Diquinoline Derivatives as Materials for Potential Optoelectronic

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Diquinoline Derivatives as Materials for Potential Optoelectronic Applications. Katarzyna Laba, Przemyslaw Data, Pawel Zassowski, Piotr Pander, Mieczyslaw Lapkowski, Krystian Pluta, and Andrew P. Monkman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512941z • Publication Date (Web): 19 May 2015 Downloaded from http://pubs.acs.org on May 29, 2015

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The Journal of Physical Chemistry

Diquinoline Derivatives as Materials for Potential Optoelectronic Applications. Katarzyna Laba,†,‡ Przemyslaw Data,†,‡,§,* Pawel Zassowski,† Piotr Pander,† Mieczyslaw Lapkowski,†,‡ Krystian Pluta,| Andrew P. Monkman§ †

Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice,

Poland ‡

Center of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej

34, 41-819 Zabrze, Poland. §

|

Physics Department, University of Durham, South Road, Durham DH1 3LE, United Kingdom.

Department of Organic Chemistry, The Medical University of Silesia, Jagiellonska 4, 41-200

Sosnowiec, Poland.

ABSTRACT

Here we report the characterization of diquinolines derivatives with ambipolar character. The investigated derivatives show low p- and moderately low n-doping redox systems. The derivatives were investigated and the electro-chemical and optical properties of these compounds measured, with attempts to correlate the properties with their π-electron conjugation lengths. We showed the marked influence of the heteroatom in the bridge (central ring) and geometry of the

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molecule with its electronic properties. The compounds were compared by UV-Vis-NIR and EPR spectroelectrochemistry indicating very different charge and spin properties dependent on the heteroatoms present. Finally the OLED devices were formed and characterized.

1. INTRODUCTION The OLED industry is very demanding in its search for new compounds that could be used as the emitter material or in transporting layers. The diquinoline group, often used in many different areas, has recently emerged as a potential candidate for optoelectronic applications. Quinoline and its derivatives are better known for their significant importance in biological and pharmaceutical activities (e.g. antiasthmatic, antibacterial, antiinflammatory).1-3 In the literature it is possible to find that quinolines exhibit broad chemical properties, as for example optoelectronic and non-linear optical properties, often combined with excellent mechanical properties.2,4 These make quinolines potentially important for many applications in molecular electronics and optoelectronics, particularly as active components of light emitting diodes but also as detectors, solar cells, thin film transistors and an electrochromic device.5-8 Since the first quinoline was synthesized, a variety of small molecular quinoline derivatives and polymers were tested for potential use as active materials in organic light-emitting diodes (OLEDs) with the aim of achieving stable OLEDs with high brightness and external quantum efficiency.4,6-9 Quinolines and their polymers exhibit high electron mobilities, good thermal stability, high photoluminescence efficiencies and good layer forming properties, which are significant and desirable in application in OLEDs.10 The correct operation of OLEDs based on conductive organic compounds requires maintaining a balanced distribution of injected charge carriers positive (holes) and negative (electrons).11 Thus the use of multilayer OLEDs with a layer of tris8-hydroxyquinoline as the electron-transporting layer has contributed to increasing the intensity


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of the emitted blue light and a substantial extension of working time.12-17 Some of the most thermally stable semiconducting quinolines (e.g. 4-phenyl or alkyl quinoline) are also known to be excellent electron-acceptors. Moreover in the literature it has been reported that polyquinoline may exhibit electroluminescence ranging from the blue to the red spectrum, by adapting the molecular structure and morphology of the polymer.18 The optical and electronic properties of polyquinolines have been examined extensively.4,10 Furthermore, some pyrazoloquinoline derivatives show fluorescence in the blue and bluish-green region.18-21 The disadvantages of the known quinolines and their derivatives are still: low quantum efficiency, poorly balanced electron and hole injection and transport. These properties required improvement.4,12 Combined pentacyclic heterocycles, both linear heteropentacenes and angular heteropentacenes have recently been reported in the literature as new types of electron donors showing significant conductivity and photoelectric properties.22,23 Based on this, we took by analogue to the pentacene heterocyclics, three diquinoline derivatives, formed by the combination with phenothiazine and thianthrene to determine the influence of different heteroatomic bridges (sulfur, nitrogen, Figure 1) and their geometry on the basic photochemical, electrochemical, photoluminescence properties and efficiency of diquinoline based OLED devices.

