A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots.

The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles.

Evaluation of intracavity optogalvanic spectroscopy for radiocarbon measurements.

Ever since the first publication of intracavity optogalvanic spectroscopy (ICOGS) in 2008, this novel technique for measuring the (14)C/(12)C ratio in carbon dioxide has rendered considerable attention. As a result, there are currently at least five different research groups pursuing research on ICOGS. With a claimed limit of detection of 10(-15) ((14)C/(12)C), i.e., in the same order as accelerator mass spectroscopy, achieved with a relatively inexpensive and uncomplicated table-top system, ICOGS has major scientific and commercial implications. However, during the past 5 years, no research group has been able to reproduce these results or present additional proof for ICOGS's capability of unambiguous (14)C detection, including the authors of the original publication. Starting in 2010, our group has set up a state-of-the-art ICOGS laboratory and has investigated the basic methodology of ICOGS in general and tried to reproduce the reported experiments in particular. We have not been able to reproduce the reported results concerning the optogalvanic signals dependence on (14)C concentration and wavelength and, ultimately, not seen any evidence of the capability of ICOGS to unambiguously detect (14)C at all. Instead, we have found indications that the reported results can be products of measurement uncertainties and mistakes. Furthermore, our results strongly indicate that the reported limit of detection is likely to be overestimated by at least 2 orders of magnitude, based on the results presented in the original publication. Hence, we conclude that the original reports on ICOGS cannot be confirmed and therefore must be in error.

On the analyses of fluorescence depolarisation data in the presence of electronic energy migration. Part II: Applying and evaluating two-photon excited fluorescence.

Electronic energy migration within a bifluorophoric molecule has been studied by time-resolved two-photon excited (TPE) fluorescence depolarisation experiments. Data were analysed by using a recently developed quantitative approach [O. Opanasyuk and L. B.-Å. Johansson, On the Analyses of Fluorescence Depolarisation Data in the Presence of Electronic Energy Migration. Part I: Theory and General Description, Phys. Chem. Chem. Phys., submitted]. The energy migration occurs between the 9-anthrylmethyl groups of the bifluorophoric molecule, bis-(9-anthrylmethylphosphonate) bisteroid. These groups undergo local reorientations, while overall tumbling of the bisteroid is strongly hampered in the used viscous solvent, 1,2-propanediol. To solely obtain information about local reorientations of the 9-anthrylmethyl group, also the mono-(9-anthrylmethylphosphonate) bisteroid was studied, which enabled modelling of the ordering potential shape. The analysis of data is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process was described by an extended Förster theory (EFT). A reasonable value of the distance between the 9-anthrylmethyl groups, as well as for the mutual orientation of the ordering potentials, is found. Furthermore, values of the two-photon tensor components were obtained.

Luminescence enhancement from silica-coated gold nanoparticle agglomerates following multi-photon excitation.

Multi-photon absorption induced luminescence (MAIL) from bare gold nanoparticles, silica-coated particles, as well as silica-coated agglomerated gold nanoparticles suspended in aqueous solution was studied by using time-resolved and steady-state luminescence spectroscopy. The nanoparticles were excited by femtosecond pulses of wavelengths ranging from 630 nm to 900 nm. The luminescence from the particles exhibits a broad spectrum in the UV and VIS region. The time-resolved measurements indicate a luminescence lifetime of a few ps, limited by the response of the experimental system. The studied dependence of the MAIL efficiency on the excitation wavelength showed that the luminescence from silica-coated agglomerates was enhanced over the whole range of excitation wavelengths, when compared to the luminescence from individual gold nanoparticles. The agglomerates show an almost excitation wavelength independent efficiency of the MAIL, while for individual nanoparticles a rapid decrease of the MAIL efficiency was observed with increasing excitation wavelength. The observed enhancement of the MAIL from the agglomerated nanostructures can be attributed to the presence of localized surface plasmon resonances in the spectral region corresponding to the excitation wavelengths. The high MAIL efficiency from the agglomerated nanoparticle structures in the near-infrared could be an advantage in the expanding field of luminescence-based-imaging, as well as in biosensor technology.

Radiologists' Knowledge and Attitudes towards CT Radiation Dose and Exposure in Saudi Arabia-A Survey Study.

