New 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids as electron transporting materials for blended-layer organic light emitting diodes

Abstract

We describe the synthesis of 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrid molecules, e.g. 2,7-bis[2-(4-tert-butylphenyl-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene 6, 2,7-bis{4-[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 10, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene 11, 2,7-bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-spirobifluorene 13 and analogue 16, comprising the 9,9-dihexylfluorene or spirobifluorene core units to which are attached aryl- or diaryl-oxadiazole units to provide linearly extended π-conjugated systems. The X-ray crystal structure is reported for compound 11. We have fabricated single-layer organic light-emitting diodes (OLEDs) using blends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as the emissive material with the electron transport (ET) compounds 6, 10, 11, 13 and 16 added to enhance electron injection. For all the devices studied electroluminescence originates exclusively from the MEH-PPV material. The external quantum efficiencies of the devices increased with increasing concentration of the ET compound up to 95% by weight, and are greatly enhanced (>two orders of magnitude) compared to pure MEH-PPV reference devices. Further improvements have been achieved by adding a layer of PEDOT : PSS and efficiencies reach ca. 0.4% at 30 mA cm⁻² for devices in the configuration ITO/PEDOT : PSS/MEH-PPV–13 (5 : 95% by weight)/Al.

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Introduction

The discovery of electroluminescence (EL) in low molecular weight organic molecules¹ and in conjugated polymers² has led to intense worldwide interest in the synthesis and properties of new materials for incorporation into organic light emitting diodes (OLEDs) for display applications. The most widely studied emissive polymers, such as poly(phenylenevinylene) and poly(fluorene) and their derivatives, are predominantly hole-transporting materials, thus hole injection predominates. Additionally charge recombination may occur too close to the polymer/cathode interface, hence, the device efficiency is lowered due to the quenching of excitons by the metal electrode. For efficient EL, a means must be found to increase the number of electrons in the material. Ways of balancing charge injection which have met with notable success are as follows:³

(i) A low work function metal, such as calcium, can be used as the cathode to lower the energy barrier to electron injection into the polymer film. The drawback of this strategy is that such metals are highly reactive and are unstable in the atmosphere.

(ii) Multilayer structures can be assembled with an electron-transporting hole-blocking (ETHB) layer placed on top of the emissive polymer film (by spin-coating or thermal evaporation) before deposition of the cathode. This approach requires more complex fabrication procedures than those used for single-layer devices.

(iii) Electron-deficient segments can be covalently bound to the emissive polymer, either by insertion into the main-chain, as end-capping groups, or as pendant side-groups. The synthesis of these polymers can be very challenging, often requiring multi-step routes and/or specific cross-coupling reactions.

(iv) Electron-transport materials can be blended into the emissive polymer prior to deposition. Single-layer devices of this type have the advantage that their manufacture requires only a single spin-coating process.⁴

2,5-Diaryl-1,3,4-oxadiazole (OXD) derivatives are electron-deficient systems which possess good thermal and chemical stabilities and high photoluminescence quantum yields.⁵ This combination of properties has led to their use as ETHB materials in OLEDs.⁶ We have previously reported on single-layer devices using blends of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) and non-emissive, electron-transporting materials containing covalently-linked diaryloxadiazole and pyridine units.⁷ The present study concerns new OXD derivatives 6, 10, 11, 13 and 16 and their blends with MEH-PPV. Our strategy was to combine the electron-transporting properties of OXD with 9,9-dihexylfluorene units to provide blue emission,⁸ and to improve further the efficiency of the OLEDs through energy or charge transfer processes within the blended layer film.⁹

Results and discussion

Synthesis

The syntheses of the OXD–fluorene hybrid molecules 6, 10, 11, 13 and 16 are shown in Schemes 1–3. The readily-available compound 1¹⁰ was converted into the corresponding dicarboxylic acid derivative 2 by lithiation and reaction with carbon dioxide; esterification of 2 gave the diester 3 which reacted with hydrazine hydrate to give compound 4. Condensation with 4-tert-butylbenzoyl chloride gave the bis(dihydrazide) 5 which underwent dehydrative cyclisation in phosphorus oxychloride to give compound 6 (44% overall yield from 1) (Scheme 1). To explore the effects of inserting additional phenyl rings into the conjugated backbone, we synthesised compounds 10 and 11 by two-fold reaction of compounds 7 and 8^6d with 9,9-dihexylfluorene-2,7-diboronic acid 9¹⁰ under standard palladium-catalysed Suzuki cross-coupling conditions (Scheme 2). The spirobifluorene core of 13 was chosen to eliminate the formation of fluorenone defects which is a factor that can quench emission and impair device performance.¹¹ Dipinacolboronate reagent 12¹² was used for this synthesis. To achieve a further extension of the π-electron framework, Suzuki cross-coupling of 8 with 4-formylbenzene boronic acid gave 14 which underwent two-fold Wittig reaction with the bis(triphenylphosphonium) salt 15¹³ to afford product 16 (Scheme 3).

Scheme 1 i, n-BuLi, −78 °C, then CO₂, 91%; ii, methanol, H₂SO₄, reflux, 94%; iii, hydrazine monohydrate, MeOH, reflux, 96%; iv, 4-tert-butylbenzoyl chloride, pyridine, rt; v, POCl₃, 110 °C, 54% for two steps.

Scheme 2 i, Compound 9, Pd[PPh₃]₄, Na₂CO₃, THF, reflux, 10 11%, 11 44%. ii, compounds 8 and 12, Pd[PPh₃]₄, PBu^t₃, K₂CO₃, toluene, reflux, 40%.

Scheme 3 i, 4-Formylbenzene boronic acid, Pd[PPh₃]₄, Na₂CO₃, THF, reflux, 78%; ii, EtOH–THF, NaOEt then HCl, 43%.

