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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 25561−25569

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Spirally Configured (cis-Stilbene) Trimers: Steady-State and TimeResolved Photophysical Studies and Organic Light-Emitting Diode Applications Shiang-Fu Hung,† Po-Hsun Fang,† Yi Wei,‡ Fang-Yuan Tsai,† Chien-Tien Chen,*,† Takumi Kimura,§ Shingo Samori,§ Mamoru Fujitsuka,§ Tetsuro Majima,*,§ Chun-Hao Lin,∥ Shiang-Hau Peng,∥ and Jwo-Huei Jou*,∥ Downloaded via COLUMBIA UNIV on January 15, 2019 at 15:37:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry and ∥Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC ‡ Department of Chemistry, Tamkang University, 151, Yingzhuan Rd, Tamsui Dist., New Taipei City, Taiwan, ROC § The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan S Supporting Information *

ABSTRACT: This article reports for the time-resolved photophysical studies of spirally configured (cis-stilbene) trimers and their spincoated organic light-emitting diode (OLED) device performances. Transient absorption profiles of spirally configured, ter-(cis-stilbene) were studied by pulse radiolysis. The emission profiles after charge recombination of their incipient radical ions in benzene provides insights into the emission mechanism and efficiency in OLED devices. Blue-, sky blue-, and green-emitting OLED devices for a maximum external quantum efficiency are 4.32%, 4.70%, and 2.77%, respectively, by solution process.

KEYWORDS: pulse radiolysis, transient absorption, OLED, spirally configured (cis-stilbene), trimers, transient emission

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INTRODUCTION In the past two decades, highly emissive fluorescent molecules caught much attention because of their potential uses in organic light-emitting diodes (OLED)1,2 which constitute one of the most effective technologies in high-resolution display and lighting.3 On the other hand, the morphological defect of vacuum-deposited films using small molecules somewhat restricts their applications in flexible devices.4 Friend et al. thus reported an emissive polymer-based LED by solution process to address this issue.5 However, the difficulty associated with the consistent purity of polymers often leads to considerable decay in the operational efficiencies and a lifetime of the resulting devices. Oligomers constructed by phenyl vinylene,6−9 thiophene,10−12 fluorene,13−22 or quinoline/quinoxaline23,24 repeating units became a major focus because of their high film-forming property and ease of purification. In the past 11 years, we have developed a unique class of molecular materials based on spirally configured, doubly ortholinked, cis-stilbene/fluorene (STIF) hybrids25 and their quinoxaline-fused analogs26−28 in OLED, dye-sensitized solar cell applications29 and bulk heterojunction solar cells.30 Among them, the 3,7-bis(diphenylamino)-substituted system served as nondoped, sky-blue fluorescent material, © 2018 American Chemical Society

showing a maximum external quantum efficiency (EQE) up to 7.9% in bilayer devices (Scheme 1).25 The significantly higher EQE than the theoretical limit of 6.5% was attributed to the enhanced fluorescence through facile triplet−triplet annihilation.31−34 It was believed that the cis-stilbene moiety in the central, chairlike, seven-membered ring could potentially stabilize the incipient singlet or triplet biradicals at the pseudo-axial and Scheme 1. 3,7-Bis(diphenylamino)-Substituted and Quinoxaline-Fused STIF Systems

Received: April 17, 2018 Accepted: July 10, 2018 Published: July 20, 2018 25561

DOI: 10.1021/acsami.8b06137 ACS Appl. Mater. Interfaces 2018, 10, 25561−25569

Research Article

ACS Applied Materials & Interfaces

found highly fluorescent and exhibited excellent film morphology in spin-coated device fabrications. Herein, we report their syntheses, time-resolved transient absorption and emission profiles during the pulse radiolysis and their promising OLED applications by solution process.

pseudo-equatorial positions of C10 and C11 in their lowest lying excited states (Scheme 2). Time-resolved, transient Scheme 2. Singlet or Triplet Biradicals at the C10 and C11 Positions of the STIF Template

