Aligned Inclusion of Hemicyanine Dyes into Silica Zeolite Films for Second Harmonic Generation

Abstract

Silicalite-1 films (thickness = 400 nm) supported on both sides of glass plates (SL/G) were prepared, and hemicyanine dyes (HC-n) with different alkyl chain lengths (n, n = 3, 6, 9, 12, 15, 18, 22, and 24) were included into the silicalite-1 films by dipping SL/Gs into each methanol solution of HC-n (1 mM) for 1 d. The included numbers of HC-n per channel (N(C)) generally decreased with increasing n; that is, they were 6.4, 23.1, 15.4, 8.2, 5.7, 3.5, 0.9, and 1.2 molecules per channel, respectively. The d(33) value gradually increased with increasing n but decreased when n > 18; that is, they were 1.12, 0.50, 2.25, 3.59, 4.99, 5.30, 1.71, and 2.57 pm V(-1), respectively. However, d(33)/N(C) progressively increased with increasing n. The d(31) values were approximately 100 times smaller than the corresponding d(33) values, and the average d(33)/d(31) ratio was 109, which is higher than those of Langmuir-Blodgett (LB) films and poled polymers of nonlinear optical (NLO) dyes, by approximately 2-5 and approximately 30-50 times, respectively. The estimated average tilted angle of the dyes with respect to the channel direction was 7.7 degrees, and the calculated average order parameter was 0.97, which is approximately 480 times higher than the values observed from poled polymers. The degree of uniform alignment (DUA) generally increased with increasing n. The progressive increase of both DUA and d(33)/N(C) with n is attributed to the increase in the tendency of HC-n to enter hydrophobic silicalite-1 channels with the hydrophobic alkyl chain first. A more than 134-fold increase in DUA was observed upon increasing n from 6 to 24. The DUA of HC-24 in the silicalite-1 film reached close to 1. Although the observed d(33) values were lower than those of the LB films of NLO dyes due to very small dye densities of the silicalite films, this methodology bears a great potential to be developed into the methods for preparing practically viable NLO films.

Determination of the Hyperpolarizability Components of Hemicyanine Dyes by Measuring the Anisotropic Fluorescence and Second Harmonic of the Dyes Uniformly Aligned within Zeolite Channels

“…The appearance of intense color only from oblique incidence indicates at once that the dipole moments of the molecules are mainly aligned along the surface normal. As it is known that the silicalite-1 film grows with its vertical (and straight) channel normal to the surface, 28 it can be inferred that the hemicyanine molecules are adsorbed only into the vertical channel of the silicate film, as shown schematically in Figure 1c. The pores parallel to the surface are not straight, and presumably the size of the hemicyanine molecule is too big to fit into that channel.…”

“…Previous SHG experiment found the alignment of the dyes is not only uniform, but also quite unidirectional when the alkyl chain at one end of the molecule becomes long enough. 28 With the very unidirectional angular distribution as well as the molecules in isolation with minimal interaction with the host matrix, 26,27 our sample is a very good system for studying various optical properties. We utilized it to determine the hyperpolarizability components of the embedded hemicyanine molecules.…”

“…28 The thicknesses of the obtained films were ∼400 nm. The synthesis of hemicyanine dyes (4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide, hereafter denoted HC-n) was described in previous reports.…”

“…The synthesis of hemicyanine dyes (4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide, hereafter denoted HC-n) was described in previous reports. 28 The hemicyanine dyes were incorporated into the channels of silicalite films by immersing SL/G plates into a vial containing a methanol solution of HC-n (10 mL, 1 mM), and the vial was kept at room temperature for a desired period of time. The immersion period of 4 h was enough to reach the maximum number densities of the dyes in SL/Gs.…”

“…On glass plates, they grow with the straight channels (b-axes) orienting normal to the glass substrates, and hemicyanine molecules enter into the straight channels. 28 Figure 1 grown on a glass (SL/G) plate, and (c) the schematic illustration of the directions of straight channels and hemicyanine incorporation in the silicalite layer of a SL/G plate.…”