Figure 1. Structures of the investigated compounds.


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2. EXPERIMENTAL The diquinoline derivatives, thiazinodiquinoline (1,4-thiazino[2,3-c;6,5-c’)diquinoline, THDQ), thioquinanthrene (1,4-dithiino[2,3-c:5,6-c']diquinoline, THQ) and isothioquinanthrene (1,4-dithiino[2,3-c:6,5-c']diquinoline, ITHQ) were synthesized according to the procedures previously described in the literature.24,25 Electrochemical investigations were carried out using an Autolab PGSTAT20 potentiostat (Metrohm Autolab). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for electrochemical measurements to investigate the redox behavior of the diquinolines and to estimate their ionization potentials (IP) and electron affinities (EA). In all electrochemical and spectroelectrochemical measurements, the diquinolines (2.0mM) were dissolved in 0.1 M solutions of Bu4NBF4, 99% (Sigma Aldrich) in dichloromethane (DCM) solvent, CHROMASOLV®, 99.9% (Sigma Aldrich). Solvents were used without further purification. Electrochemical and spectroelectrochemical measurements were carried out under argon atmosphere at room temperature. The electrochemical cell comprised a platinum disk working electrode with 1 mm diameter working area, Ag/AgCl reference electrode and a platinum coil auxiliary electrode. The potential of the reference electrode was calibrated versus the ferrocene/ferrocenium redox couple. UV-Vis spectroscopy and spectroelectrochemistry analyses were recorded by HP 8453 (Agilent) spectrometer. The measurements were carried out in a standard 2 mm quartz cuvette equipped with ITO-coated quartz working electrode. Fluorescence properties were measured in dichloromethane using a FL2500 (Hitachi) spectrometer. Electron paramagnetic resonance (EPR) measurements were carried out using Jeol JES-FA200 spectrometer. G-factor value was determined using JEOL internal Mn(II) standard.


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Spin densities were calculated at the DFT/uB3LYP/6-31G(d) level of theory, using Firefly 7.1.G QC package,26 which is partially based on the GAMESS (US)27 source code. Plots of spin density were created with Gabedit 2.4.7.28 OLED devices were fabricated using pre-cleaned indium-tin-oxide (ITO) coated glass substrates purchased from Visiontek Ltd with a sheet resistance of 15 Ω/cm2 and ITO thickness of 100 nm. The OLED devices had a pixel size of 5 mm by 5 mm. The small molecule and cathode layers were thermally evaporated in a Kurt J. Lesker Spectros II deposition chamber at 10-6 mbar. All organic evaporated compounds were purified by Creaphys organic sublimation system, m-MTDATA - 4,4′,4′′-tris[phenyl(mtolyl)amino]triphenylamine (Sigma Aldrich), BCP - bathocuproine (Sigma Aldrich), NPB - N,N'Di-1-naphthyl-N,N'-diphenylbenzidine (TCI-Europe), TAPC - 4,4′-Cyclohexylidenebis[N,Nbis(4-methylphenyl)benzenamine] (Sigma Aldrich), TPBi - 2,2',2"-(1,3,5-Benzinetriyl)-tris(1phenyl-1-H-benzimidazole)

(LUMTEC),

OXD-7

-

2,2'-(1,3-Phenylene)bis[5-(4-tert-

butylphenyl)-1,3,4-oxadiazole] (LUMTEC), LiF (99.995%, Sigma Aldrich), Aluminium wire (99.9995%, Alfa Aesar). All organic materials and aluminium were deposited at a rate of 1 Å s-1, the LiF layer were deposited at 0.1 Å s-1. Fluorescence quantum yields (QY) of the solutions were estimated by using fluorescein standards.

