Aligned Inclusion of n‐Propionic Acid Tethering Hemicyanine into Silica Zeolite Film for Second Harmonic Generation

Abstract

Organization of dipolar second-order nonlinear optical (NLO) dyes in uniform orientations is not only an interesting subject by itself but also an essential step towards a successful application of the dyes for second-harmonic generation (SHG). Zeolites have been tested as the hosts for aligned inclusion of dipolar organic NLO dyes. However, the previously tested dyes have been limited to p-nitroaniline and those that have low molecular second-order hyperpolarizability constants (b ∼ 3 × 10 electrostatic units (esu; 1 esu∼ 3.33564 × 10 C)) and the zeolites have mostly been limited to powders. As a result, the potential of NLO dye-including zeolites as practically viable SHG materials has not been fully explored. It is therefore necessary to develop methods to include high-b dipolar NLO dyes with a high degree of uniform orientation (DUO) into transparent zeolite films with uniformly oriented channels. We recently reported that hemicyanine (HC) with an octadecyl tail (HC-18; Scheme 1A; b ≥ 77 × 10 esu) preferentially enters the vertically aligned channels of transparent silicalite (SL) films (thickness 400 nm) supported on glass (SL/Gs) with the octadecyl chain first because of the hydrophobic interaction between the long alkyl chain and the silicalite channel. Interestingly, despite the fact that the average number of incorporated molecules (NC) in each 400 nm long channel was only 3.5, the measured second-harmonic intensity (I2x) was remarkably high, equivalent to 7.9 % of that of 3 mm thick quartz (along the y-axis). Indeed, the measured d33 value (a tensor component of the quadratic nonlinear susceptibility) was 5.3 pm V, which was ca. 18 times larger than the d11 value of quartz (0.3 pm V ), and was even larger than the d33 value of potassium dihydrogen phosphate (KDP; ca. 3 pm V), one of the widely used NLO materials, by roughly a factor of two. Although HC-18-incorporating SL/Gs were still unusable for practical applications because of the very small NC of HC-18 in the channels, the above result clearly demonstrated the potential of NLO-dye-incorporating SL/Gs, as they could be developed into practically useful SHG films by increasing the NC. In the case of HC, it was found that the NC increases substantially upon decreasing the alkyl chain length. Unfortunately, however, the decrease in the alkyl chain length led to a sharp decrease in I2x because of a larger negative effect caused by the decrease in the DUO than the positive effect caused by the increase in the NC. Thus, increasing both the NC and DUO has been a big challenge. We now report that the DUO of HC with a propionic acid tail (commonly designated as either HC-PA or HC-2-CO2H; Scheme 1B) in SL channels increased by a factor of about four when the dye was included in SL/Gs in the presence of tetrabutylammonium hydroxide (TBAOH), leading to a ca. 16-fold increase in I2x. The procedures for preparation of HC-PA and SL/Gs are described in the Experimental Section. The scanning electron microscope (SEM) image of a typical SL/G is shown in Figure S1 in the Supporting Information. HC-PA was introduced into SL/G from the aqueous solutions (20 mL) of HC-PA (0.1 mM) and TBAOH (see Experimental for detail); the concentration of TBAOH ([TBAOH]) varied from 0.0 to 3.0 mM. HC-PA had a maximum absorption (kmax) at 460 nm in the aqueous solution regardless of the [TBAOH] (Supporting Information Fig. S2), indicating that the base did not affect the C O M M U N IC A TI O N