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EXPERIMENTAL SECTION

The requite coupling precursor, 3,7-bis[(pinacolato)boryl]-5,5spirofluorenyl-5H-dibenzo-[a,d]cycloheptene 6, was prepared by treatment of the in situ-generated 3,7-dilithio-STIF with 2-isoproxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane [i-PrOB(OR)2] at −78 °C in tetrahydrofuran (THF). Another precursor 7 was made by Hartwig coupling of 5 with diphenylamine.35 The target materials 9−11 were then synthesized from 6 with respective aryl bromide (8, 4, and 7) via Suzuki coupling reaction in 46−78% yields (Scheme 3).36,37 Their structural identities were confirmed by 1 H and 13C NMR spectroscopic and mass analyses and ultimately proven by X-ray crystallographic analysis in the case of 9. As represented in the Oak Ridge thermal ellipsoid plot (ORTEP) diagram of 9 in Figure 1 (top), the two peripheral spirobifluorene (SPF) units in 9 are twisted on average by 31.1° and 42.1°, respectively, to the central STIF template. The central, 7-membered ring in STIF adopts a butterfly shape with an average dihedral angel of 17.2° between the CC and the two flanking benzo-fused rings. On the other hand, each fluorene unit is nearly perpendicular (89 ± 0.3°) to each spirally linked top moiety for all of trimers 9−11. Notably, the glass transition temperature (Tg) is increased by 5 °C by changing both terminal SPF to STIF units (cf. 226 and 231 °C for 9 and 10, respectively). On the other hand, both 10 and 11 showed the same glass transition temperature despite 11 having a much higher molecular weight (by 336!) and a higher Td (ΔTd, 44 °C). Therefore, the diphenylamino capping groups have negligible effects on the morphology of 11. The spirally shaped and helical natures (Figure 1, bottom) of these trimers are thus responsible for the high glass transition temperature (Tg, 226−231 °C) and high thermal stabilities decomposition temperature (Td), 491−535 °C (Table 1).

absorption and emission measurements of their in situgenerated radical ions during the pulse radiolysis27 have shown that the resulting negative charge (i.e., radical anions) only resided at the central cis-stilbene moieties with distinctive half lifetimes (τ1/2). Conversely, the resulting positive charges (i.e., radical cations) localized at the electron-donating fragments (e.g., methoxy or amino group) were responsible for the hole-transporting (HT) function in OLED devices. In addition, the radiolysis-induced emission wavelength and the intensity between radical cation and anion were consistent with electroluminescent (EL) profiles and efficiencies observed in the exciton (i.e., electron and hole) recombination events in the OLED devices. Therefore, we sought to further extend the intrinsic optoelectronic merits of the STIF template to more advanced oligomeric, molecular design by appending STIF units onto both the C3 and C7 positions of the STIF (i.e., para positions of the spirally, configured, cis-stilbene template) to form a trimeric system. The resulting fluorescent (cis-stilbene) trimers (i.e., ter-STIF) with or without end-capping modification were Scheme 3. Synthetic Sequences for 6−7 (a) and 9−11 (b)

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Figure 1. (Top) ORTEP diagram of X-ray crystal structure for 9 (ellipsoid is shown at 30% probability level); (bottom) CPK model of 9.

Figure 2. Stacked plots of UV−vis and Em spectra for 9−11.

phenomenon. Based on the B3LYP functional method at 631G* basis set, the electron densities at the highest occupied molecular orbitals (HOMO) are delocalized to the whole oligomeric main chains including the CC units in STIFs in all cases but are much further shifted toward both of the diphenylamino end groups in 11 (Figure 3). Notably, the

