Electroluminescent stilbenoids - electrochemically generated luminescence in solution and organic light emitting devices on a polymer blend basis

K. Kelnhofer', A. Knorr', Z-H. Tak", H. Biissler" and 1 Daub@ + Institut ftir Organische Chemie der Universitat Regensburg, Universitatsstr. 3 1, D-93040 Regensburg, FRG

Fachbereich Physikalische Chemie, Philipps-Universitat Marburg, D-35032 Marburg, FRG ++

Organic light-emitting diodes based on molecularly doped polymer blends have been fabricated and characterized using donor-acceptor substituted stilbenoids as active dye components for hole transport, electron transport and emission. A com- parison of the electrochemically generated luminescence (ECL), the photoluminescence (PL) and the electroluminescence (EL) of the organic light-emitting devices (OLEDs) is given.

1. Introduction

Electroluminescence in molecular, supramolecular and technological systems is based on common fundamental processes: electrons (radical anions) and holes (radicalca- tions) generated by an applied voltage collapse to yield ex- citons whose radiative decay produces visible light. Elec- troluminescence can be achieved in a variety of experimen- tal setups, ranging from electrochemical activation in solu- tion (ECL) [ 13 to light-emitting cells (LECs) [2] and light- emitting diodes (LEDs) [3]. Their potential in application is in the realm of analytical and sensor chemistry and, re- cently, in material science.

Electroluminescent devices based on organic thin layers (OLEDs) are one of the most promising next-generation flat panel display systems [4]. The structure of the devices is manifold, having thin layers sandwiched between two elec- trodes, and they can be formed either by solution casting or by vacuum evaporation. Electroluminescence has been ob- served from single crystals [5], conjugated polymers [6],

photonic energy. In the present project we were mainly in- terested in the question of whether multifunctional dyes containing (a) an electron donor group, (b) an emissive sub- unit and (c) an electron acceptor group (Scheme 1) [ 101 can be tuned for application as single active components in electroluminescent devices. In order to study and describe the fundamental processes, heterocyclic fluorophores bear- ing electron-transfer active subunits were characterized (a) by means of ECL experiments in solution and (b) in a polymer-embedded single-layer OLED.

2. Experimental

The compounds SN-P and 0 0 - P were synthesized as de- scribed elsewhere [ I 13. Single-layer LED structures were manufactured by spin-coating a chloroform solution of a mixture of SN-P and 004' (20 wt.-%) in polysulfone (PSu) as the binder polymer (structures see Schemes 2 and 4) onto commercial IT0 glass (Baltacron, Balzers). After

I 1

Scheme 1. General structure of the donor-acceptor-substituted dye (D-F-A) as active component in electroluminescent devices.

oligomers [7] and low-weight molecular dyes [S]. A device configuration for light-emitting electrochemical cells (LECs) consisting of a simultaneously p- and n-type doped thin film of conjugated polymer containing additional poly- electrolyte has previously been described [9].

Comparative studies on light-emitting diodes, light-emit- ting cells and light-emitting systems with supramolecular architecture may contribute to the understanding of how molecular structure and supramolecular arrangement deter- mine the efficiency of the transformation of electrical into

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solvent evaporation an aluminum top contact, 7 mm2 in area, was vapor deposited onto the top layer (the thickness is typically 90-100 nm). Current-voltage curves were taken with a Keithley 236 source measure unit.

3. Results and discussion

3.1. Electrochemical properties

For the present study we synthesized the fluorescing stil- benoids SN-P and 0 0 - P (Scheme 2), which are easily ox- idized and reduced. The phenothiazine subunit in SN-P is a

188 Acta Polymer., 48, 188-192 (1997) 0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinheim, 1997 0323-7648/97/0303-188$17.50 + .50/0

stronger electron-donating group than the dibenzodioxine unit in 0 0 - P . The pyrene moiety is emissive and undergoes one-electron reduction. To increase the solubility in organic solvents and to impede crystallization, tert-butyl groups were attached. The electrochemical and optoelectrochemi- cal properties were studied by cyclic voltammetry [ 121 and spectroelectrochemistry [ 131.

