The kinetics of pyrolysis of pyrrole have been investigated theoretically by ab initio quantum chemical techniques and by detailed chemical kinetic modeling of previously reported experimental results. [Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, M. Int. J. Chem. Kinet. 1991, 23, 733.] The overall kinetics can be successfully modeled by a 117 step kinetic model that gives good agreement with temperature profiles of major products and also provides an acceptable fit for minor products. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio calculations carried out at CASSCF, CASPT2, and G2(MP2) levels of theory. Several reaction pathways were investigated. The major product, HCN, arises principally from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of this carbene leads to an allenic imine precursor of HCN and propyne. This is the decomposition pathway of lowest energy. Pyrolysis is preceded by the facile tautomerization of pyrrole to 2H-pyrrolenine. The latter can undergo CN fission to form an open chain biradical species, which is the precursor of the butenenitrile isomeric products, cis- and trans-crotononitrile and allyl cyanide. The biradical can also undergo facile H-fission to form cyanoallyl radical, which is an important precursor of acetylene, acetonitrile, and acrylonitrile. H2 also arises principally from H-fission of the biradical.

Rhodamine B (RhB) is widely used in chemistry and biology due to its high fluorescence quantum yield. In high concentrations, the quantum yield of fluorescence decreases considerably which is attributed to the formation of RhB dimers. In the present work, a possible mechanism of fluorescence quenching in RhB dimers is investigated with the use of time-dependent density functional theory (TD-DFT). The excited states of monomeric and dimeric RhB species have been studied both in the gas phase and in solution with the use of the TD-BLYP/6-311G* method. Results of the calculations suggest that quenching can occur via an internal conversion to the charge-transfer singlet excited states, which can be followed by an intersystem crossing with the charge-transfer triplet states. A possibility to reduce the loss of the fluorescence quantum yield is discussed.

International audienceThe photoactivity of the stable nanosized TiO2 polymorphs is challenging for many advanced applications. In the present work, brookite TiO2 nanoparticles with two different shapes have been used as building blocks for the preparation of pure brookite mesoporous layers. The layers have been characterized before and after sensitization. They have been used as photoanodes in dye-sensitized solar cells (DSSCs). The cell functioning coupled processes have been investigated by the impedance spectroscopy (IS) technique at various applied voltages and compared to a reference anatase TiO2 solar cell. The investigations of the chemical capacitance and of the charge transfer resistance, R-ct, show that, compared to anatase, the brookite surface is less active for the recombination side reaction. The larger R-ct is shown to explain the higher open circuit voltage of the brookite cells. However, the charge transport is much slower in the brookite phase due to a lower electrical conductivity. This parameter has been quantified more than 1 order of magnitude lower in the brookite layers compared to the anatase one. On the whole, the efficiency of brookite DSSCs is mainly limited by two parameters, the dye loading and the charge collection efficiency

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A simple and effective method has been developed to prepare a composite porous film that incorporates graphene sheets and anatase TiO2 nanoparticles. After sensitization, the films have been investigated as dye-sensitized solar cell photoelectrodes. The cell performances showed that the incorporation of an optimized graphene content of 1.2 wt % increases the power conversion efficiency by 12% due to the enhancement of the short-circuit current density (J sc). The photoelectrodes have been characterized by various techniques, and the cell functioning has been studied by impedance spectroscopy over a large applied potential range. The electronic structure, charge carrier lifetime (τn), transport/collection time (τtr), and electron transport parameters of the layers have been determined. We conclude that photoelectrodes with and without graphene show no limitation due to the transport of the I–/I3 – redox shuttle. The rate of the charge transfer (recombination) parasitic reaction is unchanged with the presence of graphene. The electron transport in the photoelectrode is significantly faster for the composite film due to a quantified 60% increase in the layer conductivity. However, we have also shown that the charge-carrier collection efficiency is very high even without graphene, and that this parameter is not key to explain the cell-performance enhancement. Graphene also increases the film specific internal surface area. The composite films have a higher dye loading. They exhibit a better solar light absorption and a J sc enlargement.

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