The combination of skeleton-engineering and periphery-engineering: a design strategy for organic doublet emitters.

The emergence and development of radical luminescent materials is a huge breakthrough toward high-performance organic light-emitting diodes (OLEDs) without spin-statistical limits. Herein, we design a series of radicals based on tris(2,4,6-trichlorophenyl)methyl (TTM) by combining skeleton-engineering and periphery-engineering strategies, and present some insights into how different chemical modifications can modulate the chemical stability and luminescence properties of radicals by quantum chemistry methods. Firstly, through the analysis of the geometric structure changes from the lowest doublet excited state (D1) to the doublet ground state (D0) states, the emission energy differences between the BN orientation isomers are explained, and it is revealed that the radical with a smaller dihedral angle difference can more effectively suppress the geometric relaxation of the excited states and bring a higher emission energy. Meanwhile, a comparison of the excited state properties in different radicals can help us to disclose the luminescence behavior, that is, the enhanced luminescent intensity of the radical is caused by the intensity borrowing between the charge transfer (CT) state and the dark locally excited (LE) state. In addition, an efficient algorithm for calculating the internal conversion rate (kIC) is introduced and implemented, and the differences in kIC values between designed radicals are explained. More specifically, the delocalization of hole and electron wave functions can reduce nonadiabatic coupling matrix elements (NACMEs), thus hindering the non-radiative decay process. Finally, the double-regulation of chemical stability and luminescence properties was realized through the synergistic effect of skeleton-engineering and periphery-engineering, and to screen the excellent doublet emitter (BN-41-MPTTM) theoretically.

Hydrogen Migration-Triggered Diradicaloid Singlet-Fission Switch.

We present a theoretical design of the singlet-fission (SF) interconversion between two hydrogen tautomers to attract attention to electronic devices such as switches in the SF field. We develop a tuned π-electron conjugation strategy based on single-hydrogen migration to introduce diradical character and yield low-lying E(T1) levels. Specifically, these objectives could be accomplished by moving one hydrogen from a dihydrogen-substituted pyrazine-fused ring to another unsubstituted pyrazine-fused ring in tetraazatetracenes. The predicted SF efficiency would be expected to exceed 120%. To guide future SF design development, one rule of thumb regarding the S0-state and T1-state emerges from our research: In the S0-state, single-hydrogen migration is crucial for effectively localized electrons, which are the key factor in the formation of diradicals. Conversely, single-hydrogen migration induces a large area of π-electron conjugation in the T1-state, which is completely applied to the electron-hole interaction in the S0 → T1 transition, thereby providing low-lying E(T1) levels. Furthermore, a series of hydrogen tautomers of tetraazaacenes have been proposed as diradicaloid SF switches to verify the reliability of the above rule of thumb. This study will not only help researchers in the photovoltaic field to obtain the desired E(T1) in the future but also broaden the application of hydrogen migration in photovoltaic switch research and supplement the SF database.

Exploring the Influence of Halogen Coordination Effect of Stable Bimetallic MOFs on Oxygen Evolution Reaction.

The energy crisis and environmental pollution have forced scientists to explore alternative energy conversion and storage devices. The anodic reactions of these devices are all oxygen evolution reactions (OER), so the development of efficient OER electrocatalysts is of great significance. At the same time, understanding the reaction mechanism of OER is conducive to the rational design of efficient OER electrocatalysts. In general, catalytic active centers play a direct role in OER performance. In this paper, a series of stable bimetallic metal-organic frameworks (MOFs, named as Fe3 -Con -X2 , n=2, 3 and X=F, Cl, Br) with similar structure were synthesized by changing the halogen coordinated with the cobalt metal active center, aiming to investigate the influence of halogen substitution effect on OER performance. It was found that the OER activity of Fe3 -Co3 -F2 is much better than Fe3 -Co2 -Cl2 and Fe3 -Co2 -Br2 , indicating that the regulation of the electronegativity change of the coordination halogen atom can regulate the coordination electron structure of the metal active center, thereby achieving effective regulation of OER performance.

Self-Assembly of a Phosphate-Centered Polyoxo-Titanium Cluster: Discovery of the Heteroatom Keggin Family.

