**3.2. Thermal properties**

The thermogravimetric analysis (TGA) of trityl group containing pyranylidene type compounds is used to measure their thermal decomposition temperatures (Td). Td of compounds **WK-1** and **WK-2** are determined in the temperature range from +30°C to +510°C at a heating rate of 10°C/min [32] at the level of 10% weight loss (see Fig.18).

Pyranylidene type compounds with two N,N-ditrityloxyethylamino electron donor fragments (**ZWK-2**, **DWK-2**, **JWK-2**) are slightly more thermally stable than compounds containing only one such fragment, i.e. **ZWK-1**, **DWK-1** or **JWK-1**. The increase in thermal stability of pyranylidene type compounds by adding another electron donor fragment is as high as 10°C from **ZWK-1** to **ZWK-2**, 19°C from **JWK-1** to **JWK-2** and 29°C from **DWK-1** to **DWK-2**. The most thermally stable compound is a two electron donor fragment containing derivative of pyranylidene with malononitrile as electron acceptor in it (**DWK-2**).

Differential scanning calorimetry (DSC) measurements are used to measure the glass transition temperatures (Tg) of the compounds **WK-1** and **WK-2.** Three thermo cycles are performed for the determination of Tg. The first scan was done within the temperature range

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 211

210 Organic Light Emitting Devices

detail further in this chapter.

**O**

**H3C CH3 25a**

**O**

**A = ; ;**

**O**

and **JWK-2**) [28-30, 32, 46].

**3.2. Thermal properties** 

**A**

complicated but nevertheless a large part of each product was separated by liquid column chromatography (silicagel and dichloromethane for **ZWK-1** and **ZWK-2**, dichloromethane: hexane = 4:1 for **DWK-1** and **DWK-2**, dichloromethane: ethyl acetate = 4:1 for **JWK-1** and **JWK-2**). The physical properties of compounds **WK-1**, **WK-2** and **IWK** are described in

**H3C O**

**A**

**74a-c**

**N**

**O Ph Ph Ph**

**Figure 17.** Synthesis of glass forming derivatives of pyranylidene. Py - pyridine. (See previous figures for explanation of color significance). Since compounds **74a-c** and **75a-c** are our obtained red lightemitting materials, we have assigned specific names for each (**ZWK-1**, **ZWK-2**, **DWK-1**, **DWK-2**, **JWK-1**

The thermogravimetric analysis (TGA) of trityl group containing pyranylidene type compounds is used to measure their thermal decomposition temperatures (Td). Td of compounds **WK-1** and **WK-2** are determined in the temperature range from +30°C to +510°C

Pyranylidene type compounds with two N,N-ditrityloxyethylamino electron donor fragments (**ZWK-2**, **DWK-2**, **JWK-2**) are slightly more thermally stable than compounds containing only one such fragment, i.e. **ZWK-1**, **DWK-1** or **JWK-1**. The increase in thermal stability of pyranylidene type compounds by adding another electron donor fragment is as high as 10°C from **ZWK-1** to **ZWK-2**, 19°C from **JWK-1** to **JWK-2** and 29°C from **DWK-1** to **DWK-2**. The most thermally stable compound is a two electron donor fragment containing

Differential scanning calorimetry (DSC) measurements are used to measure the glass transition temperatures (Tg) of the compounds **WK-1** and **WK-2.** Three thermo cycles are performed for the determination of Tg. The first scan was done within the temperature range

at a heating rate of 10°C/min [32] at the level of 10% weight loss (see Fig.18).

derivative of pyranylidene with malononitrile as electron acceptor in it (**DWK-2**).

**Piperidine**

**Py**

**O**

**Ph Ph Ph**

**N**

**O Ph Ph Ph**

**N**

**O Ph Ph Ph**

**O**

**Ph Ph Ph**

**O**

**Ph Ph Ph**

**O**

**75a-c**

**A**

**N N O**

**N**

**O**

**Ph Ph Ph**

**73**

**H**

**O**

**N H**

**H O**

**N**

**74a and 75a 74a and 75b 74c and 75c**

**74a = ZWK-1 ; 75a = ZWK-2 74b = DWK-1 ; 75b = DWK-2 74c = JWK-1 ; 75c = JWK-2**

**O**

**O Ph Ph Ph**

**Figure 18.** Thermogravimetric analysis of compounds **WK-1** and **WK-2.** A sample of each compound is constantly weighed during heating. At some temperature (Td) the mass of the sample starts to decrease rapidly - this indicates when the respective compound starts to decompose and is no longer thermally stable.

