Article pubs.acs.org/JPCC
Selectively Investigating Molecular Configuration Effect on Blue Electrophosphorescent Host Performance through a Series of Hydrocarbon Oligomers Zhen Zhang,†,‡ Zhensong Zhang,§,‡ Dongxue Ding,† Ying Wei,† Hui Xu,*,†,∥ Jilin Jia,† Yi Zhao,*,§ Kai Pan,† and Wei Huang*,∥ †
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials, Heilongjiang University, 74 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China § State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, Jilin 130012, P. R. China ∥ Institute of Advanced Materials (IAM), Nanjing University of Technology, Nanjing, Jiangsu 211816, P. R. China S Supporting Information *
ABSTRACT: Hydrocarbon oligomers X9F, including S9F, D9F, and T9F as monomer, dimer, and trimer, respectively, were designed and prepared on the basis of indirect linkage and 9,9-diphenylfluorene (S9F) as repeat unit to form planar, linear, and V-shaped configurations without polarity variation and function amplification. The identical optical and electrochemical properties of X9F were achieved because of the effectively blocked intramolecualr electronic interactions by indirect linkage, including the same T1 value of 2.98 eV, high enough for hosts in blue phosphorescent organic light-emitting diodes (PHOLEDs), and the approximate FMO energy levels, which established the basis for selective investigation of independent configuration effect on the optoelectronic performance of host materials. Density function theory simulation manifested the frontier molecular orbital (FMO) location extension after oligomerization and the specific T1 locations on peripheral fluorenyls in X9F, giving rise to their different carrier-transporting abilities and host-localized triplet−triplet annihilation (TTA) and triplet−polaron quenching (TPQ) effects. As a result, D9F with the linear and locally unsymmetrical configuration revealed electron-predominant characteristics for charge balance, restrained triplet interaction for TTA suppression, and partially separated FMO and T1 locations for TPQ suppression. Consequently, the low driving voltages and the favorable maximum efficiencies, such as ∼11% for external quantum efficiency (EQE), as well as reduced roll-offs less than 8% for EQE at 1000 cd m−2, were achieved by D9F-based blue PHOLEDs as the highest performance among X9F, in which device efficiencies were improved by 50% compared to that of conventional polarized host mCP. It is conceivable that molecular configuration has significant effects on electrical properties and quenching effects of organic semiconductors with remarkable influence on intermolecular interplay and excited-state locations. meso-,15 insulating16,17 or twisted18,19 configurations have been put forward to achieve the high first triplet (T1) energy levels,20 such as >2.8 eV for blue PHOLEDs, while electrically active insulating linkages, including phosphine oxide (PO)21−35 and/ or ambipolar molecular configurations,36−66 are employed to further enhance the electrical properties. We also reported a series of highly efficient unipolar and ambipolar PO hosts on the basis of the strategies named short-axis substitution,30 multi-insulating,42 and indirect linkages,28 endowing iridium(III) bis(2-(4,6-difluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic)-based blue-emitting devices with low driving voltages (10% maximum).
