**3.3. Glass forming properties**

212 Organic Light Emitting Devices

fragments in the molecules.

additional formation of intermolecular hydrogen bonds by N-H groups of barbituric acid

**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

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

of heat is absorbed (endothermic process) which appears as a small drop on the DSC curves.

**WK-2** have better glass forming properties than the isophorene type compound **IWK**.

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 other molecules enabling higher possibility to form agreggates and crystallites.

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 110°C.

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

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 215

**Figure 22.** 1) Absorption and 2) Photoluminescence spectra of compounds **WK-1** and **WK-2** in

**Figure 23.** 1) Absorption and 2) Photoluminescence spectra of compounds **WK-1** and **WK-2** in thin

The photoluminescence (PL) spectrum of the **DWK-1** solution was found to be Stokes shifted by about 115 nm (peak position at 587 nm) with respect to the absorption spectra (see Fig.22). The PL spectra of **JWK-1** and **ZWK-1** molecules exhibited similar shapes, with their

dichloromethane solution

solid films

**Figure 21.** Optical mircroscope images of the pure films of the compound **WK-1** and **WK-2**. Dots on the pure film surface represent compound crystalline state while the remaining smooth area shows amorphous solid state.

#### **3.4. Absorption and luminescence properties**

The absorption and fluorescence spectra of the synthesized compounds in diluted dichloromethane solution and pure films are shown in Figs. 22 and 23.

A **DWK-1** molecule, whose backbone consists of the laser dye 4-(dicyanomethylene)-2 methyl-6-[p-(dimethylamino)styryl]-4H-pyran (**DCM**), in dichloromethane solution has its absorption maximum at 472 nm, which is 8 nm red shifted with respect to the pure **DCM** molecule in the same solution [9]. It shows that the bulky trityloxyethyl group has only a small influence on the energy structure of the molecule. The peaks of the absorption spectra in solution of the molecules with indene-1,3-dione (**ZWK-1**) and barbituric acid (**JWK-1**) electron acceptor substituents in the backbone are red-shifted by approximately 40 nm compared to **DWK-1**. A stronger electron acceptor group gives larger red shifts.

Synthesis and Physical Properties of Red Luminescent Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 215

amorphous solid state.

**3.4. Absorption and luminescence properties** 

**Figure 21.** Optical mircroscope images of the pure films of the compound **WK-1** and **WK-2**. Dots on the

The absorption and fluorescence spectra of the synthesized compounds in diluted

A **DWK-1** molecule, whose backbone consists of the laser dye 4-(dicyanomethylene)-2 methyl-6-[p-(dimethylamino)styryl]-4H-pyran (**DCM**), in dichloromethane solution has its absorption maximum at 472 nm, which is 8 nm red shifted with respect to the pure **DCM** molecule in the same solution [9]. It shows that the bulky trityloxyethyl group has only a small influence on the energy structure of the molecule. The peaks of the absorption spectra in solution of the molecules with indene-1,3-dione (**ZWK-1**) and barbituric acid (**JWK-1**) electron acceptor substituents in the backbone are red-shifted by approximately 40 nm

pure film surface represent compound crystalline state while the remaining smooth area shows

dichloromethane solution and pure films are shown in Figs. 22 and 23.

compared to **DWK-1**. A stronger electron acceptor group gives larger red shifts.

**Figure 22.** 1) Absorption and 2) Photoluminescence spectra of compounds **WK-1** and **WK-2** in dichloromethane solution

