Date of Award

12-2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Foulger, Stephen H

Committee Member

Ballato , John M

Committee Member

Kornev , Konstantin

Committee Member

Luzinov , Igor

Abstract

The development of charge–transporting and fluorescing colloidal particles that can be directly printed into electroluminescent devices may result in a broad impact on the use of electrical energy for illumination. The objective of this work was to design and synthesize electroactive & fluorescing colloidal particles; establish their optical, electronic, and thermodynamic properties; and transition them into a device format for potential applications. The original intended application of this work was to build “better” colloidally–based organic light emitting devices (OLEDs) by creating functional particles with superior electrical and optical performance relative to commercially available technologies, but through the course of the research, the particles that were developed were found to be better suited for medical applications. Nonetheless, the global objective envisioned at the onset of this research was consistent with its final outcomes. The research tasks pursued to accomplish this global objective included:
(1) The design and synthesis of electroactive moieties and their conversion into organic light emitting devices; An electron–transporting monomer was synthesized that was structurally & energetically similar to the small molecule 2–biphenyl–4–yl–5–(4–tert–butylphenyl)–1,3,4–oxadiazole (tBu–PBD). The monomer was copolymerized with 2–(9H–carbazol–9–yl)ethyl 2–methylacrylate (CE) and the resulting copolymer was utilized in OLEDs which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2–phenylpyridine)iridium(III) [Ir(ppy)3] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3,552 cd/m2 with the C6 and Ir(ppy)3 emitters, that were stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9–vinyl–9H–carbazole)/tBu–PBD blend devices exhibited more pronounced variations in performance with thermal aging.
(2) The surface–modication of colloids with electroactive & fluorescing moieties via “click” chemistry; Aqueous–phase 83 nm poly(propargyl acrylate) (PA) nanoparticles were surface–functionalized with sparingly water soluble fluorescent moieties through a copper(I)–catalyzed azide–alkyne cycloaddition
(CuAAC) (i.e., “click” transformation) to produce fluoroprobes with a large Stokes shift. For moieties which could not achieve extensive surface coverage on the particles utilizing a standard click transformation procedure, the presence of β–cyclodextrin (β–CD) during the transformation enhanced the grafting density onto the particles. For an oxadiazole containing molecule (AO), an azide–modified coumarin 6 (AD1) and a polyethylene glycol modied naphthalimide–based emitter (AD2), respectively, an 84%, 17% and 5% increase in the grafting densities were observed, when the transformation was performed in the presence of β–CD. In contrast, a carbazolyl–containing moiety
(AC) exhibited a slight retardation in the final grafting density when β–CD was employed. Photoluminescence studies indicated that AC & AO when attached to the particles form an exciplex. An efficient energy transfer from the exciplex to the surface attached AD2 resulted in a total Stokes shift of 180 nm for the modified particles.
(3) The synthesis and characterization of near–infrared (NIR) emitting particles for potential
applications in cancer therapy. PA particles were surface modified through the “click” transformation of an azide–terminated indocyanine green (azICG), an NIR emitter, and poly(ethylene glycol) (azPEG) chains of various molecular weights. The placement of azICG onto the surface of the particles
allowed for the chromophores to complex with bovine serum albumin (BSA) when dispersed in PBS that resulted in an enhancement of the dye emission. In addition, the inclusion of azPEG with the chromophores onto the particle surface resulted in a synergistic nine–fold enhancement of the fluorescence intensity, with azPEGs of increasing molecular weight amplifying the response.
Preliminary photodynamic therapy (PDT) studies with human liver carcinoma cells (HepG2) combined with the modified particles indicated that a minor exposure of 780 nm radiation resulted in a
statistically signicant reduction in cell growth.

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