Blue fluorophores comprised of tetraphenylethene and imidazole: aggregation-induced emission and electroluminescence

By melting tetraphenylethene (TPE) and 1,2,4,5-tetraphenyl-1H-imidazole (TPI) units together through different linking positions, three new fluorophores are synthesized, and their optical, electronic and electroluminescence (EL) properties are fully studied. Owing to the presence of TPE unit(s), these fluorophores are weak emitters in solutions, but are induced to emit strongly in the aggregated state, presenting typical aggregation-induced emission characteristics. The experimental and computational results reveal that different connection patterns between TPE and TPI could impact the molecular conjugation greatly, leading to varied emission wavelength, fluorescence quantum yield and EL performance in organic light emitting diodes (OLEDs). The fluorophore built by attaching TPE unit to the 1-position of imidazole ring shows bluest fluorescence, and its EL device emits at deep blue region (445 nm; CIE = (0.16, 0.15)). And the device based on the fluorophore by linking TPE to the 2- position of imidazole ring shows EL at 467 nm (CIE = (0.17, 0.22)) with good efficiencies of 3.17 cd∙A–1, and 1.77%.


Introduction
Recently, great efforts have been devoted to develop efficient solid-state emitters for their various potential applications in organic light emitting diodes (OLEDs) [1], organic lasers [2], fluorescent sensors [3], etc. Many conventional π-conjugated fluorophores, however, suffer from the notorious aggregation-caused quenching (ACQ) effect: they emit strongly in the dilute solutions but become faintly fluorescent in the condense phase due to the formation of detrimental species such as excimers [4], which has obstructed their high-technological applications because the fluorophoric molecules are commonly used as solid films or nanoparticles in the real-world applications. To solve this ACQ problem, various molecular engineering approaches and device fabrication techniques had been proposed, but these attempts often ended with only limited success and even led to some side effects in many cases [5,6].
In 2001, an abnormal phenomenon, termed as aggregation-induced emission (AIE), was reported by Tang's group [7], which had been proved to be an effective approach to solve the ACQ problem. The AIE fluorophores are almost non-luminescent in dilute solutions but exhibit efficient emissions in the aggregated state. A series of designed experiments and the theoretical calculations were performed, and restriction of intramolecular rotations (RIR) was rationalized to be the main working mechanism behind this novel photophysical phenomenon [8]. Among the typical AIE fluorophores, tetraphenylethene (TPE) and many of its derivatives enjoy the advantages of easy synthesis and outstanding AIE characteristic. Plenty of TPE derivatives can fluoresce intensely in solid state, and have been extensively used to fabricate efficient non-doped OLEDs [9][10][11].
As a typical heterocyclic molecule, imidazole has several substitution positions (N1, C2, C4 and C5), and many of its derivatives have been used extensively in OLEDs with high electroluminescence (EL) efficiencies, and good CIE coordinates in EL spectra [12][13][14]. In this work, we designed and successfully synthesized three new fluorophores (1-3) (Scheme 1) by melting the 1, 2, 4, 5tetraphenyl-1H-imidazole (TPI) group and TPE unit in different patterns. The photoluminescence (PL) and EL properties of these new fluorophores were investigated. The OLEDs using the fluorophores as light-emitting layers were fabricated, which showed varied EL emission color from bluish green to deep blue, with moderate efficiencies.

Device fabrication
The devices were fabricated on 80 nm Indium Tin Oxide (ITO)-coated glass with a sheet resistance of 25Ω/,. Prior to loading into the pretreatment chamber, the ITO-coated glass was soaked in ultrasonic detergent for 30 min, followed by spraying with de-ionized water for 10 min, soaking in ultrasonic de-ionized water for 30 min, and oven-baking for 1 h. The cleaned samples were treated by perfluoromethane plasma with a power of 100 W, gas flow of 50 sccm, and pressure of 0.2 Torr for 10 s in the pretreatment chamber. The samples were transferred to the organic chamber with a base pressure of 7 Â 10 -7 Torr for the deposition of NPB, emitter, and TPBi, which served as hole-transport, light-emitting, and electron-transport layers, respectively. The samples were then transferred to the metal chamber for cathode deposition which composed of lithium fluoride (LiF) capped with aluminum (Al). The light-emitting area was 4 mm 2 . The current density-voltage characteristics of the devices were measured by a HP4145B semiconductor parameter analyzer. The forward direction photons emitted from the devices were detected by a calibrated UDT PIN-25D silicon photodiode. The luminance and external quantum efficiencies of the devices were inferred from the photocurrent of the photodiode. The electroluminescence spectra were obtained by a PR650 spectrophotometer. All measurements were carried out under air at room temperature without device encapsulation.

Results and discussion
3.1 Optical property Figure 1(a) shows the absorption spectra of these new fluorophores in THF solutions. 1 and 2 exhibit absorption maxima at 337 and 290 nm, respectively, indicating that 2 possesses shorter effective conjugation length compared to 1. Although the maximum absorption peak of 3 is located at 307 nm, its absorption spectral profile presents a long and strong tail at the longer wavelength region, which makes 3 and 1 have the same optical bandgaps (E opt = 3.20 eV, Table 1) estimated from the onset wavelength of their absorption spectra, indicative of the similar conjugation length of both molecules.
The emissions of these new fluorophores are very weak in solutions. Their PL spectra in dilute THF solutions (10 -5 M) only exhibit noisy signals without discernible peaks. And the fluorescence quantum yields (Φ F ) of 1, 2 and 3 are merely 0.1%, 1.3% and 1.0% (Table 1), respectively, manifesting that they are indeed weak emitters when molecularly dissolved in good solvent. However, they emit very strong fluorescence when fabricated into thin solid films. As shown in Fig. 1(b), the emission maxima of 1 and 3 in solid films are located at 490 and 482 nm, being redshifted relative to that of 2 in solid film (469 nm) due to their better conjugations. The Φ F values of 1, 2 and 3 in solid films are increased to 72.8%, 37.0% and 50.7%, respectively, revealing that they are AIE-active and excellent solid-state light emitters. This makes them promising candidates for the fabrication of efficient OLED devices. Understandably, owing to the presence of TPE units, the intramolecular rotation is active in these 1) 1 ppm = 1Â10 -6 fluorophores when they are molecularly dissolved in THF solutions, which effectively deactivates the excited state via a nonradiative relaxation channel. The intramolecular rotations, however, are restricted by steric constraint in solid films, and thus the nonradiative decay channel is blocked, rendering the molecules emissive in solid films.