from +25°C to +250°C at a heating rate of 10°C/min [32]. After the first heating scan samples of the compounds were cooled to 25°C at a rate of 50°C/min and heated for a second time from +25°C to +250°C at a rate of 10°C/min. The Tg value is obtained from the second heating scan (see Fig.19) and for almost all compounds is higher than 100°C. We could not obtain usable DSC curves for **DWK-1**. The compounds with two N,N-ditrityloxyethylamino electron donor fragments have higher Tg compared to those with only one electron donor fragment, which may be attributed to the different numbers of bulky trityloxyethyl groups attached to the two electron donor fragment. In a larger number of bulky groups Tg increases by 8°C from **ZWK-1** to **ZWK-2** and 7°C from **JWK-1** to **JWK-2**. Pyranylidene type compounds with barbituric acid as electron acceptor, e.g. **JWK-1** and **JWK-2** have the highest Tg values compared to **ZWK-1**, **ZWK-2** and **DWK-2**, which may be due to the additional formation of intermolecular hydrogen bonds by N-H groups of barbituric acid fragments in the molecules.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 213

**Figure 20.** TGA and DSC analysis of **IWK.** (Please see Fig.18 and Fig.19 for a more detailed

other molecules enabling higher possibility to form agreggates and crystallites.

**WK-1** and **WK-2** could increase this concentration limit more then 10 times.

Information obtained from the surfaces of the pure films is consistent with the measured glass transition temperatures (Tg). Glasses having higher Tg values are found to have less crystalline dots on their pure film surface. As we were unable to determinate Tg for **DWK-1**, according to above mentioned trend its glass transition temperature is expected to be below

Thin film containing only pyranylidene type compound **WK-1** and **WK-2** are amourphous despite of small crystalline dots in it. Till now only way to prepare amourphse films which contain pyranyliden derivatives was doping them in glass forming compound. In that case maximum doping concentration was considered to be 2wt% due to self crystallization [11- 12]. However, incorporation of bulky trityloxy groups in their molecules or using glasses

Thin films are deposited on quartz glass by the spin-coating technique. Before the deposition of the layers, the quartz glass substrates are cleaned in dichloromethane. The solutions are spin-coated onto the substrates for 40 s at 400 rpm and acceleration 200 rpm/s. In all cases, pure films obtained from two electron donor fragment containing pyranylidene compounds (**ZWK-2**, **DWK-2** and **JWK-2**) have an almost pure smooth and amorphous surface, but pyranylidene compounds with one electron donor fragment (**ZWK-1**, **DWK-1** and **JWK-1**) show several crystalline state areas (see Fig.21). Both glasses containing barbituric acid as an electron acceptor fragment (**JWK-1** and **JWK-2**) show the least amount of small crystal formations on their pure film surface. The higher stability of their amorphous state could be explained by an enchancement of N-H group hydrogen bonds in the molecules. Pure films obtained from malononitrile electron acceptor fragment containing compounds (**DWK-1** and **DWK-2**) contain small crystal dots, especially **DWK-1**. This could be due to small steric dimensions of malononitrile group, which allows more **DWK-1** molecules to be concentrated in the same volume to allow closer interaction with

explanation).

110°C.

**3.3. Glass forming properties** 

**Figure 19.** DSC thermogramms of compounds **WK-1** and **WK-2.** Since amorphous compounds have several solid state phase modifications, the glass transition temperature (Tg) indicates when compound solid structure transitions from a more kinetically stable phase (with more free volume) to a more thermodynamically stable phase (with less free volume). During such phase transitions some ammount of heat is absorbed (endothermic process) which appears as a small drop on the DSC curves.

The TGA analysis of **IWK** is conducted as previously described [32]. The thermal decomposition temperature (Td) of **IWK** is found to be even higher than that of pyranylidene type compounds **WK-1** and **WK-2** (see Fig.20). However its glass transition temperature (Tg) is lower by 18°C to 35°C degrees compared to that of pyranylidene type glasses. Despite the lower thermal stability, the pyranylidene type compounds **WK-1** and **WK-2** have better glass forming properties than the isophorene type compound **IWK**.

**Figure 20.** TGA and DSC analysis of **IWK.** (Please see Fig.18 and Fig.19 for a more detailed explanation).
