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For many years now the idea of including alkalis in a Portland cement matrix has been regarded as a daft or inexcusably erroneous proposition: despite its absurdity, that opinion has been widely accepted as a basic premise by the scientific and technical community working in the area of the chemistry of cement. In 1957 Glukhovsky proposed a working hypothesis in which he established a close relationship between alkalis and cementitious materials. That hypothesis has become consolidated and has served as a basis for developing a new type of binders, initially called "alkaline cements". The present paper reviews the most significant theoretical interpretations of the role played by alkalis in the formation of the "stony" structure of cement. It ends with a broad overview of the versatility of this type of materials for industrial applications and a discussion of the possibility of building on the existing legislation to meet the need for the future regulation of alkaline cement and concrete manufacture. RESUMEN: Activación alcalina: Revisión y nuevas perspectivas de análisis.Hace algunos años, la sola idea de la presencia de álcalis en una matriz de cemento Portland se consideraba casi como una aberración, o como un error imperdonable; convirtiéndose en un postulado básico (absurdo) ampliamente aceptado por la comunidad científica y técnica vinculada a la química de los cementos. En 1957 Glukhovsky propuso una hipótesis en la que se establecía una estrecha relación entre los álcalis y los materiales cementantes. Hoy día nadie duda de que dicha hipótesis ha servido de base para el desarrollo de una nueva clase de materiales cementantes: "cementos alcalinos". En el presente trabajo se hace una revisión sobre los aspectos teóricos más relevantes del papel de los álcalis en la formación de estos conglomerantes. También se da una visión genérica de su versatilidad, desarrollo industrial y estado de la normativa actual para regular en el futuro la fabricación de cementos y hormigones alcalinos.

Energy transfer in antenna systems, ordered arrays of chromophores, is one of the key steps in the photosynthetic process. The photophysical processes taking place in such multichromophoric systems, even at the single molecule level, are complicated and not yet fully understood. Instead of directly studying individual antenna systems, we have chosen to focus first on systems for which the amount of chromophores and the interactions among the chromophores can be varied in a systematic way. Dendrimers with a controlled number of chromophores at the rim fulfill those requirements perfectly. A detailed photophysical study of a second-generation dendrimer, containing eight peryleneimide chromophores at the rim, was performed 'J. Am. Chem. Soc., 122 (2000) 9278'. One of the most intriguing findings was the presence of collective on/off jumps in the fluorescence intensity traces of the dendrimers. This phenomenon can be explained by assuming a simultaneous presence of both a radiative trap (energetically lowest chromophoric site) and a non-radiative trap (triplet state of one chromophore) within one individual dendrimer. It was shown that an analogue scheme could explain the collective on/off jumps in the fluorescence intensity traces of the photosynthetic pigment B-phycoerythrin (B-PE) (Porphyridium cruentum). The different values of the triplet lifetime that could be recovered for a fluorescence intensity trace of B-PE were correlated with different intensity levels in the trace, suggesting different chromophores acting as a trap as function of time.

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/journals/jacsat.The optoelectronic properties of dyes embedded in thin polymer films are currently under intense study due to the use of such materials in nanotechnology 1 and nano-electronics. 2 The fluorescence lifetime of a single fluorescent molecule (SM) embedded in a thin film can fluctuate as a direct manifestation of the SM nanoenvironment (e.g., as a result of polymer dynamics). [3][4][5] Furthermore, it has been shown that the fluorescence lifetime strongly depends on the position and orientation of the SM with respect to the boundaries of the film, an effect known as the influence of the electromagnetic boundary conditions (EBC). [6][7][8][9][10][11][12][13] This effect is especially important in very thin polymer films (<30 nm). Therefore, several groups have devoted efforts to determine the 3D orientation of a SM in thin polymer films. 14-17 Techniques such as annular illumination (probing the orientation of the absorption transition dipole) 15,16 and wide-field defocusing (probing the emission transition dipole moment) have been proposed and exploited. 17 In this Communication, we report on the fluorescence behavior of a single first generation multichromophoric dendrimer with four perylene imide chromophores at the rim (G1R4) embedded in a thin polymer film (Chart 1). 18,19 A model developed to describe energy transfer in this system 13 invokes energy hopping between the different chromophores. It was suggested that, at any moment in time, the emission of a single dendrimer originates from the chromophore that has the lowest energy (fluorescent trapping site) due to the inhomogeneous nature of the polymer matrix used for immobilization. 20 Due to polymer dynamics and/or consecutive photobleaching, each of the chromophores in the dendrimer can in time become the fluorescent trapping site. Since the chromophores in the dendrimer have a well-defined 3D orientation with respect to each other (tetrahedral for G1R4), probing the dipole orientation of the emitting chromophore of a single G1R4 molecule using the two above-mentioned complementary observables, namely emission patterns and the EBC effect on the fluorescence lifetime, should allow us to validate the hopping model.To do so, G1R4 dendrimers at single-molecule concentration were embedded in thin (10-25 nm) zeonex (poly(norbornene)) films. To probe the orientation of the emitting chromophore emission dipole, we used defocused (1 µm defocusing distance with respect to the glass-polymer interface) wide-field imaging. 6,17 On the basis of the characteristic intensity distribution of the defocused images, the determination of the emission dipole orientation becomes feasible. Böhmer et al. 17 provided exact wave-optical calculations of these defocused images. Upon comparing the experimental data with calculated results, we obtained the emission dipole orientation.The patterns shown in Figure 1 were observed sequentially during the indicated period. One sees that th...