Probing the Surface Polarity of Various Silicas and Other Moderately Strong Solid Acids by Means of Different Genuine Solvatochromic Dyes

Reichardt's E T (30) as well as Kamlet−Taft's α (hydrogen-bond-donor acidity) and π* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and alumina-functionalized 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 eicosafluorine-substituted derivative of 1a ( 1b ) as surface polarity indicators. Kamlet−Taft's α 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 α 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 (α = 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 α and π* values of the solid acids according to E T (30) = [ E T (30)] o + a α + s π*. The respective dependence of the α 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 well-behaved 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.