Title

Carbazole-Based, Self-Assembled, Π-Conjugated Systems As Fluorescent Micro And Nanomaterials - Synthesis, Photophysical Properties, Emission Enhancement And Chemical Sensing

Date of Award

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers, Ph.D (Committee Chair)

Second Advisor

Thomas Kinstle, Ph.D

Third Advisor

Marshall Wilson, Ph.D

Fourth Advisor

Mikhail Zamkov, Ph.D

Abstract

Fluorescent micro- and nanomaterials based on π-conjugated systems have gained considerable attention during last two decades in biological applications, optoelectronic devices and chemical sensing .π-Conjugated systems serve as highly fluorescent materials and are used extensively. While nano and micrometer-sized crystals often exhibit different physical properties relative to macrocrystalline solids, strong fluorescence from such assemblies is uncommon. Aggregation caused by strong intermolecular π-π stacking interactions in the solid or aggregated states can often cause emission to be quenched. Some compounds exhibit enhanced fluorescence in the aggregated or solid state that differs from the expected, common fluorescence quenching.

We have investigated the carbazole-based, π-conjugated organic compounds for their self-assemble properties as microcrystals and nanoparticles as well as potential application as detection in oxidative chemical species like Cr(VI) in water. We have reported the Aggregation-induced enhanced emission (AIEE) for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 μm-sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue shift in emission and enhances the intensity. X-ray cryatallographic analysis revealed key factors causing high luminescence efficiency in the crystal are a lack of strong co-facial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seems to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for unusual emission blue shift in the aggregate.

lately fluorescent, inorganic semiconductors or metal nanoparticles have been extensively studied for various potential applications such as in fluorescent biological labels, photovoltaic cells, light emitting diodes (OLED) and optical sensors. Even though fluorescent organic nanoparticles (FONs) are expected to show higher potentials due to their variability, flexibility in synthesis and ease of nanoparticle preparation, the detailed investigation of such FONs is still in the initial stages. Contrary to the inorganic or metal nanoparticles, FONs normally possesses a critical drawback against a highly fluorescent emission, because the emission of organic fluorophore is often quenched by aggregate formation in the solid state. Intermolecular vibronic interaction such as excitonic coupling, excimer formation, and excitation energy migration to impurity traps are some of the reason that responsible for this.

We have investigated the possibility of carbazole-based cyanostyryl benzene derivatives to self-assemble as highly fluorescent organic nanoparticals with color-tuned emission. Four novel carbazole-based compounds: PPPCB, (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(4-(9H-carbazol-9-yl)phenyl)-acrylonitrile), PPPt-BuCB, (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(4-(3,6-di-tert-butyl-9-H-carbazol-9-yl)phenyl)acrylonitrile), TPTCB, (2Z,2'Z)-2,2'-(1,4-phenyl-ene)bis(3-(5-(9H-car-bazol-9-yl)thiophen-2-yl)acrylonitrile) , and TPTt-BuCB, (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(5-(3,6-di-tert-butyl-9H-carbazol-9-yl)thiophen-2-yl) acrylonitrile were synthesized and characterized by means of steady-state UV-Vis and fluorescence spectroscopy. Such compounds are introduced as probes for Aggregation Induced Enhanced Emission (AIEE), where the emission color was effectively tuned throughout the visible spectrum. Emission enhancement factors as high as 88 were obtained in the aggregate state.

Although fluoresce...