New fluorene-acceptor random copolymers: Towards pure white light emission from a single polymer

Two kinds of fluorene-acceptor copolymers containing three emitting segments have been synthesized and explored as the emissive layer in white-fight-emitting devices (WLEDs). The light-emitting copolymers PFQTP and PFBTTP either contain blue-emitting (9,9-dihexyl-fluorene, F), green-emitting (quinoxaline, Q) (or yellow-emitting (2, 1, 3-benzothiadiazole, BT)), and red-emitting (thieno[3,4-b]-pyrazine, TP) units in the backbone. The energy-transfer processes between the emitting units are investigated by their absorption, photoluminescence, time-resolved photoluminescence, and electro-luminescence. The experimental results suggest that only a relatively small faction of the acceptor moiety incorporated into the fluorene can achieve white light emission through energy transfer. The energy transfer from BT to TP is more efficient that that of Q to TP because of the degree of spectroscopic overlap between absorption and luminescence. The electroluminescent device with PFBTTP1 (0.1% of BT and 0.25% of TP) as an emissive layer exhibits the Commission Internationale de L'Eclairage (CIE) 1931 coordinates of (0.33, 0.34), a luminance of 1870 cd(.)m(-2) under the condition of maximum luminance yield of 1.92 cd(.)A(-1) and without significant variation with driving voltages. The present study suggests that the single polymers based on three fluorene-acceptor segments could be potentially used for pure-white-light-emitting diodes.