Student Project Abstracts 2005 - Pluto - University of Washington

Characterization of the Photodecomposition of the CF3-FTC ChromophoreJillian ThayerOlympic CollegeJason Benedict, Dawn Cohen and Dr. Bart KahrUniversity of WashingtonINTRODUCTIONDemands for improved electro-optic devices support ongoingresearch in photonics. Non-linear optical (NLO) materials haveso far been the focus of meeting such demands, as they show significantoptical effects when an electrical field is applied. NewerNLO materials are organic chromophores, light absorbing moleculeswhich consist of an electron donor, pi system conjugationbridge, and an electron acceptor. Chromophores are often studiedwhile suspended in a polymer, which is spin coated and hardenedon a glass slide. They have increasingly complex structuresand proportionally increased susceptibility to decomposition.For instance, many bonds can be broken in an organic moleculewith a molecular weight of about 850 atomic mass units. Amongthe ten characteristics of ideal NLO materials are concerns overstrength, stability, and laser damage thresholds 1 . These areas ofinterest prove to be a problem for NLO chromophores, and theredeveloped the idea of a non-linearity/stability tradeoff. For chromophores,inherently sensitive to light, photodecomposition is ofparticular concern.Figure 1.1. A new NLO chromophore, CF3-FTCUnderstanding the mechanisms of photodecomposition isimperative if improvement of chromophores and the lengtheningof electro-optic device lifetime are expected. Photodecompositionoccurs when light energy absorbed by a chromophore is sufficientto break molecular bonds, or allow high energy reactions withtheir environment to occur. Controlled experiments are requiredto research photodecomposition. In this project linearly polarizedlight was directed at a sample of chromophores doped into a polymer.In the polymer matrix, molecules are randomly dispersed.Only those molecules whose dipole moment is approximatelyparallel to the direction of polarization should be destroyed. Thiswill result in a sample having differential absorption for mutuallyorthogonal input polarization, known as linear dichroism orabsorption anisotropy.Previous experiments conducted in the Kahr lab with polarizedlight however, resulted in isotropic decay of the chromophore,and no dichroism. This unexpected behavior implied thepresence of interactions amongst the chromophores. One hypothesisproposed a transfer of energy from molecules excited by thepolarized light to molecules out of the plane of polarization, unableto be excited by the polarized light directly. Polarized lightabsorption spectrophotometry was the primary tool in assessingdecay of chromophores, while various forms of spectroscopywere used to examine resulting photodecomposition products.METHODS AND MATERIALSMethods of analysis involved using slides spin coated with athin film of varying concentrations of dye-doped polymer. Electrodesattached to conductors on both sides of the film subject aregion of the chromophores to electric field poling and partialreorientation parallel to the plane of the applied electric field. Thepoled region should then be anisotropic relative to the unpoledregion.For analysis of photodecomposition products, previouslypoled films of AJL-8 (chromophore) were used and subjectedto various forms of spectroscopy. In order to look for clues asto the structure of photodecomposition products, clean spectraand convenient use with the thin films were important requirements.Flakes of polymer doped with chromophore were scrapedfrom poled slides, separated by poled and unpoled regions, andmixed with KBr to form pellets. The pellet was used with an infrared(IR) spectrometer, and a spectrum was obtained for boththe poled and unpoled regions. Next, poled slides were left intact,and IR spectra were obtained for both poled and unpoled regionson an attenuated total reflectance spectrometer. Finally, intactpoled slides were viewed under a Raman microscope coupled toa Raman spectrometer, and Raman spectra were taken again forboth the poled and unpoled regions.The next group of experiments also used thin films dye dopedpolymer, specifically containing the chromophore CF3-FTC, designedand synthesized by the University of Washington’s DaltonCMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 111