Synthesis, properties, and solid-state assemblies of β-alkyl-substituted dipyrrolyldiketone BF2 complexes

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Abstract

In this paper, the synthesis and solid-state structures of β-alkyl-substituted dipyrrolyldiketone BF2 complexes are discussed; these complexes can function as π-conjugated acyclic anion receptors (“molecular flippers”) and are potential building subunits of anion-responsive π-conjugated oligomers or polymers. Modes of the solid-state assemblies in these complexes determined by performing single-crystal X-ray analysis are found to depend on the lengths of the alkyl chains at the β-positions. A β-methyl-substituted receptor has a highly planar structure, which efficiently forms stacking assemblies in the core π-plane in the solid state, while a β-ethyl-substituted receptor shows no stacking assemblies due to the presence of bulky ethyl moieties, which form only dimeric structures by hydrogen bonding. β-Alkyl-substituted anion receptors can be used not only as monomer units of stimuli-responsive polymeric macromolecules but also as components of solid-state electronic materials.

Introduction

Recently, π-conjugated polymers comprising hetero-aryl moieties have attracted considerable attention due to their tunable conducting and photophysical properties. Among π-conjugated compounds, pyrrole is a well known compound and it shows unique electrical and optical properties when it is organized by means of covalently linked polymerization (oligomerization) and following interpolymer aggregation by noncovalent interactions [1]. The synthesis of pyrrole-based anion receptors has been investigated in the past, and many of them have low planarity; therefore, they are not suitable for forming stacking assemblies [2]. Among π-conjugated acyclic anion receptors with flat structures, boron complexes [3], [4] (“molecular flippers”, e.g., 1a–e and 2a,b in Fig. 1a) of 1,3-dipyrrolyl-1,3-propanediones [5] act as efficient acyclic anion receptors to form the negative-charged planes by binding an anion with inversion of pyrrole rings (Fig. 1b). Various assemblies can be formed by using the planar structures as anion-free receptors and anion complexes; the transition between planar states is one of the characteristics of molecular flippers. For example, α-alkyl-substituted 1b,c show different assembly modes – a slipped infinite stacking structure and a face-to-face dimer, respectively – in the solid state [3f]. Further, α-aryl-substitution affords extended π-conjugation and an enlarged stacking area and also connects various substituents [4]. α-Aryl-substituted receptors with long alkoxy chains (e.g., 1d) have shown the formation of anion-responsive supramolecular octane gels based on the π–π and van der Waals interactions [4], [6]. Further, amphiphilic receptors such as 1e form solvent-assisted organized structures such as those of nanoscale networks and vesicles, depending on the peripheral chains [4d]. Therefore, it is essential to understand the stacking systems in the π-planes of molecular flippers in rigid crystals in order to design and fabricate functional supramolecular assemblies. Here, we focus on β-alkyl-substituted receptors, 2a,b, as the building subunits of π-conjugated polymers and oligomers. We have discussed β-ethyl 2b, which has reactive pyrrole α-positions and which enables selective iodination available for following coupling reactions [3], [4]. “Protection” at β-positions is necessary in order to maintain the selectivity of iodination. β-Alkylation not only facilitates chemical modification at the core π-plane but may also influence the stacking mode in organized structures depending on the alkyl chain lengths. In this report, the synthesis of β-methyl-substituted receptor 2a and the solid-state stacking structures of β-alkyl-substituted derivatives are discussed.

Section snippets

Synthesis, characterization, and anion binding property

3,4-Dimethylpyrrole was obtained by the Barton-Zard's method; [7] β-methyl dipyrrolyldiketone (2a′) was obtained in 52% yield by adding malonyl chloride to a solution of 3,4-dimethylpyrrole in dry CH2Cl2 at 0 °C. Subsequently, complexation using BF3·OEt2 resulted in the formation of a highly fluorescent BF2 complex, 2a, in 90% yield. Chemical identification was performed by using MALDI-TOF-MS and 1H-NMR spectroscopy. The UV/vis absorption maximum (λmax) and emission maximum (λem excited at each λ

Conclusions

We have synthesized the β-methyl-substituted dipyrrolyldiketone BF2 complex 2a, which shows smaller Ka values as compared to 1a and 2b due to the electronic effect and the relative stability of the preorganized conformation for anion binding. The solid-state assembling modes, which may be essential for the maintenance of the organized structures of polymeric states, of the β-alkyl-substituted receptors 2a,b are found to be controlled by the β-alkyl chain lengths; the modes in the case of 2a are

Experimental

Starting materials were purchased from Wako Chemical Co., Nacalai Chemical Co., and Aldrich Chemical Co. and used without further purification unless otherwise stated. UV–visible spectra were recorded on a Hitachi U-3500 spectrometer. Fluorescence spectra and emission quantum yields were recorded on a Hitachi F-4500 fluorescence spectrometer and a Hamamatsu Quantum Yields Measurements System for Organic LED Materials C9920-02, respectively. NMR spectra used in the characterization of products

Acknowledgements

This work was supported by Grant-in-Aid for Scientific Research in a Priority Area “Super-Hierarchical Structures” (No. 18039038, 19022036) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2008–2013). The author thanks Prof. Atsuhiro Osuka, Mr. Shohei Saito, and Mr. Eiji Tsurumaki, Kyoto University, for the X-ray analyses, and Prof. Hitoshi Tamiaki, Ritsumeikan University, for helpful

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