Elsevier

Dyes and Pigments

Volume 157, October 2018, Pages 109-116
Dyes and Pigments

Effect of heteroatom exchange (S/Se) in the mesomorphism and physical properties of benzochalcogenodiazole-based liquid crystals

https://doi.org/10.1016/j.dyepig.2018.04.049Get rights and content

Abstract

In order to investigate the effect on the structure-property relationship by the exchange of sulfur atom by selenium, a new series of fluorescent calamitic liquid crystals based on 4,7-disubstituted-2,1,3-benzoselenadiazole was successfully synthesized and characterized. The mesomorphic properties were studied by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Thermal properties have evidenced the influence of changing sulfur by selenium atom in the formation of liquid crystalline phases and mesomorphism range. Photophysical studies were performed in chloroform solution, where these compounds displayed strong orange luminescent under UV light, with relevant quantum yields (ΦF = 0.14–0.16) and greater solvatochromic dependence. Electrochemical studies showed oxidation and reduction patterns similar to 2,1,3-benzothiadiazoles analogues, however presenting lower band gap.

Introduction

Organic molecules able to exhibit liquid crystalline phases are of great interest for many areas of applied science, for instance, as organic light-emitting diodes (OLEDs), display technology (LCD), organic field effect transistors (OFETs), organic photovoltaic devices (OPVs), anisotropic ion transportation, and photo- and semiconducting materials [[1], [2], [3]]. Among these liquid crystalline behavior, molecules exhibiting smectic (Sm) and columnar phases are good candidates for applications as charge carrier mobility due to their arrangement in the mesophase [4]. In this way, the rational molecular design of new thermotropic liquid crystals for applications in advanced functional materials involves the selection of an appropriate central core, linking and peripheral group. Over several years, a large number of LC compounds containing heterocyclic central units have been synthesized. Heterocycles rings such as oxadiazoles [[5], [6], [7], [8]], thiadiazoles [9,10], triazoles [11,12], pyrazoles [13,14] and isoxazoles [15] have been much investigated. Beyond these heterocycles, our group has experience in the synthesis and study of fluorescent thermotropic liquid crystals π-conjugated based on the 2,1,3-benzothiadiazole (BTD) [[16], [17], [18]].

In recent years, comparison of the effects caused by the presence of S or Se as heteroatom is becoming common for polymer and fluorophores [[19], [20], [21]], but extremely rare for LCs work. In terms of molecular design, selenium atom has larger size, less electronegativity, and is softer and more easily polarized than sulfur, what could have an important influence on the thermal, optical and electrochemical properties of selenium derivatives [4,22].

Mindful of the potential of benzochalcogendiazole, another interesting heterocycle is the 2,1,3-benzoselenadiazole (BSe). The BSe is similar to BTD with respect to strong and stable fluorescence, good thermal stability and good electron-withdrawing ability [23].

Herein, we describe the synthesis, characterization and study of thermotropic liquid crystals based on the heterocycle BSe with different alkoxy chains, rationalizing the impact of the exchange in the chemical and physical properties of these liquid crystals when compared to published data from BTD [24].

Section snippets

Synthesis

The synthetic route followed for the preparation of the final compounds based on 2,1,3-benzoselenadiazole 5a–d is presented in Scheme 1. The synthesis follows the same procedure already adopted in ours previous works [16,24]. It starts from the condensation of o-phenylenediamine with thionyl chloride and bromination of the heterocycle to obtain the 4,7-dibromo-2,1,3-benzothiadiazole (2). The Sonogashira coupling reaction between the aryl dibromide 2 and two equivalents of terminal aryl

Conclusions

In summary, a series of new fluorescent liquid crystal derivative from the heterocycle 2,1,3-benzoselenadiazole was synthesized and characterized. The thermal, photophysical and electrochemistry properties were studied. The compounds 5a-d displayed exclusively smectic C phases with characteristic broken fan-shaped texture. The mesophase range varied according to the length of the alkyl chains, where for 5d was observed a broader mesomorphism range (ΔTSmC = 60). All compounds exhibited similar

General

Infrared spectra were recorded on a Bruker VERTEX model 70/70 V spectrometer in KBr discs or films. 1H NMR spectra were obtained with a Varian Mercury Plus 400-MHz instrument using tetramethylsilane (TMS) as the internal standard. 13C NMR spectra were recorded on a Varian Mercury Plus 100-MHz spectrometer. The melting points and mesomorphic textures were determined using an Olympus BX50 microscope equipped with a Mettler Toledo FP-82 hot stage and a PM-30 exposure control unit.

Thermal

Acknowledgments

We thank the following institutions for financial support of CNPq (MCTI/CNPq Nº 14/2014-Universal) and FAPESB (PIBIC). The authors are also thankful to CEBIME-UFSC for the HRMS analysis. The LNLS is acknowledged for supplying the SAXS beam time (proposal 20170066).

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