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 (Φ _PLS) 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 Φ _PLS 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/m ²) with maximal current efficiency of 1.17 cd/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 5135 cd/m ² and current efficiency of 1.76 cd/A were achieved with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.09).