Guest-Host Cooperativity in Organic Materials Greatly Enhances the Nonlinear Optical Response Yuriy V. Pereverzev, † Kim N. Gunnerson, † Oleg V. Prezhdo,* ,† Philip A. Sullivan, † Yi Liao, † Benjamin C. Olbricht, † Andrew J. P. Akelaitis, † Alex K.-Y. Jen, ‡ and Larry R. Dalton* ,† Department of Chemistry and Department of Materials Science and Engineering, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: September 7, 2007; In Final Form: December 31, 2007 Some of the most highly active organic electro-optic (EO) materials developed recently rely on the combination of an EO-active (chromophore-containing) host material (dendrimer or side-chain polymer) and an EO-active (chromophore) guest. These new binary-chromophore materials exhibit EO coefficients (r 33 ) in the range of 250 to greater than 300 pm/V (currently as high as 450 pm/V). The EO activity of these binary-chromophore materials is greater the sum of their individual components. The experimentally observed increase in the nonlinear optical response of two representative classes of EO chromophore-EO dendrimer and EO chromophore-EO polymer mixtures relative to the response of the isolated components is described quantitatively herein by a physical model that accounts for cooperativity in the guest-host interactions. I. Introduction With the maturation of, and societal reliance on, information technology has come the need for increased data handling/ processing capability. 1 Photonics-based technologies are poised to meet this coming challenge by delivering a high-performance and cost-effective means of ultrafast information manipulation and transfer with lower energy consumption. 2-6 The perfor- mance characteristics of photonic microdevices, such as electro- optic (EO) modulators, are largely determined by the attributes of the active materials from which they are constructed. In the case of optical modulators, whose operation is based on the linear EO or Pockel’s effect, the first major hurdle to be overcome is the development of active materials that exhibit sufficient EO activity to avoid excessive drive voltage requirements. 7-10 In this area, organic materials currently offer a significant advantage over inorganic competitors. 4,11,12 Recent research efforts in the area of organic EO devices have yielded materials with dramatically enhanced electro-optic coefficients, r 33 (in the range of 250 to greater than 300 pm/V). Two classes of these materials, binary chromophore-containing dendrimer glasses and binary chromophore-containing polymers, rely on synergistic effects between EO active host and guest materi- als. 13,14 It has been observed that introduction of free chromophores to a dendrimer host containing covalently bound chromophores significantly increases the nonlinear optical (NLO) response of the final material as compared to the individual constituents. Similarly, introduction of free chromophores into a polymer bearing covalently attached dipolar chromophore-containing side-chains also results in a significantly improved EO re- sponse. 15 In this communication two binary chromophore systems are demonstrated. Free chromophore YLD_124 is introduced into both an EO dendrimer (PSLD_41) and an EO polymer (DR-1 co-pmma), Figure 1. The cooperativity phe- nomena observed are described herein by a physical model that involves energetically favorable organization of electrostatically bound complexes between the dendrimer and polymer hosts and the free chromophore guest. II. Experimental Studies II.1. Synthesis General. The synthesis and characterization of dendrimer PSLD_41 and free chromophore YLD_124 has been covered in detail elsewhere. 16,17 II.2. Electro-Optic Property Assessment. Thin-films of the materials to be tested were fabricated by dissolution of a mixture of the appropriate proportions of host and guest materials. Into freshly distilled 1,2-dichloroethane (30 mg sample/345 mg solvent) was dissolved 8% total solid weight. After agitating overnight, the solutions were then filtered (0.2 μm PTFE, Whatman) and spin-cast onto half etched ITO coated glass slides (dendrimer PSLD_41 ) 850 rpm, polymer ) 1100 rpm) to yield film thicknesses between 1.2-2.0 μm. Solvent traces were then removed in vacuo overnight at 60 °C. Poling and reflectance ellipsometry (r 33 measurement) electrodes were then fabricated by deposition of gold directly atop the films. Film thicknesses were measured by surface profilometry. Electro-optic coefficient measurements were performed as previously reported. 18-24 Teng-Man ellipsometry 18-24 can be subject to large errors (both positive and negative) when ITO is utilized with thin film organic samples. We have minimized such errors as discussed elsewhere 13,23,24 and have used modified attenuated total reflection 25 and other techniques more recently to attempt to improve absolute accuracy of measurements. While large errors are still possible it is not likely that they affect the fundamental conclusions of this report. This is particularly evident from the report of laser-assisted poling of the YLD_124 doped into DR1-co-pmma where photopoling of the DR1 host chromophores results in a factor of 2.5 increase in the acentric order of the guest YLD_124 chromophores. This direct observa- tion of the interconnection of guest and host order provides * Corresponding authors. E-mail: prezhdo@chem.washington.edu; dalton@chem.washington.edu. † Department of Chemistry. ‡ Department of Materials Science and Engineering. 4355 J. Phys. Chem. C 2008, 112, 4355-4363 10.1021/jp077194q CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008