**2. Synthesis**

The synthesis procedure of pyranylidene and isophorene D-π-A type luminophores (see Fig.1) with either one or two electron donor fragments can be divided into three main parts:


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

198 Organic Light Emitting Devices

are shown in Fig.1.

**A**

**R O D**

**b) -CH(CH3)2 ; c) -C(CH3)3) ;**

**A**

layer materials

**2. Synthesis** 

compounds.

parts:

Chromene type red luminiscent compounds

**O D**

**;**

and are able to react further with aromatic aldehydes.

**R = a) -CH3 ;**

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

luminiscent compounds Isophorene type red

**O**

**H3C <sup>D</sup>**

**H3C**

**A** - electron acceptor fragment **D** - electron donating fragment

**O D**

**A**

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

The synthesis procedure of pyranylidene and isophorene D-π-A type luminophores (see Fig.1) with either one or two electron donor fragments can be divided into three main

1. Synthesis of a backbone fragment: Synthesis of derivatives of 4*H*-pyran-4-one, which in their molecules contain not only a carbonyl group, but also at least one methyl group

2. Addition of an electron acceptor fragment to the backbone: Condensation reaction of 4*H*-pyran-4-one derivatives synthesized in 1) with active methylene group containing

3. Synthesis of pyranylidene and isophrene D-π-A type red emitters: Final addition of electron donor group containing aromatic aldehydes to compounds obtained in 2).

Benzopyran type red luminiscent compounds **A**

luminiscent compounds

**D D**

**A**

Pyranylidene type red

**; ;**

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

**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 (compound **2**) is formed.

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

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

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).

Synthesis and Physical Properties of Red Luminescent

**CH3**

**H3C CH3**

**(not isolated)**

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 201

**Monoglyme**

**NaH**

**16**

**H3C**

**O O O**

**O**

**H3C**

**22-85% 17**

**OH**

**O**

**O**

**R**

pyran-4-one (compound **13** in Fig.5) with a good overall yield (60%). As with 2-isopropyl-6 methyl-4*H*-pyran-4-one (compound **8** in Fig.3), the resulting 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.5) also contains one carbonyl group and one activated methyl group with the possibility of also adding **only one** aromatic aldehyde containing fragment.

> **CH3 CH3**

**15**

**O**

**H3C**

**O**

**conc. H2SO4 0....5oC**

**60%**

**Figure 5.** Improved synthesis of 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13**). NaH - sodium

One of oldest, but no less important methods known for the synthesis of 2,6-disubstituted-4*H*-pyran-4-ones is to obtain them from 3-substituted-vinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (compounds **17** in Fig.6) [33]. Compounds **17** are obtained from dehydroacetic acid (compound **1** in Fig.6), in which the methyl group in the acetyl fragment is activated to react preferentially with aromatic aldehydes (see Fig.6) giving 3-substitutedvinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (compounds **17** in Fig.6) [33, 36]. Details on the obtained compounds **17** and their dependence on substituents (R) in their molecules are given in Table 1. They serve as precursors for further synthesis of

**R**

**O**

**CHCl3 Piperidine**

**Figure 6.** Synthesis of 3-substituted-vinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (Compounds **17**). Above the arrow are different aromatic aldehydes with different substituents R (see Table 1), which

Using this approach it is possible to obtain many different mono-styryl-substituted 4*H*pyran-4-ones (compounds **17** in Fig.7). However only a few of previously synthesized compounds **17** give 2-styryl-substituted-6-methyl-4*H*-pyran-4-ones (compounds **18** in Fig.7) by acidic decarboxylation under the reaction conditions reported in [30, 33] (see Fig.7) as

**H**

all react with dehydroacetic acid (compound **1**) the same way. CHCl3 - chloroform.

**14**

**O O**

**O**

**O**

**CH3**

**H3C H3C CH3**

**CH3 H3C**

hydryde, conc. H2SO4 - concentrated sulfuric acid.

**13**

pyranylidene type compounds.

