The aggregation of alkyltrimethylammonium surfactants Ci2TA+ and CieTA+ in dilute water solutions of sodium poly(styrenesulfonate) has been investigated. Aggregation numbers were estimated with the time-resolved fluorescence quenching technique. In the calculations, results from binding isotherms and solubility measurements were used. Binding isotherms for dodecyltrimethylammonium bromide to the polyelectrolyte were determined using a surfactant-selective electrode. The aggregation numbers were found to be independent of the concentration of surfactant and type of counterion, but to increase with increasing surfactant tail length. From the kinetics of the quenching of pyrene fluorescence with hydrophobic and hydrophilic quenchers, it was concluded that compact aggregates with net negative charge were formed, in which the polyelectrolyte is intimately associated with the surfactant. The aggregates are joined by surfactant-free parts of the polyelectrolyte chain, the lengths of which depend on the amount of bound surfactant. The quencher dimethylbenzophenone was found to migrate between the aggregates at the highest concentration of the long-tailed surfactant.

The dynamics of electron-transfer processes between bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2′bypiridine-4,4′-dicarboxylato)ruthenium(II) (called N719) and nanostructured ZnO films have been investigated by femtosecond and nanosecond spectroscopy. The incident photon to current conversion efficiency (IPCE) for these dye-sensitized electrodes was 36% in the maximum of 530 nm, corresponding to a quantum efficiency of 80%. The highest IPCE values were obtained when the electrodes were prepared under conditions where formation of dye aggregates in the pores of the nanostructured films is avoided. For such films, the electron injection time was in the subpicosecond regime (<300 fs), which is comparable to the N719-TiO 2 system. The back electron-transfer kinetics between conduction band electrons and oxidized dye molecules were biexponential with time constants of 300 ns and 2.6 µs. Variation of the light intensity did not affect the time constants, but only their relative weights. The kinetics of back electron transfer in the N719-ZnO and N719-TiO 2 systems were found to be identical.

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.

The anchoring of the ruthenium dye {(C 4 H 9 ) 4 N}[Ru(Htcterpy)(NCS) 3 ] (with tcterpy ) 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine), the so-called black dye, onto nanocrystalline TiO 2 films has been characterized by UV-vis and FT-IR spectroscopies. FT-IR spectroscopy data suggest that dye molecules are bound to the surface by a bidentate binuclear coordination mode. The interfacial electron-transfer (ET) dynamics has been investigated by femtosecond pump-probe transient absorption spectroscopy and nanosecond laser flash photolysis. The electron-injection process from the dye excited state into the TiO 2 conduction band is biexponential with a fast component (200 ( 50 fs) and a slow component (20 ps). These two components can be attributed to the electron injection from the initially formed and the relaxed dye excited states, respectively. Nanosecond kinetic data suggest the existence of two distinguishable regimes (I and II) for the rates of reactions between injected electrons and oxidized dye molecules or oxidized redox species (D + or I 2 •-). The frontier between these two regimes is defined by the number of injected electrons per particle (N e ), which was determined to be about 1. The present kinetic study was undertaken within regime I (N e > 1). Under these conditions, the back-electron-transfer kinetics is comparable to that in systems with other ruthenium complexes adsorbed onto TiO 2 . The reduction of oxidized dye molecules by iodide results in the formation of I 2 •on a very fast time scale (<20 ns). Within regime I, the decay of I 2 •occurs in ∼100 ns via reaction with injected electrons (I 2 •-+ ef 2I -). In regime II (N e e 1), which corresponds to the normal operating conditions of dye-sensitized solar cells, the decay of I 2 •is very slow and likely occurs via the dismutation reaction (2I 2 •f I -+ I 3 -). Our results predict that, under high light intensity (N e > 1), the quantum efficiency losses in dye-sensitized solar cells will be important because of the dramatic acceleration of the reaction between I 2 •and injected electrons. Mechanisms for the ET reactions involving injected electrons are proposed. The relevance of the present kinetic studies for dye-sensitized nanocrystalline solar cells is discussed.

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The fluorescence decay of pyrene quenched by dimethylbenzophenone (DMBP) has been used to study the aggregation behavior of cetyltrimethylammonium halide (CTAX) in the presence of varied concentrations of two bile salts, sodium cholate (NaC) and sodium deoxycholate (NaDC). The study showed the different aggregation characteristics of mixed micelles. In both cases it was shown that the probe pyrene migrates from the palisade layer of the micelle to the interior of the mixed micelles with increase in bile salt concentration. The micelles were small for the CTAX/NaC system over the entire concentration range but tended to grow in size into rodlike micelles for the CTAX/NaDC system close to the equimolar concentrations, where phase separation into two micellar phases occurred. Dynamic light scattering and viscosity measurements also support this view.

Ever since the first publication of intracavity optogalvanic spectroscopy (ICOGS) in 2008, this novel technique for measuring the 14C/12C 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 (14C/12C), 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 14C 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 14C concentration and wavelength and, ultimately, not seen any evidence of the capability of ICOGS to unambiguously detect 14C 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.

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.