3. RESULTS AND DISCUSSION Differential pulse voltammetry (DPV) curves (Figure 2a) show absolute peaks of oxidation and reduction processes. As it is well known the oxidation peak is connected with the IP energy and the reduction peak is connected with the EA energy of the materials. Both the lowest oxidation and reduction peaks were observed for the THDQ compound. Assuming that


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the electrochemical band-gap value is in general in the range between those peaks, the THDQ should have the lowest electrochemical band-gap of the three materials. Moreover the band-gap of THQ and ITHQ should have a similar value because the oxidation and reduction peaks (see Figure 2a) are at the same potentials, as expected due to the isomeric structure of both molecules. In contrast, for THDQ a multistep oxidation process is observed in the positive potentials range, and a peak at lower potential than for ITHQ is observed in the negative potentials range. These major differences occur simply as a direct result of changing one heteroatom in the center ring from N to S.

Figure 2. Electrochemical responses of analyzed compounds. a) DPV curves, b) CV curves of 2mM investigated compounds in 0.1M Bu4NBF4 DCM electrolyte.

To provide more detailed electrochemical analysis, cyclic voltammetry (CV) was employed to check the ability for electrochemical polymerization, which affects the stability of


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organic molecules during multiple cycles of doping and dedoping and finally the redox process behavior of these compounds. Thiazinodiquinoline undergoes oxidation in two reversible steps (Figure 2b). The first oxidation peak corresponds to the creation of the radical cation at –0.08 V. Upon reversing the direction of sweep voltage from the first oxidation wave, a corresponding cathodic peak (the reduction of the oxidized THDQ species) is observed at –0.27 V. During multiple voltammetry scans, the system shows good stability at potentials limited to the value of the first redox couple. The next oxidation peak of THDQ is attributed to subsequent oxidation of THDQ to a dication species. Upon reversing the direction of the sweep voltage from the second oxidation wave, corresponding cathodic peaks are observed at -0.23 V and -0.27 V. Again, during the multiple voltammetry scans, and when the potential is limited to the value of the second redox couple, the system shows good stability. In both cases electropolimerization process was not observed (Figure 2b) indicating excellent stability of the material. Changing from a phenothiazine unit to a thiantrene species strongly influences the cyclic voltammetric

results.

Thioquinanthrene

(THQ)

and

isothioquinanthrene

(ITHQ)

electrooxidation show one step irreversible processes. Such behavior suggests that the intermediate radical cations are unstable. The potential of oxidation peaks are +1,19 V and +1,06 V, respectively, for ITHQ and ITHQ. During the reverse cycle, the presence of peaks due to the reduction of products are observed, for THQ -0.12 V and for ITHQ -0.07 V. CV measurements of the doping–dedoping process, in a wide potential range, were made for all investigated compounds (Figure 3b).


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Figure 3. Cyclic voltammograms at wide potential range of 2mM compounds solutions in 0.1M Bu4NBF4 DCM electrolyte at scan rate 100 mV/s.

The IP–EA energy levels and the electrochemical band-gap of diquinoline derivatives are given in Table 1. The approximation of IP-EA energy level values are calculated from the oxidation and reduction potential onsets according to established equations (Equation 1 and 2):29-31

IP = –(Ep + 5.1) [eV]

(1)

EA = –(En + 5.1) [eV]

(2)

where En and Ep are onset reduction and oxidation potentials versus the Fc/Fc+. Ionization potential (IP) is related to the HOMO and electron affinity (EA) is related to the LUMO. The


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electrochemical band-gap is taken as the difference between the electron affinity and ionization potential energy levels (Equation 3):

EGCV = EA – IP [eV]

(3)

