Hai-Bin Li

Density functional theory characterization and design of high-performance diarylamine-fluorene dyes with different π spacers for dye-sensitized solar cells

To rationalize the marked difference in the energy conversion efficiency of dye sensitized solar cells (DSSCs) based on organic dyes 1 and 2 different only in their π spacer, density functional theory (DFT) and time-dependent DFT calculations of the geometries, electronic structures and absorption spectra of the organic dyes before and after binding to titanium oxide were carried out. These enable us to determine factors such as dipole moments associated with the open-circuit photovoltage (Voc), and to quantify parameters such as the light harvesting efficiency, the electron injection efficiency associated with the short-circuit photocurrent density (Jsc). The results reveal that compared to 2 with a thiazole spacer, 1 with a thiophene spacer could cause a red shift of the absorption spectrum, increase the oscillator strength and improve the driving force for electron injection, thus leading to the larger Jsc, in good agreement with experimental data. As for Voc, our results stress that apart from the generally emphasized vertical dipole moment of the dyes pointing outward from the semiconductor surface, the number of photoinjected electrons from the dye to the semiconductor is also crucial to obtain high performance dyes with high Voc. After justifying the reliability of the quantum-chemical methods, we designed another four dyes with different π spacers to screen more efficient organic dyes. Fortunately, taking 1 as reference, we find that dye 4 with a thienothiophene spacer displays an enhanced Jsc and Voc, indicating that it will be a more efficient diarylamine-fluorene-based organic dye used in DSSCs, which will play a theoretical guiding role in the design and synthesis of new organic dyes.

Reversible piezochromic behavior of two new cationic iridium(III) complexes

We demonstrate that two new cationic Ir(III) complexes exhibit an interesting piezochromism, and their emission color can be smartly switched by grinding and heating. This is the first example that the Ir(III) complexes display piezochromic phosphorescence.

Controllable synthesis of iridium(III)-based aggregation-induced emission and/or piezochromic luminescence phosphors by simply adjusting the substitution on ancillary ligands

Three multifunctional cationic iridium(III)-based materials with aggregation-induced emission (AIE) and piezochromic luminescence (PCL) behavior have been rationally designed with the help of the theoretical calculations and successfully synthesized. All complexes contain the same cyclometalated ligand, 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz), while functionalized ancillary ligands with different substitution are used to control their photophysical properties. Complex 1 and 2 with ancillary ligands modified with aliphatic chains and carbazole end-capped alkyl groups, respectively, undergo remarkable and reversible changes in emission color in the solid state upon grinding or heating. In addition, 2, characterized as having the 3ILCT excited-state feature, simultaneously exhibits an interesting AIE behavior, showing almost non-emission in good solution but enhanced emission in its solid state. Further modification of 2 by attachment of a tert-butyl group on the ligand obtains complex 3, an amorphous material, which only displays AIE activity. More importantly, with the merits of reversible PCL and AIE properties of 2, the rare multi-channel color change and temperature-dependent emission behavior of the iridium(III) complex have been observed. Furthermore, the emissive nanoaggregates of 2 can be efficiently quenched by picric acid, making it a highly sensitive chemosensor for explosives, which is demonstrated in iridium(III)-based luminescent materials for the first time.

Theoretical discussions on electron transport properties of perylene bisimide derivatives with different molecular packings and intermolecular interactions

Seven perylene bisimide derivatives with different molecular packings and intermolecular interactions were investigated in detail within Marcus-Levich-Jortner formalism at the level of density functional theory (DFT). In theory, we further proved the report that different halogen substitutions in the core position of perylene bisimide lead to different molecular packings in their single crystals and thus obviously different electron transport properties. Here, insight into the geometries, the character of the frontier molecular orbitals, the decompositions of reorganization energies and transfer integrals in different directions was provided to shed light on the relationship between structures and properties. The molecular dynamics (MD) simulations and band structures calculations were also employed to give a multiscale understanding of their transport properties. The results show that there are small discrepancies of the intramolecular electron reorganization energies among these compounds and the transfer integrals determine their electron transport properties. Compounds 1a, 3a and 3b, with typical “brick” packing, π-stacked face-to-face packing and “herringbone” packing, respectively, have larger electron mobilities among these systems and possess different transport dimensionalities. Moreover, we also find there is close relationship between the intermolecular interaction energy and the transfer integral.

