Research Article www.acsami.org
Bifunctional Heterocyclic Spiro Derivatives for Organic Optoelectronic Devices Chin-Yiu Chan,† Yi-Chun Wong,† Mei-Yee Chan,*,† Sin-Hang Cheung,‡ Shu-Kong So,‡ and Vivian Wing-Wah Yam*,† †
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong
‡
S Supporting Information *
ABSTRACT: A series of heterocyclic spiro derivatives has been successfully synthesized and characterized by photophysical and electrochemical studies. Taking advantage of their excellent hole-transporting properties, highly efficient smallmolecular organic photovoltaic devices based on these heterocyclic compounds as donors with very low dopant concentrations have been prepared; particularly, a high open-circuit voltage of up to 1.10 V and a power conversion efficiency of up to 5.12% have been realized. In addition, most of these heterocyclic spiro derivatives are found to be highly emissive in solutions with photoluminescence quantum yields of up to 0.91, and high-performance deep-blue-emitting organic light-emitting diodes (OLEDs) have been achieved. Such devices exhibit a stable deep blue emission with CIE coordinates of (0.16, 0.04) and high external quantum efficiencies of up to 4.7%, which is one of the best values among the reported OLEDs with CIEy < 0.08. KEYWORDS: organic light-emitting diodes, organic photovoltaics, spiro, bifunctional materials, heterocyclic
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INTRODUCTION Organic optoelectronic devices offer numerous potential and distinct advantages including great flexibility, higher transparency, and lighter weight over their inorganic counterparts; in particular, they can be fabricated onto a flexible substrate to produce bendable electronic gadgets.1−7 Of particular interest is the development of flexible organic light-emitting diodes (OLEDs)8−13 and organic photovoltaic (OPV) devices,14−20 where OLEDs are some of the most promising flat panel technologies to produce large-area displays and solid-state lighting systems, and OPV devices are considered as an alternative solution for developing clean renewable energy. For OPV devices, the crucial mechanistic step for achieving high power conversion efficiencies (PCEs) is the effectiveness of the exciton dissociation into free holes and electrons.16,18−20 High charge carrier mobility in the photoactive materials is one of the prerequisites for an efficient exciton dissociation process, in which donor and acceptor materials with good hole- and electron-transporting properties should be designed.16,20 Particularly, fullerene and its derivatives have been widely employed as acceptor materials in OPV devices due to their excellent electron-transporting properties with electron mobilities of up to 10−2 cm2V−1s−1.21,22 However, the carrier mobilities in most of the donor materials are orders of magnitude smaller than those in fullerene.23,24 While extensive efforts have been made on the design and synthesis of high performance donor materials with high absorptivity and good hole-transporting properties, an increase in the π-conjugation for better hole mobility and a smaller optical bandgap © XXXX American Chemical Society
inherently result in a higher highest occupied molecular orbital (HOMO) level of the donor materials which is at the expense of a lower open-circuit voltage (Voc). For OLEDs, a charge recombination process within the emissive layer is the crucial step for light emission.25−28 To enhance the efficiency of the charge recombination process, the nonradiative deactivation of the emitting materials should be suppressed.25−28 Alternatively, the formation of aggregates should be prevented, given that emission quenching has been commonly observed from the intermolecular interactions of the aggregates.29−34 One of the simplest approaches is to introduce rigid chromophores into the emitting materials to suppress the nonradiative pathway by restricting the vibrations and rotations of the molecules.29−34 Bulky and steric chromophores can minimize the intermolecular interactions between the molecules and thus improve the photoluminescence quantum yields (PLQYs) of the organic materials.29−35 A number of organic materials have been designed and utilized for OPV devices and OLEDs; however, there are only a handful of examples of utilizing a single molecule as photoactive materials for OPV devices and as emitters for OLEDs at the same time, hereafter termed as “bifunctional” materials. Spirobifluorene and its derivatives have been extensively reported as functional materials for various optoelectronic applications and are a benchmark class of p-type semiReceived: July 26, 2016 Accepted: August 22, 2016