Electrochemical property
The electrochemical properties of these new fluorophores were investigated by CV. The voltammograms are presented in Fig. 2

Theoretical calculation
To further study the photophysical properties of these new fluorophores, theoretical calculations were performed using the Gaussian 09 program. The molecular geometrics were fully optimized and electronic structures were investigated at the ground state (S 0 ) in THF solvent at the B3LYP/6-31G(d,p) level [17]. The PCM was employed for taking the solvent effect into account. As we all know, the HOMO and LUMO are two very important orbitals for determining the photophysical properties of the fluorophores. Hence, the electron density contours of HOMOs and LUMOs for these new fluorophores are illustrated in Fig. 3 and the corresponding orbital energy levels are summarized in Table 1. We found that the calculated values are basically consistent with the experimental data, which indicates this computational method is reasonable. The optimized molecular structures reveal that the torsion angles between TPE and TPI vary greatly in 1 (29.4°) and 2 (84.8°). As indicated in Scheme 1, when TPE is located at the 1-position of imidazole ring, the torsion angles are much larger than those when TPE are linked to the 2positions of imidazole ring. This is indicative of a better conjugation of 1 and 3 than 2. As shown in Fig. 3, the HOMOs of 1 and 3 are mainly located on the central imidazole core and 4,5-positioned phenyl rings and 2positioned TPE unit, which endows them with similar HOMO energy levels. The HOMO of 2, however, allows the electrons to be only delocalized over the central imidazole core and 2,4,5-positioned phenyl rings. Obviously, 1 and 3 have more extended conjugation than 2, resulting in a much lower HOMO energy level of 2 than those of 1 and 3. The LUMOs of the 1, 2 and 3 are mainly localized on TPE unit(s). Based on above reasonable explanations, the much redder absorption and emission maxima of 1 and 3 relative to 2 become understandable. In comparison with TPE, TPI appears to be an electrondonating group. To confirm this, the energy levels of TPE and TPI are further calculated individually. According to the calculation results, the TPI unit has a higher HOMO energy level ( -5.22 eV) than TPE ( -5.33 eV), while TPE possesses a lower LUMO energy level ( -1.22 eV) than TPI ( -0.85 eV). Therefore, TPI and TPE units should serve as electron-donor and acceptor in these molecules, respectively.

Electroluminescence
The efficient emissions of 1, 2 and 3 films encourage us to study their EL properties. Non-doped OLED devices with a configuration of ITO/NPB (60 nm)/EML (20 nm)/TPBi  Figure  4 shows the EL spectra, luminescence-voltage-current density characteristics and current efficiency versus current density curves of the OLEDs based on 1, 2 and 3. And the EL performance data are also summarized in Table 2. The EL devices based on 1, 2 and 3 emit at 467, 445 and 495 nm with CIE chromaticity coordinates of (0.17, 0.22) (sky blue), (0.16, 0.15) (deep blue) and (0.21, 0.38) (bluish green), respectively. The EL peak of 3 is only slightly redshifted from the PL peak (482 nm) of its solid film, confirming that the EL is indeed from the emitting layer. The EL spectra of 1 and 2, however, are obviously blueshifted from their PL spectra in solid films, implying that partial crystallization may happen in the emitting layer of the devices of 1 and 2, because crystallization will cause blue-shifted emission in many TPE derivatives [18][19][20]. The devices of 1 and 3 show better performances than that of 2 due to the more efficient solid-state emission of 1 and 3. The device of 1 is turned on at a low voltage of 3.6 V and exhibits a maximum luminance (L max ) of 5560 cd$m -2 , a maximum current efficiency (η C,max ) of 3.12 cd$A -1 , a maximum power efficiency (η P,max ) of 2.72 lm$W -1 and a maximum external quantum efficiency (η ext,max ) of 1.77%.
The device based on 3 shows a similar performance compared to that of 1, with turn-on voltage (V on ), L max , η C, max , η P,max and η ext,max of 4.4 V, 5110 cd$m -2 , 3.97 cd$A -1 , 2.43 lm$W -1 , 1.58%, respectively. Since the conjugation of 2 is disrupted, its OLED device emits at deep blue region and shows inferior EL data (3620 cd$m -2 , 0.96 cd$A -1 , 0.75 lm$W -1 and 0.72%). According to the above results, the EL emission is tuned from bluish green to deep blue, realizing the control of EL emission and efficiency through a very simple strategy of minor structural modification.

Conclusions
In summary, three new fluorophores consisting of TPE and TPI units were prepared and their optical properties were investigated. They show very weak emissions when molecularly dissolved in dilute solutions, but they are induced to emit intensely in the solid films, demonstrating that they possess AIE characteristics. 1 and 3 exhibit longer effective π-conjugation lengths than 2, which results in redder PL emissions and higher fluorescence quantum yields in the solid state. Non-doped EL devices of were fabricated using these fluorophores as emitters, and blue EL emissions with varied EL efficiencies were obtained. The impacts of a minor structural alternation on the PL and EL properties of the fluorophores have been demonstrated.