Computed tomography (CT) is a key imaging technique in diagnostic radiology, providing highly sensitive and specific information. While its use has increased dramatically in recent years, the quantity and associated risks of radiation from CT scans present major challenges, particularly in paediatrics. The fundamental principles of radiation protection require that radiation quantities be as low as reasonably achievable and CT use must be justified, particularly for paediatric patients. CT radiation knowledge is a key factor in optimising and minimising radiation risk. The objective of this study was to analyse knowledge level, expertise, and competency regarding CT radiation dose and its hazards in paediatrics among radiologists in Saudi Arabian hospitals. A self-reported, multiple-choice questionnaire assessed the attitudes and opinions of radiologists involved in imaging studies using ionising radiation. Among the total respondents, 65% ± 13.5% had a good comprehension of the dangers of carcinogenicity to the patient resulting from CT scans, with 80% presuming that cancer risks were elevated. However, only 48.5%, 56.5%, and 65% of the respondents were aware of specific radiation risks in head, chest, and abdominal paediatric examinations, respectively. Regular, frequent, and specific training courses are suggested to improve the fundamental knowledge of CT radiation among radiologists and other physicians.

Environment-induced surface dynamics of a biomimetic ionomer studied using in situ second harmonic generation.

The environmental-induced surface dynamics of the biomimetic phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) ionomer has been studied and compared to its unfunctionalized counterpart using in situ second harmonic generation measurements. Whereas the nonpolar liquid n-hexane did not induce any surface dynamic processes in the ionomer under study, the presence of water initiated a Debye-type dynamic reaction at the surface of the PC ionomer, which had no equivalent in the unfunctionalized material. This first-order reaction was attributed to a surface enrichment process of the functionalized ionomer in the hydrophilic environment involving movement of the PC endgroups from aggregates in the bulk to the surface. The time constant of the process was found to be about 6 min, and the corresponding activation energy was 0.4 eV. The dehydration process of the PC-functionalized ionomer in nitrogen gas atmosphere could be described by two time constants, one slightly below 1 min and the other one just above 13 min. The results presented in this work show that SHG measurements are well suited for the study of polymer surface restructuring dynamics in response to environmental changes. Such information is very important for the successful design and implementation of biomimetic polymers intended for biomedical applications.

Excited-state symmetry and reorientation dynamics of perylenes in liquid solutions: time-resolved fluorescence depolarization studies using one- and two-photon excitation.

The excited-state symmetry and molecular reorientation of perylene, 1,7-diazaperylene, and 2,5,8,11-tetra- tert-butylperylene have been studied by different fluorescence depolarization experiments. The first excited electronic singlet state was reached through one-photon excitation (OPE) and two-photon excitation (TPE). A 400 and 800 nm femtosecond laser pulse was used for this purpose, and data were collected by means of the time-correlated single-photon counting technique. It is found that the rotational correlation times for each perylene derivative are very similar in the OPE and TPE depolarization experiments. For the determination of the two-photon absorption tensor, a recently described theoretical model has been applied (Ryderfors et al. J. Phys. Chem. A 2007, 111, 11531). It was found that the two-photon process can be described by a 2 x 2 absorption tensor for which the components are solvent dependent and exhibit mixed vibronic character. In the dipole approximation this is compatible with a parity-forbidden two-photon absorption into the first excited singlet state.

Sensitized hole injection of phosphorus porphyrin into NiO: toward new photovoltaic devices.

This paper describes the preparation and the characterization of a photovoltaic cell based on the sensitization of a wide band gap p-type semiconductor (NiO) with a phosphorus porphyrin. A photophysical study with femtosecond transient absorption spectroscopy showed that light excitation of the phosphorus porphyrin chemisorbed on NiO particles induces a very rapid interfacial hole injection into the valence band of NiO, occurring mainly on the 2-20 ps time scale. This is followed by a recombination in which ca. 80% of the ground-state reactants are regenerated within 1 ns. A photoelectrochemical device, prepared with a nanocrystalline NiO electrode coated with the phosphorus porphyrin, yields a cathodic photocurrent indicating that electrons indeed flow from the NiO electrode toward the solution. The low incident-to-photocurrent efficiency (IPCE) can be rationalized by the rapid back recombination reaction between the reduced sensitizer and the injected hole which prevents an efficient regeneration of the sensitizer ground state from the iodide/triiodide redox mediator. To the best of our knowledge, this work represents the first example of a photovoltaic cell in which a mechanism of hole photoinjection has been characterized.

A rod-like polymer containing (Ru(terpy)2) units prepared by electrochemical coupling of pendant thienyl moieties.