**H3C**

summarised in Table 2.

**O**

**1**

**OH**

**O**

**O**

**CH3**

**H3C**

**Figure 3.** Synthesis of 2-isopropyl-6-methyl-4*H*-pyran-4-one (compound **8**). TFA - trifluoroacetic acid, HCl - hidrochloric acid, AcOH - acetic acid, CO2 - carbon dioxide, conc. H2SO4 - concentrated sulfuric acid.

**Figure 4.** Conventional synthesis of 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13**). Ac2O - acetic anydryde, BF3 - bornon trifluoride, DMA - dimethylamine, HClO4 - perchloric acid, EtOH - ethanol.

Fortunately, there is another method for synthesizing 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.4) with good yields [7] using pentane-2,4-dione (compound **14** in Fig.5) as starting reactant.

In its *Aldol reaction* with methyl pivalate (compound **15** in Fig.5) a 7,7-dimethyloctane-2,4,6 trione (compound **16** in Fig.5) was formed. Without separating the compound **16** from reaction mixture it was subjected to acidic cyclization producing 2-*tert*-butyl-6-methyl-4*H*-

pyran-4-one (compound **13** in Fig.5) with a good overall yield (60%). As with 2-isopropyl-6 methyl-4*H*-pyran-4-one (compound **8** in Fig.3), the resulting 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.5) also contains one carbonyl group and one activated methyl group with the possibility of also adding **only one** aromatic aldehyde containing fragment.

200 Organic Light Emitting Devices

**OH**

**H3C O O**

acid.

**H3C**

**H3C CH3**

**O**

as starting reactant.

**CH3**

**Ac2O BF3 30-40%**

**H3C O**

**O**

**9 10**

**13 H3C**

**H3C O**

**CH3**

**O**

**Cl**

**H3C**

**O**

**TFA 120oC**

**CH3**

**Figure 3.** Synthesis of 2-isopropyl-6-methyl-4*H*-pyran-4-one (compound **8**). TFA - trifluoroacetic acid, HCl - hidrochloric acid, AcOH - acetic acid, CO2 - carbon dioxide, conc. H2SO4 - concentrated sulfuric

**O**

**Figure 4.** Conventional synthesis of 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13**). Ac2O - acetic anydryde, BF3 - bornon trifluoride, DMA - dimethylamine, HClO4 - perchloric acid, EtOH - ethanol.

Fortunately, there is another method for synthesizing 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13** in Fig.4) with good yields [7] using pentane-2,4-dione (compound **14** in Fig.5)

In its *Aldol reaction* with methyl pivalate (compound **15** in Fig.5) a 7,7-dimethyloctane-2,4,6 trione (compound **16** in Fig.5) was formed. Without separating the compound **16** from reaction mixture it was subjected to acidic cyclization producing 2-*tert*-butyl-6-methyl-4*H*-

**H3C**

**CH3**

**CH3**

**H3C CH3**

**CH3**

**H3C O O**

**(not isolated)**

**<sup>3</sup> <sup>4</sup> <sup>5</sup> <sup>6</sup>**

**O**

**O**

**CH3**

**CH3**

**0....5oC conc. H2**

**80%**

**CH3**

**O B F F**

> **HClO4 EtOH/H2O**

**<sup>7</sup> <sup>8</sup>**

**SO4**

**H3C O O**

**O O O**

**(not isolated)**

**CH3**

**CH3**

**CH3**

**HCl/AcOH -CO2**

**CH3**

**H3C**

**O**

**O B F F**

**12**

**N**

**CH3**

**CH3**

**CH3**

**2,6-Lutidine DMA 30-40%**

**H3C**

**H3C CH3**

**H3CO N**

**11**

**H3CO**

**CH3**

**CH3**

**CH3**

**TFA 120oC** **OH O**

**Figure 5.** Improved synthesis of 2-*tert*-butyl-6-methyl-4*H*-pyran-4-one (compound **13**). NaH - sodium hydryde, conc. H2SO4 - concentrated sulfuric acid.

