3D Monodisperse Oligofluorenes with Non-Conjugated Triphenylamine-Based Cores: Synthesis and Optoelectronic Properties

TL;DR: Two 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel-Crafts copolycondensation reaction and emit deep-blue fluorescence with high efficiencies in thin films.

Abstract: Two 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel-Crafts copolycondensation reaction. The oligomers, PF 3 -TPA and PF 3 -TPA 3 , consist of three fluorene pentamer arms that are connected non-conjugately through a triphenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene core (TPA 3 ), respectively, at the 9-position of the central fluorene of the pentafluorene arms. The coplanar structures of the cores and the linkages at the centre of the pentafluorene arms produced a 3D structure of oligomers. This specific structure efficiently retarded the crystallization tendencies of the pentafluorene arms and gave the materials completely amorphous morphological structures. Both oligomers emit deep-blue fluorescence with high efficiencies in thin films (Φ pl-film = 67 % for PF 3 -TPA 3 and 86 % for PF 3 -TPA). The introduction of triphenylamine units into the core promoted the hole-injection ability while not obviously scarificing the electron-injection ability of the oligomers. The multi-layer devices ITO/PEDOT-PSS/PF 3 -TPA 3 and PF 3 -TPA/TPBI/LiF/Al were fabricated to investigate the electroluminescence (EL) properties of the two oligomers. Both oligomers showed a low turn-on voltage of 4 V. The luminances reached 1946 cd/m 2 at 7.5 V in the PF 3 -TPA 3 device and 1055 cd/m 2 at 8 V in the PF 3 -TPA device. The EL efficiencies at this luminance were 1.63 and 1.57 cd/A, respectively.

Summary

Introduction

• In the past decade, fluorene-based conjugated polymers have emerged as a very promising class of blue-light-emit- ting materials for use in polymer light-emitting diodes because of their high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies, thermal sta- bility, good solubility and facile functionalization at the 9- position of fluorene. [1].
• Some researchers have demonstrated that the introduction of bulky groups at the 9-position of fluorene or introduction of cross-linkable moieties tended to suppress this emission and to improve the thermal stability of the PL spectra.[3].
• Inspired by these results, the authors recently synthesized monodisperse triphenylamine-substituted oligofluorenes[10] (shown in Scheme 1) in which the triphenylamine cyclic core serves as a non-conjugated spacer bearing oligofluorene arms in a multi-H shaped structure of oligomers.

Synthesis and Characterization

• The synthetic procedures used to prepare PF3-TPA3 and PF3-TPA are outlined in Scheme 3 and Scheme 4 respectively.
• Scheme 4. Synthetic route for the preparation of the steric monodisperse oligofluorene with a non-conjugated TPA core (PF3-TPA).
• The structures and monodispersities of PF3-TPA3 and PF3-TPA were identified by NMR, size-exclusion chromatography (SEC) and MALDI-TOF mass spectroscopy.
• The peaks at δ = 2.29 (6 H, singlet) and 2.05 ppm (16 H, multiplet) are due to the resonance of the CH3 groups in the TPA unit and the methylene protons of the octyl groups adjacent to C-9 of the fluorene units, respectively.
• The Mn values determined from the SEC analysis were based on polystyrene standards.

Thermal and Photophysical Properties

• The thermal properties of PF3-TPA3 and PF3-TPA were characterized by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and the results are summarized in Table 1.
• In addition, there is a negligible shift in the absorption and PL spectra for both steric oligomers on changing from the solution to solid state, which indicates the absence of strong interchain interactions in the solid-state films.
• The PF3-TPA3 film showed lower stability in the n-doping process.
• This result indicates that introducing a non-conjugated triphenylamine X. Zhang, Y. Quan Z. Cui, Q. Chen, J. Ding, J. LuFULL PAPER unit core into the oligomers promotes their hole-injection capability, but does not sacrifice their electron-injection capability.

Conclusions

• Two 3D steric monodisperse oligofluorenes with a TPA3 and TPA core (PF3-TPA3 and PF3-TPA) have been synthesized by the Friedel-Crafts copolycondensation reaction.
• The triphenylamine-based cores serve as non-conjugated spacers bearing oligofluorene arms in a multi-H-shaped structure of oligomers such that the optoelectronic proper- ties of the individual oligofluorene arms remain relatively unperturbed.
• Both oligomers show excellent thermal sta- bility and high photoluminescence quantum efficiency.
• Electrochemical analysis showed that the non-conjugated triphenylamine core promotes the hole-injection capability of the oligomers, but does not sacrifice their electron-injec- tion capability.
• LED devices based on these two oligomers showed comparable performances with respect to tri- phenylamine-substituted linear polyfluorene derivatives in solution-processed light-emitting devices.