Molecular Architectural Approach to Novel Electro-Optical Materials Page: 7 of 17
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spontaneously form supramolecular ribbons when mixed in a 1:1 ratio in chloroform. The
formation of H-bonds in the supramolecular ribbon was confirmed by FTIR spectroscopy
as indicated by the shift of the carbonyl, amide and amino IR bands. The resulting
supramolecular ribbon is asymmetric because ADT has twice as many alkyl chains as DPB.
Bragg diffraction (x-ray) was observed at 20 = 2.16 and 4.28 degrees, which shows that
the BDP-ADT system is a multilayered lamellar structure. The d-spacing value obtained
from x-ray diffraction is 41 A, in agreement with the -40 A supramolecular ribbon width
obtained from molecular three-dimensional modeling. The asymmetric BDP-ADT ribbon
can pack into either a symmetric structure; i.e., a head-to-head and tail-to-tail arrangement
or an asymmetric structure; i.e., a head-to-tail motif. It is found that the asymmetric BDP-
ADT ribbons assemble into a polar multilayer head-to-tail structure (vide infra). It is
extremely unusual to find a dipolar system that will self-assemble into a polar multilayer
because dipole-dipole repulsion typically leads to energy minimized head-to-head (or tail-
to-tail) structures. The formation of these supramolecular self-assemblies is driven
primarily by the interplay of encoded 6 H-bonds, and secondarily by hydrophobic chain -
chain and n -7c interactions.
The second harmonic generation (SHG) experimental procedure and method for
deducing the second-order tensors have been described elsewhere"' and here we will only
summarize the sum-frequency generation (SFG) experiment. SFG spectra were obtained
by overlapping tunable mid-IR and 1064 nm beams at the sample surface. A Nd:YAG
laser provided -50 ps pulses at 1064 nm. The energy density at the sample was -1
mJ/mm2. The mid-IR beam (100 pJ/pulse near 2.8 pm) was obtained by optical parametric
generation (OPG) and amplification (OPA) in BBO and KTP nonlinear crystals."-" All
input beams were p-polarized. The transmitted p-polarized SFG signal in the red spectral
range was spatially separated from the two input beams and detected using a photomuliplier
tube (PMT) and gated integrator.
The transmitted p-polarized SH intensity from the multilayer film grown on one
side of a silica substrate for both s- and p-polarized incident excitation is shown in Figure
2. As expected, no s-polarized SH signal was observed using s-polarized excitation. Very
little p-polarized SH signal was observed using s-polarized excitation (Figure 2(b)). Two
main features are observed in the data: the increase in SH signal at high angles of incidence
and the weak interference fringes. The fringes are thought to arise from the interaction of
the SH waves from the multilayer and the opposite surface of the silica substrate.9'0 These
films typically have uniaxial symmetry about the surface normal,l"5 thus yielding only two
independent tensor elements (d33 and d31). With the film thickness determined to be 300 A
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Li, D.; Johal, M.S.; Smilowitz, L.B. & Robinson, J.M. Molecular Architectural Approach to Novel Electro-Optical Materials, report, June 29, 1999; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc723360/m1/7/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.