Title

Design, Synthesis, and Properties of New Derivatives of Pentacene and New Blue Emitters

Date of Award

2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers

Abstract

The practical uses of pentacene, the most promising polyacene in the area of organic field-effect transistors, are limited by its sensitivity to oxygen and poor solubility in organic solvents. To overcome these disadvantages, new organic semiconductors, 2,3,6,9,10,13-hexa(trimethylsilylethynyl)pentacene, 2,3,9,10-tetrakis(triisopropylsilylethynyl)-6,13-bis(trimethylsilylethynyl)pentacene, and 2,3,9,10-tetrakis(3,5-di-t-butylphenylethynyl)-6,13-bis(trimethylsilylethynyl)pentacene were designed, synthesized, and fully characterized.

In comparison with pentacene, they are stable and soluble and have extended pi-conjugation. Each demonstrates reduction potentials that are essentially unchanged, but different oxidation potentials. The HOMO-LUMO gaps estimated from the electrochemical data are ca. 1.7 eV, which is among the lowest HOMO-LUMO gaps reported in the literature for pentacene derivatives. The compounds fluoresce in the deep red region, with the width at half height being less than 40 nm. In thin films the compounds show strong aggregation and adopt a favorable stacking geometry with the molecular long axis upright to the substrate surface.

Modifying pentacene at the central and terminal rings with ethynyl moieties proves viable in synthesizing new pentacene derivatives and tuning their electronic properties. The three new pentacene derivatives may be useful for electronic devices such as organic field-effect transistors or organic light-emitting diodes.

Six novel D-π-D fluorophores were also synthesized. They are 1,2,3-trimethoxy-5-(2-{4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, 5-(2-{3-fluoro-4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)-1,2,3-trimethoxybenzene, 1,2,3-trimethoxy-5-[2-(4-{4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}phenyl)-1-ethynyl]benzene, 1,2,3-trimethoxy-5-(2-{3-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, 1,2,3-trimethoxy-5-(2-{2-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, and 9,10-di[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]anthracene.

These compounds fluoresce in the blue-violet region in both solution and the solid state. In solution, they exhibit significant positive solvatochromism in fluorescence, which is ascribed to the intramolecular charge transfer upon excitation. The last D-π-D fluorophore is an exception: it exhibits insignificant solvatochromism in solution and in the solid state, only weak fluorescence in the green-yellow region was observed, which is probably due to aggregation. The last D-π-D fluorophore also shows strong two-photon excited fluorescence in the blue region with a two-photon absorption cross-section of 124 GM. The others do not.

These six D-π-D fluorophores may be useful in photonic applications where blue light is desired.