Evidence of Spontaneous Multilayer Formation for Disperse Red-1 at a Fused-Silica/2-Propanol Interface

Abstract

The film structure of a nonlinear optical azo dye (Disperse Red-1, DR-1) was probed by a combination of spectroscopic and microscopic techniques. Second harmonic generation (SHG) and linear dichroism provided evidence for the spontaneous formation of DR-1 multilayers at a 2-propanol/fused-silica interface under equilibrium conditions. Specifically, the adsorption isotherm measured by SHG exhibited two distinct adsorption regimes; the second regime (>10-3 M) being indicative of multilayer formation in which some degree of polar order was retained. The trend in apparent tilt angle measured by linear dichroism was consistent with multilayer formation at solution-phase concentrations above 10-3 M. Multilayer formation was directly observed, on a dip-coated substrate, by atomic force microscopy and further confirmed by ellipsometry. The significance of spontaneous multilayer formation with z-axis polar order in higher layers is the potential for growth of molecular multilayer systems without the need for covalent or sequential deposition.

Nanostructuring of Azomolecules in Silica Artificial Opals for Enhanced Photoalignment

“…This is also supported by the appearance of an absorption peak at 1516 cm −1 (Figure 6a), which corresponds to the asymmetric stretching of NO 2 , while the out‐of‐plane bending of aromatic C–H at 860 and 829 cm −1 was not observable 33. The chromophore surface density in our system is about one order of magnitude smaller than those usually reported in related systems,8, 31, 32, 37 and low enough to avoid significant adsorbate–adsorbate interactions 38. Considering a silanol density in colloidal silica of approximately 5 nm −2 ,27 the surface density corresponds to a 1:25 DR1/silanol ratio, so that about 4% of the silanol groups have adsorbed a DR1 molecule.…”

“…The tendency to spontaneous multilayer formation was confirmed by adsorption isotherm experiments (Figure 6c), in which the DR1 uptake from solution obeyed the BET equation, which models multilayer adsorption, rather than a Langmuir isotherm describing monolayers (Equations 16 and 17). Such an arrangement probably occurs by head‐to‐tail organization of the dipolar DR1 molecules (through donor–acceptor attractive interactions between –OH and –NO 2 groups), as reported in other studies 32, 33. Note that this chromophore distribution is compatible with the initial isotropy of the dye‐infiltrated opal given the spherical symmetry of the silica beads.…”

“…From the chromophore density ρ DR1 we calculate a surface density of adsorbed DR1 of 0.2 molecules per nm 2 (Equation 7), i.e., 2 × 10 13 cm −2 . Thereby, we have assumed that the molecules lie almost perpendicular to the surface, as previously observed in DR1‐deposited systems,32, 33 so that the average thickness of the first DR1 layer on the surface is approximately 1 nm, which is the length of the DR1 trans state 36. This is also supported by the appearance of an absorption peak at 1516 cm −1 (Figure 6a), which corresponds to the asymmetric stretching of NO 2 , while the out‐of‐plane bending of aromatic C–H at 860 and 829 cm −1 was not observable 33.…”

“…The most decisive factor appeared to be the hydrophilic character of the opal (Figure 4). The presence of silanol groups on the silica surface allowed physisorption of the DR1 molecules via hydrogen bond, preferentially through the –OH groups of DR1 rather than the nitro groups 31–33. On the one hand, FTIR spectroscopy absorption measurements on as‐grown silica opals showed, after DR1 infiltration, a decrease in the absorbance of bands assigned to adsorbed water.…”

Second‐harmonic generation from Langmuir–Blodgett film of cyclic peptide carrying two disperse red 1 on fused quartz surface

“…This finding suggests that some portion of the DR-1 units may have antiparallel arrangement with the neighbors to loose the NLO activity in the cyclo[8 + DR-1] monolayer where the structural regularity is low due to the large tilt angle. On the other hand, β was 65 × 10 −30 cm 5 /esu which is close to the reported value in the literature [50] (ca 45 × 10 −30 cm 5 /esu). The nonlinear susceptibilities were χ xxz = χ zxx = 5.4 pm/V and χ zzz = 1.8 pm/V for the cyclo[8 + DR-1] monolayer and χ xxz = χ zxx = 7.9 pm/V and χ zzz = 12.0 pm/V for the cyclo[8 + 2DR-1] monolayer, respectively.…”