One of oldest, but no less important methods known for the synthesis of 2,6-disubstituted-4*H*-pyran-4-ones is to obtain them from 3-substituted-vinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (compounds **17** in Fig.6) [33]. Compounds **17** are obtained from dehydroacetic acid (compound **1** in Fig.6), in which the methyl group in the acetyl fragment is activated to react preferentially with aromatic aldehydes (see Fig.6) giving 3-substitutedvinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (compounds **17** in Fig.6) [33, 36]. Details on the obtained compounds **17** and their dependence on substituents (R) in their molecules are given in Table 1. They serve as precursors for further synthesis of pyranylidene type compounds.

**Figure 6.** Synthesis of 3-substituted-vinylcarbonyl-4-hydroxy-6-methyl-2*H*-pyran-2-ones (Compounds **17**). Above the arrow are different aromatic aldehydes with different substituents R (see Table 1), which all react with dehydroacetic acid (compound **1**) the same way. CHCl3 - chloroform.

Using this approach it is possible to obtain many different mono-styryl-substituted 4*H*pyran-4-ones (compounds **17** in Fig.7). However only a few of previously synthesized compounds **17** give 2-styryl-substituted-6-methyl-4*H*-pyran-4-ones (compounds **18** in Fig.7) by acidic decarboxylation under the reaction conditions reported in [30, 33] (see Fig.7) as summarised in Table 2.


Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 203

**45%**

**O**

**O**

**H3C**

**O**

**CH3 CH3**

**O**

**CH3**

**O**

**O CH3**

**O**

R (of compounds 18) **Yield, % M.p., °C Recrystallized from**  p-Dimethylaminophenyl 82 156 ethyl acetale/petroleum ether

Some works can be found on red luminescent compounds where the pyranylidene fragment is hidden as a substructure in the molecule [8, 23-24]. For example, chromene type derivatives of pyranylidene are synthesized from 1-(2-hydroxyphenyl)ethanone (compound **19** in Fig.8) [23-24]. In the *Claisen condensation* reaction (see Fig.8) with ethyl-acetate in the presence of a strong base, 1-(2-hydroxyphenyl)butane-1,3-dione (compound **20** in Fig.8) is obtained. After separation it was subjected to acidic dehydrocyclization giving 2-methyl-4*H*-

**OH**

**CH3COOC2H5 AcOH/HCl**

**CH3**

**O**

For obtaining the benzopyran derivative of pyranylidene [8, 24], a two-stage synthesis

**O**

**19 20 21 Figure 8.** Synthesis of chromene fragment containing derivative of pyranylidene (compound **21**).

**CH3**

**<sup>22</sup> <sup>23</sup> <sup>24</sup>**

**Figure 9.** Synthesis of the benzopyran fragment containing derivative of pyranylidene (compounds **24**).

In the first stage of synthesis, treatment with morpholine gives us enamine **23** (4-(6 methylcyclohex-1-enyl)morpholine). In the second stage of synthesis in reaction with 2,2,6 trimethyl-4*H*-1,3-dioxin-4-one, a 2,8-dimethyl-5,6,7,8-tetrahydro-4*H*-chromen-4-one (compound **24** in Fig.9) is sucessfully obtained. Once the desired pyranylidene compound is

procedure is started from 2-methylcyclohexanone (compound **22** in Fig.9).

**O**

**N**

p-Diethylaminophenyl 68 128-130 methanol/water o-Nitrophenyl 53 187-189 methanol/water p-Isopropylphenyl 45 110-112 ethanol/water

R - substituents of aromatic aldehydes which also remain in the structure of compounds **17** after reactions. Yield - the practical production of the particular compound **17** in the reaction of dehydroacetic acid (compound **1** in Fig.6) with the corresponding aromatic aldehyde. M.p. - melting point of the particular compound **17**. Recrystallized

from - organic solvent, which is used for the particular compound **17** final purification.

chromen-4-one (compound **21** in Fig.8) with an overall 45% yield.

