Photochemical Sciences Ph.D. Dissertations

Design and Synthesis of Organic Materials for Optoelectronics

Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers

Second Advisor

Robert Vincent (Committee Member)

Third Advisor

Thomas Kinstle (Committee Member)

Fourth Advisor

John Cable (Committee Member)

Abstract

The design and synthesis of new π-conjugated organic semiconductors are currently at the forefront of research in order to realize stable and efficient organic electronic devices. Linear polycyclic aromatic hydrocarbons are π-conjugated organic systems widely studied for electronic applications. Pentacenes, for example, are a current choice in Organic Field Effect Transistors (OFETs). But practical applications of this class of compounds have been limited by their air (O2) and light sensitivity and poor solubility in most common organic compounds.

Structural modification increases the stability, improve the solubility, and improve the thin film packing and mobility over the parent hydrocarbon. The electronic, physical, and photophysical properties of pentacenes, can be easily modified by changing the substitution pattern on the main aromatic skeleton.

A comparative study of suitably functionalized, highly soluble tetraceno[2,3-b]thiophenes (3.1-3.3) and pentacenes (3.4-3.6) that show higher photoxidative stability than that of unfunctionalized corresponding acenes is reported. The absorption and emission of 3.1-3.3 (Amax = 624-656 nm, λmax = 634-672 nm, ΦF ≈ 10%) and 3.4-3.6 (Amax = 672-704 nm, λmax = 682-718 nm, ΦF ≈ 10%) were found to be systematically red-shifted by the substitution in the order of the tert-butylethynyl < triisopropylsilylethynyl < phenylethynyl groups. The oxidation potentials of these compounds were similar (E1/2 ≈ 0.70 V), except for 3.4, which showed lower oxidation potential (E1/2 ≈ 0.63 V).

Stability and emission in the solid state are important features of organic compounds to be used as Organic Light Emitting Diodes (OLEDs). The substantial emission of compounds in solution usually becomes weak in the solid state due to both intermolecular energy and electron transfer. Compounds containing an aromatic fumaronitrile core have attracted significant attention as candidates in electroluminescent devices because of their strong emissions in the solid state. Changing the substituents on fumaronitrile core helps to change the emission properties in the solid state. These compounds have higher propensity to form Fluorescent Organic Nanoparticles in appropriate inferior/superior solvent mixtures.

Compounds 5.1 and 5.2 show negative solvatochromic absorption behavior, but show both positive and negative solvatochromic behavior in the fluorescence spectra. In a water/THF mixture, 5.1 as well as 5.2 aggregate into 50-150 nm nanoparticles. The emission of nanoparticles of the new types of fluorescent organic nanoparticles 5.1 and 5.2 is much higher than that of either 5.1 or 5.2 in solution.

The emission spectra and morphologies of 5.3, 5.4, and 5.5 are affected by the water/THF ratio. At a high water/THF ratio (7:1), nanorods (1-3 μm x 80 nm) of 5.4 and nanofibers (0.8-1.5 μm × 100 nm) of 5.5 are observed. Nanoparticles of 5.3 retain a spherical structure. The chemical effects of the electron-donating or electron-withdrawing groups at the para positions are believed to play a major role in the formation of such nanostructures.

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