The magnitude of the band gap (EGCV) is a crucial property of organic semiconductors, which is responsible for the optical and electrochemical properties of the materials. The IP and EA levels are estimated at -4.8 eV and -3.3 eV for THDQ, and -6.0 eV and -2.9 eV for ITHQ. The EA energy difference between thiazinodiquinoline and isothioquinanthrene, 0.4 eV, indicates that changing the heteroatom at the bridge from N to S causes stronger electron withdrawing character from the central core structure. The IP and EA energy differences obtained respectively for THQ and ITHQ are as expected for isomeric forms the same even though they have different EA and IP energies, indicating that the band-gap is similar in both. Substitution by the sulfur in the central ring however causes a significant increase in the electrochemical band-gap, this is a surprising result. The lowest value of EGCV was obtained for THDQ (1.5 eV), for the compounds THQ and ITHQ, estimated values are very similar to each other (3.1 eV). This indicates conjugation through the nitrogen in the core, which forms a more conjugated species than their corresponding sulfur compounds. THDQ is thus conjugated to a far larger degree than the diquinolines. From these electrochemical results we conclude that THQ and ITHQ should be treated as two quinoline rings connected by a non-conjugated di-S-bridge. Such behavior is quite interesting from the stand point of tailoring linked dimers without changing the individual band-gap.


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To characterize our diquinoline systems in greater detail spectroelectrochemical techniques were used. Figure 4 presents the spectroelectrochemistry UV-Vis of diquinoline derivatives showing the absorption of the solution of each compound at different applied potentials between the compound’s neutral and fully oxidized states.

Figure 4. UV–Vis spectra recorded during electrochemical oxidation of 0.5mM of THDQ (a), THQ (b) and ITHQ (c) in 0.1M Bu4NBF4 DCM electrolyte.


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The neutral THDQ is characterized by strong absorption bands located at maxima 290 nm and 320 nm with a well defined broad but much weaker shoulders, onset at 410 nm and 510 nm. During increased oxidation of THDQ, the band at 290 nm diminishes and bands at 325 and 438 nm develop, further broad bands between 470 and 760 nm with two maximums max1 = 510 nm, max2 = 615 nm also grow in. Such behavior indicates the formation of radical cations (evolution indicated by red spectra in Figure 4) and dications (evolution indicated by blue spectra in Figure 4). One also observes that the behavior of the spectral bands at 325 nm and 510 nm are the same and diminish after the first oxidation step, indicating they represent the spectra of the radical cation. The bands at 438 nm and 760 nm similarly behave in a similar fashion and continue to grow during the second oxidation step, indicating that they representative of the second oxidation step and the dication. Given that the THDQ molecule is symmetric and the strong absorption bands consistent between the two oxidative regimes, this implies that the dication must be very similar to two weakly interacting radical cations on each side of the molecule. Very close inspection of the band at 325 nm shows that during the second oxidation step a clear shoulder at 335 nm persists into the second oxidation regime. The neutral THQ is characterized by two absorption maxima at 270 nm and 317 nm. During the oxidation process of THQ bands at 270 nm and 317 nm gradually lose their intensity as the applied potential increased. At the same time a new absorption band grows in (320–445 nm), owing to the formation of the radical cation of THQ. The neutral compound ITHQ showed one absorption peak located at maximum 275 nm. During the oxidation process of ITHQ, as the applied potential increased, the band at 275 nm diminished and a new broad radical cation band developed between at 290 and 460 nm. The optical band-gaps of each material were calculated from onset values of π–π* bands in the UV-Vis (λonset) by conversion of wavelength to the energy by equation (Equation 4):31,32


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EGOp = hc / λonset [eV]

(4)

where h is Planck’s constant, c is speed of light in vacuum, λonset is absorption onset. The results are shown in Table 1.