4-

4 -

-2

0 -

2 -

4 -

6 -

8 -

10

Table 1. Electrochemical data for the stilbenoids and the parent com- pounds obtained by cyclic voltammetry (Ep: peak potential and El": half wave potential in mV vs FOC).

-

-

E/mV vs FOC EP E1rz me," [VI

\

SN-P ox 255 2.53

0 0 - P ox 680 / 2.82 red -2210

red -2185 ox 910 red -2535 ox 325 2.86b

00 ox 995 3.53'

AE,, = E::: - for reversible electron transfer and Efed - EEx

SN-P

tat,-Butyl I for irreversible electron transfer behavior. Calculated from El" for reversible processes and from EP for ir- reversible processes. Calculated relative to Ell2 of P. ' 00-P

P 00 SN

Scheme 2. Formula of the push-pull-substituted stilbenes.

Both methods show that the dibenzodioxine derivative 0 0 - P is reversibly reduced to the corresponding radical anion. The oxidation to the radical cation is an irreversible process in solution. In contrast, both the one-electron oxida- tion and the one-electron reduction of SN-P are reversible (Fig. 1). The reduction potential of radical anion formation

3

of the stilbenoid SN-P (El'* = -2270 mV) is about 300 mV less negative than the reduction potential ofpyrene (P) (El'* = -2535 mV) while its oxidation potential is approximately 70 mV more positive than that of 10-methyl- 1 O-H-pheno- thiazine (SN). The electrochemical data are summarized in Table 1.

Some of the species formed on reduction and oxidation can be characterized by their absorption spectra. The radi- cal anion of the dibenzodioxine derivative 00-P' - absorbs at 596, 646 (sh), 114S1325 nm. For comparison, the ab- sorption maxima of P'- are at 488,577 (sh), 733,930, and 1030 nm. The radical cation SN-P" displays two long- wavelength absorption bands at 604, 664 (sh), 764 (weak) and ~ 1 1 1 5 nm (broad). The corresponding absorption bands of the radical cation (SN") are at 510, 757 and 846 nm. A comparison of the spectra gives evidence that the radical cation is preferentially centered at the heterocyclic substructure whereas the radical anion is mainly located in the pyrene substructure. Compared with the absorptions of the radical ions of the parent systems P'- and SN", the NIR absorption bands of SN-P" and 00-P'- are significantly shifted to lower energy, which indicates the delocalization within the more extended n-system [14].

3.2. Optical properties

The photoluminescence spectra [ 151 show typical fea- tures of the dependence on both the concentration (excimer formation), as is well known for the pyrene chromophore, and the solvent (solvatochromism) typical of donor-accep- tor substituted stilbenes. Representative emission spectra of SN-P in different solvents are given in Fig. 2 exhibiting bands in cyclohexane and in EEP at -1 96 "C at 480/5 15 (sh) nm, respectively 480/500 (sh) nm. In more polar solvents the emission maxima are shifted to longer wavelengths up to 595 nm in acetonitrile. The formation of intramolecular charge-transfer states (ICT) may account for these findings [ 161. The presence of an excimer emission can be deduced from the broad and low intensity emission of dibenzodiox- ine 0 0 - P at the low-energy wavelength band edge (Fig. 3)

ECL measurements were undertaken in acetonitrile and the radical anions and radical cations were generated se- quentially by switching alternately between the experimen- tally determined electrochemical potentials of reduction (Ered) and oxidation (E,,J. This forms an excess concentra-

~171.

Acta Polymer., 48, 188-192 (1997) Electroluminescent stilbenoids 1 89

8000 { cyclohexy

2000

0

EEP

- I I

400 600 800

wavelength [nm]

Fig. 2. Photoluminescence of SN-P in different solvents.

