Title

Carbazole-Based Emitting Compounds: Synthesis, Photophysical Properties and Formation of Nanoparticles

Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers

Second Advisor

Thomas Kinstle (Committee Member)

Third Advisor

John Cable (Committee Member)

Fourth Advisor

Miner Jeffrey (Committee Member)

Abstract

Carbazole is a heterocyclic tricyclic aromatic organic compound consisting of two six-membered benzene rings fused on either side of a five-membered nitrogen-containing ring. A large number of carbazole derivatives have been designed and synthesized and organic electronic devices based on these derivatives such as organic light emitting diodes (OLEDs), have been investigated. Ever since Tang and VanSlyke constructed electroluminescent (EL) devices using organic materials as emitters, development of efficient and stable EL materials has taken good momentum. Carbazole derivatives are widely used as materials for EL and hole-transporting layers of OLEDs which use their high charge mobility. The optical and electrical properties of carbazoles are affected by substitution on the 2-, 3-, 6-, 7- and 9H-positions. Many carbazole derivatives have sufficiently high triplet energy to make them an efficient host where they can serve as red, green, or blue triplet emitters. Highly fluorescent and stable carbazole-based compounds were synthesized and characterized. Substitution of carbazoles at 3- and 6- position by tert-butyl group enhanced the solubility. They showed high extinction coefficients of absorption (Amax 328-353 nm) and quantum yields of fluorescence ( max 386-437 nm; FF 0.72-0.89; F 2.09-3.91 ns) in dichloromethane. The quantum yields of fluorescence of these compounds in the solid state were also high ( max 385-422 nm; FF 0.40-0.85). Simple synthetic procedures were developed to prepare other stable carbazoles. These compounds emit blue, green, and orange-red light. The red-shifted emission in the solid state which can be as much as 120nm relative to that in solution is highly dependent on the nature and positions of the substituents.. The presence of a carbaldehyde or malononitrile on the carbazole moiety quenches fluorescence severely in solution and in the solid state. However, the effect is not the same for the fluorescence lifetime. Lowering the temperature from 25 0C to -10 0C causes a small but distinct red-shift in the emissions and a systematic increase in the FF values of blue and green emitters. A considerable edge excitation red shift was observed for some of these compounds. The emission of some of these compounds in solution showed both specific and general solvent effects. Nanoparticle research is currently an area of intense scientific work, due to the wide variety of potential applications in biomedical, optical, and electronic fields. Today nanotechnology has been used in various fields ranging from optoelectronic devices to sensors, in biological imaging as well as in third-order non-linear optics. A nanoparticle is a small object, sized between 1 and 100 nanometers, that behaves as a single whole in terms of its transport and properties. Suitably susbtituted carbazoles form highly stable fluorescent organic nanoparticles. The emission of these nanoparticles was reversibly switched on/off in the blue-green and orange-red regions from a change in the ratio of the tetrahydrofuran/water system used in their preparation. The size of the nanoparticles was depends on the solvent ratio and the emissions were significantly red shifted compared to those of dilute solutions in tetrahydrofuran. Similarly, highly stable composite fluorescent organic nanoparticles (CFONs) were prepared by co-reprecipitation of blue and red emitting carbazole-based organic compounds from water/tetrahydrofuran mixtures. SEM images showed diversity in particle size. Emission spectra of CFONs prepared from different ratios of red and blue emitters covered the entire visible region from 400 to 700 nm. Confocal microscopy measurements revealed composite organic nanoparticles emitting a white light. CIE coordinates of these CFONs demonstrated high color purity (CIE X, Y: 0.34, 0.35). These data indicate that these compounds and their nanoparticles have potentials as emitting m...