Synthesis and characterization of luminescent copolymers containing iminodibenzyl and divinylbenzene chromophores

New conjugated copolymers containing alternating N-hexyl-3,8-iminodibenzyl and divinylbenzene chromophores {poly(N-hexyl-3,8-iminodibenzyl-1,2-ethenylene-2,5-dihexyloxy-1,4-phenylene-1,2- ethenylene) (P1) and poly[N-hexyl-3,8-iminodibenzyl-2,5-bis(hexyloxy)cyanoterephthalidene] (P2)} were synthesized according to Wittig and Knoevenagel polymerization. A copolymer containing alternating carbazole and divinylbenzene derivatives {poly[9-(2-ethylhexyl)-3,6-carbazole-1,2-ethenylene-2,5-dihexyloxy-1,4-phenylene -1,2-ethenylene] (P3)} was also synthesized for comparison. The copolymers were soluble in common organic solvents such as tetrahydrofuran and toluene. Absorption and photoluminescence measurements revealed that cyano substitution at the vinylene moiety in P2 brought about a significant bathochromic shift and led to an electroluminescence color change from green to orange. The band edge energies of the copolymers were estimated from cyclic voltammograms and optical band P1 and P3 showed similar highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, indicating that the electron-donating abilities of the iminodibenzyl and carbazole chromophores were comparable. However, compared with those of P1 and P3, the HOMO and LUMO levels of P2 were greatly reduced because of conjugating and electron-withdrawing CN groups. The threshold electric field of an Al/P1/ITO glass single-layer light-emitting diode was approximately 10 × 10⁵ V/cm, whereas those for P2 and P3 were 7.5 and 16 × 10⁵ V/cm, respectively. The electroluminescence emission maxima of P1-P3 were 498, 514, and 559 nm, respectively.