Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

Shigeyuki Yamada,Qingzhi Zhang,Hiroki Fukumoto,Tomohiro Agou,David O’Hagan,Tsutomu Konno,Masakazu Nagata,Toshio Kubota,Masato Morita,Mitsuo Hara,Yizhou Wang

doi:10.3390/cryst11040450

Shigeyuki Yamada, Qingzhi Zhang + Show 9 more

Open Access

https://doi.org/10.3390/cryst11040450

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Abstract

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation.

Highlights

The fluorine atom possesses numerous unique characteristics [1,2,3,4], including the largest electronegativity of all elements (4.0 on the Pauling scale), the second smallest atomic size next to hydrogen, and the dissociation energy of the C–F bond (105.3 kJ mol−1 ) is higher than those of the C–H (98.8 kJ mol−1 ) and C–C (83.1 kJ mol−1 ) bonds
Our study was initiated with a theoretical assessment based on quantum chemical calculations using the Gaussian 16 software to discuss the effects of the fluoroalkyl units on the physical properties of the various compounds
The large difference in the LC behavior can be attributed to two factors, namely the dipole moment along a major molecular axis, which is induced by the electron-withdrawing fluoroalkyl units, and the construction of head-to-tail or head-to-head ordered structures in molecular aggregates through F···H

Summary

Introduction

The fluorine atom possesses numerous unique characteristics [1,2,3,4], including the largest electronegativity of all elements (4.0 on the Pauling scale), the second smallest atomic size (van der Waals radius, rvdw = 147 pm) next to hydrogen (rvdw = 120 pm), and the dissociation energy of the C–F bond (105.3 kJ mol−1 ) is higher than those of the C–H (98.8 kJ mol−1 ) and C–C (83.1 kJ mol−1 ) bonds Owing to these unique characteristics, the incorporation of fluorine atoms into organic molecules results in dramatic augmentations or alterations in their physical properties, in addition to introducing new functionalities [1,2,3,4]. As a result of intensive investigations into fluorinated PL molecules, it was revealed that fluorine atoms in these structures play an essential role in determining the electron density distribution and in the construction of rigid molecular aggregates via

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