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Aromatically C6- and C9-Substituted Phenanthro[9,10d]imidazole Blue Fluorophores: Structure-Property Relationship and Electroluminescent Application Wen-Cheng Chen, Yi Yuan, Yuan Xiong, Andrey L. Rogach, Qing-Xiao Tong, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06547 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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ACS Applied Materials & Interfaces

Aromatically C6- and C9-Substituted Phenanthro[9,10-d]imidazole Blue

Fluorophores:

Structure-Property

Relationship

and

Electroluminescent Application Wen-Cheng Chen,1,2 Yi Yuan,1,3 Yuan Xiong,4 Andrey L. Rogach,4 Qing-Xiao Tong,3,* and Chun-Sing Lee1,2,* 1

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China E-mail: [email protected] 2 City University of Hong Kong Shenzhen Research Institute, Shenzhen, Guangdong, P. R. China 3 Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, 243 University Road, Shantou, Guangdong, 515063, P. R. China E-mail: [email protected] 4 Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR, P. R. China KEYWORDS: C6,C9-substituted phenanthro[9,10-d]imidazole, aromatic modification, electron donor-acceptor, structure-property relationship, blue electroluminescence ABSTRACT In this study, a series of aromatically substituted phenanthro[9,10-d]imidazole (PI) fluorophores at C6 and C9 (no. 6 and 9 carbon atoms) have been synthesized and systematically characterized by theoretical, thermal, photophysical, electrochemical and electroluminescent (EL) studies. C6 and C9 modifications have positive influences on the thermal properties of the new materials. Theoretical calculation suggests that the C6 and the C9 positions of PI are electronically different. Theoretical and experimental evidences of intramolecular charge transfer (ICT) between two identical moieties attaching to the C6 and the C9 positions are observed. Photophysical properties of the fluorophores are greatly influenced by size and 1

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conjugation extent of the substituents as well as linking steric hindrance. It is found that the C6 and C9 positions afford moderate conjugated extension comparing to the C2 modification. Moreover, ICT characteristics of the new fluorophores increase as the size of substituted aromatic group, and are partially influenced by steric hindrance, with the anthracene and the pyrene derivatives having the strongest ICT excited properties. EL performances of the fluorophores were evaluated as host emitters or dopants in organic light-emitting devices (OLEDs). Most of the devices showed significantly improved efficiencies comparing to the OLED using the non-modified emitter. Among all the devices, a 5 wt% TPI-Py doped device exhibited excellent performances with an external quantum efficiency > 5% at 1000 cd/m2 and a deep-blue color index of (0.155, 0.065), which are comparable to the most advanced deep-blue devices. Our study can give useful information for designing C6/C9-modificated PI fluorophores and provide an efficient approach for constructing high-performance deep-blue OLEDs.

1. INTRODUCTION Organic π-conjugated materials for optoelectronic applications have attracted considerable attention owing to their facile and cost-efficient synthetic approaches as well as easily tunable electrical/photo physical properties.1–6 Understanding the structure-function relationship in these materials and mastering the methods for controllably tuning their versatile properties are important for their applications7–9 in various devices including organic photovoltaics (OPVs), organic light-emitting devices (OLEDs), organic field-effect transistors, molecular sensor and non-linear 2


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optics etc. In the recent years, OLEDs have attracted increasing interest as a next generation of full-color display and solid-state lighting technology.10,11 Much effort has been devoted to establish the structure-property relationship of organic electroluminescent (EL) emitters.12–14 Although commercially available green and red light-emitting materials have been well developed, efficient blue emitters with Commission Internationale de L’Éclairage (CIE) coordinates matching the National Television System Committee (NTSC) blue standard of (0.14, 0.08) are still rare.3 Furthermore, efficient deep-blue emitters are especially important since their use can effectively cut down power consumption of full-color display devices15 and excite other emitters with longer wavelength emissions.11,16 Nevertheless, their inherent large energy gaps (typically > 3 eV) often hinder the charge injection and disturb the balance of hole/electron flow and eventually undermine the device performances. Several reports have pointed out that selection of building blocks plays a key role on the evolution of emitters with desirable physical properties,17–19 e.g. photophysical, thermal, electrical properties, which determine the EL performances. Therefore, systematic investigation on the structure-property relationship of the deep-blue molecular materials deserves preferred consideration. Phenanthro[9,10-d]imidazole (PI) is an important building block,20 which was demonstrated it versatility in design of molecular materials for different research areas, such as OLEDs,21,22 OPVs,23,24 memory devices,25 light-emitting electrochemical cells,26 ﬂuorescent sensors,27 drug therapy,28 organic electro-synthesis29 etc. Scheme 1 shows the chemical structure of PI. The fused phenanthrene scaffold attached at the 3



