Abstract
Surface-enhanced Raman spectra of 4-biphenylcarboxylic acid (BPCA) and ethyl 4‘-[4‘-oxy-4-biphenylcarbonyloxy]-4-biphenylcarboxylate with a poly(propylene oxide) coil of 12 (hereafter named P-12-4) adsorbed on two different metal substrates, Ag and Au colloid monolayers, were examined, to determine the relationship between adsorption strength and geometry. A significant downshift of νCOO- by 60 cm-1 and lack of a C−H aromatic stretching vibration band indicated that BPCA lies flat on the Au surface, while a smaller shift of 40 cm-1 to a lower wavenumber and weak appearance of the C−H aromatic stretching mode implied adsorption through mainly oxygen lone pair electrons of carboxylate with the benzene ring tilted on the Ag surface. These orientations were confirmed by the desorption behavior of a liquid crystalline (LC) oligomer, P-12-4, which contains the BPCA moiety, on the Au and Ag colloid monolayers with increasing temperature.
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