X-Ray crystal structure of compound 11

The asymmetric unit of 11 contains one molecule (Fig. 1), which has neither crystallographic nor approximate symmetry.§ The fluorene moiety is planar within ±0.03 Å and forms dihedral angles of 21.9° and 30.3° with the adjacent benzene rings i and iv. In contrast, the angles between the oxadiazole and adjacent benzene rings are small, viz.i/ii 5.0, ii/iii 6.7, iv/v 11.4, v/vi 9.5°, due to the absence of repulsion between peri-H atoms. Of the two n-hexyl substituents at C(9), one is fully ordered and the other has its two orientations, A and B. The C₁₃ fragment [n-hexyl-C(9)-n-hexyl] adopts an all-trans conformation, except one terminal CH₂CH₂CH₃ moiety, which is disordered equally between trans (A) and gauche (B) conformations. The n-dodecyl chain attached to O(3) adopts an all-trans conformation, except the terminal methyl group which has a gauche orientation. In the second n-dodecyl chain, four methylene groups are disordered between two positions; in each case one CCCC torsion angle corresponds to a gauche conformation and the rest to a distortedtrans conformation [165.5(2)° to 179.4(4)°, average 171(4)°]. As a result, the former n-dodecyl chain is roughly coplanar with the polycyclic part of the molecule and the latter is bent out of this plane. The longest dimension of the molecule (in the crystal) is ca. 54 Å.

Fig. 1 Molecular structure of compound 11 as determined by X-ray diffraction.

Theoretical calculations

DFT calculations were performed to look at the geometry and the electronic state of the molecules. To decrease the computational time we calculated the molecules 11a, 13a, 16a (where index “a” means that in structures 11, 13 and 16 C₁₂H₂₅O was replaced by CH₃O, and in structures 11 and 16 C₆H₁₃ was replaced by C₂H₅; see ESI†) as well as compound 6a (where C₆H₁₃ was replaced by C₂H₅) and, for comparison, 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazole-5-yl]benzene (OXD-7), which is widely used as an ETHB molecule in OLEDs.

Comparison of calculated LUMO energies for compounds 6a, 11a, 13a and 16a with that for OXD-7 (Chart 1) shows that compounds 11a and 13a as acceptors match very well with OXD-7 and compounds 6a and 16a are even better acceptors (by ca. 0.2 and 0.3 eV, respectively; Fig. 2). So, from an energy point of view, compounds 6a, 11a, 13a and 16a are good alternatives to OXD-7 as electron transport materials. On the other hand, all these compounds (6a, 11a, 13a and 16a) have higher HOMO levels, so their function as hole blocking layers in OLEDs could be less efficient [although their HOMOs are still much lower than that of common EL polymers like MEH-PPV (∼−5.0 eV)].

Fig. 2 B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) orbital energy level diagrams for compounds 6a, 11a, 13a and 16a in comparison with the ETHB material OXD-7.

Chart 1 Chemical structure of OXD-7.

Compound 16a showed the smallest HOMO–LUMO gap (2.97 eV) and a feature is that its LUMO orbitals are delocalised between the central fluorene moiety and adjacent phenylene units (no LUMO population on the oxadiazole rings), whereas for all other oxadiazole derivatives (6a, 11a, 13a and OXD-7) substantial localisation of the LUMO is observed on the oxadiazole moieties (see ESI†).

The calculated HOMO energy for OXD-7 (−6.27 eV) is quite close to its ionisation potential measured by photoemission studies (I_p ≈ 6.5 eV, i.e. the difference is only ≈0.23 eV),¹⁴ whereas the calculated LUMO energy (−2.03 eV) is higher than found experimentally (by subtraction of the optical gap energy: E_A ≈ I_p − 3.7 eV ≈ 2.8 eV) by ca. 0.77 eV. In the next section we will use this difference between the experimental and calculated LUMO energies to moderate conduction band levels when discussing the energy diagram of OLEDs to compare LUMO levels of oxadiazole derivatives with that for MEH-PPV.

Optical properties and device performance

For all the blended layer studies, the EL output was found to be characteristic of MEH-PPV. The device configuration was ITO/MEH-PPV : ETHB/Al. The current density versus electric field (J–E) and light output versus electric field (L–E) characteristics of polymer blend devices using compound 6, 11, 13, and 16 are shown in Fig. 3 (positive bias applied to the ITO electrode). The polymer blends all contained 70% electron transport materials by weight. The J–E and L–E characteristics of an OLED based on a pure (non-blended) MEH-PPV layer are also shown for comparison. It is evident that the EL emission from the blend devices was significantly greater than the light output of the pure MEH-PPV; at the same time, the current through the blend films was lower. However, within experimental errors, it was difficult to discern any particular trends between the different blended layers. The onset voltage for light emission for the blend devices was ca. 4.5 V, compared with ca. 6 V for the pure MEH-PPV device.

Fig. 3 Current density versus electric field and light output versus electric field characteristics for MEH-PPV polymer blend OLEDs incorporating 6, 11, 13 and 16. The polymer blends each contain 70% by weight of the electron transport materials. The data for a device based on pure MEH-PPV are shown for reference.

Fig. 4 shows the external quantum efficiency of the devices whose optoelectrical behaviour are shown Fig. 3. The efficiency of the pure MEH-PPV device was about 0.001% while that of the blend devices (70% by weight) was about 0.05%. The increase in the efficiency has resulted from both an increase in the light output as well as a decrease in the current; however, Fig. 3 shows that the effect of the former is more significant.

Fig. 4 The external quantum efficiencies of MEH-PPV polymer blend OLEDs incorporating 70% by weight of 6, 11, 13 and 16. The data for a device based on pure MEH-PPV are shown for reference.

External quantum efficiencies of OLEDs containing different amounts of 13 are shown in Fig. 5. These data are similar to those previously obtained by ourselves for blends incorporating 6.⁹ The device efficiency increased with the concentration of the electron transport material over the range of composition investigated. The efficiency for the 95% blend device was more than two orders of magnitude greater than that of the pure MEH-PPV device. To investigate if this efficiency could be increased even further, a device incorporating 99.99% of 6 was fabricated. However, the EL emission was poor and the device quantum efficiency did not exceed 1 × 10⁻⁵%, indicating an upper limit to the efficiency of our blended layer structures.

Fig. 5 External quantum efficiency of blended MEH-PPV OLEDs incorporating compound 13. Data are shown for blend devices with 20%, 50%, 70%, and 95% of 13 by weight.

