Imidazole derivatives: Thermally stable organic luminescence materials

https://doi.org/10.1016/j.matchemphys.2006.01.025Get rights and content

Abstract

A series of novel imidazole derivatives as optoelectronic materials were designed and synthesized. Photoluminescence (PL) properties of solution were investigated. The thermal analysis result indicated that these organic materials have better thermal stability with better PL properties. Due to the molecular structure introduced by imidazole heterocycles. The molecular structure not only changes the optoelectronic properties of the materials, but also enhances the thermal stability of the materials. The approach is helpful for the systematic investigation of structure–property relationship and thus for the design of novel and efficient organic optoelectronic materials.

Introduction

Organic luminescent compounds (organic luminophors) are still the focus of research investigations in the field of organic electronics, due to their fascinating applications such as electroluminescent devices, sensors, lasers and other semiconductor devices [1], [2], [3]. Organic luminescent compounds, however, still have stability problems and the correlation between structure and fluorescence efficiency is still far from satisfactorily understood. While many chromophores have been designed and synthesized [4], it is still essential to find molecules that exhibit high fluorescence, pure color and high stability. Thermal and chemical instability of organic materials are considered a major factor in the poor operation stability under high-temperature [5], [6], [7], Therefore, approaches for improving the thermal stability of organic molecules without reducing fluorescence are needed. Recently, our group developed a series of blue fluorescent polyflurorene [8] and the related p–n diblock copolymer [9]. We have come to realize that it is possible to improve the thermal stability and tune energy levels by modifying the structure of luminophores.

The idea presented in this paper is to the imidazole and polyflurenes with conjugated chemical connections, leading to through-bond energy transfer. They includes oligofluorene, benzene rings and imidazole rings, because that the imidazole have better thermal stability and the oligofluorene have better luminescence property, the designed organic compounds may be have better thermal stability without reducing fluorescence.

Furthermore, the fluroene unit and benzene rings can efficiently suppress the aggregation of imidazole rings. From simple MOPAC molecular modeling, two benzene rings twist out of the imidazole plane, which may help prevent intermolecular electrostatic interactions between the molecules [10]. Although thus structure may decrease the degree of the conjugation of the molecule, the imidazole moieties are electron rich and electron donating due to the lone–pair electrons in the sp3 nitrogen atoms. As is known an electron donating group contributes to the charge transporting of a molecule [11], [12], [13], so we have designed and synthesized a series of imidazole with oligofluorene segments and investigated the thermal and optical properties.

Section snippets

General information

Chemicals and solvents (AR quality) were purchased from Aldrich Ltd. Tetrahydrofuran (THF) and ether were distilled over sodium. Dimethyl formamide (DMF) was dried over molecular sieves. Column chromatography was carried out on silica gel (200–300 mesh). 1H NMR spectra were recorded in d-DMSO solution on a 400 MHz Varian Unity 400 spectrometer. TGA was undertaken on a Shimadzu DTG-60H simultaneous DTA-TG apparatus with a heating rate of 5 °C min−1 in the nitrogen atmosphere. UV–vis spectra were

UV–vis and photoluminescence spectra

Fig. 2 shows the absorption and PL spectra of 14 in dioxane solution. In the their UV–vis absorption spectra, the absorption of 14 around 338–365 nm. The normalized emission spectra of 14 are in the ultraviolet and blue regions i.e. 400–433 nm. Such a blue-shift compared with polyflurenes, that attributed to the two benzene rings twisted out from the imidazole plane in the molecules, which leads to an increased torsion of the conjugated chain and a decreased effective conjugation length. On

Conclusion

We have presented a convenient and efficient approach towards soluble imidazole derivatives with oligofluorene. The emission spectra of the materials in dioxane solutions can be easily tuned by varying the number of fluorine rings. The thermal stability of the organic materials was investigated by TGA-DTA. It was found that the imidazole rings can help improve the thermal stability of the organic materials and further help investigated the structure–property relationship to the design of novel

Acknowledgements

The work was financially supported by Shanghai Commission of Science and Technology under Grants 022261042 and 0216nm040, National Natural Science Foundation of China under Grant 60477037.

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