Reichardt's E T (30) as well as Kamlet-Taft's R (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and aluminafunctionalized silica particles, are presented. The E T (30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorinesubstituted derivative of 1a (1b) as surface polarity indicators. Kamlet-Taft's R and π* parameters for the various solid acids are analyzed by means of Fe(phen) 2 (CN) 2 [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler's ketone [4,4′-bis(N,N-dimethylamino)benzophenone] (3) as solvatochromic surface polarity indicators. The chemical interpretation of the R and π* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe pyrene (literature data) is also discussed. It is evident that the solid surface environments observed by the various indicators are moderately strong dipolar/polarizable (π* ) 0.38 to 1.04) and are fairly strong hydrogen-bond donors (R ) 1.00 to 1.99). Theoretical E T (30) values of solid acids are calculated by applying linear solvation energy (LSE) relationships using the independently measured R and π* values of the solid acids according to E T ( 30) ) [E T (30)] o + aR + sπ*. The respective dependence of the R and π* terms, expressed by the coefficients a and s, upon the measured E T (30) value for solid acids is discussed in comparison to related multiple LSE correlations known for wellbehaved regular solvents and functionalized silicas. The results show in general that values of polarity parameters of strong HBD and dipolar surfaces are strongly influenced by the experimental conditions applied.
Empirical polarity parameters are recommended as useful characteristics for describing the internal and external surface properties of various solid materials, e. g. synthetic polymers, native polymers, inorganic oxides, sol‐gel hybrids, and composites. The polarity properties of a macromolecule have been expressed by three independent terms: the α value (the hydrogen bond donating, HBD, capacity or acidity), the β value (the hydrogen bond accepting, HBA, capacity or basicity), and the π* value (the dipolarity/polarizability). These terms can be defined using the Kamlet‐Taft solvents parameter set as the reference system. A complex property, XYZ, of a macromolecular material under study, with reference to a standard system (XYZ)0 (i. e. gas phase or a nonpolar polymer), can then be described by a simplified Kamlet‐Taft LSE (linear solvation energy) equation: XYZ = (XYZ)0 + sπ* + aα + bβ. a, b, and s are coefficients reflecting the susceptibility of the polarity terms upon XYZ. Empirical solvatochromic polarity parameters [α, β, π*, ET (30)] for synthetic polymers, copolymers, native polymers, inorganic oxidic materials, functionalized silica particles, hybrids, and composite materials have been determined by means of the following solvatochromic probe dyes: 2,6‐diphenyl‐(2,4,6‐triphenyl‐1‐pyridinio)‐4‐phenolate (1 a), Michler's ketone (2), dicyano‐bis(1,10‐phenanthrolin)iron II (3), and a novel aminobenzodifuranone dye (7). The solvatochromic band shifts of these indicators correlate precisely with the Kamlet‐Taft solvent parameters α, β, and π*. The results are compared with each other, with related solvent model compounds, and literature values. The relation of the well established ET (30) solvent polarity scale to the Kamlet‐Taft parameters α and π* of solid materials is demonstrated. Hence, a general polarity scale for solid materials is suggested.
Linear solvation energy (LSE) relationships are employed to characterize the internal polarities in the channels of a siliceous MCM-41 material and of a silica gel synthesized by a sol-gel process from tetramethoxysilane (TMOS). The polarity of the surfaces inside the silica gel and the mesopores of the MCM-41 can be quantitatively described by three independent terms: the dipolarity/polarizability (π*), the HBD (hydrogen-bond-donating) acidity (R), and the HBA (hydrogen-bond-accepting) ability (β). These terms are defined by means of the Kamlet-Taft solvent parameters R, β, and π* as a reference system. The following indicators have been encapsulated by the sol-gel process within the silica matrix or adsorbed to MCM-41: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1), dicyanobis-(1,10-phenanthrolin)iron(II) (2), and Michler's ketone (3). The solvatochromic UV/vis spectroscopic band shifts of these indicators correlate with the Kamlet-Taft solvent parameters R and π*. The hydrogen-bond-donating acidity, R, within MCM-41 increases significantly with decreasing temperature, whereas the dipolarity/polarizability term, π*, is temperature-independent. With 2 as indicator, an average hydrogen-bond acidity (R) is measured, whereas 3 is suitable to distinguish between differently acidic silanols within MCM-41 as shown by surface titration of the adsorbed indicator 3 with a sterically hindered base.
The solvatochromism of the novel dye 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-D 3 -thiazoline (1) has been investigated in 26 solvents of different polarity. 1 exhibits a positive solvatochromism, its solvent-induced bathochromic UV/Vis absorption band shift ranges from n-hexane (l max ¼ 566 nm) to dimethyl sulfoxide (l max ¼ 640 nm). The molar absorption energy of the solvatochromic band shift of 1 can be well correlated with Kamlet-Taft's and Catala `n's solvent polarity scales. 1 is mainly sensitive to the solvent's dipolarity/polarizability (p* of SPP N term) rather than of its HBD (hydrogen-bond donating) or HBA (hydrogen-bond accepting) property. It is emphasized that the UV/Vis absorption band maximum of 1 in the strong HBD solvents acetic acid, 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol fit well in the LSE (linear solvation energy) relationship with Kamlet and Taft's p* and Catala `n's SPP N scale, respectively, which makes this dye important as a dipolarity/polarizability indicator for various solid acids.
Metal-coated polyamide threads and filaments were chosen as substrate electrodes to deposit highly porous ZnO films for photovoltaic application. The films were electrodeposited at 70 degrees C from oxygen-saturated aqueous zinc salt solutions containing EosinY as a structure directing agent. The current density during deposition was increased compared with planar electrodes by enhanced diffusion at the filaments operating as cylindrical microelectrodes. Analysis by scanning electron microscopy showed an influence of geometrical constraints within the textiles and the hydrodynamic flow rate in the deposition solution on the film morphology. Photoelectrochemical characterization of sensitized films revealed the feasibility of the presented approach and indicated further steps needed for electrode optimization.
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