**1. Introduction**

196 Organic Light Emitting Devices

[65] Chen, T-R. Luminescence and electroluminescence of bis (2-(benzimidazol-2-yl) quinolinato) zinc. Exciplex formation and energy transfer in mixed film of bis (2- (benzimidazol-2-yl) quinolinato) zinc and N,N'-bis-(1-naphthyl)-N, N'-diphenyl-1,1'-

biphenyl-4,4'-diamine. Journal of Molecular Structure 2005; 737(1) 35-41.

Low molecular mass organic compounds with internal charge transfer properties are widely adopted for organic photonics such as materials for the creation of molecular electronics elements, organic magnets, solar cells and organic light emitting diodes (OLEDs) for full display panels [1-3]. One of the most widely used red light-emitting materials contains pyranylidene (4*H*-pyran-4-ylidene) or isophorene (5,5-dimethylcyclohex-2-enylidene) fragments as backbone of the molecule (see Fig.1), which are conjugated in a system with electron acceptor and electron donor fragments [1,4-24]. In many cases the light-emitting layer from such commercially available compounds is prepared by thermal evaporation in vacuum [1-2, 25-27]. Some of them are used as dopants in a polymer matrix and spin-coated onto a hole transport layer from solution [1,12]. However the doping amount of luminescent compound is limited by self crystallization and photoluminescence quenching at higher concentrations which reduce the quantum efficiency of fabricated devices significantly [11- 12]. Therefore it is important to synthesize low molecular mass light-emitting organic compounds which do not crystallize and form thin amorphous solid films from volatile organic solvents. Such compounds, which can make a solid-state glassy structure prepared from solutions, could facilitate technological processes in the production of many devices in optoelectronics, for example, light emitting devices by low-cost deposition such as wet casting methods and easier light-emitting material synthesis. Some of these red lightemitting compounds have been introduced by us [28-32].

In this chapter we present complete synthesis, thermal, optical, photoelectrical and glass forming properties of new organic glass-forming pyranylidene and isophorene fragment containing derivatives with bulky trityloxy groups in their molecules. The optical properties, both in solution and solid state, are compared. The dependance of

© 2012 Zarins et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Zarins et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

photoelectrical properties and energy structure of glassy films on molecular structure will be discussed. The most popular derivatives of pyranylidene and isophorene used in OLEDs are shown in Fig.1.

Synthesis and Physical Properties of Red Luminescent

**O**

**CH3**

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 199

**86% H3C O**

**2.1. Synthesis of the backbone fragment: 2,6-disubstituted-4***H***-pyran-4-ones** 

in Fig.2) by acidic rearrangement with following decarboxylation (see Fig.2) [32-33].

**O conc. HCl or**

**H3C O O**

(compound **2**) is formed.

**OH**

**CH3**

**backbone** of pyranylidene derivative **8** (shown in Fig.3).

The simplest of 2,6-disubstituted-4*H*-pyran-4-ones is 2,6-dimethyl-4*H*-pyran-4-one (compound 2 in Fig.2), which is obtained in 86% yield from dehydroacetic acid (compound 1

**10% H2SO4**

**-CO2**

**1 2**

2,6-Dimethyl-4*H*-pyran-4-one (compound **2** in Fig.2) has one carbonyl group which can further react with active methylene group containing compounds in *Knoevenagel* condensation reactions. It also has two activated methyl groups, which can react in the same

Another method for the syntheis of 2,6-disubstituted-4*H*-pyran-4-ones, which contain at least one active methyl group, is using 4-hydroxy-6-methyl-2*H*-pyran-2-one (compound **3** in Fig.3) as starting material [11,34]. Its further reaction with isobutyryl chloride (compound **4**  in Fig.3) in trifluoroacetic acid (TFA) gives 6-methyl-2-oxo-2*H*-pyran-4-yl isobutyrate (compound **5** in Fig.3). Without separating the compound **5** from the reaction mixture it was subjected to *Fries rearrangement* resulting in 4-hydroxy-3-isobutyryl-6-methyl-2*H*-pyran-2 one (compound **6** in Fig.3). In its decarboxylation and further acidic cyclization reactions 2 isopropyl-6-methyl-4*H*-pyran-4-one (compound **8** in Fig.3) is obtained with 80% yield. Compound **8** also contains a carbonyl group, just as the previously synthesized 2,6 dimethyl-4*H*-pyran-4-one (compound **2** in Fig.2). Since it now contains just one activated methyl group, only one aromatic aldehyde containing fragment can be added to the

One of the most preferred 2,6-disubstituted-4*H*-pyran-4-ones is 2-*tert*-butyl-6-methyl-4*H*pyran-4-one (compound **13** in Fig.4) [7,11,35]. The first synthesis method starts from 3,3 dimethylbutan-2-one (compound **9** in Fig.4). Treating it with acetic anhydryde (Ac2O) and boron trifluoride (BF3) a boron enolate (compound **10** in Fig.4) is obtained. Its further condensation reaction with 1,1-dimethoxy-N,N-dimethylethanamine (compound **11** in Fig.4) produces N,N-dimethylamino-vinyl group containing boron enolate (compound **12** in Fig.4). Then following an acidic treatment gives 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.4). However this method has a drawback because two synthetic reactions towards our target compound had low yields (30-40%), which results in a very low overall yield for synthesis of 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.4).

type of condensation reactions with one or two molecules of aromatic aldehydes.

**Figure 2.** Synthesis of 2,6-dimethyl-4*H*-pyran-4-one. Dehydroacetic acid (compound **1**) is suspended either in concentrated hidrochloric acid (conc. HCl) or 10% aqueous sulfuric acid (10% H2SO4) and heated. During the heating carbon dioxide (CO2) is liberated and 2,6-dimethyl-4*H*-pyran-4-one

**Figure 1.** Most widely used pyranylidene and isophorene type red-emitters used as OLED emission layer materials

**;**