**Figure 23.** 1) Absorption and 2) Photoluminescence spectra of compounds **WK-1** and **WK-2** in thin solid films

The photoluminescence (PL) spectrum of the **DWK-1** solution was found to be Stokes shifted by about 115 nm (peak position at 587 nm) with respect to the absorption spectra (see Fig.22). The PL spectra of **JWK-1** and **ZWK-1** molecules exhibited similar shapes, with their maxima red-shifted to 635 and 627 nm, respectively. The photoluminescence spectra are unstructured and strongly Stokes shifted in accordance with intramolecular charge-transfer nature of the excited states [39]. For compounds containing two 4-((N,N-ditrityloxyethyl) amino)styryl electron donor fragments the absorption and luminescence spectra of the solution are observed to be red-shifted and have larger extinction coefficients, which is due to the larger absorption cross section of these molecules. The peaks of the absorption spectra of **DWK-2** and **ZWK-2** are red shifted by 17 and 11 nm, respectively, compared to molecules with a single electron donor fragment. A similar red shift has been reported for the molecule with two electron donor fragments **bis-DCM** compared with **DCM** molecules with a single electron donor fragment [40]. It is observed that molecules with two electron donor fragments have a larger conjugation length. A second reason could be simultaneously functioning two donor groups which give stronger electron donor properties. The shape of the absorption spectrum of **JWK-2** is found to be different from that of **JWK-1** and the oscillator strength of the absorption band of **JWK-2** at about 502 nm becomes more intense (see Fig.22(1)).

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 217

In the case of **IWK**, the absorption and luminescence spectra of thin solid films are also found to be practically unchanged compared to its solutions spectra as shown in Fig.24.

**Figure 24.** 1) Absorption and 2) Emission of **IWK** in solutions and thin solid film

compounds, which may limit the usefulness of **IWK** in OLED applications.

**3.5. Photoluminescence quantum yields** 

The same relation is observed for **IWK** emission properties in solution as well as in thin films. However, in solid state its emission is very weak compared to pyranylidene type

Photoluminescence quantum yield (PLQY) of the investigated compounds in solution and in thin films is measured by using an integrating sphere (Sphere Optics) coupled to a CCD spectrometer [41]. PLQY thus measured for all compounds are summarised in Table 3. Compounds with more polar groups attached exhibit PLQY up to 0.54 in dilute solutions, which is slightly higher than for **DCM** dye in similar surroundings [42, 43]. PLQY depends slightly on the acceptor group as can be seen from Table 3. That means that compounds with a stronger electron acceptor group have higher PLQY. **JWK** and **ZWK** molecules with two electron donor groups have lower PLQY in comparison with one electron donor group. However, the opposite is observed with **DWK** compounds, as molecules with two electron donor groups exhibit larger PLQY. This may be due to the shielding of the acceptor group by bulky trityloxyethyl groups. PLQY of pure films is found to be more than one order of magnitude lower than that in solution. This reduction is particularly strong in the case of molecules with two donor groups. PLQY values of these compounds correlate with the intensity of the long wavelength fluorescence band, as PLQY is lower in materials with a stronger low energy fluorescence band. Molecular distortions taking place during formation of solid films are probably responsible for both of these effects. Compound molecules with

The fluorescence spectra of molecules with two electron donor fragments are broader and further Stokes shifted than molecules with only one electron donor fragment. This may be attributed to the different conjugation lengths as indicated by the absorption spectra. The peak positions of **DWK-2**, **ZWK-2** and **JWK-2** are observed at 640, 678 and 701 nm, respectively. The red shift of the absorption spectra of solutions increases corresponding sequentially to **ZWK**, **JWK** and **DWK**, as stronger electron acceptor fragments induce larger red shifts. This could be explained by their electron withdrawing properties, which differ among our investigated electron acceptor fragments. The shift of luminescence spectra did not maintain the same sequence due to the larger Stokes shift for the **JWK** molecules.

The absorption spectra of thin solid films of the molecules with one electron donor fragments are practically unchanged with respect to the solutions spectra. They are slightly broader with small red-shift indicating a weak excitonic interaction in the solid state, which is typical for glass-forming amorphous materials. For the molecules with two electron donor fragments **ZWK** and **DWK** the absorption spectra are found to shift by 21 nm and 22 nm, respectively. The peak positions of the absorption spectra for **JWK** molecules remain unchanged by the incorporation of a second electron donor fragment. However, the fluorescence spectra of all films are red-shifted in comparison with those of solution.

For molecules with one electron donor fragment, the shape of the fluorescence spectra of thin films is very similar to that in solution, which confirms that for these compounds the excited states in the aggregates in the solid state are not very different from those in molecules. However, the derivatives with two electron donor fragments exhibit an additional band at longer wavelengths in thin films, which becomes more intense going from weaker to stronger electron acceptor fragments in the studied molecules. In the case of **ZWK-2** in thin films the additional band even becomes dominant.

In the case of **IWK**, the absorption and luminescence spectra of thin solid films are also found to be practically unchanged compared to its solutions spectra as shown in Fig.24.