Despite the relatively high external quantum efficiencies of some of the blended layer OLEDs, the brightness and power efficiencies of the OLEDs are low compared to state-of-the-art polymer displays. For example, the brightness of a 70% blend device incorporating 11 was 210 cd m⁻² at a current density of 62 mA cm⁻². However, it should be noted that our devices have not been optimised in any way, e.g. by use of a low work function cathode.

The HOMO and LUMO energy levels for MEH-PPV determined from oxidation and reduction potentials in CV experiments are E_HOMO = −4.98 eV and E_LUMO = −2.89 eV.¹⁵ This HOMO energy value is much higher than experimental I_p values for OXD-7 or calculated values for OXD-7, 6a, 11a, 13a, and 16a (Fig. 2). Using the figure of 0.77 eV (the difference between the experimental value E_A ≈ 2.8 eV and calculated HOMO energy level for OXD-7, and assuming that it is similar for other oxadiazole derivatives) we can estimate LUMO energies for compounds 6, 11, 13 and 16 for direct comparison with the values for MEH-PPV (3.01, 2.83, 2.81 and 3.09 eV, respectively). The LUMO levels for compounds 6 and 16 lie ∼0.1–0.2 eV below that of MEH-PPV, whereas for compounds 11 and 13 they are comparable to that for MEH-PPV. Thus, holes are more likely to be transferred from the ITO anode to the MEH-PPV (Fig. 6), but electrons will move from the Al cathode to 6 or 16 more easily than to the MEH-PPV. The electrons can then easily move to the LUMO level of the host polymer, subsequently recombining with holes to produce EL characteristic of MEH-PPV. In the case of blends of MEH-PPV with compounds 11 and 13 electrons can also be injected directly into the LUMO of MEH-PPV as well as into the LUMO of oxadiazole derivatives (depending on the ratio in the blend). Nevertheless, in these cases EL also occurs only from MEH-PPV because of the substantial barrier for hole transfer from MEH-PPV into the HOMO of oxadiazoles.

Fig. 6 Energy band diagram of ITO, MEH-PPV, compound 6, and Al. For MEH-PPV, data are from Ref. 15; for compound 6, the calculated energies of LUMO and HOMO levels for 6a have been corrected by adding the difference between the calculated and experimental values for OXD-7 (see Ref. 14) [ΔE^calc−exp ≈ 0.77 eV (LUMO), 0.23 eV (HOMO)].

When large amounts of 6 are blended with MEH-PPV it will become more difficult to inject holes (the majority carriers) from the ITO into the blended film and the device current (majority carrier hole current) will decrease. In contrast, electron injection from the Al will increase, thereby increasing the EL. The light output will only decline when the supply of holes from the anode becomes limited. From our experimental work with blended layers based on 13, this seems to occur for concentrations of the electron transport material greater than 95% by weight.

The EL emission from OLEDs depends on the injection and transport of carriers, the generation of singlet excitons and their survival from non-radiative deactivation. For the blend devices in this work, the electron injection and transport are increased as noted above. A ‘dilution’ effect may also be a contributory factor to the increase in our OLED efficiencies with increasing electron transport material concentration. As the polymer molecules become separated by the electron transporting molecules, concentration quenching—intermolecular non-radiative decay of singlet excitons—will be reduced resulting in an enhanced light output. Kang et al.¹⁶ have previously noted that the external quantum efficiency increased over seven times when MEH-PPV was blended with 90 wt% poly(methyl methacrylate) (PMMA), an electro-optically inert material. A much larger increase in efficiency (almost 500 times) was noted by mixing the MEH-PPV with another electroactive polymer. In our own studies, no improvement in the OLED efficiency was found using a 90% PMMA and MEH-PPV blend. An increase in the photoluminescence (PL) efficiency was noted in some of the films formed from mixtures of MEH-PPV with the electron transport compounds. For example, the PL efficiency of blends based on 6 increased by a factor of three as the concentration of 6 was increased from 20% to 90% by weight. Over the same composition range, the EL efficiency increased by a factor of 40, suggesting that dilution effects do not play a major role in determining the efficiency of our blended layer OLEDs.

The external quantum efficiency of our blend devices could be increased further by incorporating a PEDOT layer between the ITO and the blend film. The external quantum efficiency of the blend devices was increased two to three times regardless of the composition. The external quantum efficiency of 95% blend devices using a PEDOT layer was about 0.4%. Table 1 shows the external quantum efficiencies of OLEDs based on 6 and 13. Thermal annealing⁹ could also be used to extend the efficiencies of the devices to around 0.5%. The results of this work will be reported separately.

Table 1 External quantum efficiencies of MEH-PPV polymer blend OLEDs incorporating compound 6 and compound 13, with and without a PEDOT : PSS layer. The current density was 30 mA cm⁻² unless designated in parentheses

	Compound 6			Compound 13
wt%	Without PEDOT	With PEDOT	wt%	Without PEDOT	With PEDOT
40%	0.01%	0.04%	50%	0.02%	0.08%
70%	0.03%	0.10%	70%	0.06%	0.15%
95%	0.11% (6.7)	0.31% (12.7)	95%	0.16%	0.38%

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For all the electron transport compounds studied, the EL spectra were found to be independent of the blend composition. Fig. 7 compares the EL spectra of a pure MEH-PPV device with blend devices, 50% by weight, incorporating 6, 11, and 16: the emission of all the blend devices corresponded to that from MEH-PPV. No emission was evident from the electron transport materials, all of which are blue emitters as observed from the EL spectra of the pure compound devices for 6, 11, 13 and 16 (λ_max = 484, 430, 433 and 487 nm, respectively). The OLED containing compound 6 possessed a main peak at 570 nm, exactly the same as that of the pure MEH-PPV device. However, the spectra of the devices incorporating 11 and 16 were red-shifted relative to pure MEH-PPV, with the main EL peak located at 590 nm. Blended devices based on 13 also exhibited a red-shifted EL spectrum (data not shown).