A
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Scheme for Compounds 1−4 and Chemical Structures of the Reported Compounds 5−8
conductors in organic field-effect transistors.29,36−41 Due to the rigid spiro-conjugation, spirobifluorene and its derivatives
exhibit excellent hole mobilities by hopping processes in the order of 10−2 cm2V−1s−1, much higher than those of the B
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces nonspiro derivatives.42,43 Meanwhile, the rigid, bulky, and three-dimensional spirobifluorene is an ideal building block for constructing light-emitting materials with high PLQYs, specifically for deep blue emission.29,31,34 In order to design organic materials with a large optical bandgap, the πconjugation length should be shortened.44 The trade-off between the length of π-conjugation and the optical bandgap of the emitters makes it difficult to produce a true deep blue emission with the Commission Internationale de L’Eclairage (CIE) y-coordinate of 450 °C (i.e., Td is defined as the temperature at which the material shows a 5% weight loss under a nitrogen atmosphere). The thermal data of the spiro compounds have been tabulated in Table 3. Photovoltaic Properties. While the previous studies by our group have demonstrated that efficient OPV devices based on the spirothioxanthene compounds (i.e., 5−8) with high Voc and PCEs could be achieved, it would be interesting to extend this work for other heterocyclic spiro compounds. To test the applicability of the heterocyclic spiro compounds as donor materials, bulk heterojunction OPV devices with the configuration of indium tin oxide (ITO)/molybdenum oxide (MoO3) (2 nm)/x % donor:C70 (60 nm)/bathophenanthroline (BPhen)
Figure 1. Electronic absorption spectra of 1−8 (a) in dichloromethane solution and (b) in neat films.
observation in the electronic absorption spectra as aforementioned. Similarly, the emission bands of 6 (λ = 427 nm) and 7 (λ = 449 nm) are found to undergo hypsochromic shift when compared to that of 5 (λ = 460 nm). The hypsochromic shift in the emission maxima is probably due to the replacement of the triphenylamine group in 5 with the less electron-rich carbazole groups in 6 and 7, resulting in the lowering of the HOMO energy levels and larger energy bandgaps. Notably, among the spirothioxanthene compounds, the emission band of 6 is found to be the most blue-shifted (λ = 427 nm), which is consistent with the larger energy bandgap determined by the cyclic voltammetry. The introduction of a sulfone group in 5 results in a red-shifted emission (λ = 460 nm) when compared to that of 8 (λ = 457 nm), which is consistent with their differences in energy bandgaps. Alternatively, the emission properties of the sulfone derivatives in the solid state at 298 K are found to be slightly different from those in the solution state. The order of the emission wavelength maxima of the compounds in solutions is 6 (427 nm) < 7 (449 nm) < 5 (460 nm); while compound 6 shows the most red-shifted emission band in the solid state. Particularly, the order of the solid state emission wavelength maxima of the compounds at 298 K is 7 (427 nm) < 5 (455 nm) < 6 (471 nm). The discrepancy could be attributed to the enhanced π−π stacking of the planar carbazole moiety in 6 in the solid state. Indeed, the solubility of 6 in organic solvents is the lowest, being only soluble in chloroform solution. By substituting the 3,6-positions of carbazole moiety with bulky tert-butyl group, the extent of π−π stacking of the carbazole D
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Table 1. Photophysical Properties of 1−8 compound
absorption λmax/nm (εmax/dm3 mol−1cm−1)
medium (T/K)
1
267 (26070), 310 (19670)
CH2Cl2 (298) solid (298) CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd CH2Cl2 (298) solid (298) thin filmd
2
3
4
5
6
7
8
301 (45120), 372 (74170)
302 (40610), 372 (79730)
309 (59210), 345 (64770)
300 (43420), 380 (87320)e
294 (54540), 346 (88140)e
298 (58040), 353 (79470)e
298 (42630), 375 (76390)e
emission λem/ nm (τo/ns) a a
451 (1.2) 449 432 457 (1.2) 457 441 419 (1.2) 425 407 460 (1.5) 445 448 427 (0.9) 471 412 449 (1.8) 427 434 457 (1.2) 457 438
PLQY a a
0.82b c
0.86 0.91b c
0.90 0.47b c
0.45 0.83b c
0.90 0.81b c
0.86 0.88b c
0.83 0.81b c
0.88
a
Nonemissive. bMeasured in dichloromethane at room temperature using quinine sulfate in 1.0 M H2SO4 as the reference (excitation wavelength = 365 nm, ϕlum = 0.55). cNot measured. dAbsolute thin-film PLQY of 10% doped into MCP by spin-coating (measured using 300 nm as the excitation wavelength). eFrom ref 50.