**Figure 24.** 1) Absorption and 2) Emission of **IWK** in solutions and thin solid film

The same relation is observed for **IWK** emission properties in solution as well as in thin films. However, in solid state its emission is very weak compared to pyranylidene type compounds, which may limit the usefulness of **IWK** in OLED applications.

#### **3.5. Photoluminescence quantum yields**

216 Organic Light Emitting Devices

(see Fig.22(1)).

molecules.

maxima red-shifted to 635 and 627 nm, respectively. The photoluminescence spectra are unstructured and strongly Stokes shifted in accordance with intramolecular charge-transfer nature of the excited states [39]. For compounds containing two 4-((N,N-ditrityloxyethyl) amino)styryl electron donor fragments the absorption and luminescence spectra of the solution are observed to be red-shifted and have larger extinction coefficients, which is due to the larger absorption cross section of these molecules. The peaks of the absorption spectra of **DWK-2** and **ZWK-2** are red shifted by 17 and 11 nm, respectively, compared to molecules with a single electron donor fragment. A similar red shift has been reported for the molecule with two electron donor fragments **bis-DCM** compared with **DCM** molecules with a single electron donor fragment [40]. It is observed that molecules with two electron donor fragments have a larger conjugation length. A second reason could be simultaneously functioning two donor groups which give stronger electron donor properties. The shape of the absorption spectrum of **JWK-2** is found to be different from that of **JWK-1** and the oscillator strength of the absorption band of **JWK-2** at about 502 nm becomes more intense

The fluorescence spectra of molecules with two electron donor fragments are broader and further Stokes shifted than molecules with only one electron donor fragment. This may be attributed to the different conjugation lengths as indicated by the absorption spectra. The peak positions of **DWK-2**, **ZWK-2** and **JWK-2** are observed at 640, 678 and 701 nm, respectively. The red shift of the absorption spectra of solutions increases corresponding sequentially to **ZWK**, **JWK** and **DWK**, as stronger electron acceptor fragments induce larger red shifts. This could be explained by their electron withdrawing properties, which differ among our investigated electron acceptor fragments. The shift of luminescence spectra did not maintain the same sequence due to the larger Stokes shift for the **JWK**

The absorption spectra of thin solid films of the molecules with one electron donor fragments are practically unchanged with respect to the solutions spectra. They are slightly broader with small red-shift indicating a weak excitonic interaction in the solid state, which is typical for glass-forming amorphous materials. For the molecules with two electron donor fragments **ZWK** and **DWK** the absorption spectra are found to shift by 21 nm and 22 nm, respectively. The peak positions of the absorption spectra for **JWK** molecules remain unchanged by the incorporation of a second electron donor fragment. However, the

fluorescence spectra of all films are red-shifted in comparison with those of solution.

**ZWK-2** in thin films the additional band even becomes dominant.

For molecules with one electron donor fragment, the shape of the fluorescence spectra of thin films is very similar to that in solution, which confirms that for these compounds the excited states in the aggregates in the solid state are not very different from those in molecules. However, the derivatives with two electron donor fragments exhibit an additional band at longer wavelengths in thin films, which becomes more intense going from weaker to stronger electron acceptor fragments in the studied molecules. In the case of Photoluminescence quantum yield (PLQY) of the investigated compounds in solution and in thin films is measured by using an integrating sphere (Sphere Optics) coupled to a CCD spectrometer [41]. PLQY thus measured for all compounds are summarised in Table 3. Compounds with more polar groups attached exhibit PLQY up to 0.54 in dilute solutions, which is slightly higher than for **DCM** dye in similar surroundings [42, 43]. PLQY depends slightly on the acceptor group as can be seen from Table 3. That means that compounds with a stronger electron acceptor group have higher PLQY. **JWK** and **ZWK** molecules with two electron donor groups have lower PLQY in comparison with one electron donor group. However, the opposite is observed with **DWK** compounds, as molecules with two electron donor groups exhibit larger PLQY. This may be due to the shielding of the acceptor group by bulky trityloxyethyl groups. PLQY of pure films is found to be more than one order of magnitude lower than that in solution. This reduction is particularly strong in the case of molecules with two donor groups. PLQY values of these compounds correlate with the intensity of the long wavelength fluorescence band, as PLQY is lower in materials with a stronger low energy fluorescence band. Molecular distortions taking place during formation of solid films are probably responsible for both of these effects. Compound molecules with

