Phosphine Oxide Derivatives as Hosts for Blue Phosphors: A Joint Theoretical and Experimental Study of Their Electronic Structure

Abstract

We report on a joint theoretical and experimental investigation of the electronic structure of a series of bis(diphenylphosphine oxide) derivatives containing a central aromatic core with high triplet energy. Such molecules can serve as host material in the emissive layer of blue electro-phosphorescent organic devices. The aromatic cores considered in the theoretical study consist of biphenyl, fluorene, dibenzofuran, dibenzothiophene, dibenzothiophenesulfone, or carbazole, linked to the two phosphoryl groups in either para or meta positions. With respect to the isolated core molecules, it is found that addition of the diphenylphosphine oxide moieties has hardly any impact on the core geometry and only slightly reduces the energy of the lowest triplet state (by, at most, ∼0.2 eV). However, the diphenylphosphine oxide functionalities significantly impact the ionization potential and electron affinity values, in a way that is different for para and meta substitutions. Excellent comparison is obtained between the experimental UPS and IPES spectra of the para biphenyl and meta dibenzothiophene and dibenzothiophenesulfone compounds and the simulated spectra. In general, the phosphine oxide derivatives present triplet energies that are calculated to be at least 0.2 eV higher than those of currently widely used blue phosphorescent emitters.

Electronic Structure of Carbazole-Based Phosphine Oxides as Ambipolar Host Materials for Deep Blue Electrophosphorescence: A Density Functional Theory Study

“…For the sake of completeness, computational data on N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD), as a representative holetransport layer (HTL) material, (Figure 1d) and on FIrpic, a widely used sky-blue phosphorescent emitter (Figure 1e), were reproduced from our previous work. 9,22 All the calculations were conducted at the DFT level of theory using the B3LYP hybrid functional with the TurboMole package (version 6.0); 23−25 we employed the default effective core potential (ECP) to deal with the core electrons of Ir and the split-valence SV(P) basis set for the valence electrons of Ir and all the electrons of the other atoms. The ground-state (S 0 ) geometries were optimized via spin-restricted calculations while the optimal structures in the lowest triplet state (T 1 ) and radical-ion states were evaluated via spin-unrestricted calculations.…”

“…This is due to the difference in mesomeric effect of directly appended phosphoryl groups on the hole and particle wave functions of NTOs. 9 On the other hand, in the cases of hosts 1−3, a phenyl group is inserted in between the phosphoryl group and carbazole and actually serves as a buffer, 33 in particular, vs. the mesomeric effect of the phosphoryl group. Therefore, to keep the triplet energy intact, it is crucial to minimize the substituent effect on the NTO wave functions that determine the triplet state; our results show that this can be achieved via the insertion of a buffer moiety.…”

Hosts for High‐Performance Phosphorescent Organic Light‐Emitting Diodes Based on Carbazole Derivatives

“…No carbazole derivative is absolutely electron-rich or deficient, which is strongly determined by the types of units and specific intermolecular linkage model because of "inductive" and "resonance" effects of different units, [63][64][65] and multifunctional hosts, which can be ambipolar, hole-blocking, or electron-transporting hosts, seem to be more power- ful for simplifying device structure and improving device performance. [66] As for organic electroluminescence, blue light is still lagging behind red and green lights in efficiency and stability.…”

Comparative Study of the Semiconducting Properties of Benzothiadiazole and Benzobis(thiadiazole) Derivatives Using Computational Techniques

“…In addition, to assess the charge-carrier injection easiness we also estimated the ionization potential and electron affinity by taking the energy differences between the ground and ionized states (non-relaxed ionized states give vertical IP/EA and relaxed ionized states give adiabatic IP/EA). [40] To obtain the adiabatic singlet-triplet (ST) gap, the lowest tripletstate geometries were calculated at the UB3LYP/cc-PVTZ level of theory. We also calculated the ST gaps using the recommended spin-flip (SF) method with the 50-50 functionals.…”