the C70 matrix, the Jsc is found to increase from 1.25 mA cm−2 to 9.07 mA cm−2. Meanwhile, the FF is increased from 0.37 to 0.41, and the Voc is slightly increased from 0.97 to 1.10 V. With the increases in Jsc, Voc, and FF, a dramatic PCE enhancement results from 0.45% to 4.09%. By further increasing the dopant concentration to 5%, the PCE is further increased to 4.49% for the optimized device fabricated with 1. The device based on 1 outperforms the TAPC-doped device (4.38%) together with an additional gain in Voc. The incorporation of two additional triphenylamine groups attached on the spiroacridine core (i.e., 2) could further improve the photovoltaic responses, in which the optimized device based on 2 demonstrates a higher Jsc of 10.84 mA cm−2 and a higher FF of 0.50. Although the Voc of the device prepared from 2 (0.94 V) is slightly lower than that of the device prepared from 1 (1.10 V), a significant increase in PCE from 4.49% to 5.09% is obtained for the optimized device. By replacing the spiroacridine core in 2 to spiroxanthene core in 3, a similar performance is obtained with a PCE of 5.12%. It is worth noting that the Jsc measured is in good agreement with the Jsc calculated from the integration of the incident photonto-current efficiency (IPCE) spectra with the AM 1.5 G (AM: Air Mass; G: Global) solar spectrum. Figure 4 depicts the current density−voltage (J−V) curve and the IPCE spectra of the devices based on 3, in which the Jsc measured from the J−V curves are within 10% error from those estimated from the IPCE spectra. However, the regio-substitution of the triphenylamine groups on the upper six-membered ring of the spiroxanthene core in 4 would cause a negative influence on the OPV performance. Devices based on 4 show a poor performance, especially a low FF, when compared to those of the devices made with 3. It is believed that the change in geometry would negatively affect the hole-transporting properties of 4, as reflected by the low FF of the devices based on 4.
Figure 2. Emission spectra of 2−8 in dichloromethane solution at room temperature.
(8 nm)/aluminum (Al) (100 nm) are fabricated, in which 1−4 are used as donor materials; while ITO, MoO3, C70, BPhen, and Al are used as the anode, anodic buffer layer, acceptor, exciton blocking layer, and cathode, respectively. Two control devices with the structures of ITO/MoO3/C70/BPhen/Al (i.e., C70only device) and ITO/MoO3/7% TAPC:C70/BPhen/Al (i.e., TAPC-doped device) have also been prepared for comparison. Under light illumination of 1 sun, the C70-only device shows a poor performance with a short-circuit current density (Jsc) of 1.25 mA cm−2, a Voc of 0.97 V, and a fill factor (FF) of 0.37. These correspond to a low PCE of 0.45%, consistent with other reports.24 The introduction of heterocyclic spiro compounds as donor materials can dramatically improve the photovoltaic responses of the OPV devices. By simply doping 3% of 1 into E
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. Cyclic voltammograms showing oxidative scans of (a) 1, (b) 2, (c) 3, and (d) 4 in N,N-dimethylformamide (0.1 M nBu4NPF6). Scan rate: 100 mV s−1.
