Molecular ordering of PAH/MA-co-DR13 azopolymer layer-by-layer films probed by second-harmonic generation
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Journal of Chemical Physics
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Molecular orientation within azopolymer thin films is important for their nonlinear optical properties
and photonic applications. We have used optical second-harmonic generation (SHG) to study the
molecular orientation of Layer-by-Layer (LbL) films of a cationic polyelectrolyte (poly(allylamine
hydrochloride)) and an anionic polyelectrolyte containing azochromophore side groups (MA-co-
DR13) on a glass substrate. The SHG measurements indicate that there is a preferential orientation of
the azochromophores in the film, leading to a significant optical nonlinearity. However, both the signal
strength and its anisotropy are not homogeneous throughout the sample, indicating the presence of
large orientational domains. This is corroborated with Brewster angle microscopy. The average SHG
signal does not increase with film thickness, in contrast to some reports in the literature, indicating
an independent orientational order for successive bilayers. Analyzing the SHG signal as a function of
the input and output polarizations, a few parameters of the azochromophore orientational distribution
can be deduced. Fitting the SHG signal to a simple model distribution, we have concluded that the
chromophores have an angular distribution with a slight in-plane anisotropy and a mean polar angle
ranging from 45◦ to 80◦ with respect to substrate normal direction, with a relatively large width
of about 25◦. These results show that SHG is a powerful technique for a detailed investigation of
the molecular orientation in azopolymer LbL films, allowing a deeper understanding of their selfassembling
mechanism and nonlinear optical properties. The inhomogeneity and anisotropy of these
films may have important consequences for their applications in nonlinear optical devices.
Como referenciar
SILVA, HEURISON S.; LOPES, FABIO J.S.; MIRANDA, PAULO B. Molecular ordering of PAH/MA-co-DR13 azopolymer layer-by-layer films probed by second-harmonic generation. Journal of Chemical Physics, v. 145, n. 10, p. 104902-1 - 104902-9, 2016. DOI: 10.1063/1.4962341. Disponível em: http://repositorio.ipen.br/handle/123456789/26784. Acesso em: 20 Mar 2026.
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