Shuping Huang

Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes

Efficient thermally activated delayed fluorescence (TADF) has been characterized for a carbazole/sulfone derivative in both solutions and doped films. A pure blue organic light emitting diode (OLED) based on this compound demonstrates a very high external quantum efficiency (EQE) of nearly 10% at low current density. Because TADF only occurs in a bipolar system where donor and acceptor centered (3)ππ* states are close to or higher than the triplet intramolecular charge transfer ((3)CT) state, control of the π-conjugation length of both donor and acceptor is considered to be as important as breaking the π-conjugation between them in blue TADF material design.

Anthraquinone-Based Intramolecular Charge-Transfer Compounds: Computational Molecular Design, Thermally Activated Delayed Fluorescence, and Highly Efficient Red Electroluminescence

Red fluorescent molecules suffer from large, non-radiative internal conversion rates (k(IC)) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (k(F)) as well as a small energy difference between the lowest singlet and triplet excited states (ΔE(ST)) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔE(ST) and large k(F). Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The k(F) and ΔE(ST) values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected time-dependent density functional theory and are approximatively proportional to the square of the cosine of the theoretical twisting angles between each subunit. However, the introduction of a Ph-bridge was found to enhance k(F) without increasing ΔE(ST). Molecular simulation revealed a twisting and stretching motion of the N-C bond in the D-A-type molecules, which is thought to lower ΔE(ST) and k(F) but raise k(IC), that was experimentally confirmed in both solution and doped film. OLEDs containing D-Ph-A-Ph-D-type molecules with diphenylamine and bis(4-biphenyl)amine donors demonstrated maximum external quantum efficiencies of 12.5% and 9.0% with emission peaks at 624 and 637 nm, respectively.

Computational Prediction for Singlet- and Triplet-Transition Energies of Charge-Transfer Compounds.

Our work reveals a high dependence on charge-transfer (CT) amounts for the optimal Hartree-Fock percentage in the exchange-correlation functional of time-dependent density functional theory (TD-DFT) and the error of a vertical transition energy calculated by a given functional. Using these relations, the zero-zero transition energies of the first singlet and first triplet excited states of various CT compounds are accurately reproduced. (3)CT and locally excited triplet ((3)LE) states are well distinguished and calculated independently.

KMBP2O8 (M = Sr, Ba): a new kind of noncentrosymmetry borophosphate with the three-dimensional diamond-like framework.

Two new isotypic anhydrous borophosphates, KMBP(2)O(8) (M = Sr, Ba), have been prepared under high temperature solution growth (HTSG). Investigation of single-crystal X-ray diffraction shows that they crystallize in noncentrosymmetric space group I42d with the following lattice parameters: a = 7.1095(18), c = 13.882(5)A for KSrBP(2)O(8) and a = 7.202(2), c = 14.300(6)A for KBaBP(2)O(8). The fundamental building block (FBB) of title borophosphates with B:P ratio = 1:2 is 12square:<12square> (symbol square represents BO(4) and PO(4) tetrahedra). The FBBs are further interlocked together to form the final 3D diamond-like architecture. Second-harmonic generation (SHG) on powder samples has been measured using Kurtz and Perry technique, which indicates that they are phase-matchable materials, and their SHG coefficients are measured to be about 1/5 (for KSrBP(2)O(8)) and 1/3 (for KBaBP(2)O(8)) times as large as that of KDP.

Molecule-substrate interaction channels of metal-phthalocyanines on graphene on Ni(111) surface

Molecule-substrate interaction channels of metal-phthalocyanines (MPcs, including NiPc, CuPc, ZnPc, FePc, and CoPc) on graphene on Ni(111) were investigated by employing high-resolution electron energy loss spectroscopy (HREELS). Except the expected IR-active modes, some Raman-active modes were also observed in all of MPcs, which are considered in this study. From the origination of the Raman-active features, it was deduced that MPcs are coupled with the substrate mainly through their central metal atom. The Raman-active modes appear as symmetric peaks in the HREELS in the case of MPcs with Ni, Cu, and Zn, whereas they are asymmetric and appear as a Fano line shape in the case of MPcs with Fe and Co. This spectroscopic difference indicates that the molecule-substrate coupling is completely different in the two cases mentioned above. The molecule-substrate interaction strength is considerably weak and comparable with the π-π interaction between molecules in the case of MPcs with Ni, Cu, and Zn, whereas it is much stronger in the case of MPcs with Fe and Co. From the HREELS observations, it can be suggested that the whole molecule can be effectively decoupled from the underneath Ni(111) by inserting a single layer of graphene between them in the case of MPcs with Ni, Cu, and Zn, whereas only benzene rings can be completely decoupled in the case of MPcs with Fe and Co.

