Elsevier

Journal of Molecular Structure

Volume 1086, 15 April 2015, Pages 216-222
Journal of Molecular Structure

Influence of substituent position on thermal properties, photoluminescence and morphology of pyrene–fluorene derivatives

https://doi.org/10.1016/j.molstruc.2015.01.018Get rights and content

Highlights

  • New position-dependent conjugated hydrocarbon fluorenyl–pyrenes were synthesized by Suzuki cross-coupling reaction.

  • All compounds exhibited deep blue fluorescence with excellent quantum yields (≈78% in dichloromethane).

  • The substituent number and positions affected thermal properties, solubility, optical properties and the morphology.

Abstract

New position-dependent conjugated hydrocarbon dyes containing a pyrene core and multi-fluorene moieties 3 have been synthesized and characterized by 1H/13C NMR spectroscopy, as well as by optical and theoretical studies. The solubility of mono-, bis- and tetra-fluorenyl–pyrene 3 tends to decrease and leads to varied thermal properties. The results of the optical studies and DFT calculations revealed that the energy gap can be easily modified or fine-tuned by either varying the substituent number or position. Remarkably, such pyrene–fluorene materials exhibited deep blue fluorescence (λmax = 400–458 nm in CH2Cl2) with excellent quantum yields (≈78%). These results suggest that these new pyrene–fluorene derivatives have potential application in OLED technology as blue host materials.

Introduction

The study of polycyclic aromatic hydrocarbons (PAH) has attracted considerable attention, due to their intriguing molecular structures [1], [2], [3] and potential application in the emerging area of molecular electronics [4], [5], [6], [7], including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPVs).

Pyrene [8], [9] is a member of the PHA family that may act as an electron donor and electron acceptor [10], and exhibits excellent fluorescence (FL) intensity with good quantum yields in solution. In general, pyrene and its derivatives tend to form dimers in condensed media, this intrinsic phenomena has been widely used for probing the structural properties of macromolecular systems in supramolecular chemistry [11]. On the other hand, pyrene is an ideal organic electro-luminescence material and has found use in efficient OLEDs/OFETs/OPVs devices [8].

Fluorene-based [12], [13], [14], [15] materials have been widely used as promising blue emitter for OLEDs. The fluorene groups are electron-rich aromatic units that can play a role in enhancing the absorption properties and lowering the oxidation potential [16]. Furthermore, the bulk and rigidity of the fluorene moiety is beneficial for improving the color purity and suppresses aggregation and excimer formation. Huck et al. [17] reported an efficient synthetic approach for preparing heterobis-functionalized fully π-conjugated polymers based on fluorene bridges. It was found that the conformation of specific push–pull molecules could exhibit intramolecular interactions by introducing two different functional end groups on both sides of the conjugated polymer.

As mentioned previously, if a pyrene and a fluorene group are integrated into one unit, the construction of functional materials based on the pyrene–fluorene framework not only enriches the category of conjugated hydrocarbons compounds [18], but also provides new perspectives on advanced material applications. For example, when a pyrene moiety is a substituent located at the terminal position of a spirobifluorene [19], [20], this type of material exhibits good electrochemical stability and strong blue emission, both in solution and in the solid state. Thomas et al. [21] reported pyrene–fluorene hybrids in which the pyrene core was connected to fluorene chromophores by acetylene units, and which were useful as color-tunable emitting materials for OLEDs.

In our laboratory, we are focusing on selectively synthesizing pyrene-based deep blue light-emitting materials for OLED applications. Herein, this article presents a series of pyrene–fluorene derivatives that incorporate pyrene with the fluorene moiety linked by a carbon–carbon single bond. Substituents at four position of the conjugated monomer of the pyrene–fluorene derivatives were synthesized by a Suzuki–Miyaura coupling reaction.

Section snippets

General procedures

1H/13C NMR spectra (400 MHz) were recorded on an Agilent NMR System 400 spectrometer and referenced to 7.26 and 77.0 ppm respectively for chloroform-D solvent with SiMe4 as an internal reference: J-values are given in Hz. IR spectra were measured for samples as KBr pellets in a Shimadzu FTIR-8400 spectrophotometer. Mass spectra were obtained with a Nippon Denshi JMS-HX110A Ultrahigh Performance Mass Spectrometer at 75 eV using a direct-inlet system. Elemental analyses were performed by Yanaco

Synthesis

The synthetic route to the four dyes 3ad is displayed in Scheme 1. The bromo-substituted pyrenes (1) were prepared according to the reported procedures; [8], [22], [23] 2-[9,9-bis(3-methylbutyl)-9H-fluoren-2-yl]-4,4,5,5-tetramethyl[1,3,2] dioxaborolane (2) [25] was obtained from 2-bromo-9,9-bis(3-methylbutyl)-9H-fluorene [26] in 70% yield as a colorless powder. For comparison, the absorption and emission properties of 3c are summarized herein [25]. In this system, the branched 3-methylbutyl

Conclusions

This article presents a series of pyrene–fluorene derivatives, which were synthesized in a facile synthetic approach, and which exhibit great stability, considerable solubility, as well as deep blue fluorescence with a high quantum yield. The SEM/TEM data revealed that the intermolecular interactions have been effectively inhibited as the number of fluorenyl groups and substituted position the substituent number and positions varied. We conclude that the incorporation of different substituent

Acknowledgements

This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)”. We would like to thank the OTEC at Saga University and the International Collaborative Project Fund of Guizhou province at Guizhou University for financial support. We also would like to thank the EPSRC (overseas travel grant to C.R.), The Royal Society and The Scientific Research Common Program of

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