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To compare homoleptic [2]catenanes and single rings consisting of gold(i) thiolates, the geometric and electronic structures, absorption spectra, binding energy, interaction energy and composition of intramolecular forces were studied theoretically.

Molecular isomeric engineering is employed to construct efficient orange-red Ir(iii) complexes for WOLEDs and understand the intrinsic structure–property relationship.

DFT and TD-DFT calculations were performed to turn conventional non-TADF units into high-lying reverse intersystem crossing D–A–D-type TADF emitters.

Spin-unrestricted DFT and spin-unrestricted TDDFT calculations were performed to systematically investigate the correlation between the electron donating ability of donors and photophysical properties in D–A luminescent radicals.

To gain a deep understanding on structure-property relationship of all-small-molecule nonfullerene organic solar cells, donor molecules SM1 and SM2 with and without cyano group substituents, respectively, were selected, for which SM1-based device has obvious higher power conversion efficiency (10.11%) than SM2-based device (5.32%) in the same condition. Here a systemic study combining quantum chemistry with molecular dynamics was conducted to explore what improve the performance of SM1-based organic solar cells from the theoretical perspective. The results indicate that the cyano substituents in SM1 not only could induce a redshift absorption spectrum through decreasing the energy gap but also lead to better intramolecular charge transfer, which is beneficial to charge transfer at donor/acceptor interface. More interestingly, the SM1/acceptor interface presents an additional charge transfer state (CT state) below the first optically Frenkel exciton state (FE state) except that its FE state has stronger absorption compared with SM2/acceptor interface, which plays an important role in improving its charge transfer rate (kinter-CT) through increasing electronic coupling of charge transfer (VDA-CT). Thus it is speculated that molecular design considering additional CT states below FE state is a promising strategy in organic solar cells like SM1-based device with good performance.

Organic solar cells (OSCs) have made rapid advances in power conversion efficiency during the past decades, which is boosted partly by the various designs of new materials, especially in donor mate...

As compared with the promising near-infrared (NIR) absorbing trans-squaraines (SQ), cis-squaraines (HSQ) possess special absorption spectra showing better performance and are being attracted as promising NIR chromophores in dye-sensitized solar cells (DSSCs). For further extending the absorption toward the NIR region, a rigidified dithiophene spacer 4H-Silolo[3,2-b:4,5-b’]dithiophen/e,4,4,-dimethyl (DTS) was introduced into the skeleton of HSQ (HSQ-DTS). A comprehensive analysis on the light harvesting ability, intramolecular charge separation, interfacial electron injection and dye regeneration were conducted by means of density functional theory (DFT) and time-dependent DFT calculations on the dye@(TiO2)38 system. Our results demonstrated that as compared with HSQ, HSQ-DTS could exhibit panchromatic absorption with more favorable charge separation ability, more injected electrons with inhibited charge recombination process due to the larger distance between the cation hole and the semiconductor surface. The superior performance of dye HSQ-DTS satisfied our original design intent. Therefore, based on these inspiring results, another six different rigidified dithiophene spacers (1-6) were also investigated to screen out more efficient cis-squaraine sensitizers. We find 3 with 4H-Phospholo[3,2-b:4,5-b’]dithiophene,4-ethynyl-,4-oxide possesses the best performance with the maximum short-circuit current ( J s c m a x ) achieved 31.60 mAcm−2, which is expected to support future experimental and theoretical studies.

Two o-carborane derivatives 1 and 2 exhibiting both aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties were investigated by combining density functional theory with molecular dynamics simulation. The reason for their TADF property was explored. Besides, taking 1 as an example, the AIE phenomenon was also probed by simulating the aggregation process and comparing the photophysical properties of the molecules in different aggregation states (including crystals, water, and films) with the isolated molecule. The results manifest that the separated orbital occupations of the HOMO and LUMO at the respective donor and acceptor units result in a small singlet–triplet energy gap and a favorable TADF feature, which is similar to many typical TADF molecules. In the AIE phenomenon, a much smaller structural change of donor and acceptor units between S1 and S0 states was experienced for the molecules in all aggregate states compared with that for the isolated molecule, decreasing the energy dissipation substantially and enhancing the luminescence efficiency. In addition, o-carborane is conjectured to help in the free rotation of donor and acceptor units for the isolated molecule, which may contribute to AIE. What's more, on the basis of 1 and 2, we designed 3–6 by means of replacing the triphenyltriazine group with other electron-withdrawing groups having lower LUMO energy levels, aiming to obtain this kind of red light emitting material. The results indeed proved our assumption that 3–6 not only present the feature of red light emission but also have both TADF and AIE properties in films.

Quantum mechanical and molecular dynamics simulations have been carried out on a series of anthracene-o-carborane derivatives (ANT-H, ANT-Ph, ANT-Me and ANT-TMS) with rare red-light emission in the solid state. The simulation of the heating process of the crystals and further comparison of the molecular structures and excited-state properties before and after heating help us to disclose the thermochromic behavior, that is, the red-shift emission is caused by elongation of the C1-C2 bond in the carborane moiety after heating. Thus, we believe that the molecular structure in the crystal is severely affected by heating. Transformation of the molecular conformation appears in the ANT-H crystal with increasing temperature. More specifically, the anthracene moiety moves from nearly parallel to the C1-C2 bond to nearly perpendicular, causing the short-wavelength emission to disappear after heating. As for the aggregation-induced emission phenomenon, the structures and photophysical properties were investigated comparatively in both the isolated and crystal states; the results suggested that the energy dissipation in crystal surroundings was greatly reduced through hindering structure relaxation from the excited to the ground state. We expect that discussion of the thermochromic behavior will provide a new analysis perspective for the molecular design of o-carborane derivatives.