Fig. 7 The EL spectra of a pure MEH-PPV OLED and blended layer devices incorporating compounds 6, 11, and 16. The composition of all the blends was 50% by weight.

It is interesting to note that the three compounds exhibiting red-shifted EL spectra (11, 13 and 16) have C₁₂H₂₅O terminal groups. These groups are replaced by tert-butyl groups in the case of compound 6. To study the influence of the terminal groups on the EL spectrum, a further compound, 10 was synthesised. This has basically the same chemical structure as 11, but with tert-butyl terminal groups instead of C₁₂H₂₅O. The EL spectra for blended layer OLEDs incorporating 10 and 11 (both 50% compositions by weight) are contrasted in Fig. 8. The EL output of the device based on 10 peaks at the same wavelength as the pure MEH-PPV device, providing very strong evidence that the red-shifts noted in our studies are associated with the C₁₂H₂₅O chains interacting with the emissive MEH-PPV.

Fig. 8 EL spectra of blended layer MEH-PPV polymer OLEDs incorporating compounds 10 and 11. The composition of both blends was 50% by weight.

The emission of conjugated polymers such as MEH-PPV can be changed by varying the chain conformation. Schwartz and co-workers reported that the photoluminescence of MEH-PPV solutions varies according to the polarity of the solvents.¹⁷ These workers noted that MEH-PPV chains in non-polar solvents such as chlorobenzene are more extended than in polar solvents such as tetrahydrofuran. This results in a red-shift of the PL spectrum of the chlorobenzene solution relative to the tetrahydrofuran solution. The same group demonstrated red shifts for MEH-PPV oriented on silica porous composite materials, in which the energy initially deposited on randomly oriented polymer segments exterior to the pores was driven to the aligned segments in the channel interior where the PL emission occurred.¹⁸ In a similar way, the MEH-PPV molecules in our blend films may adopt different conformations depending on the properties of the other component of the blend and consequently the emission from MEH-PPV varies.¹⁹

The results described above suggest a good mixing of the MEH-PPV with the electron transport compounds. No sign of phase separation was evident using atomic force microscopy, although this is not a definitive method of determining the distribution of the two components. The fact that no direct EL from the electron transport materials could be measured, even at high concentration (up to 95%), implies that the molecules of these materials are well distributed among the MEH-PPV polymer chains. Strong evidence for intimate mixing is also provided by the red-shifted EL curves for devices incorporating 11, 13 and 16. For these OLEDs, no unshifted MEH-PPV spectrum was detected, supporting the view that the MEH-PPV is able to interact with the electron transport compounds.

Conclusions

In conclusion, we have synthesised and investigated the OLED performance of new ETHB molecules 6, 10, 11, 13 and 16 comprising linearly π-extended fluorene and 1,3,4-oxadiazole moieties. The external quantum efficiences of blended-layer devices containing the electron transport compound increased significantly compared to those fabricated using pure MEH-PPV. A striking feature of this work is that electroluminescence originates exclusively from the MEH-PPV material, even when the ET compound is 95% by weight of the blend. The EL spectra of devices incorporating compounds 11, 13 and 16, which all bear terminal dodecyloxy groups, are red shifted by ca. 20 nm compared to the devices with compounds 6 and 10 which have terminal tert-butyl groups. This provides evidence for intimate mixing of the polymer and ET compounds. Further synthesis of new oxadiazole-based small molecules and polymers, and device optimisation using selected 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids both as emitting layers and as ETHB compounds will be reported in due course.

Experimental

General

Elemental analyses were obtained on a Carlo-Erba Strumentazione instrument. Melting points were determined in open-end capillaries using a Stuart Scientific melting point apparatus SMP3 at a ramp rate of 2.5 °C min⁻¹ without calibration. Solution ¹H NMR and ¹³C NMR spectra were recorded on Mercury 200, Varian Unity 300, Bruker Avance 400 and Varian Inova 500 spectrometers operating at (¹H) 199.99, 299.91, 400.13, 499.99 and (¹³C) 50.29, 75.42, 100.62, 124.99 MHz respectively. Chemical shifts are reported in ppm downfield of TMS. Mass spectra were obtained on a VG7070E instrument operating in EI mode at 70 eV. Electrospray high resolution mass spectra were obtained on a Micromass LCT (TOF). MALDI-TOF spectra were obtained on an Applied Biosystems Voyager-DE STR operating in reflector mode.

Device fabrication

MEH-PPV was purchased from Aldrich whereas electron transport materials were synthesised in Durham as described. Indium-tin-oxide (ITO) coated glass from Merck with sheet resistance of 9 Ω □⁻¹ was used as the anode. This was cleaned by ultrasonication in acetone and isopropyl alcohol for 30 min each and dried with a nitrogen gun. The polymer and electron transport materials were dissolved in chloroform to provide the blend solution, which was spin-coated onto the ITO. The concentration of electron transport material was changed from 20% to 95% of the total weight. Following the spin-coating, Al top electrodes, in the form of dots (radius 1 mm; thickness 150 nm) were thermally evaporated at a pressure of about 10⁻⁶ mbar. In some cases, poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT : PSS), purchased from Bayer AG, was spin coated onto the ITO prior to the deposition of the polymer blend. These PEDOT layers (40 nm in thickness) were dried for 12 h in 10⁻¹ mbar vacuum at room temperature to remove residual solvent.

Electrical measurements were undertaken in a vacuum chamber (10⁻¹ mbar). The d.c. bias was applied and the current measured by a Keithley 2400 Source Meter. The light emitted from the device was collected by a large area photodiode (1.1 cm diameter) connected to a Keithley 485 Digital Picoammeter. For external quantum efficiency measurements, the light power was calculated using the photocurrent and the conversion factor (wavelength dependent) of the photodiode (ampere/watt). Electroluminescence (EL) spectra were measured using an Ocean Optics USB2000 Miniature Fibre Optic Spectrometer.