two bulky acceptor groups are probably strongly distorted in solid films, so that molecular chains connecting acceptor and donor moieties are twisted. Such twisting usually leads to a red-shift in the molecular fluorescence and to fast non-radiative relaxation [44]. The twisted molecules form energy traps in solid films, which may be populated during the excitation diffusion. Therefore, even a small fraction of distorted molecules may significantly affect the fluorescence spectrum and PLQY. We were unable to measure PLQY in **IWK** pure thin solid films. Moreover, it also shows the lowest value in solution and therefore cannot be used as a light-emitting material.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 219

At higher concentrations (>3 wt%) the **DWK-1** molecule shows negligible photoluminescence quenching dependence on concentration. On the other hand molecules with two donor groups exhibit pronounced quenching. Fluorescence efficiency of the polymer film doped with 10 wt% of **DWK-2** molecules decreases 2-times compared to that of films doped with 10 wt% **DWK-1** molecules. The reason for the lower PLQY could be the same as for different PLQY of the pure films. The laser dye **DCM** dispersed in the polymer matrix at high concentration shows a remarkable fluorescence quenching. For example, at a 10 wt% concentration of **DCM** molecules, up to a 4-time decrease of quantum yield is observed in comparison with the same concentration of **DWK-1** molecules. Thus, incorporation of bulky trityloxyethyl groups prevents the formation of aggregates of the dye molecules and remarkably reduces the fluorescence quenching dependence on

**DCM** molecule is a well known laser dye. In a previous work light amplification was

**N**

**O**

**Alq3**

In order to test the light amplification prospects of our synthesized compounds, we prepared pure thin films of all the compounds on a quartz substrate and measured their amplified spontaneous emission (ASE). Such emission was observed only for four of six

**DWK-1 ZWK-1 DWK-2 JWK-1**

**N**

**O Al O**

**N**

**Figure 26.** Tris(8-hydroxyquinolinato)aluminium (**Alq3**) is a well known light-emitting material.

compounds, **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1,** as shown in Fig.27 [46].

**Figure 27.** ASE spectrum in pure films of compounds **ZWK-1**, **DWK-1**, **DWK-2** and **JWK-1**

610 630 650 670 690 710 730 750 **Wavelength (nm)**

0 0.2 0.4 0.6 0.8 1 1.2

**ASE Intensity (r.u.)**

concentration, enabling the use of higher doping levels in emissive layers.

demonstrated in **DCM:Alq3** (see Fig.25 and Fig.26) thin films [45].

**3.6. Amplified spontaneous emission properties** 


**Table 3.** Photoluminescence quantum yield of investigated molecules in dichlormethane solutions and pure thin films.

It is worth mentioning that **DCM** molecules do not show any photoluminescence from pure films due to the small distance between molecules which results in high molecular interaction. Therefore, host-guest films of transparent polymethylmethacrylat (**PMMA**) polymer with varying dye doping were prepared in order to observe the impact of concentration on photoluminescence quenching. The dependence of PLQY on concentration of **DWK-1** and **DWK-2** molecules is shown in Fig.25.

**Figure 25.** The dependence of PLQY on concentration of **DWK-1**, **DWK-2** and **DCM** dyes in **PMMA** matrix.

For comparison the PLQY of **DCM** in **PMMA** are also included in Fig.25. **PMMA** films doped with **DWK-1** and **DWK-2** at low concentration (<1 wt%) exhibit somewhat lower PLQY as compared to that obtained in solution (See Fig.24 and Table 3). This discrepancy may be attributed to the sensitivity of molecules to the polarity of the surrounding media. At higher concentrations (>3 wt%) the **DWK-1** molecule shows negligible photoluminescence quenching dependence on concentration. On the other hand molecules with two donor groups exhibit pronounced quenching. Fluorescence efficiency of the polymer film doped with 10 wt% of **DWK-2** molecules decreases 2-times compared to that of films doped with 10 wt% **DWK-1** molecules. The reason for the lower PLQY could be the same as for different PLQY of the pure films. The laser dye **DCM** dispersed in the polymer matrix at high concentration shows a remarkable fluorescence quenching. For example, at a 10 wt% concentration of **DCM** molecules, up to a 4-time decrease of quantum yield is observed in comparison with the same concentration of **DWK-1** molecules. Thus, incorporation of bulky trityloxyethyl groups prevents the formation of aggregates of the dye molecules and remarkably reduces the fluorescence quenching dependence on concentration, enabling the use of higher doping levels in emissive layers.

