1
Room-Temperature Liquid Dyes

Bhawani Narayan1,2 and Takashi Nakanishi1

1National Institute for Materials Science (NIMS), Frontier Molecules Group, International Center for Materials Nanoarchitectonics (WPI-MANA), 1-1 Namiki, Tsukuba, 305-0044, Japan

2St. Joseph's College, Department of Chemistry, 36, Langford Road, Langford Gardens, Bangalore, 560027, Karnataka, India

1.1 Introduction

Optoelectronically active functional molecules derived from p-conjugated chromophores [1] are becoming increasingly desirable. Their flexibility, light weight, processability, and low cost of fabrication provide them with an advantage over their inorganic counterparts, which are intrinsically hard and have no deformability [2]. The tailorable chemical functionalization on the p-conjugated unit, leading to tunable properties, makes them suitable for various applications in the field of optoelectronics [3]. Due to the strong p-p interactions between the p-conjugated units, these molecules generally exist as solids. As solids cannot be directly fabricated on the devices, they are either dissolved in organic solvents and coated onto a substrate, or used as gels [4] or as self-assembled phases [5]. They have been also applied as interfacial films. However, defect-free fabrications and homeotropic alignments are rather difficult due to the strong ordering and tendency to crystallize. The most common soft materials in this regard are liquid-crystalline () materials. LCs are mesophases existing between the crystalline and isotropic phases [6]. As LC materials exhibit long-range order (micrometer scale), they are advantageous over locally ordered nanomaterials. However, LC phases only form on a special class of molecules and can be either thermotropic or lyotropic. They need the assistance of temperature/solvent to exhibit their properties different from that of the intrinsic chromophores in the monomeric form. Thus, simple prediction of material properties from LCs is almost impossible, and deep investigation is needed in holding the perfectly ordered state.

A clever strategy to retain the intrinsic characteristics of the chromophore, more often in the monomeric form, in a solvent-free paradigm is by designing functional molecular liquids (s) [7]. FMLs are uncharged, solvent-free liquids at room temperature and can be categorized as one of the organic soft materials (Scheme 1.1). A close-knit member in the family is the class of "ionic liquids" [8]. Ionic liquids, which are molten salts at a relatively low temperature (below 100?°C), result from weak ionic interaction and comprise bulky cationic and anionic counterparts [9]. They possess many useful properties, such as low volatility, low melting point, noninflammability, high thermal stability, and ionic conductivity. They have mainly been explored as solvents for carrying out reactions and as catalysts, and the recent trend of their utilization is in battery research.

Scheme 1.1 Venn diagram representation of organic soft materials.

FMLs are easily processable due to their fluidic nature and are environment friendly, as they do not require solvents in further fabrications and are nonvolatile. As most studied systems are based on chromophores, these liquids possess luminescent properties. Tunable emission and facile incorporation of other emissive materials into the liquid matrix have led to a deep understanding of phenomena such as excited-state energy transfer and white light emission. They are desired in the isotropic phase; however, interchromophoric (p-p) interactions may lead to local ordering, equipping them with properties such as liquid crystallinity at lower temperatures. This chapter discusses the strategy of designing room-temperature FMLs and gives detailed insights into the applications of these materials developed so far.

1.2 Design Strategy: Alkyl Chain Engineering

Organic molecules and polymers containing p-conjugated backbones are widely used in organic/polymeric optoelectronic devices by virtue of their suitable optical, semiconducting, and electronic properties. However, in general, these p-conjugated substances tend to exist as solids at room temperature due to the strong p-p interactions. Although conventional solution-processing techniques such as spin coating and solvent-assisted methods have been used for fabrication of ordered films, few molecules form poorly soluble random aggregates. This makes the rich semiconducting p-conjugated moieties unsuitable for showing proper optoelectronic performances such as charge transport. Furthermore, aerial oxidation and photooxidation have often been reported to cause photodimerization/polymerization, resulting in the loss of the semiconducting property of the p-conjugated backbone.

A simple synthetic technique to address the aforementioned challenges is to attach solubilizing linear or branched alkyl chains [10]. The introduction of flexible and bulky alkyl chains on the functional p-conjugated units gives us a tool for fine-tuning the balance between p-p interactions among neighboring chromophores and van der Waals (vdW) interactions governed by the attached alkyl chains. This technique not only softens the intrinsically rigid p-molecular-based material but also allows for unique phase transition ranging from solids to thermotropic LC to room-temperature liquids. Furthermore, the bulky alkyl chains can wrap/cover the p-conjugated units, preventing them from aerial oxidation/or photodimerization/polymerization, and thus can guarantee them a longer lifetime with their advanced functions.

The detailed discussions of the organic liquid dyes presented in the following sections have been organized in the order of the size of their functional cores (Scheme 1.2).

Scheme 1.2 Functional p-conjugated molecular units organized according to ascending size, as discussed in this chapter. These functional p-units have been appended with alkyl chains (from left to right; carbazole, azobenzene, naphthalene, anthracene, pyrene, oligo-(p-phenylenevinylene), oligo-(p-phenyleneethylene), benzothiadiazole, porphyrin and C60 fullerene) or alkylsilane chains (from left to right; triarylamine, phthalocyanine, and oligofluorene) to develop functional liquid dyes at ambient temperature.

1.3 Alkylated p-Molecular Liquids

Substitution of multiple linear or branched alkyl chains has dramatically led to the phase transformation in many p-conjugated systems. This has made them useful for solvent-free direct applications in optoelectronics. In this section, the impact of the alkyl chain substitution on the phase behavior of several p-conjugated molecular systems has been briefly discussed. This section presents a detailed account of room-temperature FMLs. It covers the aspects of the synthetic strategies adopted to tune the physical properties of FMLs. An attempt is made to cover the development of these functional materials so far and the advances made in deriving functions from them.

1.3.1 Carbazoles

The first reported liquid carbazoles applied in an application was from the group of Peyghambarian and coworkers. 9-(2-ethylhexyl)carbazole (1, Figure 1.1a), the room-temperature liquid, was utilized as the solvent in ellipsometry measurements for determining the birefringence induced by electric field in photorefractive polymer composites [12]. Further insights into the charge-transport properties of 1 were provided by the research group of Wada and coworkers [13]. They demonstrated 1 to be a p-type semiconductor and determined the drift hole mobility by time of flight () method to be 4?×?10-6 cm2 V-1 s-1. Liquid carbazoles have more recently been employed as the liquid-emitting layer of organic light-emitting diodes (s) [14]. The liquid host 1 was used along with 6-(1)-naphthalene-2-carboxylic acid hexyl ester (BAPTNCE) as the guest emitter and tetrabutylammonium hexafluorophosphate () as the electrolyte. Using a liquid emitter had two major advantages over conventional OLEDs. Firstly, the liquid-emitting layer and the electrodes do not lose contact upon significant bending of the device. Secondly, the liquid emitters that degraded in the OLED can be easily replenished by a fresh flow of the liquid, thus increasing the lifetime of the OLED over the existing ones in the device developed by Adachi and coworkers. The maximum external electroluminescence quantum yield (FEL) achieved is 0.31?±?0.07%, and the maximum luminance achieved is approximately 100?cd m-2. The FEL and the maximum luminance are 10 and 100 times, respectively, higher than the OLEDs reported so far. Thus, examples of this kind are advantageous over the conventionally applied materials in optoelectronic devices.

Figure 1.1 Chemical structures of (a) liquid carbazole...