Table 2. Electrochemical Properties of 1−8a oxidation E1/2/Vb vs SCE
compound
reduction E1/2/Vb vs SCE
b
[Epa/V vs SCE] (ΔEp/mV)
LUMO (eV)d
−5.45 −5.30 −5.30 −5.31 −5.31 −5.70 −5.55 −5.31
−1.70 −2.19 −2.22 −1.71 −2.30 −2.38 −2.36 −2.24
[Epc/V vs SCE] (ΔEp/mV)
[+1.10]e +0.95 (83), + 1.10 (87) +0.95 (81) +0.96 (80) +0.96 (86) [+1.35]e +1.20 (82) +0.96 (80)
1 2 3 4 5g 6g 7g 8g
HOMO (eV)c
b
[−2.65]f −2.16 (83), [−2.47],f −2.13 (76), [−2.44],f [−2.64],f [−2.78]f −2.05 (82), [−2.37]f −1.97 (66), [−2.31]f −1.99 (63), [−2.32],f −2.11 (82), [−2.42],f
[−2.75]f [−2.82]f
[−2.61]f [−2.73]f
0.1 M nBu4NPF6 (TBAH) as supporting electrolyte at room temperature, scan rate 100 mV s−1. bE1/2 = (Epa + Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively; ΔEp for Fc+/0 reference couple is 66 mV. cEHOMO = −(Eox + 4.35) eV. dELUMO= −(Ered + 4.35) eV. e Irreversible oxidation wave. The potential refers to Epa, which is the anodic peak potential. fIrreversible reduction wave. The potential refers to Epc, which is the cathodic peak potential. gFrom ref 50. a
(see discussion below). It is worth noting that the common HOMO−LUMO gap rule is still valid for the present OPV devices, in which a higher HOMO level of heterocyclic spiro compounds yields a larger Voc. However, it is not applicable for compound 6 given the irreversible nature of the oxidation wave, which renders an estimation of the HOMO level difficult as a result of the introduction of uncertainty due to the presence of an overpotential. It is well established that hole mobility of donor materials plays a crucial role in determining the photovoltaic responses, specifically Jsc and FF, of the bulk heterojunction OPV devices.20−22 To correlate with the photovoltaic properties, we have determined the hole mobilites of the heterocyclic spiro compounds in an organic thin film transistor (OTFT) configuration at room temperature. The hole mobility of TAPC has also been studied for a fair comparison. Figures S2− S8 show the output and transfer characteristics of the fabricated OTFTs. It is worth noting that the hole mobility of 6 has also been investigated; however, very weak current signals were detected. Table 5 lists the linear and saturation hole mobilities in heterocyclic spiro compounds and the threshold voltages of
Table 3. Thermal Properties for 1−8 compound
Td (°C)
1 2 3 4 5b 6b 7b 8b
244 466 440 427 464 500 461 472
a
Determined by thermogravimetric analysis. Heating rate: 10 °C min−1 under a nitrogen atmosphere. Td was determined at 5% weight loss. bFrom ref 50. a
Table 4 summarizes the key data for the photovoltaic responses of the as-prepared devices. Apparently, all the devices with 7% donor concentration yield J−V characteristics similar to that of the TAPC:C70 device with Jsc of ∼10 mA cm−2, except for devices based on 6 and 7. The exceptionally low Jsc values were ascribed to the considerably lower hole mobilities in 6 and 7 F
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
(BmPyPhB) (30 nm)/LiF (0.8 nm)/Al (100 nm) have been fabricated, where MCP is used as the host material due to its relatively large bandgap and HOMO level. NPB and BmPyPhB are used as the hole- and electron-transporting layers, respectively; whereas TCTA is used as a carrier confinement layer. Figure 5 shows the normalized EL spectra of devices made with 10% 2−8 at a current density of 20 mA cm−2. All devices doped with heterocyclic spiro compounds display broad and structureless emission bands with peak maxima ranging from 408 to 439 nm. Notably, devices based on compound 7 give a true deep blue emission with CIE coordinates of (0.16, 0.04), satisfying the requirements on the y-coordinates for both the NTSC and European Broadcasting Union systems. Interestingly, it is noticed that the emission maxima of all devices are blue-shifted when compared with those of the solidstate photoluminescence (PL) spectra. In particular, the device made with 6 shows a larger extent of hypsochromic shift in the emission energy (i.e., 3278 cm−1) when compared to the emission band in the solid. Figure 6 shows the characteristics of OLEDs made with different heterocyclic spiro compounds. All devices, except for 4 with a relatively low PLQY, have been found to demonstrate a promising performance. The device made with 6 shows a maximum current efficiency of 0.5 cd A−1 and a maximum EQE of 1.6%. With the incorporation of bulky tert-butyl groups in the carbazole moiety, the device doped with 7 is found to exhibit a maximum current efficiency of 1.8 cd A−1 and a maximum EQE of 4.7%. The performance enhancement might be attributed to the incorporation of bulky tert-butyl groups, which could rigidify the molecular structure and effectively suppress the nonradiative decay pathway. Such high EQE is very close to the theoretical limit of ∼5% for fluorescent OLEDs (given that the PLQYs in thin films are equal to unity) and is one of the highest values of devices with NTSC blue emission.35 The relatively low EQE of the device with 4 may be attributed to the considerably lower PLQY of 4 and the small overlap between the absorption spectrum of 4 and the emission spectrum of MCP. It should be mentioned that the effects of dopant concentrations of heterocyclic spiro compounds also have been studied, in which devices doped with different concentrations of 7 (i.e., 4, 6, 8, 10, and 12%) are prepared. Unlike other fluorescent devices, in which the device efficiency significantly drops at high dopant concentration, the EQEs of devices with 7 are almost the same when the dopant concentration is increased up to 10%. This smaller efficiency roll-off may be attributed to the bulky structure of the heterocyclic spiro compounds that can reduce the concentration-induced quenching (Figure S9). Table 6 summarizes the key data of the OLED devices based on various heterocyclic spiro compounds and Figure S10 depicts their current density− voltage−luminance (J−V−L) curves plotted in log-scale.
