Original synthesis and spectroscopic study of thiophene triazine derivatives with enhanced luminescence properties

https://doi.org/10.1016/j.saa.2019.117708Get rights and content

Highlights

  • Development of new molecules by concerted metalation deprotonation pathway.

  • Modification of polythiophenes by diaminotriazine to boost emissive properties.

  • Identification of a strongly emissive molecule (ΦF = 0.78 ± 0.05).

  • Molecules Characterization by NMR, IR, UV-Vis, Fluorescence and HRMS.

Abstract

A straightforward access to π-conjugated oligothiophenes bearing amino-rich groups was developed. Palladium-catalyzed C–H arylation applied in the main step of the synthesis allowed to couple 2-thiophenecarbonitriles and aryl bromides with moderate to excellent yields (35–93%). Then, to improve their basic fluorescence properties, these compounds were transformed into their 2,4-diamino-1,3,5-triazine derivatives, also with good to excellent yields (74–98%). UV-Visible absorption and fluorescence studies identified a strongly emissive molecule (fluorescence quantum yield: ΦF = 0.78 ± 0.05), which could find use in sensors for applications in biology and in material chemistry. We observed an antagonistic effect in the spectroscopic properties of oligothiophenes bearing 2,4-diamino-1,3,5-triazine, resulting in improved absorptive and emissive properties for more constrained structures having shorter oligothiophenes chains.

Introduction

Oligothiophenes (OTs) have proven to be one among the most promising functional materials for a wide range of applications. The most notable properties of these compounds are related to their electrical conductivity and light emitting characteristics sensitive to environmental stimuli [1]. Both color and conductivity changes, induced by the same twisting mechanism of their mostly linear molecular structures, have been intensely studied in conducting films for electrochromics [[2], [3], [4]] or in solution for sensing applications [[5], [6], [7]].

2,4-Diamino-1,3,5-triazines (DATs) are polar, nitrogen-containing heterocycles widely used in many domains of chemistry. They are involved in the development of conducting films [[3], [4]], fire resistant resins [8], polymers [9], therapeutic drugs [10], herbicides [11], gas storage [12] and some DATs containing catalysts benefit from their nitrogen-rich content and high stability [13].

As discussed in our former work, the fluorescence properties of OT-DAT compounds might find applications in biology as fluorescent sensitizers, due to their activity towards Prion proteins [14]. The synthesis of OTs bearing amino-rich groups is a challenge due to their lack of solubility in the most of laboratory common solvents, what obviously hinders purification. To catalytically couple thiophenes, conventional cross-coupling reactions as those of Stille, Kumada, Negishi and Suzuki are most frequently used [6,15]. But such protocols are often time-consuming and require expensive and/or toxic organometallic catalysts (e.g. Sn-based).

In order to improve the synthesis of OTs bearing amino-rich groups, we have developed an adapted Fagnou’s approach [[16], [17], [18]] using a palladium-catalyzed C–H arylation to promote C–C bond formation. This pathway allows largely increased reactivity in the concerted metalation deprotonation mechanism (CMD) of the reaction between 2-thiophenecarbonitrile and aryl bromides (Scheme 1). The final compounds synthesized by this approach have been fully characterized and their basic absorption and photoluminescence properties have been determined and compared.

Section snippets

A new and efficient synthetic route for the preparation of OT-DAT compounds

The aryl bromide precursors (Scheme 2 – left side) were commercially available or prepared according to literature data [18]. 5-bromothiophene-2-carbonitrile (1) was obtained by reacting 2-thiophenecarbonitrile with bromine (Br2) and N-bromosuccinimide (NBS) in a solvent mixture of acetic anhydride / acetic acid 50/50 (v/v %) during 14 h at room temperature. The final product was then isolated by column chromatography using cyclohexane as eluent. The 5,5′-dibromo-2,2′-bithiophene (2) was

Conclusions

We report a straightforward access to π-conjugated oligothiophenes and phenylthiophenes bearing amino-rich groups. The compounds were synthesized using a palladium-catalyzed C–H arylation in the main step of the synthesis, leading to nitrile derivatives (36) in moderate to excellent yields. Further cycloaddition reactions with 2-cyanoguanidine led to DAT derivatives (3a-6a) in good to excellent yields. UV-Vis absorption and fluorescence studies were performed with all compounds to determine

Experimental

All reactions were carried out in anhydrous or freshly distillated solvents in oven dried Schlenk flasks under argon atmosphere or air (when not mentioned). Commercial reagents have been used without any further purification.

Analytical thin layer chromatography (TLC) was performed using Merck silica gel 60 F254 pre-coated plates. Chromatograms were observed under UV light at 254 nm and/or 366 nm. NMR spectroscopic data were obtained with a Bruker Advance 300 and 600 MHz and chemical shifts were

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank the “Ministère de l’Éducation Nationale, de l’Enseignement Supérieur et de la Recherche” (MENESR) and The “Ecole Nationale Supérieure de Chimie de Montpellier” (ENSCM) for supporting this work.

Note: The authors declare no competing financial interest.

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