Computational procedures

The ab initio computations of geometries of compounds 6a, 11a, 13a, 16a and OXD-7 were carried out with the Gaussian 98²⁰ package of programs at density-functional theory (DFT) level using Pople's 6-31G split valence basis set supplemented by d-polarisation functions on heavy atoms. DFT calculations were carried out using Becke's three-parameter hybrid exchange functional²¹ with Lee–Yang–Parr gradient-corrected correlation functional (B3LYP).²² Thus, the geometries were optimised with B3LYP/6-31G(d) and for all compounds and electronic structures were then calculated at both B3LYP/6-31G(d) and B3LYP/6-311G(2d,p) levels of theory. Contours of HOMO and LUMO orbitals were visualised using Molekel v.4.3 program.²³ No constraints of bonds/angles/dihedral angles were applied in the calculations and all the atoms were free to optimise.

Synthesis of materials

Starting materials were purchased from Aldrich and Lancaster and were used without further purification. Pyridine was dried over NaOH pellets for three days and used without distillation. THF was freshly distilled under argon over potassium metal. Methanol and ethanol were dried by refluxing over Mg turnings, then distilled under argon immediately before use. Column chromatography was carried out on silica (40–60 µm).

9,9-Dihexylfluorene-2,7-dicarboxylic acid (2)

2,7-Dibromo-9,9-dihexylfluorene (1)¹⁰ (4.92 g, 10 mmol) was dissolved in dry THF (150 cm³) and the solution was cooled to −78 °C. n-Butyllithium–hexane (2.5 M solution, 10 cm³, 25 mmol) was syringed in with stirring under argon atmosphere at the low temperature. The reaction was stirred with cooling for 5 h to obtain a white thick suspension. Carbon dioxide gas (dried through a P₂O₅ column) was continuously bubbled through the stirred mixture for 1 h at low temperature and an additional 0.5 h with the cooling bath removed to yield a white thick slurry. Water (100 cm³) was added then the solution was acidified (pH ca. 1) using conc. HCl. The mixture was partitioned and the aqueous phase was extracted once with diethyl ether (50 cm³). The combined organic phase was extracted with 5% NaOH solution (3 × 50 cm³). The combined alkaline solution was acidified with conc. HCl then extracted with ethyl acetate (2 × 50 cm³). After removal of ethyl acetate in vacuo the white solid residue was recrystallised from MeOH–H₂O to afford compound 2 as white plates (3.84 g, 91%), mp: 249.0–250.4 °C. MS (EI): m/z 422 (M⁺, 42%), 253 (100%); Anal. Calcd. for C₂₇H₃₄O₄: C, 76.74; H, 8.11. Found: C, 76.96; H, 8.10%. δ_H (DMSO-d₆, 300 MHz) 7.98 (m, 3H), 2.03 (m, 2H), 0.94 (m, 6H), 0.66 (t, J 6.7, 3H), 0.42 (m, 2H). δ_C (DMSO-d₆, 75 MHz) 167.5, 151.2, 143.8, 130.4, 128.8, 123.8, 120.8, 55.0, 30.8, 28.8, 23.3, 21.9, 13.8.

Dimethyl 9,9-dihexylfluorene-2,7-dicarboxylate (3)

Compound 2 (3.66 g, 8.66 mmol) was dissolved in methanol (50 cm³). Conc. H₂SO₄ (0.5 cm³) was added and the solution was refluxed for 12 h. Methanol was removed by distillation and water (100 cm³) was added to the cooled residue. A white solid was collected by suction filtration and washing with water. Recrystallisation of the solid from a methanol–water mixture afforded compound 3 as white needles (3.66 g, 94%), mp: 80.5–81.5 °C. MS (EI): m/z 450 (M⁺, 73%), 365 (100%); Anal. Calcd. for C₂₉H₃₈O₄: C, 77.30; H, 8.50. Found: C, 77.37; H, 8.45%. δ_H (CDCl₃, 300 MHz) 8.07 (d, J 8.1, 1H), 8.03 (s, 1H), 7.79 (d, J 7.8, 1H), 3.96 (s, 3H), 2.04 (m, 2H), 1.00 (m, 6H), 0.75 (t, J 6.7, 3H), 0.54 (m, 2H). δ_C (CDCl₃, 75 MHz) 167.3, 151.8, 144.4, 129.6, 128.9, 124.1, 120.3, 55.5, 52.2, 40.1, 31.5, 29.6, 23.7, 22.5, 13.9.

9,9-Dihexylfluorene-2,7-dicarboxylic dihydrazide (4)

Compound 3 (3.65 g, 8.1 mmol) was dissolved in methanol (25 cm³) with heating. Hydrazine monohydrate (20 cm³) was added and the mixture was refluxed for 12 h. The clear yellow solution was cooled to rt and water added (∼50 cm³). A suction filtration and washing with water, then recrystallisation from methanol–water yielded compound 4 as white crystals (3.50 g, 96%), mp: 182.9–184.0 °C. Anal. Calcd. for C₂₇H₃₈N₄O₂: C, 71.97; H, 8.50; N, 12.43. Found: C, 71.69; H, 8.42; N, 12.33%. δ_H (DMSO-d₆, 200 MHz) 9.83 (s, 1H), 7.90 (m, 3H), 3.41 (s, 2H), 1.99 (m, 2H), 0.96 (m, 6H), 0.68 (t, J 6.4, 3H), 0.43 (m, 2H). δ_C (DMSO-d₆, 100 MHz) 165.9, 150.8, 142.4, 132.5, 126.3, 121.5, 120.3, 54.9, 48.6, 30.9, 28.9, 23.4, 22.0, 13.8.

