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Optimizing LED properties of 2,7-Bis(phenylethenyl)fluorenes

2006-01-24
RATHNAYAKE, Hemali P.
Çırpan, Ali
LAHTI, Paul M.
KARASZ, Frank E.
(E,E)-2,7-Bis(3,4,5-trimethoxyphenylethenyl)fluorene 1, and a segmented copolymer 2 composed of the same chromophore alternated with nonconjugated 1,6-hexanediyl (alt-oligo(2,6-dimethoxylphenylene-4-vinylene-[9,9-diethylfluoren-2-yl-7-vinylene]-3,5-dimethoxy-phenylene-4-[1,6-hexanedioxyl]) were synthesized. They have solution photoluminescence emission maxima at 420-460 nm, with quantum efficiencies of 0.93 and 0.68, respectively, in chloroform. Electroluminescent spectra in an LED configuration ITO/PEDOT-PSS/(1 or 2)/Ca-Al both showed maxima at 470-480 nm, although the spectrum from 2 was significantly broader. The luminance of LEDs with 1 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. The crystallography of I showed no chromophore pi-stacking; this absence should limit tendencies for emission wavelength shifts due to solid state interchromophore interactions. When 1 was heated in air before incorporation into an LED emissive layer, a 540 nm emission component was produced, which did not occur if 1 was not heated before use. Emissive layers of 1 with PMMA gave stronger luminance than either neat 1 or 2, with only a modest increase in turn-on voltage. A 10% (w/w) I:PMMA based LED emission showed a maximum at 444 nm (blue emission with CIE color coefficients of (0.153, 0.312)), with a luminance efficiency of 4.5 cd/A and a turn on voltage of 4.5 V.