Table.1. Electrochemical and optical band-gap. Compound

IP

EA

EGCV EGOp

(eV) (eV) (eV)

(eV)

Thiazinodiquinoline (THDQ) -4.8

-3.3

1.5

2.4

Thioquinanthrene (THQ)

-6.2

-3.1

3.1

3.4

Isothioquinanthrene (ITHQ)

-6.0

-2.9

3.1

3.3

THQ and ITHQ optical and electrochemical band-gaps differ within the error margin so it could be assume that they represent similar values. Electronic transition observed in the UVVis spectra at maximum 290, 270, 275 nm for THDQ, THQ and ITHQ, respectively, are assumed to be π -π* transitions. The problem is when values were compared to THDQ derivative. The values of the optical band gap, EGOp is much bigger than those which have been estimated from the electrochemical experimental results. The optical and electrochemical bandgap are different by 0.9 eV, the optical band-gap being much higher which means that the difference cannot be an exciton binding energy for example. The CV oxidation peak gives a correct value of IP because during oxidation we observed generation of radical cations (see ESR section). A possible explanation is that the reduction CV curve does not represent the formation of radical anions, but instead is only the redox process which don’t involve negative charge


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carriers and cannot be used in the EA calculation. This is very important as throughout the literature this process to estimate the EA is dogmatically followed without question. To be sure that charged species are formed in the organic structures on oxidation EPR spectroelectrochemistry was conducted. In all three examples (Figure 5) it was possible to record a signal originating from the electrochemically formed radical cations, even in examples of THQ and ITHQ, where oxidation was described as irreversible.

Figure 5. Experimental and simulated EPR spectra of thiazinodiquinoline (a), thioquinanthrene (b) and isothioquinanthrene (c). Inset of spectrum shows spin density calculated with DFT.


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In the case of THDQ, oxidation results in the formation of a stable radical cation with well resolved hyperfine structure and a g-factor value of 2.0054, indicating that spin density mainly resides on the nitrogen atoms, rather than on carbon or sulfur. The EPR spectrum was simulated with a model containing three nitrogen and five hydrogen atoms (Table 2).

Table 2. Hyperfine data for compounds investigated in this study, where J is the spin number of nucleus, n is the number of atoms, while hfcc stands for hyperfine coupling constant. Thiazinodiquinoline (THDQ) hfcc

Atom J

n

N(1)

1

1

N(2)

1

H(1)

Thioquinanthrene (THQ) hfcc

Atom J

n

5.23

N(1)

1

2

2

1.88

H(1)

0.5

0.5

1

0.57

H(2)

0.5

H(2)

0.5

2

0.03

H(3)

0.5

2

0.02

(G)

Isothioquinanthrene (ITHQ) hfcc

Atom J

n

0.68

N(1)

1

2

0.67

2

0.65

H(1)

0.5

2

0.86

2

0.45

H(2)

0.5

2

0.65

(G)

(G)

Hfcc values indicate that spin density is located mainly on the heteroatoms of the central ring, with some density located on the two nitrogen atoms in the side rings. Similar conclusions can be made from the spin density plot calculated with DFT theory. For THQ and ITHQ, the recorded EPR signals possessed a g-factor value of 2.0079 and 2.0081 respectively, which is typical for organic radicals with spin located on the sulfur atoms as expected.32 Furthermore, signal intensity was significantly lower compared to THDQ (despite using identical concentration of compounds) with no clear hyperfine splitting. This would indicate low stability of the radical cations. Strong delocalization of spin on the thianthrene group is mainly responsible for lack of


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hyperfine structure. Based on DFT calculated spin density, model with nitrogen atoms and four hydrogen atoms were proposed (Table 2). Turning again to the UV-Vis spectra, fig 4. There is a clear difference in absorption between THDQ, and ITHQ or THQ. The THDQ has a weak red edge absorption feature peaked at 420 nm, shown in the inset of figure 4. The fluorescence emission and excitation spectra at maximum emission were collected from all three, fig 5, to compare if we observe an effect of this extra THDQ band between 370-500 nm. Similar fluorescence emission spectrum (Figure 6) and fluorescence yields were observed in all of the molecules (Table 3), with THDQ being the more emissive material, no new emission band is seen in the THDQ, which would support the identification of the broad weak absorption band centered at 420 nm in THDQ as being due to a weakly allowed (n, π*) transition and what was also observed on UV-Vis spectra.


