) clusters are based on the carbon atoms attaching in turn to the metal faces of the tetrahedral Ta₃Zr cluster. In summary, the work presented in this thesis demonstrates that the structures of metalcarbide clusters can be inferred by the determination of their IPs through PIE experiments in combination with DFT calculations on candidate structural isomers.Thesis (Ph.D.) - University of Adelaide, School of Chemistry and Physics, 200

Infrared spectra of mass-selected Mg+-H2 and Mg +-D2 complexes

Rotationally resolved infrared spectra of Mg(+)-H(2) and Mg(+)-D(2) are recorded in the H-H (4025-4080 cm(-1)) and D-D (2895-2945 cm(-1)) stretch regions by monitoring Mg(+) photofragments. The nu(HH) and nu(DD) transitions of Mg(+)-H(2) and Mg(+)-D(2) are red-shifted by 106.2 +/- 1.5 and 76.0 +/- 0.1 cm(-1) respectively from the fundamental vibrational transitions of the free H(2) and D(2) molecules. The spectra are consistent with a T-shaped equilibrium structure in which the Mg(+) ion interacts with a slightly perturbed H(2) or D(2) molecule. From the spectroscopic constants, a vibrationally averaged intermolecular separation of 2.716 A (2.687 A) is deduced for the ground state of Mg(+)-H(2) (Mg(+)-D(2)), decreasing by 0.037 A (0.026 A) when the H(2) (D(2)) subunit is vibrationally excited

The Ontario psychologist

In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M+-H-2 and M+-D-2 complexes in the gas-phase, with targets that include Li+-H-2, B+-H-2, Na+-H-2, Mg+-H-2, Al+-H-2, Cr+-D-2, Mn+-H-2, Zn+-D-2 and Ag+-H-2. Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M+-H-2 complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M+ photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M+-H-2 complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H-2 molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M+-H-2 and M+-D-2 infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M+center dot center dot center dot H-2 interaction

Attachment of molecular hydrogen to an isolated boron cation: An infrared and ab initio study

Structural properties of the B(+)-H(2) electrostatic complex are investigated through its rotationally resolved infrared spectrum in the H-H stretch region (3905-3975 cm(-1)). The spectrum, which was obtained by monitoring B(+) photofragments while the IR wavelength was scanned, is consistent with the complex having a T-shaped structure and a vibrationally averaged intermolecular separation of 2.26 angstrom, which decreases by 0.04 angstrom when the H(2) subunit is vibraitionally excited. The H-H stretch transition of B+-H(2) is red-shifted by 220.6 +/- 1.5 cm(-1) from that of the free H(2) molecule, much more than for other dihydrogen complexes with comparable binding energies. Properties of B(+)-H(2) and the related Li(+)-H(2), Na(+)-H(2), and Al(+)-H(2) complexes are explored through ab initio calculations at the MP2/aug-cc-pVTZ level. The unusually large red-shift for B+-H(2) is explained as due to electron donation from the H(2) sigma(g) bonding orbital to the unoccupied 2p(z) orbital on the B(+) ion

Electron Injection and Energy-Transfer Properties of Spiropyran–Cyclodextrin Complexes Coated onto Metal Oxide Nanoparticles: Toward Photochromic Light Harvesting

The photochromic and spectroscopic properties of spiropyran−γ-cyclodextrin inclusion complexes coated onto the surface of zirconia and titania nanoparticles (NPs) are examined. After ultraviolet (UV) irradiation, the colorless spiropyran (SP) isomer contained within the cyclodextrin (CD) is converted to the colored merocyanine (MC) isomer, which absorbs light in the 400–600 nm region. A significant increase in the excited-state lifetime for MC-CD on zirconia NPs (τ_MC = 1.32 ns) is observed compared to MC in solution (τ_MC = 0.21 ns), indicating that MC → SP photoisomerisation is hindered by the surrounding CD. Excitation of MC-CD on titania NPs results in electron injection into the titania conduction band with a quantum yield of 0.70. Excitation of MC-CD on zirconia NPs when a squaraine-based dye sensitizer is also attached to the NP surface results in energy transfer to the dye sensitizer with a quantum yield of 0.65. The prolonged excited-state lifetime of the encapsulated MC-CD is responsible for the high electron injection and energy-transfer quantum yields. The SP-CD/MC-CD system has potential applications for photochromic, light-harvesting electrodes in dye-sensitized solar cells