#### **3.6. Amplified spontaneous emission properties**

218 Organic Light Emitting Devices

light-emitting material.

pure thin films.

matrix.

two bulky acceptor groups are probably strongly distorted in solid films, so that molecular chains connecting acceptor and donor moieties are twisted. Such twisting usually leads to a red-shift in the molecular fluorescence and to fast non-radiative relaxation [44]. The twisted molecules form energy traps in solid films, which may be populated during the excitation diffusion. Therefore, even a small fraction of distorted molecules may significantly affect the fluorescence spectrum and PLQY. We were unable to measure PLQY in **IWK** pure thin solid films. Moreover, it also shows the lowest value in solution and therefore cannot be used as a

Solution Thin film

DWK-1 0.32 0.026 DWK-2 0.43 0.009 JWK-1 0.47 0.011 JWK-2 0.32 0.007 ZWK-1 0.54 0.01 ZWK-2 0.4 0.003 IWK 0.098 -

**Table 3.** Photoluminescence quantum yield of investigated molecules in dichlormethane solutions and

It is worth mentioning that **DCM** molecules do not show any photoluminescence from pure films due to the small distance between molecules which results in high molecular interaction. Therefore, host-guest films of transparent polymethylmethacrylat (**PMMA**) polymer with varying dye doping were prepared in order to observe the impact of concentration on photoluminescence quenching. The dependence of PLQY on concentration

**Figure 25.** The dependence of PLQY on concentration of **DWK-1**, **DWK-2** and **DCM** dyes in **PMMA**

For comparison the PLQY of **DCM** in **PMMA** are also included in Fig.25. **PMMA** films doped with **DWK-1** and **DWK-2** at low concentration (<1 wt%) exhibit somewhat lower PLQY as compared to that obtained in solution (See Fig.24 and Table 3). This discrepancy may be attributed to the sensitivity of molecules to the polarity of the surrounding media.

of **DWK-1** and **DWK-2** molecules is shown in Fig.25.

**DCM** molecule is a well known laser dye. In a previous work light amplification was demonstrated in **DCM:Alq3** (see Fig.25 and Fig.26) thin films [45].

**Alq3**

**Figure 26.** Tris(8-hydroxyquinolinato)aluminium (**Alq3**) is a well known light-emitting material.

In order to test the light amplification prospects of our synthesized compounds, we prepared pure thin films of all the compounds on a quartz substrate and measured their amplified spontaneous emission (ASE). Such emission was observed only for four of six compounds, **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1,** as shown in Fig.27 [46].

**Figure 27.** ASE spectrum in pure films of compounds **ZWK-1**, **DWK-1**, **DWK-2** and **JWK-1**

From the other two samples of **JWK-2** and **ZWK-2** no ASE signal has been observed. The peak positions of ASE are red shifted as compared to the fluorescence band maxima (see Fig.27 and Fig.22). The red shift values were found to be 14, 18, 10 and 31 nm for **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1,** respectively. Variations in the peak intensity of ASE spectra as a function of the pump beam pulse energy are shown in Fig.28, from which ASE threshold values are estimated to be 90±10, 330±20, 95±10, 225±20 μJ/cm2 for **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1**, respectively. These values are larger in comparison with the threshold values (of the order of micro joules per square centimeter) reported for some other materials [46, 47].