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imidazole ring invokes the photophysical activity and the thermal stability due to the enhanced rigidity and planarity. The unique molecular framework of PI stemming from its two different nitrogen entities – pyrrole-like nitrogen atom (N1) and pyridine-like nitrogen atom (N3) endows this building block with bipolar nature.30,31 Despite its excellent electrical and photophysical properties, non-modified PI suffers from several deficiencies, which hinders its application as an active EL material. On one hand, absorption and emission spectra of PI are confined within UV region, whereas majority of the optoelectronic materials are required to have response in the visible region. Besides, the planar framework of PI tends to form bimolecular excimer in solid state, and the N3 may further induce intermolecular hydrogen bonds (nitrogen-hydrogen interaction),32 which have negative influences on the emission properties in solid state. Therefore, efforts have been devoted to address these issues by functional substitution at various positions as shown in Scheme 1. It is reported that incorporating functional groups to PI via C2 position (no. 2 carbon atom) induces direct conjugated extension and may lead to intense intramolecular charge transfer (ICT).33–35 Li et al. demonstrated that linking an electron rich arylamine group to PI via

C2

position

can

redshift

the

UV

emission

(369

nm)

of

1,2-diphenyl-1H-phenanthro[9,10-d]imidazole to deep-blue region (438 nm) and the resulting emitter exhibited a typical ICT characteristic evidenced by positive solvatochromic effect in emission.33 On the contrary, N1 substitution does not lead to significant photophysical variation mainly due to perpendicular configuration between the substituents and the PI framework.36 Therefore, some PI derivatives are designed 4


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with bulky moieties, such as tert-butyl phenyl group, at N1 position, aiming at controlling the intermolecular interaction in solid state.22,32,37

Scheme 1. Chemical structure of phenanthro[9,10-d]imidazole and the main features of substitutions at various positions. To date, majority of the papers have been published on substituting the PI group at the C2 and the N1 positions. In contrast, significance of substitution at the C6 and the C9 positions has not been noticed until 2016.38,39 So far, there is no systematic studies on the structure-property relationship on C6/C9 modified PI EL emitters. To fully exploit the potential of C6/C9 substituted fluorophores, it is significant to understand the structure-property relationship in the C6 and the C9 substitutions which is so far unavailable. In this study, we designed and synthesized a family of aromatically substituted deep-blue PI fluorophores via C6 and C9 positions. Different aromatic hydrocarbons, i.e. benzene, naphthalene, phenanthrene, anthracene and pyrene, with different size, shape and conjugation, are connected to the C6 and the C9 of PI (Scheme 2), as a stereotype to systematically investigate the structure-property relationship. It is found that C6,C9-substitution provides a moderate conjugated extension comparing to the C2-modification. Theoretical calculation indicates that 5



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asymmetry HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) distributions are resulted even when the same groups are attached to both the C6 and the C9 sites. Such asymmetry is attributed to the two different nitrogen atoms in the imidazole ring. Photophysical study demonstrates that ICT characteristics of the new molecules increase as the size of substituted aromatic groups, and are partially influenced by steric hindrance. The EL performances of the materials were evaluated both as host emitters and dopant emitters in OLEDs. It is worth noting that the OLEDs employing TPI-Ph show violet-blue emissions with very low CIEy values (0.034 to 0.050) and decent performances. Among all the devices, a 5 wt% TPI-Py doped OLED showed the highest performance with an EQE > 5% at 1000 cd/m2 and deep-blue EL (color index: (0.155, 0.065)).

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization The new blue fluorophores were synthesized by a facile three-step procedure, as shown

in

Scheme

2.

The

precursor

9-dibromo-1,2-bis(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (TPI-2Br)39 was

prepared

by

pre-brominating

the

phenanthrene-9,10-dione

to

3,6-dibromophenanthrene-9,10-dione,40 followed by a "one-pot" reaction to construct the PI skeleton. At last, the new fluorophores were obtained by a Suzuki coupling reaction between TPI-2Br and varying aromatic boric acids, and separated by column chromatography with good yields. We also synthesized the non-modified

6


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1,2-bis(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (TPI) as a reference for comparison. Detail synthesis of the new compounds is descripted in Supporting Information. Chemical structures of the blue fluorophores were determined with 1H, 13

C NMR and mass analysis.

Scheme 2. Synthesis of the new molecules.