**OH**

**O**

**O**

**CH3**

**CH3**

**EtOH/Na**

**N**

**H**

**O**

**Table 2.** Fries rearrangement possibility [33] of pyranylidene precursors **17** (Fig.7).

R - substituents of aromatic aldehydes which also remain in the structure of compounds **17** after reactions. Yield - the practical production of the particular compound **17** in the reaction of dehydroacetic acid (compound **1** in Fig.6) with the corresponding aromatic aldehyde. M.p. - melting point of the particular compound **17**. Recrystallized from - organic solvent, which is used for the particular compound **17** final purification.

**Table 1.** Synthetic information on pyranylidene compounds **17** (see Fig.6).

**Figure 7.** Synthesis of 2-styryl-substituted-6-methyl-4*H*-pyran-4-ones (compounds **18**).

#### Synthesis and Physical Properties of Red Luminescent Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 203


R - substituents of aromatic aldehydes which also remain in the structure of compounds **17** after reactions. Yield - the practical production of the particular compound **17** in the reaction of dehydroacetic acid (compound **1** in Fig.6) with the corresponding aromatic aldehyde. M.p. - melting point of the particular compound **17**. Recrystallized from - organic solvent, which is used for the particular compound **17** final purification.

**Table 2.** Fries rearrangement possibility [33] of pyranylidene precursors **17** (Fig.7).

202 Organic Light Emitting Devices

**R (of compounds 17) Yield, % M.p., °C Recrystallized from**  Phenyl 55 130-132 methanol o-Nitrophenyl 65 161-163 acetic acid/water m-Nitrophenyl 60 192 chloroform

p-Nitrophenyl 22 165-167 chloroform/ethyl acetate

p-Dimethylaminophenyl 71 198-200 Chloroform, ethyl acetate, benzene p-Diethylaminophenyl 58 150 Chloroform, ethyl acetate

p-Nitrophenyl 47 246-247 dioxane

o-Hydroxyphenyl 67 186-188 methanol m-Hydroxyphenyl 61 181-183 ethanol p-Hydroxyphenyl 69 260-262 dioxane p-Methoxyphenyl 73 153-154 ethanol 2,3-Dimethoxyphenyl 47 147 ethyl acetate

3,4-Dimetoxyphenyl 46 185 benzene/ethyl acetate 3,4-Diethoxyphenyl 43 163 ethyl acetate o-Chlorophenyl 36 116-117 ethanol p-Chlorophenyl 54 155-156 ethanol

p-Isopropylphenyl 65 139-141 methanol

R - substituents of aromatic aldehydes which also remain in the structure of compounds **17** after reactions. Yield - the practical production of the particular compound **17** in the reaction of dehydroacetic acid (compound **1** in Fig.6) with the corresponding aromatic aldehyde. M.p. - melting point of the particular compound **17**. Recrystallized

from - organic solvent, which is used for the particular compound **17** final purification. **Table 1.** Synthetic information on pyranylidene compounds **17** (see Fig.6).

**R**

**Figure 7.** Synthesis of 2-styryl-substituted-6-methyl-4*H*-pyran-4-ones (compounds **18**).

**O**

**17**

**H3C**

**OH**

**O**

**O**

1-Naphtyl 62 190 ethyl acetate

3,4-Dichlorophenyl 46 185 ethyl acetate, benzene/chloroform

β-styryl 57 185 chloroform/ethyl acetate 2-Furyl 85 144 benzene/ethyl acetate

**2**

**AcOH**

**45-82%**

**O**

**18**

**-CO H3C R**

**O**

Some works can be found on red luminescent compounds where the pyranylidene fragment is hidden as a substructure in the molecule [8, 23-24]. For example, chromene type derivatives of pyranylidene are synthesized from 1-(2-hydroxyphenyl)ethanone (compound **19** in Fig.8) [23-24]. In the *Claisen condensation* reaction (see Fig.8) with ethyl-acetate in the presence of a strong base, 1-(2-hydroxyphenyl)butane-1,3-dione (compound **20** in Fig.8) is obtained. After separation it was subjected to acidic dehydrocyclization giving 2-methyl-4*H*chromen-4-one (compound **21** in Fig.8) with an overall 45% yield.

