2.1 General information and instrumentation

Solvents were distilled prior to use. THF and ether were dried over sodium/benzophenone, and distilled under Argon. All the reactions were carried out under argon atmosphere. ¹H, ¹³C and ²⁹Si NMR spectra were recorded on a Bruker Advance 200 DPX spectrometer, the FT-IR spectra on a Thermo Nicolet Avatar 320 spectrometer, the UV–visible spectra on a Secomam Anthelie instrument and the MS spectra on a Jeol JMS-DX 300 spectrometer. The emission and excitation spectra were performed on a SLM Amenco 8100 spectrometer using degassed 10^–5 M solutions, the quantum yields were determined using perilene as standard (Φ_em = 0.94). I–V characteristics were recorded with a Keithley 2400 Sourcemeter, L–V with a photodiode placed under the OLED and coupled to a HP 3457A multimeter. Electroluminescence spectra were measured using an Ocean Optics PC2000 CCD spectrometer. All EL devices were kept and characterized in a glove box under nitrogen.

2.2 Synthesis of p-2,2′-dipyridylaminobromophenyl (2)

A mixture of 1,4-dibromobenzene (4.939 g, 20.9 mmol), di-2-pyridylamine (1.2 g, 6.96 mmol), K₂CO₃ (1.12 g, 8.37 mmol) and CuSO₄ (0.174 g, 0.697 mmol) in water (20 ml) and CH₂Cl₂ (100 ml) was stirred well and evaporated to dryness in vacuum. The mixture was ground in a mortar and three to five drops of CH₂Cl₂ were added to this mixture. The mixture was heated in a sealed tube at 200 °C for 6 h. After being cooled at room temperature, the mixture was dissolved in CH₂Cl₂ (100 ml) and water (100 ml) and extracted. After evaporation of the solvent, the residue was subjected to column chromatography CH₂Cl₂ to afford compound 2 (yield: 83%). M.p.: 124 °C. ¹H NMR (CDCl₃, δ, ppm): 8.35 (dd, J₁ = 7 Hz, J₂ = 2 Hz, 2H), 7.60 (td, J₁ = 7 Hz, J₂ = 2 Hz, 2H), 7.51 (d, J = 9 Hz, 2H), 7.09 (d, J = 9 Hz, 2H), 7.05–6.95 (m, 4H). ¹³C NMR (CDCl₃, δ, ppm): 157.32, 149.06, 144.81, 138.11, 133.14, 129.03, 118.93, 118.93, 117.44, 114.31. HRMS (FAB+, m-nitrobenzyl alcohol matrix) m/z: calc. for C₁₆H₁₂N₃Br 326.0293, found 326.0271.

2.3 Synthesis of 1,1-dimethyl-2,5-bis(4-phenyl-di-2-pyridylamine)-3,4-diphenylsilole (3)

A mixture of lithium (0.055 g, 8 mmol) and naphthalene (1.03 g, 8 mmol) in THF (15 ml) was stirred at room temperature under argon for 5 h to form a deep green solution of lithiumnaphthalenide. To this mixture was added bis(phenylethynyl)dimethylsilane (1) (0.5 g, 2 mmol) in THF (10 ml). After stirring for 10 min, the reaction mixture was cooled to 0 °C and [ZnCl₂(tmen)] (tmen = N,N,N′,N′-tetramethylenediamine) (2.01 g, 8 mmol) was added, followed by an addition of THF (20 ml). After stirring for an hour at room temperature, a solution of 4-(2,2′-dipyridylamino)bromobenzene (2) (1.304 g, 4 mmol) in THF (20 ml) and [PdCl₂(PPh₃)₂] (0.100 g, 0.13 mmol) were successively added. The mixture was heated under reflux and stirred for 20 h. After hydrolysis by water, the mixture was extracted with Et₂O. After evaporation of the solvents, the resulting residue was subjected to a column chromatography CH₂Cl₂/THF (85:15) to give 0.560 g of 3 as a bright yellow crystalline solid (yield: 40%). ¹H NMR (CDCl₃, δ, ppm): 8.35 (dd, J₁ = 7 Hz, J₂ = 2 Hz, 4H, B rings: H-1), 7.56 (td, J₁ = 7 Hz, J₂ = 2 Hz, 4H, B rings: H-3), 7.07–7.02 (m, 6H, A, B and C rings), 6.97–6.95 (m, 20H, A, B and C rings), 0.57 (s, 6H, Si(CH₃)₂).¹³C NMR (CDCl_3, δ, ppm): 158.54, 154.58, 148.93, 142.85, 139.65, 137.89, 137.03, 130.55, 130.27, 127.93, 126.65, 126.47, 123.49, 118.56, 117.62, –2.72. ²⁹Si NMR (CDCl₃, δ, ppm): 8.038. HRMS (FAB+, m-nitrobenzyl alcohol matrix) m/z: calc. for (M⁺ + 1H) 753.3162 C₃₈H₃₂N₆Si, found 753.3196.

