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.

About: This article is published in European Journal of Organic Chemistry.The article was published on 2010-04-01 and is currently open access. It has received 12 citations till now. The article focuses on the topics: Triphenylamine.

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.