2.2. Thermal Properties Thermal properties of the compounds were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 10 °C/min. As shown in Figure 1a, decomposition temperatures (Td, 5% weight loss) of the compounds increase as the size of the substituents. TPI-1Na and TPI-2Na are two isomers obtained by connecting PI to respectively 1- and 2-positions of the naphthalene moieties (Scheme 2). TPI-2Na has a higher Td than TPI-1Na (502 versus 456 °C), mainly due to the better conjugation between PI and 2-substituting naphthyls

7



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with less steric hindrance. Further discussion on this is given in later part of the paper in light of results from theoretical calculation. The relatively low Td of 479 °C for TPI-An is also attributed to steric hindrance which limits the conjugation. Glass transition temperatures (Tgs) of the compounds also increase with the size of the substituents. For compounds with the same substituents such as TPI-1Na and TPI-2Na, higher Tg (168 versus 138 °C) was observed when the substituent has more steric hindrance. For the pyrene derivative, Tg of TPI-Py is high up to 253 °C. All the compounds exhibit much higher Tds and Tgs than the reference molecule TPI. Furthermore, comparing to the C2-substituted counterpart Py-BPI41 (Td = 431 °C, Tg = 137 °C), TPI-Py shows much better thermal stability, indicating that C6 and C9 substitutions of PI is a possible approach for improving thermal property.

8


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Figure 1. (a) TGA and (b) DSC analysis of the new compounds.

2.3. Theoretical Calculation Theoretical calculations were performed based on density functional theory (DFT) on a B3LYP/6-31g(d,p) level. The energy minimized geometries of the molecules are shown in Figure S1. It can be seen that changing the substituents at the C6 and the C9 positions have little influences on the twisting angles between the PI plane and the 4-(tert-butyl)pheny groups (i.e. θ1 and θ2). As expected, much larger variations in θ3 and θ4 are observed. For example, TPI-1Na displays a more twisted configuration (θ3 = 56.2°; θ4 = 53.9°) than its isomer TPI-2Na (38.8° and 36.5° respectively) due to the

9



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larger steric hindrance provided by the repulsion of hydrogen at the 8 position of naphthalene. To some extent, this difference is also responsible for the much higher Tg of TPI-1Na over that of TPI-2Na. For the same reason, TPI-An with two highly twisted anthracene substituents shows twisting angles larger than 75°.

Figure 2. Frontier molecular orbitals and energy levels of the new molecules.

The calculated energy levels and frontier molecular orbital distributions are shown in Figure 2. The HOMO energy levels of the modified TPI derivatives are nearly the same as that of TPI, which are calculated to be ~5 eV. On the other hand, aromatic substitution at C6 and C9 would deepen the LUMO levels, which can lead to easier electron injection. Except for TPI-An, the HOMO is mainly delocalized over the PI moiety, whereas the LUMO distribution tends to concentrate on the C9-linking aromatic groups, with much less contribution from the C6 substituents. This asymmetry is likely to be originated from the obvious difference in LUMO contributions of the C6 and the C9 atoms in TPI. In a recent study by Justin Thomas 10


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et al. concerning asymmetry C4/C7 modified benzimidazole compounds, they found that C4 and C7 brominated sites in benzimidazole have different reactivities in Suzuki coupling reaction, which are originated from the two different nitrogen atoms in the imidazole ring;7 thus similar properties can be expected in the case of PI. Imidazole is an aromatic group featured with a sextet of electron in the conjugated system satisfying the Hückel's (4n + 2) rule (Scheme 3). The lone pair electrons attached at N1 participates in the formation of conjugated system, while the pair on N3 has no contribution. In the case of PI, the conjugated lone pair electrons can be transferred toward C6 or C9 according to the resonance theory. The electron migration towards C6 is more favorable since it results in the largest charge separated distance (red arrow in Scheme 3) than the case of C9 (blue arrow). This preferred electronic structure leads to charge accumulation at the C6 position. Therefore, this unique electronic structure of PI induces that HOMO and LUMO tend to distribute at the C6 and the C9 substitutions respectively.

Scheme 3. Electronic structure of imidazole ring and the proposed electronic preference effect in 11



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C6 and C9 substituted PI molecules.

