Design, Synthesis and Investigations of Liquid Crystalline Organic Semiconductors

Abstract

This thesis entitled “Design, Synthesis and Investigations of Liquid Crystalline Organic Semiconductors” describes simple and straightforward synthetic pathways, characterizations and applications of new LC/non-LC organic materials, with their potential applications in acid sensing, optoelectronic devices mainly OLEDs and OFETs applications. A brief overview of the chapters is given. Chapter 1, gives a general introduction to liquid crystals, characterization techniques and their applications. Chapter 2a addresses the synthesis and characterization of pyrazino[2,3-g]quinoxaline (PQ) derivatives where the central pyrazino[2,3-g]quinoxaline central core is substituted with eight peripheral flexible tails of varying lengths. The compounds with very short/branched peripheral chains did not stabilize any liquid crystalline phase, while the medium to long-chain homologs exhibited columnar phases. All the compounds exhibited a high molar extinction coefficient and bright greenish-yellow emission behavior in solution and solid state. One of the columnar liquid crystalline materials was used in the fabrication of host-guest OLED exhibited higher efficiency and bright green emission. Chapter 2b describes the sensing ability of the pyrazino[2,3-g]quinoxaline derivative to volatile trifluoroacetic acid in trace amounts (in parts per billion levels). The reversible detection of acid-sensing behavior was visually perceivable in both solutions as well as in the drop-casted film on a TLC paper-based strip. Chapter 3 explains new design strategies for the synthesis of donor-acceptor-donor pyrazino[2,3-g]quinoxaline carbazole derivatives with straight chain and branched alkyl peripheral chains. The electron-donating carbazole moieties that have been placed in the periphery showed a strong electron-donating ability, high molar extinction coefficients and lower band-gap, bright emission behavior in solution and solid-state, along with positive solvatochromism. One of the branched chain compounds was used in yellow and white OLEDs device fabrications. Chapter 4 describes the detailed synthesis of naphthalene and perylene-based imidazole derivatives with a new donor-π-acceptor-π-donor architecture containing ten flexible chains for solution-processable organic field effect transistor (OFETs) applications. These compounds showed a wide range of columnar LC behavior. One of these naphthalene and perylene bisimide derivatives were evaluated for their application in OFET devices exhibited high hole mobility values.