Figure 4. (a) J−V characteristics and (b) IPCE spectra of devices doped with 3 at different concentrations under light illumination of 1 sun. Inset: The product of the IPCE data and the AM 1.5G solar spectrum for devices doped with 3.
the TFTs. Notably, most of the heterocyclic spiro compounds exhibit high linear and saturation mobilities in the range of 10−4 ∼ 10−3 cm2V−1s−1, comparable to those of the prototypical hole-transporting TAPC. Except for compound 7, both linear and saturation mobilities are only ∼10−6 cm2V−1s−1, much lower than those of other heterocyclic spiro compounds.42,43 Such small hole mobilities inevitably result in low FF and thus PCE for devices with 7. This is also true for devices with 6, where the hole mobilities are considerably lower than the threshold limit of the instrument (i.e., 10−7 cm2V−1s−1). In correlating with the photovoltaic responses, it seems that hole mobilities of donor materials are not the main determining factor governing the OPV performance, provided that the hole mobilites of the donor materials have already reached or exceeded 10−4 cm2V−1s−1. Electroluminescence (EL) Properties. As demonstrated in the emission studies, most of the heterocyclic spiro compounds exhibit a deep blue emission with high PLQYs of up to 0.91. It is envisaged that efficient deep blue-emitting OLEDs can be achieved using the heterocyclic spiro compounds as emitters. Except for nonemissive compound 1, all compounds have been employed to serve as a fluorescent dopant in the fabrication of multilayer OLEDs, due to the fact that they are strongly emissive in the deep blue region in solutions with PLQYs. Devices with the configuration of indium tin oxide (ITO)/α-naphthylphenylbiphenyldiamine (NPB) (70 nm)/4,4′,4″-tris(carbazole-9-yl)-triphenylamine (TCTA) (5 nm)/10% 2−8: N,N′-dicarbazolyl-3,5-benzene (MCP) (30 nm)/1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene
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CONCLUSIONS In summary, a series of bifunctional heterocyclic spiro compounds has been successfully synthesized and applied in the fabrication of both OPV devices and OLEDs. The effect of the structural modifications of different spiro cores, including spiroacridine, spiroxanthene, and spirothioxanthene, on the device performance has been studied. It is demonstrated that the heterocyclic spiro compounds are capable of serving as donor materials in OPV devices and fluorescent dopants in OLEDs. Particularly, high performance OPV devices with high Voc of up to 1.10 V and a PCE of up to 5.12% have been G
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Table 4. Key Photovoltaic Responses for the As-Prepared Devices without and with Donor Materiala compound
dopant conc. (%)
Jsc calculated (mA cm−2)
C70-only TAPC 1
0 7 3 5 7 9 3 5 7 9 3 5 7 9 3 5 7 9 3 5 7 9 3 5 7 9 3 5 7 9 3 5 7 9
1.15 10.62 8.90 9.13 9.36 9.14 9.30 9.56 9.84 9.96 9.27 9.85 10.00 10.10 8.62 9.56 9.74 9.60 9.78 9.77 10.00 9.84 2.86 3.12 3.70 4.27 6.44 7.42 7.94 8.15 9.81 10.03 9.99 9.97