2.4. Photophysical Properties UV-vis absorption and photoluminescence (PL) spectra of the fluorophores measured in THF solution are shown in Figure 3 and the key data are listed in Table 1. TPI has the highest energy absorption bands in THF solution, with peaks at 314 and 363 nm ascribed to the π-π* electronic transitions for the PI and C2-substituent to PI moiety,22 respectively. Upon modifying with benzene, naphthalene and phenanthrene groups, the absorption spectra display bathochromically shifted and structureless bands. On the other hand, TPI-An mainly shows characteristic anthracene's absorption bands (350, 368 and 386 nm). With pyrene substitutions, TPI-Py has the broadest absorption peaking at 346 nm with high intensity. The disappearance of the intrinsic pyrene/PI vibrational bands suggests the existence of ICT electronic interaction in TPI-Py. For the small size aromatic benzene or naphthalene and non-linear phenanthrene derivatives, PI moiety dominates the absorption, yet TPI-An and TPI-Py have redshifted absorption due to the highly conjugated anthracene/pyrene. The absorption spectra in thin films are shown in Figure S2. The thin films exhibit comparable absorption to those in solutions. With the absorption onset of the films, we estimated the optical energy gaps (Egs) to be 3.27, 3.12, 3.06, 3.16, 3.16, 2.97 and 2.96 eV for TPI, TPI-Bz, TPI-2Na, TPI-1Na, TPI-Ph, TPI-An and TPI-Py, respectively. The effect of C6- and C9-modification on PL spectra basically follows the variations observed in the absorption spectra. TPI-Bz, TPI-2Na, TPI-1Na and TPI-Ph show analogically fine emission profiles to TPI with different degree of redshifts. TPI-2Na (λf = 398 nm) 12


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emits violet-blue light with longer wavelength than its isomer TPI-1Na (λf = 386 nm) and even TPI-Ph (λf = 389 nm) with a larger conjugation of substituents, which is mainly due to the more planar conjugation in TPI-2Na. The emissive profiles of the fluorophores can be tuned from violet-blue to deep-blue region via modifying TPI with anthracene and pyrene. Most of the materials emit prompt fluorescence with lifetimes shorter than 10 ns (Figure S3 and Table 1). The fluorescence lifetimes of C6,C9-substituted compounds are comparable to that of TPI, indicating that C6,C9-modification does not effects the fluorescence decay significantly. TPI-Bz shows a relatively long fluorescence lifetime (10.1 ns), possibly ascribed to the small size benzene substituents, which may easily suffer from configuration changes during the relaxing process. Bathochromically shifted, wide and structureless emission bands are observed in TPI-An and TPI-Py solutions with maxima at 429 and 437 nm, respectively. Figure 4a shows PL photographs of 30 nm thin films of the fluorophores under UV light excitation. Except for the TPI film, all the samples exhibit structureless and redshifted emission bands (Figure 4b) when comparing to those of the solutions (Figure 3b). The film of TPI-An has a broad emission profile and a long tail in long wavelength region mainly due to the close intermolecular stacking induced by the peripheral anthracene groups. The absolute PL quantum yields (PLQYs) of the films are measured using an integrating sphere. After C6 and C9 modifications, all the fluorophores show improved PLQYs in solid films (Table 1). The PLQY of TPI-Py film is up to 0.93. In order to assess the substitution effects on the photophysical properties, we used 13



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TPI-Py as a model C6,C9-substituted derivative to compare the C2-modified molecule Py-BPI.41 The absorption and PL spectra of the compounds are shown in the Figure S4. The absorptions of the two emitters are analogical, peaking at around 350 nm. Although one more pyrene ring is incorporated to the PI moiety in TPI-Py, its absorption does not redshift significantly, showing a comparable Eg to that of Py-BPI. Besides, because one more pyrene is incorporated in TPI-Py, it shows intense absorption with molar extinction coefficient (ε) of 7.1 × 105 M-1 cm-1, which is much higher than that of Py-BPI (ε = 4.5 × 105 M-1 cm-1). The PL spectra of TPI-Py and Py-BPI exhibit similar profiles having full-widths at half-maximum of 68 and 67 nm, respectively. The Py-BPI has a shorter wavelength PL peak than that of TPI-Py (440 versus 450 nm), mainly due to the benzene spacer resulting in extra twisting. We also compare the PL peaks of TPI-An and TPI-Py to the C2-substituted anthracene and pyrene derivatives from other literature.42 It is found that our compounds show shorter-wavelength emissions compared to the corresponding C2-substituted compounds. Overall, substitutions at C6 and C9 positions of PI can induce moderate π conjugated extension comparing to the C2-modified ones.

14


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Figure 3. (a) Absorption and (b) PL spectra in THF solution.

Figure 4. (a) PL photographs of the thin films prepared on quartz substrates under UV light excitation and (b) their corresponding PL spectra. 15



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Table 1. Photophysical properties of the new compounds. Compd.

λaa [nm]

λfb [nm]

PLQYc

τd [ns]

TPI

314, 346, 363/316, 349, 366

374, 391, 415/378, 396, 418

0.15

6.4

TPI-Bz

312, 359, 377/338, 365, 383

383, 409, 434/403, 420

0.43

10.1

TPI-2Na

337, 380/347, 387

398, 416/432

0.61

6.6

TPI-1Na

325, 374/332, 378

386, 405/412

0.31

7.4

TPI-Ph

326, 374/337,379

389, 405/412

0.72

7.0

TPI-An

317, 332, 350, 368, 386/354, 374, 391

429/456

0.69

6.1

TPI-Py

346/356

437/468

0.93

3.8

a

b

Absorption peak measured in THF solution / solid film, respectively. Fluorescence peak measured in THF solution / solid film,

respectively. cPhotoluminescence quantum yield of thin film measured by using integrating sphere. dFluorescence lifetime measured in CH2Cl2 solution.