2,7-Bis[2-(4-tert-butylphenyl-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene (6)

To a solution of compound 4 (0.90 g, 2 mmol) in pyridine (30 cm³) was added 4-tert-butylbenzoyl chloride (1.18 g, 6 mmol) dropwise and the mixture was stirred at rt for 12 h to afford a clear yellow solution. Pyridine was removed by vacuum distillation and the oily residue was recrystallised once from methanol–water to afford a white solid of the crude compound 5. The dried solid of 5 was mixed with POCl₃ (10 cm³) and the mixture was heated with an oil-bath to 110 °C with stirring for 12 h. POCl₃ was removed by careful distillation under vacuum using a water-pump and methanol (100 cm³) was added to the cooled residue. Recrystallisation of the solid from ethanol yielded compound 6 as a white solid (0.80 g, 54%), mp: 179.8–181.0 °C. MS (EI): m/z 734 (M⁺, 100%); Anal. Calcd. for C₄₉H₅₈N₄O₂: C, 80.07; H, 7.95; N, 7.62. Found: C, 79.97; H, 7.96; N, 7.64%. δ_H (CDCl₃, 300 MHz) 8.15 (m, 4H), 7.92 (d, J 8.7, 1H), 7.58 (d, J 8.4, 2H), 2.15 (m, 2H), 1.39 (s, 9H), 1.08 (m, 6H), 0.73 (t, J 6.9, 3H), 0.63 (m, 2H). δ_C (CDCl₃, 75 MHz) 164.8, 164.7, 155.4, 152.3, 143.3, 126.8, 126.2, 126.1, 123.4, 121.4, 121.1, 121.0, 74.7, 55.9, 40.3, 35.1, 31.5, 31.1, 29.6, 23.8, 22.5, 14.0.

General procedure for the cross-coupling reactions

The boronic acid, the halide and the catalyst [tetrakis(triphenylphosphino)palladium] (5 mol% relative to the boronic acid) were added sequentially to degassed THF or toluene and the mixture was stirred at 20 °C for 0.5 h. Degassed aqueous Na₂CO₃or K₂CO₃ solution was added and the reaction mixture was heated under Ar at 80 °C until TLC monitoring showed that the reaction was complete (48–96 h). Solvent was evaporated in vacuo and the crude products were extracted into organic solvent. The organic layer was washed with H₂O, separated and dried over MgSO₄. Products were purified by column chromatography.

2,7-Bis{4-[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene (10)

Following the general procedure, compound 7²⁴ (0.3 g, 0.74 mmol), compound 9¹⁰ (0.20 g, 0.48 mmol), tetrakis(triphenylphosphino)palladium (55 mg, 0.05 mmol), THF (30 cm³) and Na₂CO₃ (1 M, 4 cm³); reaction time 48 h; extracted with DCM. Chromatography eluent: dichloromethane–ethyl acetate, initially 9.9 : 0.1 v/v then 1 : 1 v/v gave a yellow oil, recrystallisation from a cyclohexane–hexane mixture gave a yellow solid impurity which was removed by suction filtration. The filtrate was evaporated in vacuo to yield 10 as a white solid (70 mg, 11%), mp: 187–189 °C. HRMS (ES+): m/z 886.5184 (M⁺) (calcd. for C₆₁H₆₆N₄O₂ 886.5186). δ_H (CDCl₃, 500 MHz,) 8.24 (d, J 8.5, 4H), 8.09 (d, J 8.5, 4H), 7.83–7.86 (m, 6H), 7.65–7.68 (m, 4H), 7.57 (d, J 8, 4H), 2.08 (m, J 8, 4H), 1.39 (s, 18H), 1.06 (m, 12H), 0.75 (m, 10H). δ_C (CDCl₃, 125 MHz,) 164.68, 164.31, 155.34, 151.98, 144.70, 140.68, 138.92, 127.70, 127.35, 126.78, 126.25, 126.06, 122.63, 121.46, 121.11, 120.40, 55.45, 40.36, 35.09, 31.43, 31.11, 29.64, 23.79, 22.54, 13.98.

2,7-Bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}-9,9-dihexylfluorene (11)

Following the general procedure, compound 8^6d (0.44 g, 0.91 mmol), compound 9 (0.23 g, 0.54 mmol), tetrakis(triphenylphosphino)palladium (63 mg, 0.05 mmol), THF (20 cm³) and Na₂CO₃ (2 M, 0.82 cm³); reaction time 96 h; extracted with ethyl acetate. Chromatography eluent: DCM–EtOAc (24 : 1 v/v), followed by recrystallisation from cyclohexane gave 11 as a white solid (0.27 g, 44%), mp: 161.5–162.0 °C. MS (MALDI-TOF) m/z Calcd. for C₇₇H₉₈N₄O₄: 1143.63 (M⁺). Found 1143.74; Anal. Calcd. for C₇₇H₉₈N₄O₄: C, 80.87; H, 8.64; N, 4.90. Found: C, 80.59; H, 8.60; N, 4.77%. δ_H (CDCl₃, 300 MHz) 8.2 (d, J 8.4, 4H), 8.0 (d, J 8.7, 4H), 7.8 (m, 6H), 7.6 (d, J 7.8, 2H), 7.5 (s, 2H), 7.0 (d, J 8.7, 4H), 4.0 (t, J 6.6, 4H), 2.0 (m, 4H), 1.8 (m, 4H), 1.3–1.2 (m, 36H), 1.0 (m, 12H), 0.8 (t, J 6.9, 6H), 0.7 (m, 10H). δ_C (CDCl₃, 100 MHz) 164.87, 164.28, 162.23, 152.23, 144.84, 140.92, 139.19, 128.94, 127.34, 127.52, 126.50, 122.94, 121.69, 120.64, 116.40, 115.225, 68.53, 55.71, 40.63, 32.16, 31.70, 30.01, 29.91, 29.88, 29.84, 29.81, 29.62, 29.60, 29.37, 26.25, 24.05, 22.94, 22.81, 14.38, 14.25. A crystal for X-ray analysis was grown from a hexane–DCM mixture.