3.1 Synthesis

Silole 3 was conveniently prepared by the method described by Tamao et al. that involves the one-pot reductive intramolecular cyclization of bis(phenylethynyl)silane (1) and subsequent Pd(0)-catalyzed cross-coupling reaction with an arylbromide as depicted in Scheme 1. The synthesis of 4-(2,2′-dipyridylamino)bromobenzene (2) that was originaly described by Jia et al. [12] was improved by carrying out the Ullmann's condensation in sealed tubes. Silole 3 was recovered as a bright yellow solid with 24% yield.

3.2 Photoluminescent properties

In the Fig. 2 are shown the selected UV–visible absorption and fluorescence spectra recorded in solution of compounds 2, 3 and 1,1-dimethyl-2,3,4,5-tetraphenylsilole (4), for comparison. At the first glance, the absorption spectrum of 3 may be described as the superposition of the spectra of its constitutive chromophores; the absorption bands observed at ca. 275 nm originate from π–π* transitions of the silole substituents that is to say, the phenyl groups at the three and four positions and the phenyldipyridylamino groups at the two and five positions, whereas the absorption band observed at 391 nm originates from the π–π* transition of the silole ring [3]. However, when compared with the values reported for the reference compound 4 (absorption: λ_max = 359 nm, emission: λ_max = 467 nm) [3], the red-shifts that are observed for 3 both in the absorption and emission spectra (absorption: λ_max = 391 nm, emission: λ_max = 516 nm) are indicative of a substantial perturbation of the HOMO–LUMO levels provided by the presence of the terminal dipyridylamino groups. If we consider now both the emission spectrum of 2 (λ_ex = 290, λ_max = 385 nm) and the absorption spectrum of 3, one can anticipate that an intramolecular energy transfer may take place since the dipyridylamine emission greatly overlaps the silole π–π* transition (λ_max = 391 nm). As a first consequence of such a process, one of the more striking effect afforded by the presence of the dipyridylamino groups is reflected by a substantial increase of the emission quantum yield values that jumps from 0.0014 for 4 to 0.06 for 3.

3.3 Fluorescent chemosensor properties

Preliminary trials have been carried out to estimate the potential of silole 3 as chemosensor for metals ions such as Zn²⁺ and Cu²⁺ since their analytical determination is of biological particular interest [13]. Emission spectra (λ_ex = 388 nm) of free silole 3, 3·ZnCl₂ and 3·CuCl₂ complexes are presented in Fig. 3. 3·ZnCl₂ and 3·CuCl₂ complexes were prepared by reacting equimolar amounts of 3 and the desired metal ion in a dichloromethane/methanol mixture and isolated as small crystals by layering the resulting solution with hexanes. Chemical analysis of the crystals gave, in each case, a 3/M²⁺ ratio of 1:1, indicating a polymeric structure for the complexes as described elsewhere [14]. As seen clearly on the curves, the presence of Cu²⁺ ions in 1:1 ratio yields a total quenching of the fluorescence, whereas Zn²⁺ ions affect moderately the emission intensity arising from the silole π–π* transition (ca. 25%). The same phenomenon is observed when CuCl₂ is added to silole 3 in solution and both selectivity and quenching activities are currently being studied.