Steady-state photophysical properties of these blue or green fluorescent spirally configured (cis-stilbene) trimers 9−11 were obtained under atmosphere at ambient temperature. The longest absorption bands with a peak maxima (λmax) appeared at 377, 386, and 419 nm, for 9−11, respectively (Table 1 and Figure 2). Notably, dramatic bathochromic shifts both in UV−vis (42 nm) and Em λmax (80 nm) were observed for 11 presumably because of resonance delocalizations exerted by both diphenylamino groups. To investigate the property of singlet excited states in 9−11, the changes of dipole moment of excited states were monitored by solvatochromic photoluminescence experiments (Figure S1). As the solvent polarity increased from a nonpolar solvent (n-hexane) to a polar solvent (acetonitrile), the emission wavelength of 9−11 was located at the range of 431−530 nm. The absorption and photoluminescent spectra of compound 9 and 10 displayed very minor changes in different solvents, which strongly indicate that both their the ground states and excited states were not much polarized in different polarity environments (Figure S2). On the other hand, the emission profiles of compound 11 that occur with a progressive red shift from 457 to 530 nm was observed, which can be ascribed to that diphenylamino groups enable intramolecular charge transfer process in the molecule.

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RESULTS AND DISCUSSION Molecular simulations based on time-dependent density functional theory provide some insights into this photophysical

Figure 3. Optimized frontier molecular orbitals of ground (HOMO, left panel) and first excited (LUMO, right panel) states for 9 (a), 10 (b), and 11 (c).

Table 1. Morphological, Photophysical (Obtained by Steady-State Measurement), and Electrochemical Data for 9−12

Tg/Td (°C) 9 10 11 1238

226/509 231/491 231/535 -/450

Abs λmax (nm)a,b 309 386 309 310

(37), 377 (32) (43) (44), 419 (57) (50.1), 351 (90.8)

Em λmax (nm)a,c

Φfa (%)

Φfd (%)

Φfe (%)

433, 460 (54) 441, 462 (52) 513 (81) 393, 412 (44)

99 99 85 99

91 94 76 90

94 97 82

Eoxa/Eredf (V) +0.84/−2.44, −2.64 +0.84/−2.53 +0.38, +0.81/−2.46 +1.32, +1.56/−2.01, −2.21

EHOMOg, eV

ELUMOh, eV

Egi, eV

5.6 5.6 5.2

2.7 2.8 2.6

2.9 2.8 2.6

a Measured in 1,2-dichloroethane (DCE). bData in parentheses correspond to ε × 10−3. cData in parentheses correspond to full width at half maximum. dSingle-layer thin-film PL quantum yields. eThin-film PL quantum yields (15 wt % 9−11: CBP). fMeasured in THF. gEHOMO = Eox + 4.8. hELUMO = EHOMO − Eg. iCalculated from the onset of the absorption spectrum.

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Figure 4. Stacked plots of time-resolved transient absorption spectra obtained during the pulse radiolysis of 9−12 in Ar-saturated DCE, DMF, and Bz.

lowest occupied molecular orbital (LUMO) in 9 exhibited higher electron densities in the central dibenzosuberene (DBE) template of STIF. In contrast, the LUMO electron densities in 10 are evenly distributed to the three templates among the three STIF units. Furthermore, the electrondonating diphenylamino groups in 11 were responsible for the more localized electron density distributions toward the central DBE moiety in its LUMO, and thus resulting in lowering its lowest-lying transition energy. The electrochemical behaviours of 9−11 were evaluated by cyclic voltammetry experiments by using ferrocene as an internal reference in 1,2-dichloroethane (DCE) (Table1). The oxidation redox couples observed at +0.84 (quasi-reversible), +0.84 (quasi-reversible), and +0.38 V (reversible) for 9, 10, and 11 (Figure S3a), respectively, were ascribed to be the oxidation of SPF (9 and 10) and diphenylamine (11) units, respectively. Additionally, differential pulse voltammetry in the oxidation region that clearly showed at +0.81 V (quasireversible) for compound 11 (Figure S3b) was ascribed to be the oxidation of SPF units. Conversely, all of these materials showed quasi-reversible reduction profiles. The reduction redox couples observed at −2.44 to −2.53 V (Ered) for 9−11 correspond to the reductions of their central DBE units. The second reduction observed at −2.64 V for 9 is due to the reduction of one fluorene unit along the trimeric chain. The results suggest that the DBE unit can stabilize an incipient radical anion much easier than fluorene unit because of the more extensive resonance stabilization exerted by the central cis-olefin unit. To gain insights into the origin of exciton emissions both in solution and film states in the OLED device, these materials were subjected for pulse radiolysis experiments to determine their transient absorption and τ1/2 of in situ-generated radical cations and anions in solutions. Moreover, their emission behaviours from the charge recombination of their radical ions were also determined. It has been established the radical cation and anion of an organic molecule can be selectively formed in DCE and DMF, respectively, during the pulse radiolysis. On the other hand, the absorption and emission spectra of 1M* and 3M* generated from the charge recombination of radiolysis-induced radical cation and radical anion pairs can be measured in benzene (Bz).39−49 As indicated in Figure 4 and Table 2, there was a significant red shift in the transient absorption peak (λmax) from 550 to 620 nm in DCE by just changing the central unit from SPF to

