Asymmetrically twisted phenanthrimidazole derivatives as host materials for blue fluorescent, green and red phosphorescent OLEDs

The electroluminescent properties of asymmetrically twisted phenanthrimidazole derivatives comprised of fluorescent anthracene or pyrene unit namely, 1-(1-(anthracen-10-yl)naphthalen-4-yl)-2-styryl-1H-phenanthro[9,10-d]imidazole (ANSPI), 1-(1-(pyren-1-yl) naphthalene-4-yl)-2-styryl-1H-phenanthro[9,10-d]imidazole (PNSPI), 4-(2-(4-(anthracen-9-yl) styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (ASPINC) and 4-(2-(4-(pyren-1-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (PSPINC) for blue OLEDs have been analyzed. The asymmetrically twisted conformation interrupt π-conjugation effectively results in deep-blue emission. The pyrene containing PSPINC based non-doped blue device (476 nm) shows maximium efficiencies (current efficiency (ηc)-4.23 cd/A; power efficiency (ηp)-2.86 lm/W; external quantum efficiency (ηex)-3.48%: CIE (0.16, 0.17) at 3.10 V. Among the doped blue devices, An(PPI)2:ASPINC shows high efficiencies (ηc-12.13 cd/A; ηp-5.98 lm/W; ηex-6.79%; L-23986 cd m−2; EL-458 nm) at 3.15 V with CIE (0.15, 0.17) than An(PPI)2:PSPINC based device which is inconsistent with non-doped device performances. The green and red PhOLEDs show higher efficiencies with Ir(ppy)3: ASPINC (ηc-50.6 cd/A; ηp-53.4 lm/W; ηex-17.0%; L-61581 cd m−2; EL-501 nm, CIE (0.31, 0.60) at 3.32 V and (bt)2Ir(dipba): ASPINC (ηc-15.2 cd/A; ηp-16.5 lm/W; ηex-14.5%; L-13456 cd m−2; EL-610 nm), CIE (0.63, 0.36) at 3.20 V, respectively. The complete energy transfer between the host and dopant molecules improved the efficiency of PHOLEDs.

The phenanthrimidazole ring coupled with anthracene/pyrene moieties and styryl fragment at N 23 and C 25 position to form an asymmetrically twisted structure enhanced the thermal stability. The blue emissive materials exhibit maximum thermal stability as evidenced by decomposition temperature (T d ) (corresponding to 5% weight loss): 400°-ANSPI, 452°-PNSPI, 412°-ASPINC and 460°C -PSPINC. This will prevent the decomposition of these materials during vacuum deposition and device operation processes. The high T d indicates the high resistance of fused aromatic ring on thermolysis and the high T d could enhance the device lifetime ( Fig. 3) [65][66][67][68][69] . These materials has the ability to form an amorphous glass with a high glass-transition temperature (T g ) of 120° -ANSPI°, 132° -PNSPI, 123° -ASPINC and 139 °C -PSPINC which is beneficial for the formation of stable, homogeneous and amorphous film upon thermal evaporation and decreases the phase separation of host-guest system when used as host material (Fig. 3: Table 1).
The thermal morphological stability of ANSPI, PNSPI, ASPINC and PSPINC thin film were examined by AFM measurement (30° and 110 °C for 10 h). The RMS (root-mean-square roughness) of ANSPI (0.41 nm), PNSPI (0.46 nm), ASPINC (0.38 nm) and PSPINC (0.31 nm) thin-film surface reveal that there is no substantial changes before and after annealing (Fig. 3). The excellent thermal and amorphous stability indicates that phenanthrimidazole moiety comprised of anthracene and pyrene fragments may influence the arrangement of the molecules in the thin film and supports the suitability of these emissive materials for fabrication of blue OLEDs 65-67 . Electrochemical and photophysical properties. The onset oxidation potential (E ox ) for ANSPI, PNSPI, ASPINC and PSPINC are 0.90, 0.92, 0.81 and 0.86 eV versus ferrocenium/ferrocene redox couple, respectively (Fig. 3). Thus, the HOMO energies were estimated to be −5.70, −5.72, −5.61 and −5.66 eV, respectively [70][71][72] , the E HOMO of these materials is higher than that of fluorescent host material 4,4′-N,N′-dicarbazolylbiphenyl (CBP~ −6.0 eV) and matches well with widely used hole transporting material NPB implying that only little hole-injection barrier between NPB and these materials. The lower energy barrier between emitting layer, ANSPI, PNSPI, ASPINC and PSPINC and hole transporting layer will facilitate effective hole injection into emission layer.
