A covalent/electrostatic layer-by-layer self-assembly method was used to achieve polar ordering of a water soluble, reactive dye in the fabrication of nonlinear optical (NLO) films. We observed a quadratic relationship between the second harmonic intensity I 2ω and bilayer number for all films made with Procion Brown MX-GRN, demonstrating that the polar ordering of the chromophores is consistent in each successive bilayer. As the ionic strength of the dye deposition solution was increased to 0.5 M NaCl, the of the films increased by approximately 250% to 50 × 10-9 esu, with a corresponding average chromophore tilt angle of 38°. This was attributed to increased shielding of the dye charges which led to higher chromophore density in the bilayers. The electrooptic coefficient for films of 50 bilayers fabricated at 0.5 M NaCl was 14 ± 2 pm/V. Importantly, these films exhibited excellent thermal stability, with only a 10% decrease in (I 2ω)1/2 after 36 h at 85 °C and then 24 h at 150 °C. Furthermore, the (I 2ω)1/2 recovered completely upon cooling to room temperature. These results with a commodity textile dye point to the potential value of this class of reactive chromophores and this self-assembly method for fabrication of electrooptic materials at ambient conditions from aqueous solutions.

Materials that exhibit second-order nonlinear optical (NLO) properties are key components in electrooptic modulators and frequency-doubling devices. [1] Second-harmonic generation (SHG), in which incident light at one frequency is converted into light at twice that frequency, is one example of second-order NLO phenomena and is often used as an experimental probe of the second-order susceptibility (c (2) ). A material must have a non-centrosymmetric structure to possess a nonzero c (2) . Electrooptic modulators have traditionally employed ferroelectric inorganic crystals, such as lithium niobate or potassium dihydrogen phosphate, which are formed at high temperatures. However, organic NLO materials offer several advantages in performance, such as higher nonlinear susceptibilities, higher modulation rates, and potentially lower device fabrication costs. [2] Organic films exhibiting nonzero c (2) values have been fabricated using a variety of methods, including electric field poling, [3] Langmuir±Blodgett (LB) films, [4] and covalent self-assembly. [5] Both poled polymer systems and LB films have been made with non-centrosymmetric structures that exhibit relatively high values for c (2) , but poor temporal or mechanical stability restrict their potential applications. [6] Deposition processes using reactive silane compounds require organic solvents and high temperatures. [5] There is a large and growing body of literature on the use of layer-by-layer (LBL) methods for fabricating nanostructured films for a variety of applications. The LBL technique, which relies on purely electrostatic interactions, was first developed by Iler [7] and further elaborated upon by Decher et al. [8] Several research groups have demonstrated that the NLO films made by this technique have greater thermal and temporal stability than poled polymer systems. [9] A related approach that could be employed to fabricate NLO materials involves the use of low-molecular-weight dye molecules and polyelectrolytes as film constituents. Yamada et al. made films of poly(diallyldimethylammonium chloride) and Erichrome Black T that exhibited an SHG intensity that increased only for the first five bilayers and then reached a plateau. [10] Other research groups, including ourselves, have found that ionic interactions alone are not sufficient for constructing LBL films with low-molecular-weight chromophores. [11] A combination of low-molecular-weight chromophoric molecules and polyelectrolytes that could be used to construct stable NLO films with the large number of bilayers needed for electrooptic devices is yet to be demonstrated.The objective of this work was to demonstrate that LBL films made under ambient conditions with a water-soluble, monomeric chromophoric molecule can possess high net polar ordering in each bilayer. By alternating the methods of deposition for each monolayer (covalent reaction and electrostatic interaction) and decoupling the chromophore orientation from the steric constraints of a polymer chain, we hypothesized that the no...

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Materials that exhibit second-order nonlinear optical (NLO) properties are key components in electrooptic modulators and frequency-doubling devices. [1] Second-harmonic generation (SHG), in which incident light at one frequency is converted into light at twice that frequency, is one example of second-order NLO phenomena and is often used as an experimental probe of the second-order susceptibility (c (2) ). A material must have a non-centrosymmetric structure to possess a nonzero c (2) . Electrooptic modulators have traditionally employed ferroelectric inorganic crystals, such as lithium niobate or potassium dihydrogen phosphate, which are formed at high temperatures. However, organic NLO materials offer several advantages in performance, such as higher nonlinear susceptibilities, higher modulation rates, and potentially lower device fabrication costs. [2] Organic films exhibiting nonzero c (2) values have been fabricated using a variety of methods, including electric field poling, [3] Lang-muir±Blodgett (LB) films, [4] and covalent self-assembly. [5] Both poled polymer systems and LB films have been made with non-centrosymmetric structures that exhibit relatively high values for c (2) , but poor temporal or mechanical stability restrict their potential applications. [6] Deposition processes using reactive silane compounds require organic solvents and high temperatures. [5] There is a large and growing body of literature on the use of layer-by-layer (LBL) methods for fabricating nanostructured films for a variety of applications. The LBL technique, which relies on purely electrostatic interactions, was first developed by Iler [7] and further elaborated upon by Decher et al. [8] Several research groups have demonstrated that the NLO films made by this technique have greater thermal and temporal stability than poled polymer systems. [9] A related approach that could be employed to fabricate NLO materials involves the use of low-molecular-weight dye molecules and polyelectrolytes as film constituents. Yamada et al. made films of poly(diallyldimethylammonium chloride) and Erichrome Black T that exhibited an SHG intensity that increased only for the first five bilayers and then reached a plateau. [10] Other research groups, including ourselves, have found that ionic interactions alone are not sufficient for constructing LBL films with low-molecular-weight chromophores. [11] A combination of low-molecular-weight chromophoric molecules and polyelectrolytes that could be used to construct stable NLO films with the large number of bilayers needed for electrooptic devices is yet to be demonstrated.The objective of this work was to demonstrate that LBL films made under ambient conditions with a water-soluble, monomeric chromophoric molecule can possess high net polar ordering in each bilayer. By alternating the methods of deposition for each monolayer (covalent reaction and electrostatic interaction) and decoupling the chromophore orientation from the steric constraints of a polymer chain, we hypothesized that the n...