Infrared attenuated total reflection spectroscopy studies of aprotic condensation of (EtO)3SiRSi(OEt)3 and RSi(OEt)3 systems with carboxylic acids

Aprotic hydrolysis and condensation reactions of bis end-capped trialkoxysilanes ((EtO)3SiRSi(OEt)3), linked via the organic chain R containing urea groups chemically bonded to a poly(propyleneglycol) (PPG) chain, in the presence of carboxylic acids, i.e. acetic-, chlorodifluoroacetic- and trifluoroacetic acids, were studied using infrared attenuated total reflection (IR ATR) spectroscopy. IR and 29Si NMR spectral analysis revealed solvolysis reactions: the carboxylic acids interacted with ethoxysilane groups forming silyl esters leading to the formation of bridging SiOSi groups and carboxylic acid ester by-products. These results were compared with those obtained on simpler single capped methyltriethoxysilane (MeSi(OEt)3, MTEOS) condensed with trifluoroacetic acid. Gelation of (EtO)3SiRSi(OEt)3 (catalyzed with acetic acid) encapsulated between a transparent conductive oxide (TCO) glass and a conductive and IR transparent silicon wafer was followed with the help of IR reflection-absorption spectroscopy. The results revealed that aprotic solvolysis of the hybrid precursor with acetic acid led to the formation of non-aqueous gels with low silanol content, confirming the advantages of aprotic solvolysis of organic-inorganic hybrids used as redox electrolytes in hybrid electrochromic (EC) and dye-sensitized photoelectrochemical (DSPEC) cells. Some comments regarding the accuracy of IR ATR spectral measurements compared to IR transmission spectra are also given.