2,7-Bis{4-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]phenyl}spirobifluorene (13)

Compound 8 (0.40 g, 0.82 mmol), compound 12¹² (0.23 g, 0.4 mmol), tetrakis(triphenylphosphino)palladium (58 mg, 0.05 mmol), tri-tert-butylphosphine (0.02 g, 0.1 mmol), toluene (10 cm³) and K₂CO₃ (2 M, 2.17 cm³); reaction time 48 h; extracted with chloroform. Chromatography eluent: DCM–EtOAc (19 : 1 v/v) followed by recrystallisation from a cyclohexane–ethanol mixture gave compound 13 as a white solid (0.18 g, 40%), mp: 138.4–139.6 °C. MS (MALDI-TOF) m/z Calcd. for C₇₇H₈₀N₄O₄: 1125.48 (M⁺). Found 1125.66; Anal. calcd. for C₇₇H₈₀N₄O₄: C, 82.17; H, 7.16; N, 4.98. Found: C, 81.71; H, 7.28; N, 4.96%. δ_H (CDCl₃, 400 MHz) 8.08–8.04 (m, 8H), 8.00 (d, J 8, 2H), 7.93 (d, J 8, 2H), 7.72 (dd, J_ab 8, J_ac 1.6, 2H), 7.60 (d, J 8, 4H), 7.44 (td, J_ab 7.6, J_ac 0.8, 2H), 7.17 (td, J_ab 7.6, J_ac 0.8, 2H), 7.03–7.01 (m, 6H), 6.86 (d, J 7.6, 2H), 4.04 (t, J 6.8, 4H), 1.83 (m, 4H), 1.5–1.3 (m, 36H), 0.89 (t, J 6.8, 6H). δ_C (CDCl₃, 100 MHz) 164.51, 163.93, 161.98, 150.21, 148.29, 147.51, 143.83, 141.88, 141.25, 139.84, 128.69, 128.05, 127.60, 127.17, 127.09, 124.22, 122.78, 122.64, 120.73, 120.25, 116.16, 114.98, 68.30, 66.14, 31.93, 29.67, 29.64, 29.60, 29.57, 29.38, 29.36, 29.14, 26.00, 22.70, 14.13.

4′-[5-(4-Dodecyloxyphenyl)-1,3,4-oxadiazol-2-yl]-biphenyl-4-carbaldehyde (14)

Following the general procedure, compound 8 (1.46 g, 3.0 mmol), 4-formylbenzene boronic acid (0.58 g, 3.9 mmol), tetrakis(triphenylphosphino)palladium (0.23 g, 0.2 mmol), THF (25 cm³) and Na₂CO₃ (1 M, 12 cm³); reaction time 96 h; extracted with chloroform. Chromatography eluent: CHCl₃–EtOAc (9 : 1 v/v) and recrystallisation from CHCl₃–ethanol gave compound 14 as a white solid (1.19 g, 78%), mp: 211.9–212.7 °C. MS (EI): m/z 510 (M⁺); Anal. Calcd. for C₃₃H₃₈N₂O₃: C, 77.61; H, 7.50; N, 5.49. Found: C, 77.15; H, 7.34; N, 5.40%. δ_H (CDCl₃, 200 MHz) 10.10 (s, 1H), 8.25 (d, J 7.8, 2H), 8.09 (d, J 8.0, 2H), 8.01 (d, J 7.8, 2H), 7.80–7.84 (m, 4H), 7.04 (d, J 8.0, 2H), 4.05 (t, J 6.2, 2H), 1.83 (m, 2H), 1.28 (m, 18H), 0.89 (t, J 6.4, 3H). δ_C (CDCl₃, 100 MHz) 191.70, 162.12, 145.74, 142.64, 141.47, 135.81, 130.37, 128.75, 127.98, 127.78, 127.44, 124.06, 116.07, 115.05, 99.99, 68.34, 31.91, 29.65, 29.63, 29.58, 29.56, 29.37, 29.34, 29.13, 26.00, 22.68, 14.10.

Compound 16

Compound 14 (0.28 g, 0.55 mmol) and compound 15¹³ (0.28 g, 0.27 mmol) were dissolved in a mixture of distilled anhydrous ethanol (30 cm³) and THF (15 cm³). Sodium metal (0.03 g) in anhydrous ethanol (15 cm³) was then added dropwise over 0.5 h. The reaction was stirred for 12 h and then HCl (0.1 M, 1 cm³) was added. The solvents were removed in vacuo and the crude product was extracted with toluene, washed with water and dried over MgSO₄. After removal of toluene in vacuo the crude product was purified by column chromatography [silica eluent; CHCl₃–diethyl ether (11.5 : 1 v/v)] and recrystallised from CHCl₃–ethanol to yield compound 16 as a yellow solid (0.16 g, 43%). mp: 171.3–172.7 °C. MS (MALDI-TOF) m/z Calcd. for C₉₃H₁₁₀N₄O₄: 1347.89 (M⁺). Found 1347.76; Anal. calcd. for C₉₃H₁₁₀N₄O₄: C, 82.87; H, 8.23; N, 4.16. Found: C, 82.38; H, 8.23; N, 4.05%. δ_H (CDCl₃, 400 MHz) 8.21 (d, J 7.2, 4H), 8.10 (d, J 7.6, 4H), 7.73 (d, J 7.6, 4H), 7.70 (m, 10H), 7.52–7.56 (m, 4H), 7.25–7.29 (m, 4H), 7.04 (d, J 7.6, 4H), 4.05 (t, J 6.6, 4H), 2.05 (m, 4H), 1.83 (t, J 8.1, 4H), 1.28 (m, 36H), 1.09–1.13 (m, 12H), 0.89 (t, J 6.0, 6H), 0.71–0.78 (m, 10H). δ_C (CDCl₃, 100 MHz) 164.62, 164.00, 162.02, 156.66, 151.66, 143.61, 140.82, 138.64, 137.53, 136.22, 129.97, 128.70, 127.36, 127.32, 127.22, 127.05, 125.82, 122.87, 120.89, 120.02, 116.21, 115.01, 68.32, 55.05, 40.57, 31.92, 31.53, 29.77, 29.67, 29.64, 29.60, 29.57, 29.38, 29.35, 29.15, 26.00, 23.83, 22.68, 22.60, 14.11, 14.01.

Acknowledgements

This work was funded by EPSRC, Durham County Council under the Science and Technology for Business and Enterprise Programme SP/082, and the Postdoctoral Fellowship Programme of Korea Science and Engineering Foundation (KOSEF) (for one of the authors, J. H. A.).