To further investigate the properties of the excited states, solvation effects on the photophysical properties were studied in different solvents. Twelve solvents with different polarity were used in the measurement. The absorption and the PL spectra in the solutions are shown in Supporting Information (Figure S5-11), and the related data are listed in Table S1 and S2. From non-polar hexane to highly polar acetonitrile (ACN), the absorption spectra of all the compounds remain unchanged, indicating imperceptible ICT properties in their ground states. The emission spectra for TPI-Bz, TPI-2Na, TPI-1Na and TPI-Ph do not redshift significantly and their vibrational profiles are retained even in polar solutions, showing comparable excited properties to those of the non-modified TPI. By contrast, the emissions of TPI-An and TPI-Py not only show large redshifts, but also gradually widened and structureless profiles along with increment of orientation polarization (∆f) of the medium. This may due to the ICT electronic interaction in the excited states of TPI-An and TPI-Py.43 Such photophysical properties are also in accord with the results from DFT calculations. 16


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Based on the above analyses, it is indicated that the PI group in TPI-An and TPI-Py does not serve as the central emitting core; instead, it becomes an ICT inducer. We further plotted the curves of ∆f against the Stokes shift (νa-νf), and estimated the dipole moments of the excited state (µes) according to the Lippert-Mataga model,44,45 as shown in Figure S12. It can be observed that TPI, TPI-Bz, TPI-2Na, TPI-1Na and TPI-Ph display only one linear relation each. Their µes are relatively small, which are calculated to be 10.94, 11.70, 16.01, 13.66 and 17.05 D, showing characteristic locally excited (LE) properties. Obviously, the µes increase with the conjugation or the size of substitution at C6 and C9 positions, which implies progressively enhanced ICT properties of the molecular excited states, while LE features are still dominating. On the other hand, the anthracene and the pyrene substituted derivatives show evident ICT excited states. TPI-An and TPI-Py show two-section linear relations in their ∆f versus (νa-νf) curves. Although small µes (10.07 and 10.77 D for TPI-An and TPI-Py, respectively) are obtained in solvents with low ∆f (< 0.17), the slopes in the high ∆f (> 0.17) region are steep, corresponding to µes of 25.14 and 23.27 D. This ICT property is expected to enhance the EL performance.46–48

2.5. Electrochemical Properties The electrochemical properties of the fluorophores measured by cyclic voltammetry (CV) are illustrated in Figure 5. The oxidation scans were carried out in CH2Cl2 to estimate the HOMO levels which were deduced from the onset oxidation potentials with regard to the HOMO level of ferrocene (-4.8 eV), while the reduction scans were conducted in the DMF solutions. All the compounds display similar onset oxidation 17



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potentials but distinguishable CV curves originated from the different substituted aromatic rings. The HOMO levels were calculated to be -5.62, -5.52, -5.49, -5.55, -5.54, -5.53 and -5.50 eV for TPI, TPI-Bz, TPI-2Na, TPI-1Na, TPI-Ph, TPI-An and TPI-Py, respectively, which are slightly lower than that of the hole transporter N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB, ~-5.40 eV), implying their good hole injection capacity. By contrast, the reduction curves are similar and have less features compared to the oxidation waves. Combined with the Eg values, their LUMO levels are estimated to be -2.35, -2.40, 2.43, -2.39, -2.38, -2.56 and -2.54 eV, respectively (LUMO = Eg + HOMO).

Figure 5. Cyclic voltammetry measurements of the new compounds.

2.6 Electrical Properties Hole- and electron-only devices (HODs and EODs) were fabricated and their current density-voltage (J-V) characteristics (Figure S13) are used to evaluate the carrier 18


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transport properties. The HODs and EODs have configurations of respectively ITO (indium tin oxide)/NPB (10 nm)/one of the compounds (70 nm)/NPB (10 nm)/Al (100 nm) and ITO/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB, 10 nm)/one of the compounds (70 nm)/TmPyPB (10 nm)/LiF (1 nm)/Al (100 nm). It is demonstrated that the compounds featuring planar molecular configuration show high electron currents in the corresponding EODs, such as TPI-Bz and TPI-2Na. TPI-An and TPI-Py, are expected to have good electron transport properties inherited from their aromatic substituents, and also suggested by the close packing in solid state which have been discussed in the photophysical section. By contrast, because TPI-1Na and TPI-Ph have larger twisting angles induced by substituents, they are hard to form good intramolecular contact to facilitate charge hopping, and thus resulting in low electron currents in their EODs. Most of the compounds show good hole conductivity due to the excellent hole transport property of PI moiety.49 TPI-1Na shows the worst electrical property. Apart from the high twisting configuration, the small volume of π electron of naphthalene comparing to phenanthrene and anthracene may be the other reason.

