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Organic luminophores in molecular and polymeric materials
This work has involved designing and synthesizing conjugated molecular and polymeric materials which are highly functionalized through structural modifications in order to enhance their electronic, photonic and morphological properties. The main objective is to synthesize novel organic luminophores which have efficient photoluminescence, as well as to optimize electroluminescence properties for organic light emitting diodes (OLEDs). ^ A series of well-defined 2,7-bis(phenylethenyl)fluorenes/fluorenones (OFPVs/OFOPV) and 2,7-bis(phenyl)fluorenes/fluorenones (OFPhs) have been synthesized. The main synthetic methodologies used are palladium-catalyzed Heck coupling, Suzuki coupling and Knoevenagel condensation. The structure-property relationships---especially the effect of the fluorenone moiety on the photophysical and electroluminescent properties of these OFPVs, OFPhs and OFOPVs---were systematically investigated. The origin of an unwanted low-energy emission (g-band) in fluorene derivatives was evaluated by single-molecule and controlled luminophore/impurity blending photoluminescence studies (SMPL). ^ OFPV and its segmented copolymer, poly-FPV (1-2) exhibit highly blue fluorescence in chloroform solution with quantum efficiencies of 0.93 and 0.68 respectively. Electroluminescence characteristics in an LED configuration ITO/PEDOT-PSS/(1 or 2)/Ca-Al of both were evaluated and OFPV gave much better EL performances with compared to its analogous segmented copolymer. The luminance efficiency of LEDs with OFPV was over 10-fold higher than those with copolymer 2, 0.515 versus 0.040 cd/A, with turn-on voltages of 3 and 5 V, respectively. Blends of 1 with PMMA gave stronger luminance than neat 1, with only a modest increase in turn-on voltage. A 10%(w/w) 1:PMMA based LED showed a maximum luminance of 450 cd/m2 at 7 V, with a luminance efficiency of 4.5 cd/A and a turn on voltage of 4.5 V. ^ Among the structurally functionalized OFPVs, terminal heptyloxy substitution to make OFPV (8) gave the best electroluminescence performance as a blue emitter. It shows maximum luminance efficiency of 1.02 cd/A with maximum brightness of 1500 cd/m2 at 12 V. Unlike the OFPVs, shorter conjugation length OFPhs gave poor LED performance with an undesired longer wavelength green component at 500-550 nm in EL spectra. Blending the OFPhs with PMMA eliminated this long wavelength component and gave better color purity blue LED emission up to 25%(w/w) blends, even though these still gave poor overall LED emission efficiencies compare to the OFPVs.^
Hemali P Rathnayake,
"Organic luminophores in molecular and polymeric materials"
(January 1, 2007).
Electronic Doctoral Dissertations for UMass Amherst.