Over the past 200 years, the most famous and important heteroatom Keggin architecture in polyoxometalates has only been synthesized with Mo, W, V, or Nb. Now, the self-assembly of two phosphate (PO4 3- )-centered polyoxo-titanium clusters (PTCs) is presented, PTi16 and PTi12 , which display classic heteroatom Keggin and its trivacant structures, respectively. Because TiIV has lower oxidate state and larger ionic radius than MoVI , WVI , VV , and NbV , additional TiIV centres in these PTCs are used to stabilize the resultant heteroatom Keggin structures, as demonstrated by the cooresponding theoretical calculation results. These photoactive PTCs can be utilized as efficient photocatalysts for highly selective CO2 -to-HCOOH conversion. This new discovery indicates that the classic heteroatom Keggin family can be assembled with Ti, thus opening a research avenue for the development of PTC chemistry.

After the electronic field: structure, bonding, and the first hyperpolarizability of HArF.

In this work, we add different strength of external electric field (E(ext)) along molecule axis (Z-axis) to investigate the electric field induced effect on HArF structure. The H-Ar bond is the shortest at E(ext) = -189 × 10(-4) and the Ar-F bond show shortest value at E(ext) = 185 × 10(-4) au. Furthermore, the wiberg bond index analyses show that with the variation of HArF structure, the covalent bond H-Ar shows downtrend (ranging from 0.79 to 0.69) and ionic bond Ar-F shows uptrend (ranging from 0.04 to 0.17). Interestingly, the natural bond orbital analyses show that the charges of F atom range from -0.961 to -0.771 and the charges of H atoms range from 0.402 to 0.246. Due to weakened charge transfer, the first hyperpolarizability (ß(tot)) can be modulated from 4078 to 1087 au. On the other hand, make our results more useful to experimentalists, the frequency-dependent first hyperpolarizabilities were investigated by the coupled perturbed Hartree-Fork method. We hope that this work may offer a new idea for application of noble-gas hydrides.

Quantum chemical characterization and design of host materials based on phosphine oxide-substituted (triphenylamine) fluorene for (deep) blue phosphors in OLEDs.

We studied the electronic structures of a series of fluorene derivatives (p/mPODPFs and p/mPOAPFs) using density functional theory calculations and investigated their performances as host materials in organic light-emitting diodes from three aspects, i.e. triplet energy, ability of charge injection from neighboring organic layer or electrode, and match of the hosts and the reference guests (FIrpic and FCNIr) for efficient energy transfer (EF). Especially for the last aspect, the singlet/triplet (S(1)/T(1)) energies as well as the simulated host emission and guest absorption spectra are investigated to predict the possible emission mechanisms in the host-guest system and therefore to pursue the most suitable host for (deep) blue guest. From the investigated results, we deduced that pPODPF and pPOAPF are suitable for sky-blue FIrpic due to feasible Förster/Dexter energy transfers from pPODPF/pPOAPF to FIrpic, which agrees well with the experimental results. Furthermore, the higher external quantum efficiency (20.6%) of the pPOAPF-based device than that of the pPODPF-based device (13.2%) in experiments was inferred to be attributed to the matching S(1) energies between pPOAPF and FIrpic as well as good hole/electron injection abilities of pPOAPF in spite of a smaller overlap between the pPOAPF emission and FIrpic absorption spectra. By contrast, mPOAPF and mPODPF, designed in the work, may match with deep-blue FCNIr. In particular, mPOAPF may exhibit good performance as a host material for deep blue FCNIr as a consequence of its own balanced hole/electron injection ability and the matching S(1)/T(1) energies between mPOAPF and FCNIr.

Regulating and Predicting the Polyhedral Crystal Morphology in Spirofluorene Molecular Systems.

