Design and synthesis of organic chromophores for imaging, lithography and organic electronics

Abstract

The absorption and emission maxima, photostabilities and photoreactivities of small-molecule organic chromophores can be tailored by (a) the choice of an appropriate parent structure and (b) the deliberate introduction of substituents that predictably alter the optical properties and photochemistry of this parent structure. Suitably-designed chromophores can be used in a variety of applications, such as imaging (for example, as fluorescent labeling agents or as indicators for specific analytes), optical lithography and as active components in organic electronic devices. In Chapter 1, a fluorogenic chemosensor to detect saturated nitramine and nitrate ester explosives was devised based on a photochemical reduction reaction. 10-Methyl-9,10- dihydroacridine (AcrH2) was found to transfer a hydride ion equivalent to the high explosives RDX and PETN upon irradiation at 313 nm in degassed acetonitrile solutions. Mechanistic photophysical studies indicated that the photoreduction of RDX proceeded via a two-step electron-hydrogen atom transfer reaction, whereas PETN photoreduction proceeded via a threestep electron-proton-electron transfer sequence. A zinc analog was synthesized and found to display an 80- or 25-fold increase in 480 nm emission intensity upon reaction with RDX or PETN, respectively; moreover, the Zn analog was found to be unresponsive to TNT and other common contaminants, in addition to being photostable under ambient conditions. In Chapter 2, the nitramine-containing explosive RDX and the nitroester-containing explosive PETN were shown to be susceptible to photodegradation upon exposure to sunlight. The products of this photodegradation were identified as reactive, electrophilic NOx species, such as nitrous and nitric acid, nitric oxide, and nitrogen dioxide. NN-Dimethylaniline was capable of being nitrated by the reactive, electrophilic NOx photodegradation products of RDX and PETN. A series of 9,9-disubstituted 9,10-dihydroacridines (DHAs) were synthesized from either N-phenylanthranilic acid methyl ester or a diphenylamine derivative and were similarly shown to be rapidly nitrated by the photodegradation products of RDX and PETN. An increase in the emission signal at 550 nm was observed upon nitration of DHAs due to the generation of fluorescent donor-acceptor chromophores. Using fluorescence spectroscopy, the presence of ca. 1.2 ng of RDX and 320 pg of PETN could be detected by DHA indicators in the solid state upon exposure to sunlight. In Chapter 3, optical lithography with organic photochromes is demonstrated. In the past, the formation of microscale patterns in the far field by light has been diffractively limited in resolution to roughly half the wavelength of the radiation used. We demonstrated lines with an -4- average width of 36 nm, about one-tenth the illuminating wavelength ( 11 = 325 nm), made by applying a film of thermally-stable photochromic molecules above the photoresist. Simultaneous irradiation of a second wavelength (k= 633 nm) rendered the film opaque to the writing beam except at nodal sites, which let through a spatially constrained segment of incident 1 light, allowing subdiffractional patterning. In Chapter 4, rylene dyes functionalized with varying numbers of phenyl trifluorovinylether (TFVE) moieties were subjected to a thermal emulsion polymerization to yield shape-persistent, water-soluble chromophore nanoparticles. Perylene and terrylene diimide derivatives containing either two or four phenyl TFVE functional groups were synthesized and subjected to thermal emulsion polymerization in tetraglyme. Dynamic light scattering measurements indicated that particles with sizes ranging from 70 - 100 nm were obtained in tetraglyme, depending on monomer concentration. The photophysical properties of individual monomers were preserved in the nanoemulsions and emission colors could be tuned between yellow, orange, red, and deep red. The nanoparticles retained their shape upon dissolution into water and the resulting water suspensions displayed moderate to high fluorescence quantum yield, thus making them attractive candidates for bioimaging applications. In Chapter 5, a series of substituted 6,6-dicyanofulvenes (DCFs) were synthesized starting from masked, dimeric or monomeric cyclopentadienones. DCFs lacking sufficient steric bulk around the fulvene core tended to reversibly undergo a [4+2] dimerization. In addition to being highly crystalline, DCFs were darkly-colored compounds due to the presence of weak electronic transitions in the visible region of the electromagnetic spectrum. DCFs displayed two distinct, reversible one-electron reductions by cyclic voltammetry. Based on their high crystallinity and suitable electron affinities, and buoyed by their relatively cheap and straightforward synthesis, DCFs are interesting candidates for organic electron-transport materials.