Houyu Zhang

Electroluminescence from triplet metal—ligand charge-transfer excited state of transition metal complexes

Abstract Triplet metal—ligand charge-transfer (MLCT) excited-state emissions of some osmium(II) complexes Os(CN)2(PPh3)2X(X = bipyridine derivatives or anthroline derivatives) are enhanced upon incorporation into the poly(N-vinyl carbazole) (PVK) matrix. By employing a cell of indium—tin oxide (ITO)-coated glass/Os complex:PVK/2-(4-biphenyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD)/AI configuration, a stable and uniform red electroluminescence is observed over 8 V d.c. bias.

Tight intermolecular packing through supramolecular interactions in crystals of cyano substituted oligo(para-phenylene vinylene): a key factor for aggregation-induced emission

Strong supramolecular interactions, which induced tight packing and rigid molecules in crystals of cyano substituent oligo(para-phenylene vinylene) (CN-DPDSB), are the key factor for the high luminescence efficiency of its crystals; opposite to its isolated molecules in solution which have very low luminescence efficiency.

Theoretical Studies of Blue-Emitting Iridium Complexes with Different Ancillary Ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.

The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation†

Herein, we describe the molecular electronic structure, optical, and charge-transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) with triphenylamine (TPA)-substituted anthracene derivatives. The high performance of OLEDs with TPA-substituted anthracene is revealed to derive from three original features in comparison with aryl-substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority-carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole-electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built-in electric field to prompt the balance of charge-carrier injection.

Theoretical Characterization of a Typical Hole/Exciton-Blocking Material Bathocuproine and Its Analogues

The structural, electronic, and carrier transport properties of bathocuproine (BCP), which is a typical hole/exciton-blocking material applied in organic light-emitting diodes (OLEDs), have been investigated based on density functional theory (DFT) and ab initio HF method. The detail characterizations of frontier electronic structure and lowest-energy optical transitions have been studied by means of time-dependent density functional theory (TD-DFT). Five BCP analogues, o-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-diphenyl-1,10-phenanthroline (3), 4,7-diphenyl-1,10-phenanthroline (4), and 2,9-bis(trifluoromethyl)-1,10-phenanthroline (5) have also been studied in order to select more suitable candidates of efficient hole-blocking materials. The calculated results showed that rigid planar structures, conjugate degrees, and substitute groups play crucial roles in the hole/exciton-blocking and electron-transport properties of these materials. The calculated geometries, ionization energies (IP), and energy gap between the singlet ground state and triplet excited state (E(T1)) were well in agreement with the experimental results. On the basis of the incoherent transport model, the calculated electron mobility of BCP is 1.79 x 10(-2) cm(2)/(V s), which is comparable to experimental results of 1.1 x 10(-3) cm(2)/(V s). The electron mobilities for compounds 1, 4, and 5 are 3.45 x 10(-2), 2.90 x 10(-2), and 1.40 x 10(-2) cm(2)/(V s), respectively. The calculated results indicated that compounds 1, 4, and 5 may be more effective hole/exciton-blocking materials than BCP.

A partially incoherent rate theory of long-range charge transfer in deoxyribose nucleic acid

A quantum chemistry based Greens function formulation of long-range charge transfer in deoxyribose nucleic acid (DNA) double helix is proposed. The theory takes into account the effects of DNAs electronic structure and its incoherent interaction with aqueous surroundings. In the implementation, the electronic tight-binding parameters for unsolvated DNA molecules are determined at the HF/6-31G* level, while those for individual nucleobase-water couplings are at a semiempirical level by fitting with experimental redox potentials. Numerical results include that: (i) the oxidative charge initially at the donor guanine site does hop sequentially over all guanine sites; however, the revealed rates can be of a much weaker distance dependence than that described by the ordinary Ohms law; (ii) the aqueous surroundings-induced partial incoherences in thymine/adenine bridge bases lead them to deviate substantially from the superexchange regime; (iii) the time scale of the partially incoherent hole transport through the thymine/adenine pi stack in DNA is about 5 ps

Unified approach to the Bloch-Redfield theory and quantum Fokker-Planck equations

By using a rather simple algebraic approach, we revisit and further bridge between two most commonly used quantum dissipation theories, the Bloch–Redfield theory and a class of Fokker–Planck equations. The nature of the common approximation scheme involving in these two theories is analyzed in detail. While the Bloch–Redfield theory satisfies the detailed-balance relation, we also construct a class of Fokker–Planck equations that satisfy the detailed-balance relation up to the second moments in phase-space. Developed is also a generalized Fokker–Planck equation that preserves the general positivity of the reduced density operator. Both T1-relaxation and pure-T2 dephasing are considered, and their temperature dependence is shown to be very different. Provided is also an analogy between the quantum pure-T2 dephasing and the classical heat transport.

Morphology-dependent fluorescence ON/OFF of a beryllium complex: ACQ in amorphous solids, AEE in crystalline powders and the dark/bright fluorescence switch

Here we report a bis(2′-hydroxychalcone)beryllium complex Be(HC)2 that displays yellow fluorescence (λem = 557 nm; Φf = 0.10) in solution. Notably, the solution of this complex produces a non-emissive amorphous thin film (ACQ effect; fluorescent “OFF” state) but brightly emissive crystalline powders (AEE-active; fluorescent “ON” state) with deep red (λem = 678 nm; Φf = 0.27) or near infrared (λem = 700 nm; Φf = 0.20) emission colors and the fluorescent “ON” and “OFF” states can be smoothly transformed into each other by simple engineering processes: mechanical grinding and solvent fuming.

Supramolecular interaction-induced self-assembly of organic molecules into ultra-long tubular crystals with wave guiding and amplified spontaneous emission

Ultra-long tubular crystals (length up to 10 mm and diameter of 70 μm) of a small organic functional molecule, 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB), are successfully prepared through a physical vapor transport (PVT) method. On the basis of crystal structural analysis and density functional theory calculations, we show that the formation of such tubular crystals is drawn by the branched hydrogen bonds between the BCPEB molecules. High crystalline quality, highly ordered molecular orientation, and hollow-like topological structure enable the crystal to exhibit optical wave guided emission behaviors with low optical loss (3 dB mm−1) and highly polarized emission (the polarized ratio is about 20). Amplified spontaneous emission (ASE) characteristics of the tubular crystals were also studied; the net gain coefficients at the peak wavelength and the threshold are 91.7 cm−1 and 36 kW cm−2, respectively.

Bipolar host molecules for efficient blue electrophosphorescence: a quantum chemical design.

On the basis of density functional theory (DFT) calculations, a new series of bipolar host molecules for efficient blue electrophosphorescence devices are designed by linkage of hole-transporting moiety carbazole (CZ) and electron-transporting unit diphenylphosphoryl (ph(2)P horizontal lineO) to the core molecules with high triplet energies. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of the designed molecules have been studied with emphasis on triplet energies, spin density distributions, ionization potentials, electron affinities, and the influence of molecular topology. Designed bipolar host molecules possess the following features: (1) relatively higher highest occupied molecular orbital (HOMO) for hole injection and, relatively lower lowest unoccupied molecular orbital (LUMO) for electron injection; (2) HOMO and LUMO separation and localization in the respective hole- and electron-transporting moieties; (3) dramatic bond length changes in ionic states occurring at different parts of the bipolar molecules with respect to their neutral states; (4) keeping higher triplet energy. The DFT results provide deep insight into the nature of bipolar molecules and show that the designed molecules are feasible to meet the requirements of the host materials for blue triplet emissions.