2.7. Electroluminescence performances We firstly fabricated a set of non-doped OLEDs to evaluate the EL performances of the new fluorophores, with a general structure of ITO/NPB (70 nm)/TCTA (4,4',4"-tris(carbazol-9-yl)triphenylamine, 5 nm)/one of the emitters (30 nm)/TPBi (2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), 30 nm)/LiF (1 nm)/Al 19



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(150 nm). Figure 6a shows the energy levels of the materials used in the non-doped devices. Low energy barriers among the device are conducive to charge injection. Exciton can be well confined within the emissive layers (EMLs) because of the large Egs of TCTA and TPBi. TPBi layer also functions to block hole owing to its deep HOMO level (-6.3 eV). All the non-doped devices show almost linear J-L (luminance) characteristics (Figure S14), showing typical photoelectric characteristics of general emitting diodes. J-V-L characteristics are shown in Figure 6b. Current density of the OLEDs employing TPI-An and TPI-Py show the fastest increases with voltage. It is partly attributed to their smaller Egs and relatively deeper LUMO levels. Nevertheless the worst electrical characteristics are observed in the devices based on TPI-1Na and TPI-Ph. These are in accord with the results from the HODs and EODs. The EL performances are summarized in Table 2. The turn-on voltages (Vons) of the non-doped devices using TPI-2Na, TPI-An and TPI-Py are very low (2.8, 2.6, 2.5 V respectively) mainly due to the good charge transport and the small Egs of emitters. It is worth noting that practical brightness of 100 cd/m2 (1000 cd/m2) can be obtained at low voltages of 3.3 V (4.2 V) and 2.9 V (3.8 V) for the devices using TPI-An and TPI-Py respectively. Among all the emitters, TPI-Py exhibits the best efficiencies with an external quantum efficiency (EQE) up to 5.69%. Low driving voltage is accompanied by an excellent power efficiency (PE) of 8.46 lm/W. However, the corresponding non-doped device emits sky-blue emission with CIE coordinates of (0.157, 0.177). The OLED using TPI-An also shows greenish blue EL emission (color index: (0.200, 0.222) at 6 V) with a moderate EQE of 3.39% due to the formation of 20


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excimer. There is a sub-peak at ~640 nm in the device based on TPI-1Na, which significantly undermines the blue color purity with CIE coordinates of (0.184, 0.116) (at 6 V). At this stage we cannot give a precise explanation, it may be due to the formation of electromer.50 Figure S15 shows the EL and PL spectral comparison. Profiles of the EL spectra of the non-doped devices are similar to those of the PL spectra in the corresponding thin films. The bathochromic (for shorter-wavelength light emitting TPI, TPI-Bz, TPI-1Na and TPI-Ph) or hypochromatic (TPI-2Na, TPI-An and TPI-Py) shifts in the EL spectra comparing to the PL spectra, are probably due to weak microcavity effects.51 Figure S16 shows the EL spectra of the non-doped devices under different driving voltages. Due to the non-intrinsic emission from the fluorophores in the TPI-1Na and the TPI-An based OLEDs as discussed above, their EL spectra and the corresponding CIE coordinates are not stable under different driving voltages (Figure S16d and f). The OLEDs using the other emitters have more stable color purities as the bias increases. The EL spectra show slight variety in the initial bias, mainly due to the inefficient charge injection leading to unstable recombination zone in the EMLs. As voltage further increases, charge injection becomes more efficient, resulting in a more stable recombination zone for better EL spectra stability. The devices employing TPI-Bz, TPI-2Na and TPI-Ph emit violet-blue EL with color indexes of (0.162, 0.043), (0.161, 0.063) and (0.158, 0.050), respectively, outperforming that of the OLED using non-modified TPI (0.163, 0.076). The EL peak of the TPI-based device is only 420 nm, whereas the emission is featured with a relatively long tail in longer wavelength region, leading to slightly higher CIEy 21



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value. Among the violet-blue emitting non-doped OLEDs, the TPI-Ph based device has the best efficiency, exhibiting a maximum EQE of 3.72%, mainly owing to its relatively high PLQY.