2
3
4
5b
6b
7b
8b
a
Jsc (mA cm−2) 1.25 10.73 9.07 9.59 9.74 9.42 8.79 10.28 10.60 10.84 8.25 10.53 10.68 10.78 7.34 9.88 10.87 10.79 9.78 10.86 10.83 10.60 2.90 3.04 3.86 4.17 6.62 7.67 8.57 8.35 10.14 10.94 10.81 10.67
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.03 0.47 0.27 0.10 0.26 0.08 0.23 0.25 0.15 0.07 0.29 0.11 0.06 0.27 0.16 0.45 0.12 0.39 0.22 0.16 0.14 0.11 0.12 0.11 0.17 0.09 0.04 0.08 0.12 0.02 0.17 0.41 0.11 0.05
Voc (V) 0.97 0.85 1.10 1.09 1.09 1.08 0.91 0.94 0.94 0.94 0.93 0.93 0.95 0.95 0.95 0.94 0.98 0.98 0.96 0.95 0.94 0.92 1.06 1.10 1.08 1.08 1.16 1.17 1.16 1.16 0.96 0.95 0.94 0.93
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
FF 0.37 0.48 0.41 0.43 0.42 0.41 0.41 0.50 0.50 0.50 0.41 0.41 0.49 0.50 0.50 0.38 0.39 0.41 0.47 0.50 0.53 0.54 0.27 0.26 0.27 0.29 0.34 0.35 0.36 0.37 0.43 0.48 0.49 0.49
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
PCE (%) 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.45 4.38 4.09 4.49 4.46 4.17 3.28 4.83 4.98 5.09 3.15 4.01 4.97 5.12 3.49 3.53 4.15 4.34 4.41 5.16 5.40 5.27 0.83 0.87 1.13 1.31 2.61 3.14 3.58 3.58 4.19 4.99 4.98 4.86
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.13 0.11 0.07 0.12 0.06 0.18 0.06 0.08 0.11 0.15 0.15 0.15 0.04 0.13 0.11 0.24 0.10 0.10 0.03 0.06 0.07 0.04 0.03 0.05 0.04 0.09 0.03 0.02 0.01 0.09 0.08 0.05 0.09
Standard deviations were obtained from device-to-device variation of four identical devices. bFrom ref 50.
Table 5. Hole Mobilities of the Heterocyclic Spiro Compounds Measured by TFT Method compound TAPCa 1 2 3 5a 7 8a a
linear mobility (cm2V−1s−1) 2.5 1.5 1.3 2.5 2.0 5.3 1.4
× × × × × × ×
10−3 10−4 10−3 10−3 10−4 10−6 10−3
saturation mobility (cm2V−1s−1) 3.1 2.2 1.6 2.7 1.4 7.7 8.5
× × × × × × ×
10−3 10−4 10−3 10−3 10−4 10−6 10−4
VT (V) −2.4 −13.7 −5.5 −5.4 −15.5 −27.7 −10.1
From ref 50.
achieved. More importantly, highly efficient OLEDs with true deep blue emission and high EQEs of up to 4.7% have been realized, which is one of the highest values among devices with NTSC y-coordinates reported in the literature. These successful demonstrations provide insights and guiding principles on the design of multifunctional organic materials based on heterocyclic spiro compounds.
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Figure 5. EL spectra of OLEDs made with 2−8 doped in MCP host at current density of 20 mA cm−2. (Diphenylamino)phenylboronic acid,58 (4-(9H- carbazol-9-yl)phenyl)boronic acid,59 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)-phenyl)boronic acid,60 2′,7′-dibromo-10-phenyl-10H-spiroacridine-9,9′-fluorene,48 2,7dibromospirofluorene-9,9′-xanthene,52 and 2′,7′-dibromospirofluorene-9,9′-xanthene52 were synthesized according to the procedures
EXPERIMENTAL SECTION
Materials. All chemicals used for synthesis were of analytical grade and were purchased from Sigma-Aldrich Chemical Co. 4H
DOI: 10.1021/acsami.6b09211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 6. Characteristics of OLEDs based on different heterocyclic spiro compounds.