Synthesis and Physical Properties of Red Luminescent

*N n* (1)

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 221

 

<sup>0</sup> ( ) \* [( ( ) \* ( )] ( \*) ( ) *em a n*

where *n*\* is the density of excited molecules, *N* is the total density of molecules, 0(),em()and \*() are cross-sections of the ground state absorption, stimulated emission and excited state absorption, respectively. As it can be seen from Eq. (1) even weak ground state absorption may strongly reduce the amplification coefficient or make it negative. This is because only a small fraction of molecules is usually excited even under high intensity excitation conditions, i.e., N>>n\*. Thus, the absorption band tails, which overlap with fluorescence band, are evidently responsible for the red shifts in ASE spectra in comparison with the maxima of the fluorescence. Note, that the light propagation length is limited by the film thickness in the absorption measurements, while ASE emission can propagate a

**3.7. Photoelectrical properties and energy structure of glassy thin films** 

from the spectrum of the quantum efficiency of photoconductivity β(hυ) [49]:

 

Information about the location of energy levels enables one to determine the best sample structure for electroluminescence measurements. To characterize the energy gap in organic solids several methods are applied. In organic crystals as well as amorphous solids charge carriers do not emerge as "bare" quasi-free electrons and holes but as a polaron type quasiparticle, dressed "in electronic and vibronic polarization clouds" [48, 49]. Electronically relaxed charges may be formed far enough from each other which give rise to a wider optical band gap EGOpt [49, 50]. The optical energy gap EGOpt may be obtained from the low energy threshold of the absorption spectra of organic thin films. The vibrationally and electronically relaxed charge carrier states contribute to the adiabatic energy gap EGAd. It could be attributed to the threshold energy of photoconductivity Eth which can be estimated

> ( ,) ( ,) ( )( ) ( ) *ph j hU h U*

where jph is the density of photocurrent at a given photon energy hand applied voltage U, I(h) is the intensity of light (photons/cm2s), k(h) is the transmittance of the semitransparent electrode and g(h) is the coefficient which characterizes the absorbed light

Eth can be determined from a sample where the organic compound is sandwiched between two semitransparent electrodes, which in our case are ITO and thermally evaporated aluminum. The sample is irradiated through the electrodes and current changes are measured as shown in Fig.29(1). Efficiency of photoconductivity at different light energy is calculated using Eq. (2) and is plotted as a function of the photon energy in Fig.29(2). The sample is illuminated from both aluminum and ITO side when positive and negative voltage is applied to them. Eth is determined by plotting 2/5 as a function of the photon energy. The intersections of tangents at low photon energy on the curve of 2/5 plotted as a

function of the photon energy and photon energy axis gives Eth as shown in Fig.29(3).

*kh Ih gh* 

 

(2)

 

much longer way along the film.

in the organic layer.

However, a direct comparison is difficult because the ASE threshold, in addition to material properties, depends also on the sample and excitation geometries, film thickness, optical quality and excitation pulse duration.

Nevertheless it has not been observe ASE in pure **DCM** films, but we have measured it in **DWK-1** which is the same **DCM** with additional trityloxyethyl group. It should also be noted that some sample degradation has been observed at the highest excitation intensities; however no noticeable degradation is observed when excitation intensity is 1.5 - 2 times exceeding the ASE threshold.

**Figure 28.** ASE intensity as a function of irradiation pulse energy in **DWK-1**, **DWK-2**, **JWK-1**, **ZWK-1** compounds in thin solid film. Lines are guides for the eye.

ASE develops in the spectral position where the light amplification coefficient has the maximal value. The amplification coefficient may be described as:

Synthesis and Physical Properties of Red Luminescent Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 221

$$n(\mathcal{k}) = n^\* \left[ (\sigma\_{cm}(\mathcal{k}) - \sigma^\*(\mathcal{k})) - (N - n^\*)\sigma\_0(\mathcal{k}) \right] \tag{1}$$

where *n*\* is the density of excited molecules, *N* is the total density of molecules, 0(),em()and \*() are cross-sections of the ground state absorption, stimulated emission and excited state absorption, respectively. As it can be seen from Eq. (1) even weak ground state absorption may strongly reduce the amplification coefficient or make it negative. This is because only a small fraction of molecules is usually excited even under high intensity excitation conditions, i.e., N>>n\*. Thus, the absorption band tails, which overlap with fluorescence band, are evidently responsible for the red shifts in ASE spectra in comparison with the maxima of the fluorescence. Note, that the light propagation length is limited by the film thickness in the absorption measurements, while ASE emission can propagate a much longer way along the film.

