Near-infrared (NIR) Fourier transform (FT) Raman spectra have been measured for a series of 4-[4‘-[[(4‘ ‘-alkyloxy)phenyl]azo]naphthyloxy] alkyltrimethylammonium compounds [C
n
AzoNaph(1,4)C
m
N+Br-]
in a solid state, in a cast film on a CaF2 plate, and in dipping films on etching silver foils and for one- and
three-layer Langmuir−Blodgett (LB) films of C10AzoNaph(1,4)C4N+−sodium dextran sulfate (SDS) on a
CaF2 plate and etching silver foils. The one-monolayer LB film on the silver foil shows much more intense
Raman signals than the 75-layer film on the CaF2 plate, showing a great surface-enhanced Raman scattering
(SERS) effect for the LB film on the silver foil. Since SDS plays a role as a spacer, C10AzoNaph(1,4)C4N+
is not directly adsorbed on the silver surface. Therefore, the strong SERS effect for the LB films of C10AzoNaph(1,4)C4N+−SDS may be explained by an electromagnetic mechanism. The one-monolayer dipping
film of C10AzoNaph(1,4)C4N+Br- on the silver foil shows about 10 times stronger Raman signals than the
one-monolayer LB film and the spectral intensity pattern is quite different between the dipping and LB
films. It is very likely that the direct adsorption of C10AzoNaph(1,4)C4N+Br- on the silver foil gives a much
stronger SERS effect for the dipping film. The different intensity pattern between the two kinds of films
may be due to the differences in the molecular orientation and/or aggregation in the films. The dipping
film of C7AzoNaph(1,4)C4N+Br- yields a much smaller SERS enhancement factor. The alkyl tail is too
short to have well-organized molecular aggregation and/or orientation in the film, and the enhancement
factor may be sensitive to the molecular aggregation and/or orientation in the film.