3.4 Electroluminescent properties and EL devices manufacture

Cyclic voltammetry was performed on silole 3 to estimate the HOMO and LUMO energy levels. For efficient electron-transporting materials, a low reduction potential is preferred in order to facilitate electron injection from the cathode. On the contrary, for efficient hole-transporting materials, a low oxidation potential will favor hole injection from the anode. From the onset potentials of the oxidation and reduction processes, the band gap was estimated to be 2.33 eV, which is in good agreement with the values obtained from the optical absorption spectrum (2.48 eV). The HOMO and LUMO energy levels were calculated assuming the energy level corresponding to the electrochemical potential of the Saturated Calomel Electrode (E_SCE) is –4.4 eV [15]. The HOMO and LUMO levels were then estimated to be –5.22 and –2.89 eV for silole 3.

EL devices based on silole 3 were fabricated, with the structure shown in Fig. 4. The electrodes are to be chosen in order to match the HOMO and LUMO levels of the organic materials. Indium Tin Oxide (ITO) is a commonly-used anode because it is transparent and has a relatively high work function (4.8 eV), which matches quite well the HOMO level of the siloles. Between ITO and the silole a 60 nm-thick layer of poly(3,4-ethylene-dioxythiophene) doped with poly(styrene sulfonate) (also called PEDOT/PSS) was inserted. PEDOT/PSS is a conducting polymer acting as a buffer in reducing the shorting problems caused by the ITO deposition process and acts as a barrier to the oxygen and indium diffusion from ITO [16]. Moreover it slightly increases the work function of the anode, lowering the barrier to holes. PEDOT was spin-coated on top of ITO and cured at 80° for about 1 h. A 50-nm-thick silole layer was then thermally evaporated under secondary vacuum (around 10^–6 mbar). Calcium was used as the cathode as its work function (2.9 eV) is close to the electron affinity of the siloles. Ca was evaporated through a shadow mask on top of the silole and capped with a layer of aluminum to minimize its oxidation. In the ITO/PEDOT/silole/Ca device the silole acts as the emitter but also as the electron and HTL.

The normalized EL spectrum recorded for silole 3 operating in the EL device (Fig. 5) is almost superimposable with the PL spectrum obtained from a thin film of this compound upon an excitation at 400 nm. Both spectra show a similarly featured emission peak in the yellow–green region (λ_max = 545 nm) showing that the silole ring is the emissive component in each case.

Finally, to evaluate the performance of silole 3 as EL materials, we have examined the current density–voltage (J–V) and luminance–voltage (L–V) characteristics of ITO/PEDOT/silole/Ca devices (Fig. 6). Above a threshold voltage of 9 V a significant current flows, giving rise to light emission with excellent performance, reaching a luminance of 10⁴ Cd/m² at 15, a maximum efficiency of 27 Cd/A and 6 lm/W, and an efficiency of 16 Cd/A and 4 lm/W at 100 Cd/m². For comparison, this compound is more efficient than Alq, one of the most efficient EL materials reported so far [17,18] which exhibits an efficiency of 2.2 Cd/A under the same conditions.

In conclusion, we have reported in this communication the synthesis and the use in EL devices of a new diarylsilole having di-2-pyridylaminophenyl as aryl groups. This compound was designed to promote an energy transfer from the aryl side-groups to the central silole ring, and therefore, enhance the luminescence efficiency. According to this assumption, the photoluminescence quantum yield of silole 3 was found to be enhanced by a factor of 40 when compared to the parent tetraphenyl silole 4. This enhancement was also observed when used in single-layer light-emitting diodes with an efficiency of more than 16 Cd/A at 100 Cd/m², which is firstly more efficient than Alq₃, and secondly the best value reported so far in the literature for siloles derivatives. Moreover, we have briefly shown that silole 3 may be used as fluorescent chemosensors, subject that is currently under development.