Student Project Abstracts 2005 - Pluto - University of Washington

Spectroscopic Investigations of Chromophores in Dyed Salt CrystalsStacy A. OliphantEdmonds Community CollegeKristin L. Wustholz, Bart Kahr and Philip J. ReidDepartment of ChemistryUniversity of WashingtonINTRODUCTIONThe Science and Technology Center (STC) is committed tothe development of next-generation electro-optic (EO) devices,typically based on dye-doped polymers. Polymer-based switcheshave demonstrated greater device efficiencies when compared totraditional inorganics. Since the EO effect relies on the nonlinearmaterial response, the dye-doped polymers must be noncentrosymmetric.This is typically achieved via the application of apoling field. Ultimately, EO activity is given by:alignment and photophysics of single chromphores in dye-dopedcrystal hosts using confocal microscopy. Furthermore, by comparingthe orientations of dyes to their energetics, the environmentalheterogeneity among individual dye molecules within thecrystal can be studied.|Nµβ|where the EO response is proportional to the number of dye molecules(N), the molecular dipole moment (μ), hyperpolarizability(β), and the ordering parameter (cos 3 θ) that represents the extentof alignment afforded by poling. Understanding the factors thatinfluence chromophore orientation has taken a backseat to the developmentof enhanced molecular properties (i.e. β). Yet, recentwork suggests that poling is only partially effective in achievingmolecular alignment 1 – making it quite clear that the poling procedureand ordering parameter are not well understood.To experimentally test the efficacy of poling, and hencebulk EO device efficiency, it makes sense to begin with a systemin which the chromophores are intrinsically aligned by thehost. When grown in the presence of many organic dyes, aqueoussolutions of potassium acid phthalate (KAP) and sodium potassiumtartrate tetrahydrate (commonly known as Rochelle salt)frequently deposit “dyed crystals” wherein chromophores are orientedand overgrown by the host lattice. In these crystals, dyesselectively adsorb to particular sub-volumes within the latticeand bulk spectroscopic studies suggest that they are intrinsicallyaligned during the growth process. For example, Figure 1 showsthat 4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran(DCM) adsorbs to the {11 - 1} growth sector, whileViolamine R (VR) adds to the {010} growth sector of the hostcrystal, KAP.Here, a natural comparison presents itself in the study of theNLO properties of chromophore-polymer composites and dyedopedsingle crystals. We use single-molecule spectroscopy tomeasure the orientations and fluorescence spectra of embeddeddye molecules. The focus of this work was to interrogate theFigure 1. Dye-Doped KAP Crystals. Violamine R (1) and DCM(2) ad3sorb to different growth sectors of the KAP crystal host.EXPERIMENTALDyed crystals were grown by slow evaporation from aqueoussolution, in a temperature controlled air chamber (30°C) withdye concentrations of 10 -4 M to 10 -8 M. The latter concentrationwas used to ensure single-molecule resolution.Single-molecule experiments were performed using a confocalmicroscope with the following parameters: an invertedmicroscope (Nikon, TE2000). Excitation from a 532 nm solidstatelaser (NovaLux) was filtered (Chroma) and the excitationpolarization was manipulated using a half-waveplate. The laserwas focused to a diffraction-limited spot with an objective (100x,Nikon, 1.3 NA). Cleaved crystal samples (~1 mm) were mountedon a closed loop x-y piezo scanning stage (Queensgate). Emissionwas spectrally filtered by a dichroic mirror and emissionfilters (Chroma) and spatially filtered with a confocal pinhole(ThorLabs: 50 μm). The fluorescence intensity was imaged ontoa single-photon counting APD (PerkinElmer). Single-moleculefluorescence spectra were measured by adding a monochromator(Acton) attached to a liquid N 2CCD camera (Princeton Instruments)to the confocal instrument.CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 99