Liquid Crystal Dimers Liquid crystal (LC) dimers are formed by connecting two mesogenic cores through flexible spacers. They are soft materials that exhibit unique functionality of combining both order and mobility at a molecular, supramolecular and macroscopic level. Although a lot of research is happening in this area, no exclusive book describing the basic design principles, transitional properties, device fabrication and applications of dimeric LCs is available. At best a chapter on LC dimers can be seen in some books. This book comprehensively covers the design principles, synthesis and thermal behaviour of all types of LC dimers. Besides fundamental concepts, it also discusses recent advances in the field of calamitic, discotic and bent-core dimeric mesogens. Beginning with a chapter on the introduction of LC dimers, odd-even behaviour, basic classification and most common mesophases in dimers, this book focuses on the usage of LC dimers in developing new materials to study a range of interesting phenomena such as formation of biaxial nematic phase containing rod-like and disc-like mesogenic units. It covers transitional properties, molecular field theories of dimers, the dielectric, conductivity studies, modelling and simulation, fabrication and testing of display and electro-optic devices of several LC dimers in detail. It also presents technological relevance of LC dimers including dopants in LC display mixtures exhibiting faster relaxation time and strong flexoelectric coupling. Santanu Kumar Pal is an Associate Professor in the Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali (IISERM), India. His research interests include design, synthesis and characterization of soft materials, LC based biosensors and nanoparticles in LCs. Sandeep Kumar works as a Professor in Soft Condensed Matter Group at Raman Research Institute, Bangalore, India. His areas of research include design, synthesis and applications of various liquid crystalline materials. 13:50:01, subject to the Cambridge Core terms of use, available at 13:50:01, subject to the Cambridge Core terms of use, available at Liquid Crystal Dimers Santanu Kumar Pal Sandeep Kumar 13:50:01, subject to the Cambridge Core terms of use, available at University Printing House, Cambridge CB2 8BS, United Kingdom One Liberty Plaza, 20th Floor, New York, NY 10006, USA 477 Williamstown Road, Port Melbourne, vic 3207, Australia 4843/24, 2nd Floor, Ansari Road, Daryaganj, Delhi – 110002, India 79 Anson Road, #06–04/06, Singapore 079906 Cambridge University Press is part of the University of Cambridge. It furthers the University’s mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence. www.cambridge.org Information on this title: www.cambridge.org/9781107157590 © Santanu Kumar Pal and Sandeep Kumar 2017 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2017 Printed in India A catalogue record for this publication is available from the British Library Library of Congress Cataloging-in-Publication Data Names: Pal, Santanu Kumar, 1981- author. | Kumar, Sandeep, 1959- author. Title: Liquid crystal dimers / Santanu Kumar Pal, Sandeep Kumar. Description: Daryaganj, Delhi, India : Cambridge University Press, 2017. | Includes bibliographical references and index. Identifiers: LCCN 2016054803| ISBN 9781107157590 (hardback ; alk. paper) | ISBN 1107157595 (hardback ; alk. paper) Subjects: LCSH: Liquid crystals. | Dimers. | Polymer liquid crystals. Classification: LCC QD923 .P35 2017 | DDC 621.3815/422--dc23 LC record available at https://lccn.loc.gov/2016054803 ISBN 978-1-107-15759-0 Hardback Additional resources for this publication at www.cambridge.org/9781107157590 Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. 13:50:01, subject to the Cambridge Core terms of use, available at To Dipika Pal, my wife Arghya Pal, my son My father, family members and the memory of my mother Santanu Kumar Pal To My family members and friends Sandeep Kumar 13:50:16, subject to the Cambridge Core terms of use, available at 13:50:16, subject to the Cambridge Core terms of use, available at Contents Preface ix Acknowledgements xi 1. Introduction 1 1.1 Liquid Crystals 1 1.2 LC Dimers 2 1.3 Classification of LC Dimers 2 1.4 Calamitic Dimers 2 1.5 Discotic Dimers 5 1.6 Bent-Core Dimers 7 2. Calamitic–Calamitic LC Dimers 10 2.1 Introduction 10 2.2 Linear LC dimers 11 2.3 H-Shaped LC Dimers 26 2.4 T-Shaped LC Dimers 37 2.5 T-Shaped Chiral Dimers 46 2.6 U-Shaped LC Dimers 46 2.7 O-Shaped LC Dimers 51 3. Calamitic–Cholesteric LC Dimers 59 3.1 Introduction 59 3.2 Structure–Property Relationship of Cholesteric Dimers 60 4. Discotic LC Dimers 118 4.1 Discotic–Discotic LC Dimers 118 4.2 Discotic–Calamitic LC Dimers 169 13:49:59, subject to the Cambridge Core terms of use, available at viii Contents 5. Bent-Core LC Dimers 185 5.1 Symmetrical Bent-Core LC Dimers 185 5.2 Nonsymmetrical Bent-Core LC Dimers 201 5.3 Unconventional Bent-Core LC Dimers 219 6. Perspectives 225 6.1 P hotoresponsive Cholesteric Dimers for Rewritable Recording Materials 225 6.2 Discotic Dimers as Dopants for LC Display (LCD) Mixtures 227 6.3 Discotic Dimers as Organic Light-Emitting Diodes (OLEDs) 228 6.4 Bent-Core Dimers for Display Devices 229 6.5 Bent-Core Dimers for Conducting Molecular Wires 229 6.6 Bent-Core Dimers for Photo-responsive Functional Systems 229 Index 233 13:49:59, subject to the Cambridge Core terms of use, available at Acknowledgements It would not be possible to write this book without the help and support of numerous people around me including my teachers, family, friends and colleagues. At the end, it is a pleasant task to express my gratitude to all those who contributed in many ways to make this book possible and made it memorable for me. I would like to express my sincere gratitude to Professor N. Sathyamurthy, Director at IISER Mohali, for his support and constant encouragement during the course of this work. I would like to thank various publishers and authors for their permission to reproduce figures from previous publications. These figures, taken in part or adapted from other sources, are duly acknowledged wherever they appear in the book. I would like to thank my research scholars and post-doctoral fellows who contributed in drafting different chapters of this book. Shilpa Setia, a research scholar at IISER Mohali, has contributed in drafting Introduction, Cholesteric–calamitic Liquid Crystal (LC) dimers, Discotic LC dimers and the Perspectives section of this book. Harpreet Singh, a research scholar at IISER Mohali, participated in drafting Calamitic–calamitic LC dimers. Joydip De, a research scholar at IISER Mohali, has contributed in drafting Calamitic LC dimers. Golam Mohiuddin, a post-doctoral fellow at IISER Mohali, has contributed in drafting Bent-Core LC dimers. I would like to express my sincere gratitude to my students Monika Gupta, Indu Bala, Indu Verma, Sumyra Sidiq, Supreet Kaur, Vidhika Punjani, Gurdeep Singh and Manmohan from IISER Mohali for their interest and help throughout the preparation of this book. This book could not have been written without the support of my family members. I thank my father Lakshman Chandra Pal for his constant support. I would also like to thank my wife Dipika Pal and son Arghya Pal for their patience, encouragement, cooperation and moral support during the course of this work. Santanu Kumar Pal I would like to express my sincere gratitude to my students and friends for their constant support in my research career. This book could not have been written without the blessings of my mother and other elder family members. I would like to thank Navita, Navdeep, Shivani, Sudeep, Swati and my beloved granddaughter Shubhangi for their love and cooperation during the course of this work. Sandeep Kumar 13:36:21, subject to the Cambridge Core terms of use, available at 13:36:21, subject to the Cambridge Core terms of use, available at