Figure 6. Non-doped OLEDs: (a) device structures and energy levels of the used materials; (b) current density-voltage-luminance characteristics; (c) EL spectra; (d) EQE-luminance curves.

Table 2. EL performances of the fabricated OLEDs. Emitter

Conc. (wt%)

Vona (V)

λEL (nm)

CIE coordinatesb

CEcmax (cd/A)

PEdmax (lm/W)

EQEemax (%)

TPI

Neat

3.4

420

(0.163, 0.076)

0.65

0.58

0.96

TPI-Bz

Neat

3.2

420

(0.162, 0.043)

0.50

0.45

1.45

TPI-2Na

Neat

2.8

428

(0.161, 0.063)

2.17

2.27

1.98

TPI-1Na

Neat

3.8

420

(0.184, 0.116)

0.70

0.55

0.83

5

3.7

416

(0.162, 0.034)

0.70

0.61

2.37

10

3.5

416

(0.162, 0.034)

0.82

0.74

2.57

20

3.4

416

(0.161, 0.035)

0.89

0.80

2.63

TPI-Ph

TPI-An

Neat

3.1

428

(0.158, 0.050)

1.62

1.57

3.72

5

3.7

432

(0.155, 0.054)

1.95

1.71

3.96

10

3.4

436

(0.156, 0.069)

2.74

2.32

3.93

20

3.2

440

(0.158, 0.083)

4.42

4.10

4.70

22


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Neat

2.6

448

(0.200, 0.221)

5.92

4.97

3.39

5

4.1

436

(0.155, 0.065)

3.03

2.02

5.28

10

3.7

440

(0.153, 0.079)

3.81

2.70

5.21

20

3.3

444

(0.153, 0.099)

5.39

3.89

5.78

Neat

2.5

8.18

8.46

5.69

TPI-Py

a

2 b

460 c

(0.157, 0.177) d

e

Voltage at 1 cd/m . Detected at 6 V. Current efficiency, power efficiency and external quantum efficiency at maximum.

In order to further improve efficiency and color purity, we used TPI-Ph, TPI-An and TPI-Py as emissive dopants to fabricate OLEDs. In this host-guest system, where emitter molecules can be diluted in the host matrix, not only device efﬁciency can be improved via efficient Förster energy transfer,52 but also a better blue color purity can be expected. We first prepared the 20 wt% doped thin films on quartz substrates using 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) as host, and their PL spectra are shown in Figure S17. Comparing to the pristine thin films, it is evident that the doped films display hypochromatically shifted and narrow PL spectra, showing improved PLQYs of 0.75, 0.82 and 0.95 for TPI-Ph, TPI-An and TPI-Py, respectively. Then a series of doped OLEDs were fabricated with a general structure of ITO/NPB (70 nm)/TCTA (5 nm)/5, 10 or 20 wt% dopant in CBP matrix (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm). EL spectra detected at 6 V of the doped devices are shown in Figure 7. All the doped OLEDs show significantly improved blue color purities, displaying violet-blue to deep-blue EL emissions with CIEy coordinates from ~0.03 to 0.10. Figure S18 shows the EL spectra of the 10 wt% doped devices under different voltages. The EL spectra of the doped device are much more stable than the corresponding non-doped devices. Furthermore, evident blue shifted EL emissions are observed when compared to the corresponding non-doped devices, especially for the OLEDs employing TPI-An or TPI-Py. Two factors are responsible for this phenomenon. Diluting the emitter 23



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molecules in host eliminates the effect of intermolecularly close stacking, as evidenced by disappearance of the broad EL sub-band in the doped OLEDs employing TPI-An. Moreover, this can be ascribed to the decreased medium polarity,53 because CBP is generally used as a non-polar host material.54 Taking TPI-Py as an example, upon increasing the doping concentration of TPI-Py, the EL emissions constantly shift to longer wavelength region with CIEy value increases to 0.177 (non-doped) from 0.065 (5 wt%). Whereas negligible emission variation is observed in the TPI-Ph based devices, since TPI-Ph is less sensitive to the medium polarity than TPI-Py, which has been discussed in the PL section. Figure 8 shows EQE-L-PE curves of the doped OLEDs and the key data are listed in Table 2. The TPI-An and the TPI-Py based OLEDs show commensurate or improved device efficiencies comparing to that of non-doped devices. Maximum EQE of 5.28% is obtained in the OLED with EML having 5 wt% TPI-Py, while that of the corresponding non-doped device is 5.69%. The Von is raised from 2.5 to 4.1 V because of the larger Eg of the host. Electrical performance is improved upon increasing the doping level to 20 wt%, showing a decent Von of 3.3 V and a PE of 3.89 lm/W. These improved photometric efficiencies are also partly ascribed to the bathochromically shifted EL spectra in the heavily doped OLEDs.55 Analogous variation is observed in the TPI-An based OLEDs, but the efficiencies are greatly boosted upon using doped EML. An OLED using 20 wt% TPI-An shows an EQE up to 4.70%, exceeding that of non-doped counterpart (3.39%). These improved performances are benefited from elimination of the excimer emission by using doping technique comparing to the 24