1. INTRODUCTION Phosphorescent organic light-emitting diodes (PHOLEDs) are one of the most promising candidates for the next generation of energy-efficient displays1−3 and solid-state lighting4−11 because of its great advantage of theoretical internal quantum efficiency up to 100% by harvesting both singlet and triplet excitons. Suffering from worse concentration quenching12 and triplet− triplet annihilation (TTA),13 most phosphors should be dispersed in host materials with suitable optoelectronic characteristics,3 for which high excited energy levels and good electrical performance are the main indicators for exothermic host−dopant energy transfer and charge transfer balance.14 Therefore, in recent years, these two factors have received great attention when developing host materials with desired optoelectronic performance, especially for blue electrophosphorescence. As a result, effective strategies on the basis of © 2014 American Chemical Society
Received: June 30, 2014 Revised: August 10, 2014 Published: August 19, 2014 20559
dx.doi.org/10.1021/jp506513x | J. Phys. Chem. C 2014, 118, 20559−20570
The Journal of Physical Chemistry C
Article
monomer, dimer, and trimer, respectively, were designed and synthesized on the basis of indirect linkage strategy by splicing together S9F repeat units through para-linkage of their 9phenyls to form the planar, linear, and V-shaped configurations (Figure 1 and Scheme 1). The conjugation extension was
Nevertheless, it is noteworthy that when the molecular systems were further extended with more functional groups and complicated linkage styles, the materials with well-designed photophysical and electrical properties can not constantly achieve the desired device performance. In this case, we believe that except for optoelectronic characteristics, there are other factors that should be concerned. Because molecular configuration would determine stacking modes and interactions between adjacent molecules, molecular configuration is believed to contribute to charge mobility and multiparticle quenching processes. Therefore, for various applications, different molecular configurations are constructed: the planar and rigid structures are employed for high charge mobility, whereas asymmetric configurations are utilized to suppress quenching effects. However, because configuration variation is always accompanied by functionality alteration, it is rather difficult to clarify the independent influences from the molecular configuration adjustment and function enhancement. Therefore, the selective investigation of molecular configuration effect on electroluminescent (EL) performance becomes imperative. In this case, two key points should be considered to eliminate interference from other factors: (i) optical properties, including both the first singlet and triplet (S1 and T1) energy levels, should be fixed to afford similar energy transfer processes; and (ii) single molecular electrical properties should be well-controlled while adjusting functional group number and molecular components. Thus, oligomers would provide a suitable platform as a homologous series with the same functional components and identical group arrangement. Nevertheless, if highly polarized groups are involved, optoelectronic properties would be directly correlated to the repeat unit number in oligomers because of amplified molecular polarity. Consequently, nonpolar oligomer hosts might be the most suitable systems for selectively investigating the influence of molecular configuration variation on EL performance. Actually, nonpolar phosphorescent hosts are superior in (i) confining exciton on phosphors through both exothermic energy transfer and direct charge capture by dopants; (ii) suppressing multiparticle quenching effects, such as triplet−polaron quenching (TPQ)67 as inertia matrixes; and (iii) enhancing the device stability owing to the stronger photochemical stability of C−C bond compared with labile bonds (e.g., C−N, C−O etc.) in polarized hosts.68 However, because the T1 state is sensitive to the molecular conjugation to such a degree that the one phenyl extension from benzene to biphenyl of T1localized moiety leads to a remarkable decrease of T1 from 3.0 to 2.7 eV, let alone polycyclic aromatic groups,17 except for conjugation extension, constructing multidimensional oligomeric configurations provides a beneficial alternative to intermolecular interactions, while making T1 energy constant. In our recent work, the high-efficiency ambipolar PO host materials were demonstrated through indirect linkage strategy to effectively suppress the negative effects of biphenylene and carrier-transporting groups on T1,48 which gave us the inspiration to construct high energy gap nonpolar host oligomers with identical optical properties and different molecular configurations. In this contribution, a series of hydrocarbon oligomer hosts with the collective name X9F for blue PHOLEDs, namely 9,9diphenylfluorene (S9F), 4,4′-bis(9-phenyl-9H-fluoren-9-yl)biphenyl (D9F), and 9,9′-(4′,4″-(9H-fluorene-9,9-diyl)bis(biphenyl-4′,4-diyl))bis(9-phenyl-9H-fluorene) (T9F) as
Figure 1. (a) Molecular design of indirectly linked fluorene-based hydrocarbon monomer, dimer, and trimer, namely, S9F, D9F, and T9F, respectively; (b) single-crystal structure of D9F.
effectively blocked by indirect linkage to render the identical optical and electrochemical properties for X9F, including the same electronic absorption, fluorescence (FL) and phosphorescence (PH) profiles, and the close frontier molecular orbital (FMO) energy levels, which were further verified by density functional theory (DFT) simulation. An impressive T1 value as high as 2.98 eV made these materials competent hosts for blue PHOLEDs. Meanwhile, the different configurations and incorporation degrees in the FMOs of diphenylene group in X9F gave rise to their different charge-hopping modes and consequently discrepant carrier-transporting abilities. Because of the uniform T1 locations on specific fluorenyls in X9F, locally unsymmetrical configuration of D9F favored suppressing triplet−triplet annihilation (TTA). As a result, D9F endowed its blue PHOLEDs with the highest performance among X9Fbased devices, including driving voltages as low as 2.7 V for onset,