#### **3.7. Photoelectrical properties and energy structure of glassy thin films**

220 Organic Light Emitting Devices

quality and excitation pulse duration.

exceeding the ASE threshold.

From the other two samples of **JWK-2** and **ZWK-2** no ASE signal has been observed. The peak positions of ASE are red shifted as compared to the fluorescence band maxima (see Fig.27 and Fig.22). The red shift values were found to be 14, 18, 10 and 31 nm for **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1,** respectively. Variations in the peak intensity of ASE spectra as a function of the pump beam pulse energy are shown in Fig.28, from which ASE threshold values are estimated to be 90±10, 330±20, 95±10, 225±20 μJ/cm2 for **DWK-1**, **DWK-2**, **JWK-1** and **ZWK-1**, respectively. These values are larger in comparison with the threshold values (of the order of micro joules per square centimeter) reported for some other materials [46, 47].

However, a direct comparison is difficult because the ASE threshold, in addition to material properties, depends also on the sample and excitation geometries, film thickness, optical

Nevertheless it has not been observe ASE in pure **DCM** films, but we have measured it in **DWK-1** which is the same **DCM** with additional trityloxyethyl group. It should also be noted that some sample degradation has been observed at the highest excitation intensities; however no noticeable degradation is observed when excitation intensity is 1.5 - 2 times

**Figure 28.** ASE intensity as a function of irradiation pulse energy in **DWK-1**, **DWK-2**, **JWK-1**, **ZWK-1**

ASE develops in the spectral position where the light amplification coefficient has the

compounds in thin solid film. Lines are guides for the eye.

maximal value. The amplification coefficient may be described as:

Information about the location of energy levels enables one to determine the best sample structure for electroluminescence measurements. To characterize the energy gap in organic solids several methods are applied. In organic crystals as well as amorphous solids charge carriers do not emerge as "bare" quasi-free electrons and holes but as a polaron type quasiparticle, dressed "in electronic and vibronic polarization clouds" [48, 49]. Electronically relaxed charges may be formed far enough from each other which give rise to a wider optical band gap EGOpt [49, 50]. The optical energy gap EGOpt may be obtained from the low energy threshold of the absorption spectra of organic thin films. The vibrationally and electronically relaxed charge carrier states contribute to the adiabatic energy gap EGAd. It could be attributed to the threshold energy of photoconductivity Eth which can be estimated from the spectrum of the quantum efficiency of photoconductivity β(hυ) [49]:

$$\beta(h\nu, lL) = \frac{j\_{ph}(h\nu, lL)}{k(h\nu)I(h\nu)\g(h\nu)}\tag{2}$$

where jph is the density of photocurrent at a given photon energy hand applied voltage U, I(h) is the intensity of light (photons/cm2s), k(h) is the transmittance of the semitransparent electrode and g(h) is the coefficient which characterizes the absorbed light in the organic layer.

Eth can be determined from a sample where the organic compound is sandwiched between two semitransparent electrodes, which in our case are ITO and thermally evaporated aluminum. The sample is irradiated through the electrodes and current changes are measured as shown in Fig.29(1). Efficiency of photoconductivity at different light energy is calculated using Eq. (2) and is plotted as a function of the photon energy in Fig.29(2). The sample is illuminated from both aluminum and ITO side when positive and negative voltage is applied to them. Eth is determined by plotting 2/5 as a function of the photon energy. The intersections of tangents at low photon energy on the curve of 2/5 plotted as a function of the photon energy and photon energy axis gives Eth as shown in Fig.29(3).

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 223

between optical band gaps and photoconductivity threshold values with correlation coefficient 0.993. The slope of this linear relation is found to be 1 and intercept 0.28 as shown

**Figure 30.** Cyclic voltamperogramme curves of compounds **WK-1** and **WK-2**. Posistive values are

oxidation potential and negative values reduction potential.

in Fig.31

**Figure 29.** 1) Photocurrent at different wavelength for **JWK-2** compound ,2) Dependency of photoconductivity efficiency on photon energy for **JWK-2** compound,3) Determination of Eth from photoconductivity efficiency spectral dependence.

Optical band gap EGOpt, photoconductivity threshold value Eth and reduction-oxidation potential Uredox, determined from cyclic voltamperogramme, for investigated compounds are presented in Table 4.


