New class of light-emitting polymers/oligomers

Novel electro-/photoactive multifunctional dyes were synthesized based on dithienothiophene, DTT (π-center), linked by an electron D/D -or D/A pair segment; D-π-D (dye 1) or D-π-A (dye 2), to develop efficient bipolar light-emitting (LE) materials capable of balanced electron/hole capture in a single-layer LE device. A strong bipolarity manifested by electrochemical amphotericity was observed notably with dye 1, which cannot be accounted for without the involvement of the n-center, leading to the low HOMO/LUMO energy-gap and a small difference in the energy-gap between dye 1 and dye 2. Single-layer LE devices were fabricated by making a blend based on the dye (0.5 wt%), PVK (70%) as matrix and PBD (30%) as electron-transporting/hole blocking material, and by sandwiching between ITO and Al electrodes. PL intensity of the dyes by excitation at 440-450 nm (λ_max) is much weaker compared to that at 340 nm (exciting mainly PVK), suggesting that the excitation of the dyes occur by energy-transfer from PVK. When compared PL intensities between the two dyes at a concentration <1%, dye 1 is slightly higher relative to dye 2. In contrast, the EL intensity shows an opposite trend in that dye 2 is much higher relative to dye 1 under the same condition. This implies that unlike the PL, the EL is not due to the transfer of the excited-energy from PVK but carrier trapping/exciton formation on the dye chromophore, and that dye 2 is strongly bipolar and thus more capable of balanced carrier capture.