Effect of electronic and nuclear factors on the dynamics of dye-to-semiconductor electron transfer

Dye-to-semiconductor electron transfer is the initial step in many processes where light is used for the storage of information (e. g. color photography) or converted into electricity as in dye-sensitized solar cells. In the latter, interfacial charge injection occurs on a timescale spanning from tens of femtoseconds (10-15 s) to several picoseconds (10-12 s), with an efficiency approaching 100 %. In standard electron transfer theory, the parameters controlling charge transfer dynamics are divided into two categories: a) electronic factors, depending essentially upon the overlap between the electronic wavefunctions of the donor and the acceptor, and b) nuclear factors, characterized by the reorganization energies of both the reactants and the surrounding solvent molecules. Because of the ultrafast injection rates observed for dye/semiconductors couples typically used in photovoltaic applications, the hypotheses leading to these models are likely to be invalid. In this work, we make use of time-resolved spectroscopic techniques to study the influence of some electronic and nuclear parameters influencing charge injection dynamics into wide bandgap semiconductors. As a preliminary study, we investigated reductive quenching of excited dyes by iodide at the surface of nanocrystalline TiO2. Our experiments show that the quantum yield of this reaction, that is in competition with electron injection, depends strongly upon the I- concentration, but also upon the preparation and aging of the samples. We deduce that a fraction of the dye molecules are not adsorbed directly onto the surface of the semiconductor, but rather aggregated in a second adsorption layer. In the light of these results we reconsidered the issue of charge injection from the standard N3 dye into TiO2. This reaction, largely studied due to its importance for dye-sensitized solar cells, has previously been reported to take place with a fast (< 100 fs) phase, followed by a slower (1-100 ps) component. Our measurements show that the slow part is actually related to the presence of weakly adsorbed dye molecules. When aggregation is minimized, we observe monophasic injection dynamics (τ < 20 fs). This result suggests the process is beyond the scope of vibration-mediated electron transfer models and is controlled by electron dephasing in the solid. Next, we applied two different approaches to investigate the influence of the distance over which charge transfer takes place. First, forward and back electron transfer kinetics were measured for a series of bridged-sensitizers containing p-phenylene spacer groups. For both reactions the rate decays exponentially with distance. However, the damping factor is much larger for the back transfer (β = 0.5 Å-1) than for injection (β = 0.19 Å-1), suggesting that nuclear reorganization plays an important role for the former reaction. In the second approach, a thin layer of Al2O3 of controlled thickness is inserted between the sensitizer and TiO2. In this case the distance parameter β is similar for both reactions. The weak estimated value (β = 0.15 Å-1) indicates that thin layers of aluminum oxide is much less insulating that pure, crystalline Al2O3, and that electron tunneling is mediated by empty states in the thin layer according to the superexchange mechanism. Finally, the influence of the density of acceptor states has been examined by using a ruthenium complex adsorbed on Nb2O5 films. Varying the excitation wavelength made it possible to probe the bottom of the conduction band where the density of states decays exponentially. Despite the spectral width of the pump laser pulses, a weak retardation (2-3 times) of electron injection was observed. In summary, our experiments show that electronic factors, in particular variations of electronic coupling with distance, mainly control electron injection dynamics at dye/semiconductor interfaces. By revealing the role of aggregation for standard dyes adsorbed on TiO2, these results have important consequences for the development of dye-sensitized solar cells. Finally, in the case of ultrafast monophasic kinetics, standard electron transfer models are not obeyed, and the dynamics is rather controlled by electron dephasing and relaxation in the continuum of acceptor states in the solid

Tuning the wavelength of lasing emission in organic semiconducting laser by the orientation of liquid crystalline conjugated polymer

We report the optical pumping of one-dimensional distributed feedback (DFB) conjugated polymer devices using a uniaxially aligned liquid crystalline polymer, poly(9,9-dioctylfluorene). We can independently select the alignment direction (via a rubbed polyimide layer) and the DFB structure (via nanoimprinting). In comparison with unaligned film, we show that lasing threshold is substantially reduced when absorption is parallel to the aligned direction (similar to 20.0 mu J cm(-2) pulse(-1)). This is mainly due to the higher absorption coefficient estimated in the table by calculating the exciton densities at each threshold value. We also report the control of lasing wavelength through independent selection of alignment direction and DFB orientation, which is achieved through the control of the effective refractive index of waveguide (n(eff)).open171

l-Alanylglycylhistamine dihydro­chloride

In the title compound {systematic name: 4-[2-({N-[(2S)-2-ammonio­propano­yl]glyc­yl}amino)­eth­yl]-1H-imidazol-3-ium dichloride}, C10H19N5O2 2+·2Cl−, the pseudo-tripeptide l-alanyl­glycyl­histamine is protonated at both the terminal amino group and the histidine N2 atom. The resulting positive charges are neutralized by two chloride anions. In the crystal, the organic cation adopts a twisted conformation about the CH2—CH2 bond of histamine and about the C—N bond in the main chain, stabilized by a short intra­molecular C—H⋯O contact. In the crystal, N+—H⋯O and N+—H⋯Cl− hydrogen bonds link the mol­ecules into infinite sheets parallel to the (100) plane. The stacking of these sheets along the a axis is supported by Namide—H⋯Cl− hydrogen bonds