The calculated LUMO energies [−2.62 eV -ANSPI; −2.58 eV-PNSPI; −2.39 eV-ASPINC; −2.46 eV-PSPINC] are in close with 1, 3, 5-tris(N-phenylimidazol-2-yl)benzene supports the electron injection abilities. The frontier molecular orbital analysis also confirms the carrier injection abilities and they can employed as potential emitters in OLEDs 71,72 . The optimized molecular geometry reveal that the π-conjugation between the phenanthrimidazole is interrupted that could induce the blue emission. The N 23-substituent is in a perpendicular direction which tends to inhibit π-π intermolecular interaction. The HOMO of ANSPI and PNSPI is mainly localized on naphthylanthracene and pyreneanthracene units at N 23, respectively and LUMO of ANSPI and PNSPI is located on phenanthrimidazole and styryl units. The HOMO of ASPINC and PSPINC is localized on anthracene/pyrene and phenyl of styryl fragment at C 25, respectively and LUMO is located on cyanonaphthyl at N 23 of phenanthrimidazole core. The significant spatial separation of HOMO and LUMO levels suggest that the HOMO-LUMO excitation would shift the electron density distribution from donor to acceptor of ANSPI, PNSPI, ASPINC and PSPINC leading to a polarized excited state. Such separation can provide hole-and electron-transporting channels where holes and electrons can realize intermolecular hopping smoothly along their respective conducting pathways. This indicates that ANSPI, PNSPI, ASPINC and PSPINC are bipolar materials with charge transport properties which is the requirement for host materials. The balanced carrier transport properties play a key role in conducting both holes and electrons and thus, improved the efficiency 73 .
The UV-vis absorption and PL (low temperature and thin film) spectra of ANSPI, PNSPI, ASPINC and PSPINC in CH 2 Cl 2 solution (10 −5 mol L −1 ) were measured to evaluate their optical properties. A higher intensity absorption around 339 nm (354 nm-ANSPI, 339 nm-PNSPI, 358 nm-ASPINC and 341 nm-PSPINC) originates from π-π* transition of phenyl ring whereas the lower intensity absorption at 366 nm-ANSPI, 355 nm-PNSPI, 369 nm-ASPINC and 360 nm-PSPINC are attributed to π-π* transition of anthracene/pyrene units. The absorption spectra of vacuum-deposited thin film of these compounds are similar to the corresponding solution spectra in view of spectral profiles and wavelength. The PL spectra of ANSPI, PNSPI, ASPINC and PSPINC in CH 2 Cl 2 / film, ANSPI (410/445 nm), PNSPI (395/460 nm), ASPINC (430/450 nm) and PSPINC (420/465 nm) reveal the blue emission (Fig. 4). Compared with pyrenyl phenanthromidazoles (PNSPI and PSPINC), anthracenyl phenanthroimidazoles (ANSPI and ASPINC) show bathochromic shift due to conjugation 74 . The maximum emission of ANSPI, PNSPI, ASPINC and PSPINC in thin film is red-shifted compared to solution due to exciton hopping in film 75 , a gradual decreasing red-shift was observed with an increasing conjugation. It should be noted that the bulky substituent at N 23 and C 25 of phenanthrimidazole effectively limits the π-conjugation results in deep-blue emission.
Among the pyrene compounds PNSPI and PSPINC and anthracene compounds ANSPI and ASPINC, PSPINC and ASPINC exhibit maximum efficiency due to better charge carrier transporting ability and the deeper emission which can be attributed to naphthonitrile group that induced effective molecular separation. Devices based on ANSPI, PNSPI, ASPINC and PSPINC show maximum brightness of 8356, 11823, 10123 and 12568 cd m −2 with blue emission at 465, 470, 469 and 476 nm, respectively (Fig. 4). The emission of anthracene compounds ANSPI and ASPINC are red-shifted around 19 nm compared to film emission which may be caused by the intermolecular interaction at the excited state. Since the compounds fabricated in device is in a thicker solid state, inevitably, the intermolecular interaction and the electrical field polarization in the excited state must be considered. However, the EL of pyrene compounds PNSPI and PSPINC are red-shifted only around 10 nm compared to PL in solid state: the electrical field polarization induced red shift of those compounds are attributed to separated HOMO/LUMO distribution. Meanwhile, the spatial steric configuration of these compounds also affects the electrical field polarization. The electroluminescence (EL) spectra of the devices show similar trends as PL in solid state because of EL peak with narrow FWHM of ANSPI, PNSPI, ASPINC and PSPINC are of 78, 65, 70 and 60 nm, respectively. The EL spectra of pyrene devices (PNSPI and PSPINC) are red shifted compared to anthracene devices (ANSPI and ASPINC) which can be ascribed to hypochromic shift of EL spectra and broader FWHM of ANSPI and