The relative spacing of amines in 3-aminopropylsilyl-grafted silica is studied by solid-state fluorescence spectroscopy of 1-pyrenecarboxylic acid (PCA) and 1-pyrenebutyric acid (PBA) bound to traditionally prepared, deprotected benzyl- or deprotected trityl-spaced aminosilicas. Thermogravimetric analysis and FT-Raman spectroscopy results show evidence that the protected imine can be cleaved to yield the corresponding amine in essentially quantitative yield. The steady-state fluorescence spectroscopic data of either PCA or PBA indicate that the number of amine pairs on the surface separated by a distance of 1 nm or less decreases as the total amine loading decreases. Both the intensity ratio of the excimer band to the monomer band (I 470/I 384 or I exc/I mon) and lifetime decay studies of the fluorophore are useful probes of the amine spacing. Separation of amines on the surface can be achieved by either use of a protected synthesis route or through reduction of the concentration of the unprotected 3-aminopropyltrimethoxysilane used in the grafting solution. However, the two routes lead to materials with significantly different average amine spacings. Due to clustering of unprotected amines in solution before grafting or on the surface during the grafting process, amine−amine distances on the surface of materials prepared by an unprotected synthesis are on average smaller than when a protected synthesis is used. With the protected synthesis, evidence suggests that the amines are more isolated, with larger average amine−amine distances when compared to corresponding materials with a similar amine loading prepared via an unprotected synthesis. This is attributed to both the steric influence of the protecting groups and a reduction in silane clustering in solution due to protection of the amines before grafting. Thus, the mechanism of surface amine spacing when using the protection−deprotection strategy appears to involve both of these factors (especially in the case of trityl-spaced samples).

Pyrolysis studies have been conducted at 375 °C on several silica-immobilized phenethyl phenyl ether (PPE) model compounds, representative of related β-O-4 aryl ether linkages in lignin, to explore the impact of restricted mass transport on reaction pathways. As found previously for fluid-phase PPE, two competitive free-radical decay pathways are operative including a significant rearrangement pathway involving an O,C-phenyl shift for surface-attached PhCH2CH•OPh radicals. The selectivity for the rearrangement pathway is found to be sensitive to substituents and, in particular, to the structure of neighboring spacer molecules on the surface. In contrast to solution-phase behavior, dilution of PPE molecules on the surface with rigid aromatic spacers such as biphenyl or naphthalene hinder the rearrangement path. This phenomenon, attributed to steric constraints that decrease the rate of the 1,2-phenyl shift, is not observed when a more flexible spacer molecule (diphenylmethane) is employed. An improved knowledge of the pathways involved is important since this rearrangement pathway, which was also observed in the pyrolysis of α-aryl ether models, can result in the formation of valuable chemicals (aryl aldehydes and ketones) or undesirable refractory compounds (biphenyls and diphenylmethanes) during the thermochemical processing of lignin.

Optical polarization spectroscopy has been used to investigate molecular dynamics of the fluorescent probes 1-pyrenebutanol and 1-pyrenebutyric acid at the solid/air interface of cab-o-sil (fumed silica nanoparticles). Pyrenebutanol was chemically attached to the surface of cab-o-sil through a silyl ether bond, while pyrenebutyric acid was physisorbed on the surface. Dynamics of fluorescence depolarization for both molecules was studied under steady-state and time-resolved conditions. Low and high loadings of the probe molecules were used in our studies to examine the dynamics of the probes motion and excimer formation. Our data indicate that excimer formation is mostly static in nature for the adsorbed probe, but shows more dynamic character for the chemically attached probe with spacer molecules present. Fluorescence lifetimes were dependent on the concentration of the probe molecule and became shorter at higher surface loadings for both the chemically attached and physisorbed probes. For the chemically attached probe, the presence of excess co-attached biphenyl molecules was found to provide a 2-D solvent-like environment.

S-Alkylation followed by heterocyclization of trifluoromethyl-3-cyano-2(1H)-pyridinethiones was used for preparation of libraries of S-alkyl trifluoromethylpyridines and thieno[2,3-b]pyridines. The S-alkylation (in water--DMF mixtures) was successful for all 18 alkylating agents employed (yields typically > 50%). S-Alkyl derivatives were further converted to corresponding thieno[2,3-b]pyridines via heterocyclization in base conditions (yields > 65%). Structures of new compounds were elucidated by a combination of IR and 1H NMR spectroscopy and elemental analysis and were confirmed by means of single-crystal X-ray diffraction analysis.