A new rod-like coordination polymer consisting of (Ru-(terpy)2) motifs bridged by bithiophene units has been prepared by electrochemical polymerisation in acidic organic medium.

Two-photon excited fluorescence depolarisation and electronic energy migration within donor-donor pairs.

A unified theoretical description is presented for one- and two-photon excited fluorescence depolarisation and electronic energy migration within pairs of chromophores. Two weakly coupled donor groups are linked via a rigid macromolecule with the ability to undergo restricted reorienting motions. Describing these reorienting motions as well as their influence on the coupling is rather complex, but can be accounted for by using the extended Förster theory. Here explicit expressions have been derived for chromophores belonging to the point groups D(2h), D(2) and C(2v) when residing in uniaxial potentials (i.e. C(infinity v) symmetry). From the given basic equations, it is possible however, to derive the relevant equations for molecules of arbitrary symmetry in any uniaxial orienting potential. The expected time-resolved fluorescence anisotropy for different two-photon absorption tensors are compared for reorienting fluorophores in liquids, as well as in anisotropic systems. Simulated fluorescence depolarisation data are also displayed that mimic energy migration within pairs of two-photon excited donor molecules, which simultaneously undergo reorienting motions within effectively isotropic and uniaxially anisotropic environments. The obtained results demonstrate that the time-resolved fluorescence anisotropy strongly depends on the properties of the two-photon absorption tensor, as well as on using a linear or a circular polarisation of the excitation field.

Two-photon excited fluorescence and molecular reorientations in liquid solutions.

Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.

The symmetry of two-photon excited states as determined by time-resolved fluorescence depolarization experiments.

A new experimental and theoretical approach is presented for the quantitative determination and assignment of the two-photon absorption tensor of fluorophores dissolved in liquid solutions. Two linearly independent time-resolved fluorescence anisotropies and the two-photon polarization ratio were determined from experiments based on using the time-correlated single photon counting technique. The data were analyzed in a global manner under the assumption of prevailing diffusive molecular reorientations and when accounting for the influence of rapid unresolved reorientations. The method has been applied in fluorescence studies of perylene, two-photon excited at 800 nm. The analysis suggests that the two-photon transition is mediated via vibronic coupling including at least two vibrations of different symmetry, and also that the first singlet excited electronic state acts as a dominating intermediate state.

Sequential merocyanine product isomerization following femtosecond UV excitation of a spiropyran.

The ring-opening dynamics of the photochromic switch 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] in tetrachloroethene is studied with both femtosecond time-resolved ultraviolet (UV)/visible and UV/mid-infrared (IR) pump-probe spectroscopy. During the first picosecond we identify two new transient features in the UV/vis experiments, the first of which we assign to spiropyran S1 --> S(n) absorption (lifetime < or = 0.2 ps). The second feature (lifetime 0.5 +/- 0.2 ps) we tentatively assign to the merocyanine T2 state. After 1 ps both probing methods show biexponential merocyanine formation kinetics, with average time constants of 17 +/- 3 and 350 +/- 20 ps. In the UV/IR experiments, the initial dynamics show more dispersion in formation times than in the UV/vis measurements, whereas the slower time constant is the same in both. A weak transient IR signal at approximately 1360 cm(-1) demonstrates that this biexponentiality is caused by a sequential isomerization between two merocyanine species. Lifetimes provide evidence that the merocyanine S1 state is not involved in the photochemical reaction.

Energy transfer pathways in dinuclear heteroleptic polypyridyl complexes: through-space vs through-bond interaction mechanisms.

A series of homo- and heteronuclear ruthenium and osmium polypyridyl complexes with the bridging ligands 1,3-bis(5-(2-pyridyl)-1H-1,2,4-triazol-3-yl)benzene (H(2)mL) and 1,4-bis(5-(2-pyridyl)-1H-1,2,4-triazol-3-yl)benzene (H(2)pL) are reported. The photophysical properties of these compounds are investigated, and particular attention is paid to the heteronuclear (RuOs) compounds, which exhibit dual emission. This is in contrast to phenyl-bridged polypyridine Ru-Os complexes with a similar metal-metal distance, in which the Ru emission is strongly quenched because the nature of the bridging ligand allows for an efficient through-bond coupling. The results obtained for the compounds reported here suggest that energy transfer is predominantly taking place via a dipole-dipole, Förster type, mechanism, that may dominate when through-bond coupling is weak. This is in stark contrast to ground state interaction, which is found to be critically dependent on the nature of the bridging unit employed.