Table 2. Summarized Data for Time-Resolved Transient Absorption and Emission Spectra Obtained During Pulse Radiolysis of 9−12

9 10 11 12

Abs λmaxa,b (nm)

Abs λmaxb,c (nm)

620 620 570 550

670 685 690 570

(3.9) (3.5) (11.3) (7.1)

(7.1) (13.3) (14.2) (5.1)

Abs λmaxd (nm) 650 700 655 667

Em λmaxd,e/Intd (nm/%) 463 473 521 425

(63)/60 (61)/64 (89)/109 (32)/100

Measured in DCE. bData in parentheses denote τ1/2 in microsecond. Measured in DMF. dMeasured in Bz (the total amount of emission relative to that of 12 is 100%). eData in parentheses correspond to full width at half maximum. a c

STF (cf. 9 and 12). In contrast, very similar transient absorption spectra (λmax at 620 nm) and shorter τ1/2 of 3.7 ± 0.2 μs were observed for 9 and 10. The results imply that these incipient radical cations (M•+) are mainly localized in the central units, and “the confined cis-stilbene unit” in STIF tends to have a better resonance delocalization effect than “the confined biphenyl unit” in SPF. Nevertheless, 12•+ has a longer τ1/2 of 7.1 μs than the τ1/2 of 3.9 and 3.5 μs of 9 and 10, respectively, supporting that SPF can better stabilize an incipient radical cation. On the other hand, a hypsochromic shift in the transient absorption λmax from 620 to 570 nm (cf. 10 and 11) was observed with longer τ1/2 of 11.3 μs presumably because of positive-charge localization and stabilization by either diphenylamino segment. Therefore, when the positive charges of the resulting positive charge of 9•+ and 10•+ are more localized to the central STIF unit, the neutralization processes tend to be accelerated and thus shortlived. Apparently, the neutralization occurs via the collision process between the incipient radical cations and chloride ion in DCE and is more facile in 9•+ and 10•+, which have larger collision cross sections than those of the diphenylamino units in the amino-capping system-12. Because τ1/2 of a radical ion for a given ambipolar compound has already been proven to correlate nicely with its capability of charge transport in OLED device when its HOMO or LUMO energy level matches well with the work function of either electrodes,26,27 one can expect that 11 (HOMO, 5.2 eV) has a good hole-injection function of indium tin oxide (ITO work function, 4.7 eV). 25564


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ACS Applied Materials & Interfaces Table 3. EL Results of OLED Device for 9−11 device config.a

Em λmaxb (nm)

Vonc (V)

9/A 9/B 9/C 10/A 10/C 11/A 11/C 12/A

444, 472 (78) 448, 472 (85) 444, 468 (64) 460, 484 (76) 460, 488 (64) 508 (96) 496, 528 (80) 408, 424 (51)

3.2 3.2 3.1 3.2 3.5 2.9 3.0 3.2

(4.8) (6.2) (5.3) (5.0) (6.1) (4.6) (6.2) (5.5)

