Oligofluorene-based push–pull type functional materials for blue light-emitting diodes

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 343–370 nm, photoluminescence maxima from 403 to 410 nm, and absolute quantum yields (ΦPLs) 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 (2–8 nm). The main emission maxima of F1, F2, and F3 in the thin films were located at 418–420 nm, while the main emission of F4 was found to be shifted to 446 nm, followed by a shoulder peak at 421 nm. The ΦPLs 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 (4.1 V) and highest maximum luminance (2180 cd/m2) with maximal current efficiency of 1.17 cd/A. When F4 was fabricated into the optimized device of ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition, highest brightness of 5135 cd/m2 and current efficiency of 1.76 cd/A were achieved with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.09).