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Figure 6. Excitation (solid line) and fluorescence (doted line) spectra recorded at maximum absorption peak of 10µM investigated compounds in DCM. Excitation or emission wavelengths were chosen according to table 3.

This third absorption peak (350-500 nm) in THDQ arises from the “phenothiazine unit” in the molecule and the shape and height ratio of the second and third absorption peaks (330 nm and 410 nm) are similar to those seen in pure phenothiazine but with a 90 nm bathochromic shift. A replacement of the N-bridge to S-bridge results in a delicate hipsochromic shift of what absorption (in both cases THQ and ITHQ). The shape of the spectra of THQ and ITHQ are quite similar. Again, though, the simple exchange of a core bridge S and N atom leads to large differences in the electronic spectra apart from emission.

Table.3. Optical parameters of compounds dissolved in CH2Cl2 where λab is absorption maxima, λex excitation maxima, λem fluorescence maxima and ΦFl is a fluorescence quantum yield. ΦFl λex

λem

(nm)

(nm)

Thiazinodiquinoline (THDQ)

304

549

36/30

Thioquinanthrene (THQ)

282

563

25/21

Isothioquinanthrene (ITHQ)

287

557

24/21

Compound

(%) DCM/Toluene

To study excited state relaxation dynamics in diquinoline derivatives, PL decay curves were measured (Figure 7).


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Figure 7. PL decay curves of 10µM toluene solutions of the THDQ (a), THQ (b) and ITHQ (c) derivatives excited at 348 nm. Lines are multiexponential fits of the data. In dilute solution, the three derivatives exhibited biexponential decay with a main decay time constants (τ) of 1.7−3.67 ns, and a second time constant 19.6−21.83 ns. ITHQ showed the (slightly) longest τ of 3.67 ns fluorescence lifetime. Phenothiazine core of THDQ shows better conjugation co the involvement of longer time (19.6 ns) is higher (68.3%) than in similar ITHQ


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with thianthrene core (21.43 ns as 34.8% of the emission) or even less conjugated THQ (21.83 ns as 7% of the emission). The common denominator in all of these molecules is a quinoline group so it would seem that the fluorescence main emission is from this part of the molecule.

As the final part of the investigation, the following OLED devices were fabricated: ITO/m-MTDATA (50nm)/ THDQ (30nm)/BCP (30nm)/LiF (1nm)/Aluminium, ITO/NPB (40nm)/TAPC

(20nm)/THQ

or

ITHQ

(30nm)/TPBi

(20nm)/

BCP

(20nm)/LiF

(1nm)/Aluminium. Because of the IP and EA values of the new compounds, it was difficult to choose proper injecting and transporting layer to achieve optimal devices. For the THDQ device, m-MTDATA was chosen as the hole injecting (HIL) and transporting layer (HTL), because of it’s high HOMO energy (similar to THDQ), it also acts as an effective electron blocking layer (EBL) because of it’s high LUMO energy.34 Normally a thick ETL layer used to avoid pin holes which cause short circuit. As a TPBi layer cannot be used here because it’s LUMO is to high compare to THDQ, and thick BCP or OXD-7 layer would significantly decrease the electron mobilities,35,36,37 a compromise 30 nm BCP layer were chosen. In THQ and ITHQ NPB as HTL and TAPC as EBL could readily be used, together with TPBi (ETL) and BCP (HBL) were chosen experimentally which gave the best device values (Figure 8).


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Figure 8. THDQ, ITHQ and THQ OLED characteristics. a) Current density vs bias, b) EQE vs current density, c) luminous power efficiency vs current density, d) EQE vs luminance, inset, characteristics of devices at 100 cd/m2 brightness.