**Table 4.** Optical band gap EGOpt, photoconductivity threshold value Eth and red-ox potential Uredox for the compunds **DWK-1**, **DWK-2**, **JWK-1**, **JWK-2**, **ZWK-1**, **ZWK-2**.

According to Table 4, the redox potential of **DWK**, **JWK** and **ZWK** is higher for compounds with one electron donor group compared to compounds with two electron donor groups (see Fig.17). The same relation is found for optical band gap as well.

The photoconductivity threshold value cannot be obtained for **DWK-2** thin films due to the low value of photocurrent. For other compounds we obtain an excellent linear correlation between optical band gaps and photoconductivity threshold values with correlation coefficient 0.993. The slope of this linear relation is found to be 1 and intercept 0.28 as shown in Fig.31

222 Organic Light Emitting Devices

**Figure 29.** 1) Photocurrent at different wavelength for **JWK-2** compound ,2) Dependency of photoconductivity efficiency on photon energy for **JWK-2** compound,3) Determination of Eth from

Optical band gap EGOpt, photoconductivity threshold value Eth and reduction-oxidation potential Uredox, determined from cyclic voltamperogramme, for investigated compounds are

> EGOpt (eV) Eth (eV) Uredox (V) DWK-1 2.20 1.92 2.35 DWK-2 2.10 - 1.99 JWK-1 2.08 1.78 2.01 JWK-2 1.88 1.62 1.90 ZWK-1 2.08 1.78 2.04 ZWK-2 1.96 1.68 2.00

**Table 4.** Optical band gap EGOpt, photoconductivity threshold value Eth and red-ox potential Uredox for

According to Table 4, the redox potential of **DWK**, **JWK** and **ZWK** is higher for compounds with one electron donor group compared to compounds with two electron donor groups

The photoconductivity threshold value cannot be obtained for **DWK-2** thin films due to the low value of photocurrent. For other compounds we obtain an excellent linear correlation

photoconductivity efficiency spectral dependence.

the compunds **DWK-1**, **DWK-2**, **JWK-1**, **JWK-2**, **ZWK-1**, **ZWK-2**.

(see Fig.17). The same relation is found for optical band gap as well.

presented in Table 4.

**Figure 30.** Cyclic voltamperogramme curves of compounds **WK-1** and **WK-2**. Posistive values are oxidation potential and negative values reduction potential.

Synthesis and Physical Properties of Red Luminescent

Glass Forming Pyranylidene and Isophorene Fragment Containing Derivatives 225

organic compound. This is confirmed by the current voltage characteristics shown in Fig.32. A similar current is observed at the lower voltage where only holes are injected either from ITO or aluminium when biased with a positive voltage. At higher voltage current is higher

**Figure 32.** Current-voltage characteristics of pure thin films of **ZWK**, **JWK**, **DWK** compounds. Solid line – compounds with one electron donor group, dashed line - compounds with two electron donor

The temperature modulated space charge limit current (TM SCLC) method is used to analyse the charge carrier local trapping states in solid films [55]. The condition for using this method (TM SCLC) is monopolar injection, which is achieved in our case when a positive voltage is applied to the aluminium electrode. The measured activation energy is plotted as a function of the applied voltage for the investigated compounds as shown in

No charge carrier local trap states are found in films of compounds with one electron donor group due to only one plateau which reaches zero. All compounds with two electron donor groups are found to have charge carrier trap states. The additional plateau of activation energy, which can be clearly seen from Fig.33 means that the thin films contain local trap states. The hole shallow trap depths are found to be 0.1, 0.24 and 0.3 eV in **ZWK-2 JWK-2** and **DWK-2,** respectively. Such trap states decrease the efficiency of electroluminescence and should be avoided in fabricating high efficiency light emitting diodes. The activation

group.

Fig.33.

when ITO is positive in comparison with positive aluminium.

**Figure 31.** Linear correlation between optical band gap and photoconductivity threshold value. Line is the best fit with slope coefficient one.

The energy of the photoconductivity threshold is defined as the difference between the conduction levels of holes and electrons [51]. The value of the intercept implies that the optical band gap is 0.28 eV larger than the difference between the conduction levels of holes and electrons. It shows a constant energy difference between optical band gap and adiabatic gap despite the various molecule structures.