Theoretical characterization and design of small molecule donor material containing naphthodithiophene central unit for efficient organic solar cells

To seek for high‐performance small molecule donor materials used in heterojunction solar cell, six acceptor–donor–acceptor small molecules based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) units with different acceptor units were designed and characterized using density functional theory and time‐dependent density functional theory. Their geometries, electronic structures, photophysical, and charge transport properties have been scrutinized comparing with the reported donor material NDT(TDPP)2 (TDPP = thiophene‐capped diketopyrrolopyrrole). The open circuit voltage (Voc), energetic driving force(ΔEL‐L), and exciton binding energy (Eb) were also provided to give an elementary understanding on their cell performance. The results reveal that the frontier molecular orbitals of 3–7 match well with the acceptor material PC61BM, and compounds 3–5 were found to exhibit the comparable performances to 1 and show promising potential in organic solar cells. In particular, comparing with 1, system 7 with naphthobisthiadiazole acceptor unit displays broader absorption spectrum, higher Voc, lower Eb, and similar carrier mobility. An in‐depth insight into the nature of the involved excited states based on transition density matrix and charge density difference indicates that all S1 states are mainly intramolecular charge transfer states with the charge transfer from central NDT unit to bilateral acceptor units, and also imply that the exciton of 7 can be dissociated easily due to its large extent of the charge transfer. In a word, 7 maybe superior to 1 and may act as a promising donor candidate for organic solar cell.

Piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property of a new cationic iridium(III) complex

A new cationic Ir(III) complex based on a dendritic ancillary ligand has been designed and synthesized, which simultaneously exhibits piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property for the first time.

A theoretical discussion on the relationships among molecular packings, intermolecular interactions, and electron transport properties for naphthalene tetracarboxylic diimide derivatives

Three naphthalene tetracarboxylic diimide derivatives 1–3 with high electron mobilities and long-term ambient stabilities were investigated employing Marcus–Levich–Jortner formalism at the density functional theory (DFT) level. The complicated relationships among molecular packings, intermolecular interactions, and transport properties for these compounds were focused on and analyzed through investigating the sensitivities of transfer integrals to intermolecular relative orientations, the optimizations of the major transport pathways and the calculations of intermolecular interaction energies by using dispersion-corrected DFT. The results show that the transfer integrals are sensitive to the subtle changes of relative orientations of molecules, especially for core-chlorinated compounds, and there is an interplay between intermolecular interaction and molecular packing. It is found that the transfer integrals associated with the molecular packing motifs of these systems determine their electron mobilities. Interestingly, further discussions on band structures, the anisotropies and temperature dependences of mobilities, and the comparisons of mobilities before and after optimization indicate that the intermolecular packing motifs in the film state may be different from those in the crystalline state for 2. Finally, we hope that our conjecture would facilitate the future design and preparation of high-performance charge-transport materials.

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido-Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido-pyridine boron difluoride dyes 1-4, a series of scarce 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index-the charge-transfer distance. It is found that PBE0 provides satisfactory overall results. An in-depth insight into Huang-Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2, while 3 shows a relatively blue-shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1. Finally, compound 4, which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm(-1)), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light-emitting diodes. The theoretical results could complement and assist in the development of BODIPY-based dyes with both large Stokes shift and high quantum efficiency.

Intramolecular π-stacking in cationic iridium(III) complexes with a triazole–pyridine type ancillary ligand: synthesis, photophysics, electrochemistry properties and piezochromic behavior

To make Ir(III)-based complexes potentially multifunctional materials, two new cationic Ir(III) complexes with a 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (Phtz) ancillary ligand were designed and synthesized. By introducing the pendant phenyl ring into the ancillary ligand, the two complexes possess desired intramolecular π–π stacking between the pendant phenyl ring of the Phtz ligand and one of the phenyl rings of the cyclometalated ligand, which renders the complexes more stable. Density functional theory calculation indicates that the intramolecular π–π interactions in both complexes can reduce the degradation reaction in metal-centered (3MC) states to some extent, which further implies their stability. With these results in combination with their reversible oxidation and reduction processes as well as excellent photophysical properties, the stable light-emitting cells (LECs) would be expected. Furthermore, the two synthesized complexes exhibit reversible piezochromism. Their emission color can be smartly switched by grinding and heating, which is visible to the naked eye. In light of our experimental results, the present piezochromic behavior is due to interconversion between crystalline and amorphous states.

Forward molecular design for highly efficient OLED emitters: A theoretical analysis of photophysical properties of platinum(II) complexes with N-heterocyclic carbene ligands

The electronic structures and photophysical properties of eight Pt-complexes with different N-heterocyclic carbene ligands and potential to serve as light emitting diode materials were investigated by density functional theory and time-dependent density functional theory, employing the BP86 functional for geometry optimisations, SAOP potential for excited state calculations and all-electron TZ2P basis set throughout. Non-radiative and radiative decay rate constants were determined for each system through analyses of the geometric relaxations, d-orbital splitting and spin-orbit couplings at the optimised S(0) and T(1) geometries. Three Pt-systems bound to two N-heterocyclic carbenes were shown to be nonemissive, while a fourth was shown to be emissive from the T(1) excited state. Similar T(1)-initated emission was observed for three other Pt-systems investigated, each bound to four N-heterocyclic carbenes, while a fourth similarly tetra-ligated system showed T(2)-initation of emission. The results highlight the coupling of ligand-identity to photophysical properties and more importantly, the potential for rational optimisation and tuning of emission wavelengths and phosphorescent efficiencies. Encouragingly, two of the tetra-N-heterocyclic carbene ligated systems show strong potential to serve as highly-efficient blue and green light emitting materials, respectively.