ASPINC. Inspired by the efficient performance of PSPINC and its weak electrical field polarization effect, the blue electroluminescent device based on PSPINC was further optimized with the device configuration: The MoO 3 used as hole injection layer, 1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl] cyclohexane (TAPC) was used as hole transporting layer (HTL) and TPBi functioned as electron transporting and hole blocking layer (Fig. 5; Table 2 (Fig. 4) between the absorption of dopant with emission of ASPINC results energy transfer (ET) to dopants. Since triplet energy (E T~2 .60 eV) of these materials is lower than FIrpic~2.65 eV the incomplete host → dopant energy transfer leads to lower the efficiencies. ASPINC (E T ) > FIrpic (E T ), green and red emitters show triplet-triplet (Dexter) ET is possible along singlet-singlet (Forster) ET when doping green/red emitters into host ASPINC. Emission of 5 wt% of RGB dopants:host ASPINC film confirmed efficient energy transfer and λ max of film is same with λ max dopants λ max , however ASPINC λ emi was not obtained. The single-exponential decay (2.7 ns, 1.01 ns and 0.83 ns) supports complete ET www.nature.com/scientificreports www.nature.com/scientificreports/ from ASPINC host → dopants show high-performance OLED devices. The slightly red-shifted emission from doped film to neat film of corresponding dopants should be related to the tighter packing between the dopant molecules in the neat film. The above studies show that ANSPI, PNSPI, ASPINC and PSPINC could be good host materials for both fluorescent and phosphorescent OLEDs. The efficient performance of the non-doped blue OLEDs prompted us to explore the possibility of using these blue emissive materials as host in host/dopant hybrid devices. To evaluate its practical utility, a series of blue fluorescent OLEDs with simple configuration of ITO/NPB (10 nm)/ANSPI or PNSPI or ASPINC or PSPINC (40 nm): 5% An(PPI) 2 /TPBi (15 nm)/LiF (1 nm)/Al (100 nm) were fabricated and the energy diagram of these materials used in the EL devices is shown in Fig. 5. NPB was used as the hole transporting material (HTL) and TPBi was used as the electron transport/hole-blocking layer (ETL/ HBL) (Fig. 5). Similar with non-doped device, the operating voltage of pyrene compounds PNSPI and PSPINC based devices is lower than anthracene compounds ANSPI and ASPINC based devices which may be attributed  (Fig. 7). Among the doped blue fluorescent devices, An(PPI) 2 :ASPINC shows high efficiencies than An(PPI) 2 :PSPINC based device which is inconsistent with non-doped blue device performances.
The EL spectrum is consistent with the PL spectrum of An(PPI) 2 : ASPINC/ANSPI/PNSPI/PSPINC suggesting that the blue EL emission results from the intrinsic emission of An(PPI) 2 . However, the efficient overlap of emission spectra of ANSPI, PNSPI, ASPINC and PSPINC in film state with absorption spectra of the dopant (An(PPI) 2 ) enhanced the efficiency (Fig. 4). The film of Ir(ppy) 3 and (bt) 2 Ir(dipba) doped in ASPINC at 5 wt% concentration was used as emissive layer to fabricate the phosphorescent green and red devices, respectively. Similarly, green PhOLEDs with Ir(ppy) 3  at 3.20 V show higher efficiencies, such high and stable EL performance should be attributed to the balanced carrier injection/transport ability of ASPINC which result in a broad distribution of recombination region in the corresponding emissive layer. A low probability of triplet-triplet annihilation that causes an efficiency roll-off at high current density for the PhOLEDs. Charge confinement in the emissive layer is another key factor for high EL efficiency. There is a large energy barrier of ~0.6 eV prevents the hole leakage from ASPINC to TPBi together which prevents electron leakage from ASPINC to NPB. Therefore, holes and electrons can be effectively confined inside the ASPINC results in achieving high efficiency and low roll-off OLEDs. Besides, the E T (~2.60 eV) of ASPINC is high enough than the phosphorescent dopants green [Ir(ppy) 3 , E T : ~2.4 eV) 81 and red ((bt) 2 Ir(dipba), E T : ~2.1 eV] 82 for working as a host, the energy loss during the host-to dopant energy transfer process can be reduced as far as possible 59 . The ASPINC was found to be the best host material for our devices. It is notable that the excellent performances of blue, green and red OLEDs were harvested from same device configuration by adopting same host material. The simple material system and easy fabrication process are of significance and importance for reducing the cost and enhancing the process stability in commercial mass production.

Data availability
The authors declare that data in our manuscript are available.