Filtered overlap: speedup, locality, kernel non-normality and Z_A~1

We investigate the overlap operator with a UV filtered Wilson kernel. The filtering leads to a better localization of the operator even on coarse lattices and with the untuned choice $\rho=1$. Furthermore, the axial-vector renormalization constant $Z_A$ is much closer to 1, reducing the mismatch with perturbation theory. We show that all these features persist over a wide range of couplings and that the details of filtering prove immaterial. We investigate the properties of the kernel spectrum and find that the kernel non-normality is reduced. As a side effect we observe that for certain applications of the filtered overlap a speed-up factor of 2-4 can be achieved.Comment: 30 pp, 23 fig

Photochemistry of 2-butenedial and 4-oxo-2-pentenal under atmospheric boundary layer conditions

Unsaturated 1,4-dicarbonyl compounds, such as 2-butenedial and 4-oxo-2-pentenal are produced in the atmospheric boundary layer from the oxidation of aromatic compounds and furans. These species are expected to undergo rapid photochemical processing, affecting atmospheric composition. In this study, the photochemistry of (E)-2-butenedial and both E and Z isomers of 4-oxo-2- pentenal was investigated under natural sunlight conditions at the large outdoor atmospheric simulation chamber EUPHORE. Photochemical loss rates, relative to j(NO2), are determined to be j((E)-2-butenedial)/j(NO2) = 0.14 (0.02), j((E)-4-oxo-2-pentenal)/j(NO2) = 0.18 (0.01), and j((Z)-4-oxo-2-pentenal)/j(NO2) = 0.20 (0.03). The major products detected for both species are a furanone (30 – 42%) and, for (E)-2-butenedial, maleic anhydride (2,5-furandione) (12 – 14%). The mechanism appears to proceed predominantly via photoisomerization to a ketene- enol species following -H abstraction. The lifetimes of the ketene-enol species in the dark from 2-butenedial and 4-oxo-2-pentenal are determined to be 465 s and 235 s, respectively. The ketene-enol can undergo ring closure to yield the corresponding furanone, or further unimolecular rearrangement which can subsequently form maleic anhydride. A minor channel (10 – 15%) also appears to form CO directly. This is presumed to be via a molecular elimination route of an initial biradical intermediate formed in photolysis, with an unsaturated carbonyl (detected here but not quantified) as co-product. -dicarbonyl and radical yields are very low, which has implications for ozone production from the photo-oxidation of unsaturated 1,4-dicarbonyls in the boundary layer. Photochemical removal is determined to be the major loss process for these species in the boundary layer with lifetimes of the order of 10 – 15 minutes, compared to > 3 hours for reaction with OH

4-{2-[3-(2-Ammonioacetamido)propanamido]ethyl}-1H-imidazol-3-ium dichloride

Mol­ecules of the title compound, Gly-β-Ala-Histamine dihydro­chloride, C10H19N5O2 2+·2Cl−, are linked by N—H⋯O and N—H⋯Cl hydrogen bonds into two-dimensional polymeric sheets parallel to the (011) plane, forming a stacked structure along the a axis. The parallel layers are also inter­linked alternately by different N—H⋯Cl hydrogen bonds, forming a three-dimensional framework

DOTA-NOC, a high-affinity ligand of somatostatin receptor subtypes 2, 3 and 5 for labelling with various radiometals