2000

s: 500

4 g :ooo ." m E M

500

0

coo 600 e

wavelenglh nm

Fig. 3. Photoluminescence of 00 -P in acetonitrile at different con- centrations.

tion of oppositely charged radical ions, which (by inter- molecular electron transfer) leads to electronically excited species. Practically, there are several mechanistic routes possible: (a) S-route: the direct population of the first ex- cited singlet state after radical ion annihilation; (b) T-route: the population of the first excited triplet state followed by a T-T annihilation step finally leading to an up-conversion into the first excited singlet state; (c) E-route: formation of emitting excimer and exciplex states.

The phenothiazine derivative SN-P exhibits almost identical emission spectra on photochemical and electro- chemical activation [18] (Fig. 4). In the case of the diben- zodioxine derivative 0 0 - P the electrochemically generated luminescence is significantly weaker (as expected from the lower stability of the radical ions) and shifted to longer wavelength (Fig. 5) . Obviously, the excimer emission of 0 0 - P is generated preferentially. Therefore, SN-P appears to be the prime candidate for LED fabrication because it gives stable radical ions and has a comparatively more effi- cient luminescence [ 191.

I I coo 500 600 700 800

wavelenglh nm Fig. 4. Photoluminescence and electrochemiluminescence spectra of SN-P in acetonitrile.

4000

P 2 s n

2 e

.H

.- 3 2000

0

c 300 400 500 600 7

wavelength [nm]

0

Fig. 5. Photoluminescence and electrochemiluminescence spectra of 00-P in acetonitrile.

3.3. Energetics and ECL experiments

For the evaluation of the energetics of generating the ex- cited states by electrochemical activation (ea), the potential differences of the two half-reactions AGea = E,, -Ered are of significance since they offer the limiting energy which can be electrochemically achieved and which can be stored by the radical ion pairs. The values of me, of the stilbenoids SN-P, 0 0 - P and of the parent compounds pyrene (P), dibenzodioxine (00), and phenothiazine (SN) are likewise included in Table 1.

The energetics of the ECL experiment was calculated based on the following equation:

AGea 1 AlT"" - TAS with TAS - 0.1 eV [20]

AG,, equals the difference of the two half-wave potentials which can be obtained from the peak potentials of the oxi- dation and reduction half reactions [21] and M&, repre- sents the energy of the first excited states [22]. The total

190 Kelnhofer, Knorr, Tak, Bassler, Daub Acta Polymer., 48, 188-192 (1 997)

1.0 0.9

0.8 0.7

J 0.6

- -- -- - - - -

a

0.3 0.2 0.1

- - - - - -

~ ~ g $ ~ q g g s ~ m w n m n o m e n m m m * n n % 2 S % $ S 3 3 % e e Esc: wavelength [nm]

Fig. 6. Photoluminescence (thick) and electroluminescence spectra of SN-P single layer (20%): PSu.

140 - n- 120.-

0 5 10 15 30

voltage M

Fig. 7. Current-voltage characteristic of SN-P (m) and 0 0 - P (-).

IT0 SN-P Al Scheme 3. Energy diagram.

Scheme 4. Formula of the polysulfone binder polymer.

free energy (AGea) for SN-P'+/SN-P'- annihilation is calcu- lated to be 2.5 eV based on the difference between the half- wave potentials (Scheme 3). The energy necessary to popu- late the first excited singlet state (AHexc) is roughly ob- tained fi-om the maximum of the emission signal at 590 nm and amounts to approximately 2.25 eY As already men- tioned the energetic evaluation of the ECL of 0 0 - P must be treated with caution due to the irreversibility of the radical

cation formation. Nevertheless, the free energy released during the recombination of the radical ions (AG,,, = 2.8 eV) in this system is insufficient to populate the first ex- cited singlet state of 0 0 - P via the S-route (AH? + -0.1 eV = 2.85 eV), but it is presumably sufficient to produce excimers leading to the broad long wavelength emission band.