**Figure 8.** Synthesis of chromene fragment containing derivative of pyranylidene (compound **21**).

For obtaining the benzopyran derivative of pyranylidene [8, 24], a two-stage synthesis procedure is started from 2-methylcyclohexanone (compound **22** in Fig.9).

**Figure 9.** Synthesis of the benzopyran fragment containing derivative of pyranylidene (compounds **24**).

In the first stage of synthesis, treatment with morpholine gives us enamine **23** (4-(6 methylcyclohex-1-enyl)morpholine). In the second stage of synthesis in reaction with 2,2,6 trimethyl-4*H*-1,3-dioxin-4-one, a 2,8-dimethyl-5,6,7,8-tetrahydro-4*H*-chromen-4-one (compound **24** in Fig.9) is sucessfully obtained. Once the desired pyranylidene compound is

obtained, the addition of electron acceptor and electron donor fragments becomes a more simplified process, which will be described in detail below in this chapter.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 205

**A**

**37-39**

**H3C**

**O**

**O**

**39**

**N**

**H3C H3C**

Many different electron acceptor fragments (compounds **25-35** in Fig.10) can be introduced in 2,6-disubstituted-4*H*-pyran-4-ones [1,4-18, 28-30,32] using acetic anhydride (Ac2O) as solvent and catalyst. From these, malononitrile (compounds **25** in Fig.10) is the most commonly used. Since isophorene (3,5,5-trimethylcyclohex-2-enone) (compound **36** in Fig.11) is an inexpensive reagent, which can be purchased from chemical suppliers - such as ACROS and ALDRICH, all that remains is to add electron acceptor and electron donor fragments. As with 2,6-disubstituted-4*H*-pyran-4-ones, the electron acceptor fragments are added in *Knoevenagel* condensation reactions [18-21, 31, 37] with active methylene group

> **DMFA A**

> > **O**

**O**

**Figure 11.** Synthesis of electron acceptor fragment containing derivatives of isophorene (compounds 37-39). As in Figure 10, the electron acceptors are marked in red while the structure backbone remains

The electron acceptor fragment containing derivatives of isophorene (3,5,5 trimethylcyclohex-2-enone) (compounds **37-39** in Fig.11) thus obtained are not always isolated from the reaction mixture [31, 37]. Once they are formed, the electron donor fragment containing aromatic aldehyde is added in the mixture for further reaction with the

**2.3. Synthesis of pyranilydene and isophorene type red luminescent compounds** 

Once the electron acceptor fragment is introduced, the last step for obtaining a fully functional pyranylidene and isophorene red luminescent compounds is to add one or two electron donor fragment containing aldehydes. They are added in *Knoevenagel* condensation reactions with electron acceptor fragment containing derivatives of isophorene as shown in Fig.12 and pyranylidene shown in Fig.13, which contain one or two activated methyl groups.

For isophorene type compounds one electron donor fragment (**40-44**) is always introduced after an electron acceptor fragment is already in the molecule (see Fig.12) [18-21, 31, 37]. Many different structures of electron donor fragments are introduced (compounds **45-57** in Fig.13) in the pyranylidene backbone after introducing the electron acceptor fragment [1,4- 18,27-29,31]. In cases where only one methyl group reacts with the aldehyde, a mono-styryl

**H3C CH3**

**N**

**Et 38**

**S**

**N**

**Et**

**CH3**

**O**

**36**

**37**

**H3C**

**A = <sup>N</sup> <sup>N</sup>**

**by final addition of electron donor fragments** 

in black.

aldehyde.

containing compounds **37-39** (see Fig.11).