ηcc/ηpc (cd A−1/lm W−1)

EQEc (%)

EQEmaxd (%)

L20c (cd/m2)

2.5/1.6 3.4/0.7 4.6/2.6 3.3/2.1 6.4/3.3 4.4/3.0 7.9/4.0 0.7/0.4

1.80

1.87

3.69 1.83 3.74 1.40 2.65

4.32 2.06 4.70 1.71 2.77

495 667 901 646 1260 864 1564 132

CIEc (0.16, (0.16, (0.15, (0.16, (0.16, (0.27, (0.24, (0.15,

0.17) 0.21) 0.14) 0.24) 0.22) 0.56) 0.53) 0.05)

a

Device configuration: A: ITO/PEDOT:PSS (35 nm)/9−11 (by spin-coating) (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm); B: ITO/ PEDOT:PSS (35 nm)/10 (by vacuum thermo-evaporating) (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm); and C: ITO/PEDOT:PSS (35 nm)/CBP: 9−11 (15 wt %) (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm). bData in parentheses correspond to full width at half maximum. c ηc, ηp, and the data in parentheses for Von and L20 were measured at 20 mA/cm2. dMaximum EQE.

The emission spectra of 9−12 were taken during the pulse radiolysis in an Ar-saturated Bz (Figures S4 and S5). Notably, all of these materials show only monomer emissions with τ1/2 less than 8 ns, which were resulted from charge recombination of the incipient radical ion pairs and/or from triplet−triplet annihilation of 3M* (at high radiation doses). They all have essentially the same emission wavelengths as those found by electroluminescence measurements (vide infra, Table 3). Similar to our previous finding in donor-STIF-donor systems,51−56 there are also direct correlations between these emission intensities and device efficiencies in their OLED performances. Therefore, we propose the fluorescent emission pathways during the pulse radiolysis of 9−12 in Bz (Scheme 4). First,

There was an even more pronounced bathochromic shift in the transient absorption peak (λmax) from 570 to 670 nm in DMF by just changing the central unit from SPF to STF (cf. 9 and 12). In contrast, a slight red shift in the transient absorption spectra (λmax shifts from 670 to 685 nm) but with a more extended τ1/2 from 7.1 to 13.3 μs was observed by replacing both of the terminal SPF to STIF units (cf. 9 to 10). The results imply that the central STIF unit tends to stabilize the incipient radical anion (M•−) much better than the central SPF besides the better resonance delocalization effect of “the confined cis-stilbene unit”. In addition, the incipient 10•− can be delocalized from the central STIF unit independently to either of the STIF terminal units. On the other hand, very similar transient absorption profiles in terms of λmax (685 and 690 nm) and τ1/2 of 13.3 and 14.2 μs were observed for 10 and 11, respectively, supporting the resonance delocalization proposal. According to the trend for τ1/2 of 7.1, 13.3, and 14.2 μs for 9−11, respectively, 10•− and 11•− tend to be stabilized by the independent delocalization toward either STIF terminals, thus slowing down the neutralization process and exhibiting longer τ1/2 because of a more diffused collision cross section with DMF•+. Notably, the LUMO energy levels (2.6−2.8 eV)50 of 9−11 are similar to that (2.7 eV) of TPBi, which makes 9−11 as potential electron transporters and 11 as an ambipolar material. Transient absorption spectra from the recombination between M•+ and M•− were measured in Bz, and their decays were monitored by quenching in the presence of oxygen. The absorption bands for 9−12 were in the range of 650−700 nm and were assigned to be triplet absorptions from their lowest triplet excited states populated via energy transfer from 3Bz* or charge recombination between radical ions. In comparison with 12, there existed a blue shift by 17 nm in the transient absorption band of 9 (λmax, 650 nm). The transient absorption band was red-shifted from 650 to 700 nm by supplanting both terminal SPF units in 9 to two STIF units as in 10. The results indicated that the incipient triplet biradical in 3M* of 12 can be delocalized through the trimeric SPF chain. In contrast, the triplet biradical in 3M* of 9 resides mainly on the central DBE template and may shift to either of the peripheral DBE units as in the case of 10 depending on the timing of energy transfer from 3Bz* or charge recombination from the more electrondiffused 10•−. The suppression of resonance delocalization in the case of diphenylamino capping system-11 resulted in the blue shift of its transient absorption band back to 655 nm.

