Synthesis and photophysical characteristics of 2,7-fluorenevinylene-based trimers and their electroluminescence

Three new 2,7-fluorenevinylene-based trimers were synthesized and characterized. The synthesis was carried out by the Heck coupling reaction of 9,9-dihexyl-2,7-divinylfluorene with 2-(4-bromophenyl)-5-phenyl-1,3,4- oxadiazole, N,N-diphenyl-4-bromoaniline, or 3-bromopyrene to afford the trimers OXD, TPA, and PYR, respectively. All the trimers were readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, and toluene. Their glass transition temperatures ranged from 33 to 60°C. The UV-vis spectra showed an absorption maximum at λ_a,max = 379-417 nm with optical band gap of E_g = 2.47-2.66 eV. In solution, they emitted strong blue-green photoluminescence (PL) with PL maximum at λ_f,max = 455-565 nm and fluorescence quantum yield of Φ_f = 0.65-0.74. On the other hand, in their spin-coated films, the PL efficiencies significantly decreased due to the presence of concentration quenching. All samples showed nanosecond transient lifetime containing two components, suggesting excimer formation. The organic light-emitting diodes (OLEDs) with OXD and TPA showed green emission with electroluminescence (EL) quantum efficiencies of η_EL ∼ 10^-2%, while very weak EL efficiency of η_EL ∼ 10^-5% was observed with PYR. The highest occupied molecular orbital (HOMO) levels of the films were found to be 5.05-5.75 eV.