Modelo de casos de uso del sistema

Initial studies were carried out on all of the synthesised trisubstituted triazines. The optical absorbance of the compounds clearly shows that the substitution pattern on the triazines has a marked effect on the absorbance of the compounds (Figure 2.1).

Figure 2.1

Optical absorbance of the trisubstituted triazines and reference compound 2.19

A closer look at the absorption spectra of compounds 2.10, 2.9 and 2.5 shows even more clearly the effect of the substituent on the absorption spectra, with the nitrile group in 2.10 giving a strong red-shift towards the visible region spectrum in comparison with the methyl substituted 2.9 and the methoxy substituted 2.5 (Figure 2.2).

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

49 Figure 2.2

Optical absorbance of 2.5, 2.9 and 2.10.

In figure 2.3 the comparison of the absorption spectra of compound 2.19 with an aromatic benzene core with the absorption spectra of 2.17 that has a central triazine ring shows the effect of the triazine on the absorption spectra.

Scheme 2.3

Optical absorbance of compounds 2.17and 2.17.

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

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The absorption of compound 2.19 with a central benzene ring lies in the blue region, with the absorption maximum at 265 nm, whereas with the triazine ring as the core of the molecule, the absorption maximum is shifted noticeably towards the visible region with the maximum at 290 nm.

The comparison of the spectra of compounds 2.10 and 2.15, as well as compounds 2.9 with 2.14 emphasises the importance of the presence of the NH group between the triazine ring and its aromatic substituents. The optical absorbance of compounds 2.15 and 2.14, where the NH group has been replaced by an ether bridge, show dramatic decrease in the optical absorbance value.

Figure 2.4

Comparison of the optical absorbance of 2.10 with 2.15, and 2.9 with 2.14.

The importance of the NH group can be further appreciated when comparing the absorbance of compound 2.14 with an ether bridge (Figure 2.4), compound 2.17 with the aromatic substituent directly attached onto the triazine ring (Figure 2.3) and compound 2.9 with an NH bridge. From these studies it is evident that both the presence of the triazine ring and the NH bridge are essential for optimal absorbance.

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

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The theoretical and experimental absorbance spectra of compounds 2.5, 2.9 and 2.10 were well in accordance, as can be seen by comparing the theoretical absorbance shown in figure 2.5 with the experimental absorbance shown in figure 2.2.

Figure 2.5

Theoretical absorbance of compounds 2.5(OCH3), 2.9 (CH3) and 2.10 (CN).

The absorption-emission spectra of the most promising compounds, 2.5, 2.9 and 2.10 were measured (Figure 2.5). It can be seen that the methoxy substituent does not give noticeable emission spectra, whereas methyl substituted 2.9 gives an emission with the maximum around 375 nm, and nitrile substituted 2.10 has an emission maximum at 420 nm.

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

52 Figure 2.6

Experimental absorption and emission spectra of substituted triazines 2.10 with nitrile-, 2.9 with methyl-, and 2.5 with methoxyanilines as substituents.The excitation wavelength for each PL

spectrum was in correspondence of the absorption maximum.

It is important to note again the exact correspondence between the theoretical and experimental data both in the vibrational frequencies and also in the optical absorption. These data further underline the exact structural features of the organic compounds and the main optical properties of the phosphors.

For deeper understanding of the kinetic mechanism, time resolved photoluminescence (TRPL) measurement were performed in the pico-second time domain (Figure 2.7). There are two different types of photoluminescence (PL) measurements: the more common stationary PL measurements, and time resolved photoluminescence measurements. In stationary PL measurements the sample is irradiated with a continuous light and the emission spectra is recorded. Instead, in TRPL the sample is irradiated with a pulsating light, where the length of the light pulse is shorter than the extinction time of the sample.

In fact, TRPL is quite simply the measurement of the mean time of the intensities of luminescence rising from all of the recombinations from the optically active centres of the sample. TRPL allows to learn much more of the characteristics of the sample than simple stationary luminescence measurements in the way of quantum efficiency, kinetics of the excitation and time of life of the luminescence, and so even though these measurements require very sophisticated instruments, they are nevertheless very widely used.

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

53 Figure 2.7

TRPL spectra of compounds 2.9 and 2.10

The time resolved photoluminescence experiments carried out with thermally treated compounds that had been heated 200 °C show a shorter PL life by 10%, indicating a change in the crystal structure of the compounds at elevated temperatures (Figure 2.8). This change is much more evident in TRPL studies than in Raman spectra carried out at the same temperatures (Figure 2.10), and the 10% shorter PL life of the samples that have been thermally treated is a strong indicator of a structural change that takes place in these temperatures.

400 450 500 550 600 650 700 750 800

0,0 0,2 0,4 0,6 0,8 1,0

WL (nm)

Normalized sr413 Normalized sr386 Normalized 394 Trattamento termico a 200 °C

Figure 2.8

Photoluminescence experiments of 2.5 (black line), 2.9 (blue line) and 2.10 (red line).

Suvi Henna Maria Rajamäki – Synthesis of Heterocycles for OLED Applications Tesi di Dottorato in Scienze e Tecnologie Chimiche - Università degli Studi di Sassari

54