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TPI-Py based doped OLEDs with negligible excimer. It is observed that the current density in the heavily doped TPI-An or TPI-Py based OLEDs increases faster than the device with low dopant concentrations, as shown in Figure S19. By contrast, the efficiency of the non-doped OLED employing TPI-Ph (EQEmax = 3.72%) is much higher than those of corresponding doped devices (EQEmax < 2.7%). This can be attributed to the low charge transport property of TPI-Ph. This property becomes worse upon diluting TPI-Ph in host molecules (Figure S20), even increasing the doping concentration the device efficiency does not further improved. Therefore, we suggest that the larger intermolecular spacing and poor charge transport property of TPI-Ph are responsible for the deduction of performances in doped OLEDs. The doped devices with TPI-Py show the best performances (EQE > 5%) with deep-blue emissions (CIEy < 0.10) in the series. More importantly, these devices show negligible efficiency roll-offs in high brightness. Deep-blue OLED (color index: (0.155, 0.065)) using 5 wt% TPI-Py as an EML exhibits a decent EQE of 5.01% at a practical brightness of 1000 cd/m (EQEmax = 5.28%). This performance is among the best in deep-blue devices with CIEy lower than 0.08.32,35,56–59

Figure 7. EL spectra of the devices employing (a) TPI-Ph, (b) TPI-An and (c) TPI-Py with different doping concentrations. 25



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Figure 8. EQE-Luminance-PE curves of the doped OLEDs employing (a) TPI-Ph, (b) TPI-An and (c) TPI-Py, respectively.

3. CONCLUSION We designed and synthesized a series of aromatically substituted PI fluorophores at C6 and C9 positions for deep-blue OLED application, and systematically investigated the structure-property relationship. Theoretical calculations suggested that C6 and C9 positions of PI are electronically different owing to electronic preference effect, which may induce ICT properties. This also provides a guideline to design specified 26


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donor-PI-acceptor or other asymmetric molecules via C6 and C9 sites by using two successive coupling reactions. It is found that Tds and Tgs can be greatly improved after aromatic modification, depending on the size of substituent and the resulting molecular configuration. Photophysical analysis demonstrated that substitutions at C6 and C9 positions induce moderate conjugated extension comparing to C2 modification. Moreover, ICT characteristics of the new fluorophores increase as expanding the size of substituted aromatic groups, and partially influenced by the steric hindrance, with the anthracene and pyrene derivatives having the much improved ICT excited properties. The EL performances of the materials were evaluated as host emitters or dopant emitters. Most of the devices showed significantly improved efficiencies comparing to the OLED using non-modified TPI emitter. Among all the emitters, a 5 wt% doped device using TPI-Py showed the best performances with an EQE > 5% at 1000 cd/m2 and CIE coordinates of (0.155, 0.065), which are comparable to the state-of-the-art deep-blue OLEDs. This study offers useful information for designing C6/C9-modification of PI fluorophores and provides an efficient approach for constructing high-performance deep-blue OLEDs.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. General information, synthesis detail, device fabrication and measurement, optimized geometries, absorption spectra in solid thin films, transient PL decay, absorption and PL spectra of Py-BPI and TPI-Py, absorption and PL spectra of the new compounds recorded in different solvents, linear 27



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correlation of the orientation polarization of solvents with the Stokes shift, J-V characteristics of single carrier type devices, L-J characteristics of the non-doped OLEDs, comparison of the EL spectra of the non-doped OLEDs and the PL spectra of the thin films, voltage-dependent EL spectra of the non-doped OLEDs, PL spectra of the 20 wt% doped and the pristine thin films, voltage-dependent EL spectra of the 10 wt% doped OLEDs, J-V-L characteristics of the doped OLEDs, J-V characteristics of the non-doped and 20 wt% doped single carrier type devices based on TPI-Ph.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENT CSL would like to acknowledge the financial support from the National Key R&D Program of China (Project No.: 2016YFB0401002); QXT acknowledges the supports from the Natural Science Foundation of China (Project No.: 51673113) and the National Basic Research Program of China (973 Program No.: 2013CB834803). ALR acknowledges the support by a grant from the Germany/Hong Kong Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the German Academic Exchange Service (Reference No.: G-CityU103/16).

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Aromatically C6- and C9-Substituted Phenanthro[9 ... - ACS Publications

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