Crystallization of organic steric molecules often leads to multiple polyhedral crystal morphologies. However, the relationships among the molecular structure, supramolecular interaction, aggregation mode and crystal morphology are still unclear. In this work, we elaborate two model crystals formed by spiro[fluorene-9,9'-xanthene] (SFX) and spiro[cyclopenta[1,2-b : 5,4-b']dipyridine-5,9'-xanthene] (SDAFX) to demonstrate the feasibility of morphology prediction by periodic bond chain (PBC) theory based on interaction energy (IE) values in terms of single point energy. With non-directional van der Waals forces, only one PBC direction is found in SFX crystal, leading to the irregular 1D rod-like structure. Compared with SFX, the extra N heteroatoms in SDAFX can bring additional hydrogen bonds and some other interactions into the bulky molecular skeletons, inducing 3-dimensionally oriented PBCs to form the explicit F-face network in SDAFX which leads to the final octahedral structure. A simple and accurate method has been provided to quantify PBC vector on the supramolecular level in the organic molecular system, and the PBC theory has also been further demonstrated and developed in the morphology prediction of organic spiro-molecules.

Efficient electron transmission in covalent organic framework nanosheets for highly active electrocatalytic carbon dioxide reduction.

Efficient conversion of carbon dioxide (CO2) into value-added products is essential for clean energy research. Design of stable, selective, and powerful electrocatalysts for CO2 reduction reaction (CO2RR) is highly desirable yet largely unmet. In this work, a series of metalloporphyrin-tetrathiafulvalene based covalent organic frameworks (M-TTCOFs) are designed. Tetrathiafulvalene, serving as electron donator or carrier, can construct an oriented electron transmission pathway with metalloporphyrin. Thus-obtained M-TTCOFs can serve as electrocatalysts with high FECO (91.3%, -0.7 V) and possess high cycling stability (>40 h). In addition, after exfoliation, the FECO value of Co-TTCOF nanosheets (~5 nm) is higher than 90% in a wide potential range from -0.6 to -0.9 V and the maximum FECO can reach up to almost 100% (99.7%, -0.8 V). The electrocatalytic CO2RR mechanisms are discussed and revealed by density functional theory calculations. This work paves a new way in exploring porous crystalline materials in electrocatalytic CO2RR.

Various unique coordination patterns of Hg and DFT calculations to determine the formation of a 3-D supramoleculer framework by covalent and noncovalent interactions.

By combining a large pi-conjugated bidentate ligand L: 3,6-dipyrazole-N-ethylcarbazole with HgI(2), an extraordinary supramolecular coordination polymer, [Hg(4)L(2)I(8)](infinity), has been prepared. The crystal structures of the ligand and its coordination polymer were determined by X-ray crystallography, which shows three varied coordination modes especially the rare asymmetric quadruply bridged trinuclear moieties in [Hg(4)L(2)I(8)](infinity). Density functional theory (DFT) calculations (ADF) performed on model dimers show the roles of covalent and noncovalent interactions in establishing the three-dimensional architecture.

Theoretical study on a novel series of fullerene-containing organometallics Fe(eta5-C55X5)2 (X = CH, N, B) and their large third-order nonlinear optical properties.

Geometry structures, electronic spectra, and third-order nonlinear optical (NLO) properties of Fe(eta (5)-C 55X 5) 2 (X = CH, N, B) have first been investigated by time-dependent density functional theory. We analyzed the intramolecular interactions between ferrocene and the C 50 moiety. The calculated electronic absorption spectrum indicates that the short wavelength transitions are ascribed to the C 50 moiety mixed charge transfer transition of ferrocene itself, while the low energy excitation transitions are ascribed to the unique charge transfer transition from ferrocene to C 50 moiety in these systems. The third-order polarizability gamma values based on sum of states (SOS) method show that this class of ferrocene/fullerene hybrid molecule possesses a remarkably large third-order NLO response, especially for Fe(eta (5)-C 55B 5) 2 with the static third-order polarizability (gamma av) computed to be -10410 x 10 (-36) esu and the intrinsic second hypepolarizability to be 0.250. Thus, these complexes have the potential to be used for excellent third-order nonlinear optical materials. Analysis of the major contributions to the gamma av value suggest that the charge transfer from ferrocene to C 50 moiety along the z-axis (through Fe atom and the centers of two hybrid fullerenes) play the key role in the NLO response. Furthermore, boron substitution is an effective way of enhancing the optical nonlinearity compared to CH and N substitution, owing to smaller energy gap and better conjugation through the whole molecule.