3.4. Single-layer LEDs

The stilbenoids SN-P and 0 0 - P were then dissolved in a solution of polysulfone (Scheme 4) in chloroform, spin-cast on an IT0 anode and finally contacted with an aluminum cathode. The photoluminescence spectra of the film and the electroluminescent response of the single-layer device are given in Fig. 6. The current-voltage characteristics are shown in Fig. 7. The current density plotted on a logarith- mic scale versus reciprocal field strength proves the Fow- ler-Nordheim behavior for tunneling injection [23] of the charge carriers (Fig. 8). At high fields, linearity is observed and from the slope an injection barrier height between IT0 and the organic layer of @ = 0.3 eV for the majority carriers (holes) is calculated, corresponding well with the energy level scheme from cyclovoltammetric data. The external quantum yields of the device were found to be @ ( s N - ~ ) = 3.5 x % and @(oo.p) = 3.0 x lo4 %. The addition of a hole blocking layer and variation of the polymer matrix re- sult in higher quantum yields up to 0.12 %.

Acta Polymer., 48, 188-192 (1997) Electroluminescent stilbenoids 19 1

1 .ooEHa I

Fig. 8. Fowler-Nordheim plot of SN-P.

4. Conclusion

The electrochemiluminescence of the two stilbenoids SN-P and 0 0 - P in solution and their electroluminescence in OLEDs have been investigated. As already indicated by the electrochemical reversibility of the radical cation and radical anion formation in solution and by the energetical considerations, the electrochemiluminescence of SN-P is more efficient than that of 0 0 - P . The choice of the hetero- cyclic subunits allows the electron and hole transporting properties to be determined as well as the emission wave- length in polymer blend devices.

Acknowledgement

This work has been supported by the Bundesministerium fiir Bildung und Forschung (project number: 03N1004C6).

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[ 121 Experimental conditions of the cyclic voltammetry measure- ments: solvent acetonitrile; potentials in [mV] vs. ferrocene (FOC) as reference; reversible half-wave potential El,*, anodic and cathodic peak potentials Epa and Epc. Experimental condi- tions: measurement at room temperature, scan rate: 250 mV/s, working electrode: platinum disc electrode, pseudo-reference electrode: Ag/AgCl, counter electrode: platinum spiral elec- trode; supporting electrolyte: tetrabutylammonium hexafluo- rophosphate (TBAHFP). Experimental setup of the spectroelectrochemical experiment: The spectroelectrochemical measurements were performed with the use of the optical-transparent-electrode (OTE) tech- nique. The spectra were recorded by a Perkin-Elmer LAMBDA 9 spectrophotometer. Solvent: acetonitrile, (sh = shoulder).

[ 141 The degree of delocalization corresponds to the rotational bar- riers of the central double bond. See: [ 1 Oa] and H. Spreitzer, M. Scholz, G. Gescheidt, J. Daub, Liebigs Ann. 1996,2069.

[ 151 Unless otherwise indicated, the emission spectra are uncor- rected. All ECL measurements were carried out in acetonitrile at concentrations of 5 x 10-~ - 5 x lo4 M, excitation wave- length: 00-P: 398 nm, SN-P: 375 nm (these excitation wave- lengths were also used in the records of the excimer emission). Due to the low solubility the wavelength of the excimer emission can be given only approximately since this band ap- pears as a shoulder ofthe normal emission. EEP: ether/ethanol/ isopentane 5 : 2 : 5 .

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[ 191 At present, we are unable to determine the total efficiency of the electrochemically generated luminescence quantitatively; see: P. McCord, A.J. Bard, 1 Electroanal. Chem. 1991,318,91.

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Received November 11, 1996 Final version February 13, 1997

192 Kelnhofer, Knorr, Tak, Bassler, Daub Acta Polymer., 48, 188-192 (1997)