Scheme 4. Fluorescent Emission Pathways during the Pulse Radiolysis of 9−12 in Bz

Bz•+ and Bz•− are generated from the ionization of Bz, and the following electron trapping by Bz, respectively, during the pulse radiolysis to react with M to derive to the corresponding M•+ and M•−, respectively. The combination of M•+ and M•− forms both 1M* and 3M* and then convey their radiative relaxation to cause light emission. In particular, for this system, the anionic nanoclusters can evenly distribute on the molecules in the film. Furthermore, the electron hopping of the anionic nanocluster occurs to transfer to the delocalized cation cluster on another molecules. Such a phenomenon can form singlet and triplet excited-state molecules to result in the observed emissions (Scheme 5). When 12 was taken as a reference sample and compared individually to 9−11, their relative emission intensity trend [11 (109%) > 10 (64%) ≈ 9 (60%) in Table 2] is consistent with their current and power efficiency trends in OLED devices. For example, the relative emission intensity for 10 (64%) is 1.1 times stronger than those for 9 (60%). Similarly, the current and power efficiencies in device configuration A and C for 10 are 1.3−1.4 times and 1.3 times stronger than those for 9, in device A or C by using them as HT-type (dopant) emitters, 25565


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Scheme 5. Charge Recombination Mechanisms between Incipient Radical Anion and Radical Cation Moieties of (a) 11 and (b) 10

Figure 5. Stacked plots of EL spectra in device configuration A and B (a) and C (b) for 9−11.

Figure 6. Stacked plots of I−V−L curves in device configuration A and B (a) and C (b) for 9−11.

Figure 7. Energy-level diagram of the traditional nondoped (a) and doped (b) devices.

respectively. These results indicate that STIF is a promising template for oligomeric materials with HT-type hosts for highly efficient blue/green fluorescent devices by spin-coating processes. The optoelectronic performances of these compounds were measured by fabricating OLED devices with these materials as the HT- type (dopant) emitters. In these device fabrications,

poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS)57 was spin-coated onto ITO surface as the hole-injection layer. Conversely, 1,3,5-tris-(N-phenyl-benzimidazole-2-yl)benzene (TPBi)58 was chosen as an electrontransporting layer. LiF/Al double layers were sequentially deposited as the electron injection layer and cathode, respectively. To investigate the electroluminescence (EL) 25566



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CONCLUSIONS Conclusively, we have successfully synthesized the spirally configured (cis-stilbene) trimers. Their time-resolved transient spectra obtained during the pulse radiolysis are first reported as well as their optoelectronic applications in OLED. All of these spin-coated devices exhibit among the best EL efficiencies with comparable performances to the ones manufactured by thermoevaporation process to date.6−20,60−65 The excellent film ductility for this oligomeric system takes advantage in flexible device fabrication for the high-efficient flat-panel display and lighting source in the near future.