Earlier studies have shown that modification of the octapeptide octreotide in positions 3 and 8 may result in compounds with increased somatostatin receptor affinity that, if radiolabelled, display improved uptake in somatostatin receptor-positive tumours. The aim of a recent research study in our laboratory was to employ the parallel peptide synthesis approach by further exchanging the amino acid in position 3 of octreotide and coupling the macrocyclic chelator DOTA(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to these peptides for labelling with radiometals like gallium-67 or -68, indium-111, yttrium-90 and lutetium-177. The purpose was to find radiopeptides with an improved somatostatin receptor binding profile in order to extend the spectrum of targeted tumours. A first peptide, [111In,90Y-DOTA]-1-Nal3-octreotide (111In,90Y-DOTA-NOC), was isolated which showed an improved profile. InIII-DOTA-NOC exhibited the following IC50 values (nM) when studied in competition with [125I][Leu8, d-Trp22, Tyr25]somatostatin-28 (values for YIII-DOTA-NOC are shown in parentheses): sstr2, 2.9±0.1 (3.3±0.2); sstr3, 8±2 (26±1.9); sstr5, 11.2±3.5 (10.4±1.6). Affinity towards sstr1 and 4 was very low or absent. InIII-DOTA-NOC is superior to all somatostatin-based radiopeptides having this particular type of binding profile, including DOTA-lanreotide, and has three to four times higher binding affinity to sstr2 than InIII,YIII-DOTA-Tyr3-octreotide (InIII,YIII-DOTA-TOC). In addition, [111In]DOTA-NOC showed a specific and high rate of internalization into AR4-2J rat pancreatic tumour cells which, after 4h, was about two times higher than that of [111In]DOTA-TOC and three times higher than that of [111In]DOTA-octreotide ([111In]DOTA-OC). The internalized radiopeptides were externalized intact upon 2h of internalization followed by an acid wash. After 2-3h of externalization a plateau is reached, indicating a steady-state situation explained by reactivation of the receptors followed by re-endocytosis. Biodistribution studies in CA 20948 tumour-bearing rats showed rapid clearance from all sstr-negative tissues except the kidneys. At 4h the uptake of [111In]DOTA-NOC in the tumour and sstr-positive tissues, such as adrenals, stomach and pancreas, was three to four times higher than that of [111In]DOTA-TOC. Differential blocking studies indicate that this is at least partially due to the uptake mediated by sstr3 and sstr5. These very promising preclinical data justify the use of this new radiopeptide for imaging and potentially internal radiotherapy studies in patient

Dynamics of Photoinduced Interfacial Electron Transfer and Charge Transport in Dye-Sensitized Mesoscopic Semiconductors

Molecular systems designed for the conversion of solar energy offer ideal models in the study of the kinetics of light-induced electron transfer at surfaces. Due to their high porosity, nanocrystalline oxide semiconductor films allow investigations of interfacial and lateral charge transfer processes that are barely detectable on flat surfaces. Although it has proven to be very promising, the redox photochemistry of the metal oxide I molecular monolayer I electrolyte interface is still a largely unexplored scientific domain, offering huge potential for investigation and exploitation of physical and chemical processes. Carrier trapping and charge transport are also key to the efficiency of molecular photonic devices. Carrier dynamics and transport in unconventional media are studied utilizing THz time domain spectroscopy. We summarize here some aspects of the work currently carried out in these fields as part of our continued effort in the fundamental study of the dynamics of photoinduced electron transfer processes

Fixed Point Gauge Actions with Fat Links: Scaling and Glueballs

A new parametrization is introduced for the fixed point (FP) action in SU(3) gauge theory using fat links. We investigate its scaling properties by means of the static quark-antiquark potential and the dimensionless quantities $r_0 T_c, T_c/\sqrt{\sigma}$ and $r_0 \sqrt{\sigma}$, where $T_c$ is the critical temperature of the deconfining phase transition, $r_0$ is the hadronic scale and $\sigma$ is the effective string tension. These quantities scale even on lattices as coarse as $a \approx 0.3$ fm. We also measure the glueball spectrum and obtain $m_{0^{++}}=1627(83)$ MeV and $m_{2^{++}}=2354(95)$ MeV for the masses of the scalar and tensor glueballs, respectively.Comment: 45 pages, 12 figures, Late

The well-being of childless men and fathers in mid-life

Using data from the first wave of the Netherlands Kinship Panel Study conducted in 2002–03, this paper examines the economic, psychological and social wellbeing among 1,467 men aged 40–59 years with different parenthood histories and circumstances: the childless, fathers who live with their children, non-co-resident fathers, and ‘empty-nest fathers ’. The gerontological interest is whether there are variations in wellbeing by parenting, and whether they persist in old age. The results showed that fathers have higher incomes than childless men, regardless of their partner history. As regards psychological wellbeing, men’s partner history counts, not their parenthood status. Being single contributes to low levels of psychological wellbeing. The findings provide evidence of the socially integrating effects of parenthood and for men’s ‘good-provider ’ role. Childless men and nonco- resident fathers report poorer quality family relationships. In addition, childless men were least likely to report helping others in the community. Overall, more support is found for the notion that fatherhood is a transforming event than that the wellbeing benefits derive from fathering activities. The paper concludes with a discussion of the implications of the findings for inequalities in wellbeing and informal support among the male members of the cohort born during 1943–63 when they reach old age