First-principles modeling of nonlinear optical properties of C3N4 polymorphs

Using the first-principles method combined with the anharmonic oscillator model, the electronic, linear, and nonlinear optical properties of four C3N4 polymorphs are investigated. The calculations show that the third-order susceptibilities and nonlinear refractive indices increase with the increase of volume of C3N4 polymorphs below 1.5eV, whereas the plasmon frequencies and relativistic nonlinear refractive indices decrease. The obtained results indicate that the spinel C3N4 has some potential applications in nonlinear optics.

Density functional theoretical determinations of electronic and optical properties of nanowires and bulks for CdS and CdSe

The authors present density functionaltheoretical results of electronic and optical properties of wurtzite-type CdS and CdSesemiconductornanowires and bulks. The results show that quantum confinement will increase the energy gap, decrease the dielectric function, change the sign of birefringence, and increase the absolute value of birefringence at low-frequency region, as the dimensions of CdS and CdSe are reduced from the bulk to a nanowire with diameter of several nanometers. The optical spectral peaks of CdSenanowires show redshifts as compared with the corresponding ones of CdSnanowires.

Charge Transfer, Luminescence, and Phonon Bottleneck in TiO2 Nanowires Computed by Eigenvectors of Liouville Superoperator.

A nonadiabatic excited state dynamics study of ⟨001⟩ anatase TiO2 nanowire is obtained by combining density matrix in Liouville-Redfield formalism and ab initio electronic structure calculations. The properties of eigenvectors of Liouville-Redfield superoperator are investigated. The time evolutions of Kohn-Sham orbital populations are obtained for different electronic excitations. The numerical solutions of the population changes over time are in agreement with the analytical results. The analytical and numerical results on the electron and hole relaxation rates are compared. The electron nonradiative relaxation to the bottom of conduction band involves Ti 3d orbitals, whereas the hole nonradiative relaxation to the top of valence band is mainly localized in surface O 2p orbitals. The rate of relaxation in nanowire is slower than in bulk TiO2 demonstrating phonon-bottleneck effect. Calculated emission spectrum has vanishing contribution from lowest energy excitation, which indicates charge transfer.

A theoretical investigation of hyperpolarizability for small GanAsm (n+m=4–10) clusters

In this paper, the second and third order polarizabilities of small Ga(n)As(m) (n + m=4-10) clusters are systematically investigated using the time dependent density functional theory (TDDFT)6-311+G* combined with the sum-over-states method (SOSTDDFT6-311+G*). For the static second order polarizabilities, the two-level term (beta(vec.2)) makes a significant contribution to the beta(vec) for all considered Ga(n)As(m) clusters except for the Ga3As4 cluster. And, for the static third order polarizabilities, the positive channel (gamma(II)) makes a larger contribution to gamma(tot) than the negative channel (gamma(I)). Similar to the cubic GaAs bulk materials, the small Ga(n)As(m) cluster assembled materials exhibit large second order (1 x 10(-6) esu) and third order susceptibilities (5 x 10(-11) esu). The dynamic behavior of beta(-2omega; omega, omega) and gamma(-3omega; omega, omega, omega) show that the small Ga(n)As(m) cluster will be a good candidate of nonlinear optical materials due to the avoidance of linear resonance photoabsorption.

Y-doped Li8ZrO6: A Li-Ion Battery Cathode Material with High Capacity

We study--experimentally and theoretically--the energetics, structural changes, and charge flows during the charging and discharging processes for a new high-capacity cathode material, Li8ZrO6 (LZO), which we study both pure and yttrium-doped. We quantum mechanically calculated the stable delithiated configurations, the delithiation energy, the charge flow during delithiation, and the stability of the delithiated materials. We find that Li atoms are easier to extract from tetrahedral sites than octahedral ones. We calculate a large average voltage of 4.04 eV vs Li/Li(+) for delithiation of the first Li atom in a primitive cell, which is confirmed by galvanostatic charge/discharge cycling data. Energy calculations indicate that topotactic delithiation is kinetically favored over decomposition into Li, ZrO2, and O2 during the charging process, although the thermodynamic energy of the topotactic reaction is less favorable. When one or two lithium atoms are extracted from a primitive cell of LZO, its volume and structure change little, whereas extraction of the third lithium greatly distorts the layered structure. The Li6ZrO6 and Li5ZrO6 delithiation products can be thermodynamically metastable to release of O2. Experimentally, materials with sufficiently small particle size for efficient delithiation and relithiation were achieved within an yttrium-doped LZO/carbon composite cathode that exhibited an initial discharge capacity of at least 200 mAh/g over the first 10 cycles, with 142 mAh/g maintained after 60 cycles. Computations predict that during the charging process, the oxygen ion near the Li vacancy is oxidized for both pure LZO and yttrium-doped LZO, which leads to a small-polaron hole.