Table 6. Key Parameters for OLED Devices Doped with 10% of Compounds 2−8
a
compound
current efficiency (cd A−1)
power efficiency (lm W−1)
EQE (%)
2 3 4 5 6 7 8
0.7 1.1 0.3 1.0 0.5 1.8 1.9
0.4 0.9 0.1 0.6 0.4 1.2 1.3
1.2 2.1 0.7 1.2 1.5 4.7 3.4
λmax (nm) (CIE (x,y))a 428 432 412 439 408 418 437
(0.15, (0.15, (0.16, (0.15, (0.15, (0.16, (0.15,
0.07) 0.06) 0.05) 0.09) 0.05) 0.04) 0.06)
CIE coordinates are taken at a current density of 20 mA cm−2. 894.4 [M]+. Elemental analysis calcd. (%) for C67H47N3: C 90.00, H 5.30, N 4.70; found: C 90.12, H 5.31, N 4.47. TPA−O−SPIRO−TPA (3). TPA−O−SPIRO−TPA was synthesized according to a procedure similar to that for the synthesis of 2, except 2,7-dibromospirofluorene-9,9′-xanthene (490 mg, 1 mmol) was used in place of 2′,7′-dibromo-10-phenyl-10H-spiroacridine-9,9′fluorene. Yield: 556 mg (68%). 1H NMR (400 MHz, CDCl3, 298 K/ppm): δ 6.51 (d, 2H, 8.0 Hz, Spiro), 6.79 (t, 2H, 6.4 Hz, Spiro), 6.99−7.09 (m, 16H, TPA), 7.17−7.25 (m, 12H, TPA), 7.36−7.39 (m, 6H, Spiro), 7.60 (d, 2H, 8.0 Hz, Spiro), 7.83 (d, 2H, 8.0 Hz, Spiro). HRMS (Positive EI) calcd. for C61H42ON2: m/z = 818.3292; found: 818.3279 [M]+. Elemental analysis calcd. (%) for C61H42N2O· 0.5(CH2)4O: C 88.49, H 5.42, N 3.28; found: C 88.47, H 5.20, N 3.36. 2′,7′-TPA−O−SPIRO−TPA (4). 2′,7′-TPA−O−SPIRO−TPA was synthesized according to a procedure similar to that for the synthesis of 2, except 2′,7′-dibromospirofluorene-9,9′-xanthene (490 mg, 1 mmol) was used in place of 2′,7′-dibromo-10-phenyl-10H-spiroacridine-9,9′-fluorene. Yield: 532 mg (65%). 1H NMR (400 MHz, CDCl3, 298 K/ppm): δ 6.60 (d, 2H, 2.0 Hz, Spiro), 6.96−7.06 (m, 16H, TPA), 7.10 (d, 4H, 8.6 Hz, Spiro), 7.21−7.25 (m, 12H, TPA), 7.30 (d, 2H, 8.0 Hz, Spiro), 7.34−7.41 (m, 4H, Spiro), and 7.79 (d, 2H, 8.0 Hz, Spiro). HRMS (Positive EI) calcd. for C61H42ON2: m/z = 818.3292; found: 818.3271 [M]+. Elemental analysis calcd. (%) for C61H42N2O·0.5(CH2)4O: C 88.49, H 5.42, N 3.28; found: C 88.73, H 5.28, N 3.27.
reported in the literature. Compound 1 was also synthesized according to the procedures reported in the literature.56 Physical Measurements and Instrumentation. 1H NMR spectra, mass spectra, and electronic absorption spectra were recorded as reported previously.50 Elemental analyses of the newly synthesized compounds were performed on a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences, Beijing. Electrochemical measurements and thermal gravimetric analysis were performed as reported previously.50 Steady-state excitation and emission spectra were recorded at room temperature on a Spex Fluorolog-2 Model F111 fluorescence spectrofluorometer. Solid-state photophysical studies were carried out with solid samples contained in a quartz tube. Time-resolved emission studies were performed using a Horiba Jobin Yvon FluoroCube based on a time-correlated single photon counting method using a nano-LED with a peak wavelength and pulse duration equal to 371 nm with