The highest external quantum efficiency (EQE) for 100 cd/m2 luminance, was found for for ITHQ at 0.62%, half of theoretical maximum (c.a. 1.3%). Also the ITHQ device had the


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largest luminous power efficiency and current efficiency at low luminance ea.100 cd/m2 (1.05 lm/W, 1.39 cd/A respectively). The current turn-on voltage of all devices is very similar 2.4 V but due to the low luminance the emission turn-on voltage was at 2.7 V. These device parameters are not sufficient for application in OLEDs, but it’s clearly shows that there is a possibility to use such structures as base for further modification to form high efficient OLED devices.

4. CONCLUSIONS Three pentacyclic conjugated compounds containing quinoline units have been investigated with a view to introducing this stable moiety into OLED application, Electrochemical, spectroelectrochemical and EPR techniques have been used to elucidate the nature of stable charge species in these materials. The investigation has also enabled us to determine the effects of combining two quinoline units using different heteroatomic bridges and the isomeric structure on the basic photo and electrochemical properties, which are very important in practical applications. Our measurements show that surprisingly large difference to the electronic properties in this series occur when a single heteroatom is changed in the central ring bridge. The highest radical stability was observed for thiazinodiquinoline. For all compounds we estimated IP and EA energy levels as well as EG band-gap. The electron affinity energy difference between thiazinodiquinoline and isothioquinanthrene (0.36 eV) indicates that changing the heteroatom in the bridge from N to S causes strong electron withdrawing character. The IP and EA energy levels obtained for THQ and ITHQ compounds have very similar values as expected. The electron affinity energy difference between thioquinanthrene and isothioquinanthrene (0.2 eV) indicates that the isomer (THQ) has a slightly stronger electron


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withdrawing character. Introducing a nitrogen heteroatom into the bridge greatly reduces the electrochemical band gap by nearly a half. The band-gap decreases in the series thioquinanthrene > isothioquinanthrene > thiazinodiquinoline

(Table 1). However, we find large discrepancies

between the electrochemical band gap and the measured optical gap in THDQ, whereas both ITHQ and THQ show agreement within error. We strongly believe that in the case of THDQ, the first reduction wave does not represent true reduction which gives an incorrect estimate for EA and thus optical gap. In all cases fluorescence was observed with the strongest fluorescence found in thiazinodiquinoline. For all materials, the excitation and emission spectra are not mirror images and they all emit at very similar energies and have similar spectral shape. This we assume indicates that emission arises from the quinolone moiety in each molecule. Small differences in peak position and line shape indicate the slight effects of the structure geometry. Incorporation of sulfur atom instead of nitrogen in central ring influences not only the properties of neutral molecules, but also their radical cations, as shown by EPR spectroelectrochemical measurements. In the case of thiazinodiquinoline, the radical is located mainly at the nitrogen and sulfur atoms of the central ring, with some spin density located at side rings, mainly at two other nitrogen atoms. Very clear hyperfine splitting is observed. The presence of two sulfur atoms in the central ring localizes the radical strongly on central ring, as confirmed by the complete lack of hyperfine splitting on the EPR spectrum and the shift in gfactor. This is probably responsible for the electrochemical instability of the quinanthrene materials. Interestingly, the best OLED device were obtained from ITHQ, but device performance is not sufficient for applicability in OLEDs yet, but it’s clearly shows that there is a possibility to use such structures as base for further modification to form compounds for high efficient OLED


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devices. However, the major differences in electronic properties between these molecules induced simply by the changing of one heteroatom in the core bridge is surprising and gives new opportunities to produced tailored molecules for optoelectronic applications.

Corresponding Author *E-mail: [email protected] Funding Sources This work was supported by Polish Ministry of Science and Higher Education project no. IP2012 039572 and Mobility Plus project no. 932/MOB/2012.

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Diquinoline Derivatives as Materials for Potential Optoelectronic

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