References

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. McKay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature, 1990, 347, 539.
3. Reviews: (a) A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed., 1998, 37, 402; (b) R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund and W. R. Salenek, Nature, 1999, 397, 121; (c) U. Mitschke and P. Bäuerle, J. Mater. Chem., 2000, 10, 1471; (d) for a monograph see S. M. Kelly, Flat Panel Displays, Royal Society of Chemistry, Cambridge, 2000; (e) G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15 10.1039/b413249c , in press.
4. Y. Cao, I. D. Parker, G. Yu, C. Zhang and A. J. Heeger, Nature, 1999, 397, 414.
5. B. Schultz, M. Bruma and L. Brehmer, Adv. Mater., 1997, 9, 601.
6. (a) M. Thelakkat and H.-W. Schmidt, Polym. Adv. Technol., 1998, 9, 429; (b) K. Chondroudis and D. B. Mitzi, Chem. Mater., 1999, 11, 3028; (c) C. Wang, G.-Y. Jung, A. S. Batsanov, M. R. Bryce and M. C. Petty, J. Mater. Chem., 2002, 12, 173; (d) S. W. Cha, S.-H. Choi, K. Kim and J.-I. Jin, J. Mater. Chem., 2003, 13, 1900.
7. (a) P. Cea, Y. Hua, C. Pearson, C. Wang, M. R. Bryce, F. M. Royo and M. C. Petty, Thin Solid Films, 2002, 408, 275; (b) P. Cea, Y. Hua, C. Pearson, C. Wang, M. R. Bryce, M. C. López and M. C. Petty, Mater. Sci. Eng. C, 2002, 22, 87.
8. M. T. Bernius, M. Inbasekaran, J. O'Brien and W. Wu, Adv. Mater., 2000, 12, 1737.
9. For preliminary results concerning compound 6 see: J. H. Ahn, C. Wang, C. Pearson, M. R. Bryce and M. C. Petty, Appl. Phys. Lett., 2004, 85, 1283.
10. G. Hughes, C. Wang, A. S. Batsanov, M. Fearn, S. Frank, M. R. Bryce, I. F. Perepichka, A. P. Monkman and B. P. Lyons, Org. Biomol. Chem., 2003, 1, 3069.
11. (a) Y. H. Kim, D. C. Shin, S.-H. Kim, C.-H. Ko, H.-S. Yu, Y.-S. Chae and S. K. Kwon, Adv. Mater., 2001, 13, 1690; (b) C.-H. Chen, F.-I. Wu, C.-F. Shu, C.-H. Chien and Y.-T. Tao, J. Mater. Chem., 2004, 14, 1585; (c) L. Romaner, A. Pogantsch, P. Scandiucci de Freitas, U. Scherf, M. Gaal, E. Zojer and E. J. W. List, Adv. Funct. Mater., 2003, 13, 597.
12. K.-T. Wong, Y.-T. Chien, R.-T. Chen, C.-F. Wang, Y.-T. Lin, H.-H. Chiang, P.-Y. Hsieh, C.-C. Wu, C. H. Chou, Y. O. Su, G.-H. Lee and S.-M. Peng, J. Am. Chem. Soc., 2002, 124, 11576.
13. J. K. Kim, S. I. Hong, H. N. Cho, D. Y. Kim and C. Y. Kim, Polym. Bull., 1997, 38, 169.
14. D. O'Brien, A. Bleyer, D. G. Lidzey, D. D. C. Bradley and T. Tsutsui, J. Appl. Phys., 1997, 82, 2662.
15. J. H. Kim and H. Lee, Chem. Mater., 2002, 14, 2270.
16. I. N. Kang, D. H. Hwang, H. K. Shim, T. Zyung and J. J. Kim, Macromolecules, 1996, 29, 165.
17. T. Q. Nguyen, V. Doan and B. J. Schwartz, J. Chem. Phys., 1999, 110, 4068.
18. (a) B. Schwartz, Ann. Rev. Phys. Chem., 2003, 54, 141; (b) T. Q. Nguyen, J. Wu, V. Doan, B. J. Schwartz and S. H. Tolbert, Science, 2000, 288, 652; (c) T. Q. Nguyen, J. Wu, S. H. Tolbert and B. J. Schwartz, Adv. Mater., 2001, 13, 609; (d) B. J. Schwartz, T. Q. Nguyen, J. Wu and S. H. Tolbert, Synth. Met., 2001, 116, 35.
19. D. R. Baigent, A. B. Holmes, S. C. Moratti and R. H. Friend, Synth. Met., 1996, 80, 119.
20. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople, Gaussian 98, Revision A.9, Gaussian, Inc., Pittsburgh PA, 1998.
21. A. D. Becke, Phys. Rev. A, 1988, 38, 3098; A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
22. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
23. (a) P. Flükiger, H. P. Lüthi, S. Portmann and J. Weber, Molekel, Version 4.3, Swiss Center for Scientific Computing, Manno, Switzerland, 2002, http://www.cscs.ch/molekel/; (b) S. Portmann and H. P. Lüthi, Chimia, 2000, 54, 766.
24. C. Wang and M. R. Bryce, to be published.

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Footnotes

† Electronic supplementary information (ESI) available: B3LYP/6-31G(d) optimised geometries (figures and tables of coordinates) and orbital energy levels diagrams for compounds 6a, 11a, 13a and 16a and OXD-7. See http://www.rsc.org/suppdata/jm/b4/b413066a/

‡ On leave from the L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, R. Luxemburg Street 70, Donetsk 83114, Ukraine.

§ Crystal data: 11, C₇₇H₉₈N₄O₄, M = 1143.59, T = 120 K, triclinic, space group P1̄ (No. 2), a = 12.7434(9), b = 15.6535(11), c = 17.7496(13) Å, α = 90.56(1), β = 95.93(1), γ = 107.64(1)°, U = 3353.0(4) ų, Z = 2, μ = 0.07 mm⁻¹, Mo-K_α radiation ([small lambda, Greek, macron] = 0.71073 Å), SMART 1K CCD area detector, 34169 reflections with 2θ ≤ 55°, 15321 unique, R_int = 0.058, R = 0.057 [9543 data with F² ≥ σ(F²)], wR(F2) = 0.140 (all data). CCDC reference number 248570. See http://www.rsc.org/suppdata/jm/b4/b413066a/ for crystallographic data in .cif or other electronic format.

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