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A series of new oligofluorene-based push-pull type blue light-emitting
functional materials, namely,
2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9-dihexylfluorene (F1),
7-(9H-carbazol-9-yl)-7'-(4-cyanophenyl)-2,2'-bi(9,9-dihexylfluorene) (F2),
7-(9H-carbazole-9-yl)-7''-(4-cyanophenyl)-2,2':7',2''-ter(9,9-dihexylfluorene)
(F3), and
7-(9H-carbazole-9-yl)-7'''-(4-cyanophenyl)-2,2':7',2'':7'',2'''-quarter(9,9-dihexylfluorene)
(F4) were synthesized and characterized. Their onset decomposition
temperatures for the thermal bond cleavage and the glass-transition
temperatures were in general increased with increasing number of fluorene
units. In dilute toluene solution, the oligofluorenes exhibited main
absorption peaks in the range of 343-370nm, photoluminescence maxima from
403 to 410nm, and absolute quantum yields (@F"P"Ls) of higher than 87%. In
contrast, the absorption spectra of these compounds in the thin films had
no large differences from those in the solutions except for the slight peak
red-shifts (2-8nm). The main emission maxima of F1, F2, and F3 in the thin
films were located at 418-420nm, while the main emission of F4 was found to
be shifted to 446nm, followed by a shoulder peak at 421nm. The @F"P"Ls of
these thin films were estimated in the range of 59.2-68.7%. The existence
of the electron-pull and -push end groups could effectively tune the energy
levels of the oligofluorenes. By using the organic light emitting device
(OLED) configuration of ITO/PEDOT:PSS/oligofluorenes/TPBi/LiF/Al by
solution-process, F4 displayed the best performance: the lowest turn-on
voltage (4.1V) and highest maximum luminance (2180cd/m^2) with maximal
current efficiency of 1.17cd/A. When F4 was fabricated into the optimized
device of ITO/MoO"3/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition,
highest brightness of 5135cd/m^2 and current efficiency of 1.76cd/A were
achieved with the Commission Internationale de l'Eclairage (CIE)
coordinates of (0.16, 0.09).
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