performance of these new luminogens, these materials were independently applied to the top of PEDOT:PSS layer by spincoating (i.e., configuration A). For comparison, the devices with 4,4′-bis(carbazolyl)biphenyl (CBP) as host were also fabricated (i.e., configuration C). All of the EL data are compiled in Table 3, and their I−V−L correlation plots are shown in Figures 5 and 6. The device is configured as: (A): ITO/PEDOT:PSS (35 nm)/9−12 (by spin-coating) (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm). The devices from 9, 10, and 12 exhibited blue, sky-blue (λmax, 444 and 460 nm), and deep blue (λmax, 424 nm) fluorescent emissions with luminescence (L20) of 495 cd/m2 and 646 cd/m2, respectively, at 20 mA/cm2. They all showed the same turn-on voltage of 3.2 V. The EQE, current efficiency, and power efficiency (EQE/ηc/ηp) were 1.9%/2.5 cd A−1/1.6 lm W−1 and 2.1%/3.3 cdA−1/2.1 lm W−1 at 20 mA/ cm2 for devices from 9 and 10. These efficiencies were 3.5−4 and 4.7−5.3 times better than that of the device based on 12, supporting better film morphologies for the devices based on 9 and 10. Additionally, 11 exhibited green emission (λmax, 508 nm) with L20 of 864 cd/m2 and even a lower turn-on voltage of 2.9 V. This can be explained by considering the energy-level diagram, as shown in Figure 7a. Compound 11 has a significantly higher HOMO level than that of 9 and 10, implying that the holes are easily injected into 11. The EQE, current efficiency, and power efficiency (EQE/ηc/ηp) were 1.7%/4.3 cd A−1/3.0 lm W−1 for device from 11 (Figures S7 and S8). For a strict comparison, device B based on thermal vacuum deposition of 9 was also examined. The EL results for 9 in terms of L20 and ηc were 667 cd/m2 and 3.4 cd/A, respectively. They were 1.3−1.4 times better than those by solution process. However, its ηp was 0.7 lm W−1, which was only 43% of that by solution process. These results strongly support that the cis-stilbene-based trimers 9−11 have excellent film morphology with intact conductivity and similar device performance by solution process in OLED devices (Figure S9). Furthermore, triplet excitation energy levels of these compounds 9−11 are estimated as 2.26 eV (Figure S6). CBP59 was chosen as the host material in view of its high triplet excitation energy level (2.58 eV). Because triplet excitation energy of CBP is much higher than that of dopants, it can confine excitons in the emissive layer to avoid energy transfer from guest to host. Whereas 9−11 were used as guest materials to examine their best device performance (Figure 7b). The devices are configured as: (C): ITO/PEDOT:PSS (35 nm)/CBP: 9−11 (15 wt %) (25 nm)/TPBi (40 nm)/LiF (1 nm)/Al (150 nm). The 9−11 exhibited blue, sky-blue, and green emissions (λmax, 444, 460, and 496 nm) with L20 of 916, 1230, and 1564 cd/m2 with EQE/ηc/ηp of 3.7%/4.6 cd A−1/ 2.6 lm W−1, 3.7%/6.4 cd A−1/3.3 lm W−1, and 2.7%/7.9 cd A−1/4.0 lm W−1, for 9−11, respectively (Figures S7 and S8). In comparison with device A, device B using CBP as the host in the host−guest system has inhibited concentration quenching and can promote carrier balance to increase the device efficiency. Furthermore, the EQE efficiency of compound 11 is significantly weaker in all devices. It may be attributed to a 12−18% reduction in the quantum yield of compound 11 compared to those of 9 and 10 (Table 1). On the other hand, blue and sky-blue emissions (device C) in the current density of 1−10 mA/cm2has a roll-off value of 6.3% for 9 and 10.6% for 10.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06137.

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Synthetic, morphological, spectral, electrochemical, and device details for 1−11 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.-T.C.). *E-mail: [email protected] (T.M.). *E-mail: [email protected] (J.-H.J.). ORCID

Shiang-Fu Hung: 0000-0002-5829-5222 Fang-Yuan Tsai: 0000-0001-7150-1687 Chien-Tien Chen: 0000-0001-8073-6007 Mamoru Fujitsuka: 0000-0002-2336-4355 Tetsuro Majima: 0000-0003-1805-1677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the members of the Radiation Laboratory of SANKEN, Osaka University, for running the linear accelerator and the National Science Council of Taiwan for generous financial support to this research. This work has been partly supported by a Grant-in-Aid for Scientific Research (project 22245022 and others) from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) of the Japanese Government.

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REFERENCES

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