Influences of fluorine auxochrome in fluorene molecular electroluminescent material on optoelectronic property

The UV-Vis absorption and photoluminescence (PL) spectra of 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl) quinoxaline (Py), 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)-6,7-difluoroquinoxaline (F2Py) and 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)-5,6,7,8-tetrafluoroquinoxaline (F4Py), which are fluorene molecular derivatives with conjugated structure, were investigated. For further investigation of the influences of fluorine auxochrome in fluorene molecular electroluminescent material on optoelectronic property, the electroluminescence (EL) characteristics of materials were studied by double-layer organic light-emitting diodes (OLEDs) using N,N′-Di-(1-naphthalenyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (NPB) as a hole transporting layer (HTL) with the conventional vacuum deposition method. The results showed that the absorption and PL spectra of materials in film state red shifted with fluorine substituents increased in molecule configuration. The performance of OLEDs is as follows: at a bias voltage of 5 V, Py emitted a blue-green light at 508 nm with the Commission Internationale d’Eclairage (CIE) coordinates of (0.23, 0.43) and full width at half maximum (FWHM) of 100 nm. The device had a turn-on voltage (defined as the drive voltage at the luminance of 1 cd/m²) of 4.8 V, a luminance of 129 cd/m² with a current density of 59 mA/cm² at 10 V, and a maximum luminous efficiency of 0.18 lm/W at 5.4 V. F2Py and F4Py emitted a green light peaking at 544 nm and a yellow light at 570 nm at 5 V, with the CIE coordinates of (0.38, 0.56) and (0.44, 0.49), and FWHM of 103 and 117 nm, respectively. The F2Py and F4Py devices had a turn-on voltage of 4 and 2 V, a luminance of 557 and 3300 cd/m² with a current density of 100 and 880 mA/cm² at 10 V, and a maximum luminous efficiency of 0.22 lm/W at 7.6 V and 0.53 lm/W at 2 V, respectively. Supported by the National Natural Science Foundation of China (Grant Nos. 60425101, 20674049), Program for New Century Excellent Talents in University (Grant No. NCET-06-0812) and Young Excellence Project of UESTC (Grant No. 060206)