Optical Properties of Disulfide-Functionalized Diacetylene Self-Assembled Monolayers on Gold: a Spectroscopic Ellipsometry Study

Spectroscopic ellipsometry (SE) has been applied to study the optical properties of thiolate polydiacetylene
(PDA) self-assembled monolayers (SAMs) deposited on low-roughness polycrystalline gold. A systematic
investigation of methyl-terminated diacetylene (dihexacosa-7,9-diyn disulfide, DS9) SAMs is presented. The
results have been compared with experiments on carbazolyl-derivatized diacetylene (14-(9H-9-carbazolyl)tet-
radeca-10,12-diyn-1-yl disulfide, CDS9) SAMs and with findings recently obtained on SAMs of alkanethiols.
The SE measurements have been complemented with X-ray photoelectron spectroscopy and atomic force
microscopy data. The difference between SE spectra measured after and before the monolayer assembly (??
) ?SAM - ?Au and ?? ) ?SAM - ?Au) showed specific absorptions of the adsorbed molecules, including
narrow features in the 500-700 nm wavelength range which have been interpreted as markers of the SAM
polymerization state. To our knowledge, these results represent the first unambiguous optical detection of
polymer-induced absorptions in the case of diacetylene SAMs. Polymerization mainly occurred in the so-
called red phase (absorption peak around 550 nm) on very flat surface regions obtained by flame-annealing
the sample. The polymeric phase was stable against moderate UV irradiance. The detection of the blue phase
was questionable as specific absorptions (around 640 nm) overlap with an absorption band related to the
formation of the S-Au interface.