In contrast to the traditional view that the small organic molecules emit fluorescence, more and more experiments manifest their special luminescence types, such as the thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence. Why the similar organic molecules exhibit different luminescence types is focused on in this work on the basis of density functional theory/time-dependent density functional theory calculations on a series of small organic molecules with phenoxazine or carbazole as a donor and diphenyl-triazine as an acceptor. The deep analysis of the geometrical and electronic structures shows how the structure, especially for the donor–acceptor dihedral angle, determines the singlet–triplet energy gap and the property of excited states. The explorations on the electron–hole pairs of natural transition orbitals and the contribution of the key heteroatom (N) to different molecular orbitals reveal the distinct electron transition processes of excitation to singlet and trip...

Two sets of triphenylethylene/tetraphenylethylene (TPE) derivatives containing carbazole and thianthrene-9,9,10,10-tetraoxide were calculated with density functional theory/time-dependent density functional theory to explore the synergistic effect of the popular aggregation-induced emission (AIE) groups (TPE) and the thermally activated delayed fluorescence (TADF) functional groups (carbazole and thianthrene-tetraoxide). Through comparing the molecular geometric and electronic structures and the properties of the excited states, the significant influences of transition configurations and natures as well as spin-orbit coupling of the excited states on the intersystem crossing and reverse intersystem crossing (RISC) processes have been clarified. In addition, the aggregation process of molecules in water was simulated, and the transformation of molecular conformation and final proportion of each conformation were explored, whose influence on RISC rate, a determinant of TADF, was further analyzed. These investigations explain why only one set of the molecules present TADF when both sets of them have small singlet-triplet energy gap (ΔEST). The comparison of the reorganization energy between the molecule in isolated state and in solid state demonstrates that the AIE phenomenon is mainly due to the restricted low-frequency motion of the benzene ring in TPE group in aggregate surroundings, which indicates TPE plays its role as an AIE functional group. Therefore, it is necessary to comprehensively investigate the various indicators that affect the TADF performance when combine TPE with TADF groups for AIE-type TADF materials, and a small ΔEST value is not a sufficient condition.

The emission color is a crucial parameter to evaluate the performances of luminescent materials, thus seeking out the effective control method for illuminating color is an important issue. To further explore the relationship between emission colors and structure of aggregation-induced emission (AIE)-active phosphors, cationic Ir(III) complexes comprising the same ancillary ligand but different 1-phenyl-1H-pyrazole derivatives as cyclometalated ligands, were designed and synthesized. The experimental results show that all complexes exhibit outstanding AIE characters and their emission colors can be adjusted by simply changing their cyclometalated ligands. However, different from previous strategy, modifying main ligands only changes the energy level of lowest unoccupied molecular orbital rather than that of highest occupied molecular orbital. Combining this result and our reported approach, a strong blue-emitting AIE-active Ir(III) complex is successfully constructed. In addition, we observed that all complexes upon mechanical grinding exhibit remarkable and reversible mechanochromic luminescence behavior.

The case that aggregation brings a huge influence on the structure and fluorescent property of 5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2-carbaldehyde (P4TA) was detailedly investigated here by employing both quantum mechanics and molecular mechanics. Besides the isolated molecule, the aggregated molecule both in water and crystal state was studied by focusing on the comparison of photoelectronic properties, including the geometrical and electronic structures at ground and excited states, emission and internal conversation properties. For the aggregation state, the intermolecular interaction was employed to explain the difference in structure, emission color and intensity of different polymorphs. The noticeable contribution from low-frequency region, corresponding to the four phenyl rings twisting vibration, to the Huang-Rhys factor and reorganization energy, as well as the possible potential energy surface crossing between S0 and S1 states for isolated molecules was considered as the reason of its aggregation induced emission (AIE) performance. Importantly, the aggregation process in water simulated at the same time helps us to have a deeper understanding of the AIE behavior of P4TA, which also provides another perspective to explore the AIE phenomenon in theory.

Two reported Ir(iii) complexes 1a and 1b containing oxazoline and imidazoline in ancillary ligand, respectively, were investigated by DFT/TD-DFT method. In order to obtain full-color display materials, we designed a group of Ir(iii) complexes 2a-3d based on 1a, which exhibits higher quantum efficiency in phosphorescence, by introducing electron-donating/electron-withdrawing moieties to different positions of the ancillary ligand to adjust emission color. In addition to calculating the radiation rate and analyzing its determining factors, we also estimated nonradiative ability by evaluating the spin-orbit coupling matrix element between the ground state (S0) and the lowest triplet state (T1) as well as the reorganization energy from T1 to S0 to estimate quantum efficiency more accurately. In particular, an in-depth analysis on the contribution of each vibration mode to reorganization energy helped us to identify the effect of substituents on the nonradiative process. Besides, charge injection/transfer properties and energy relation of the states related to exciton quenching via the triplet metal-centered state were also examined, which provide an estimation on the OLED performance of our designed complexes. Overall, we expect 2b and 3c to be more efficient blue-emitting emitters than 1a and 3a and 3b to be efficient green and red emitters, respectively.

For a photochromic spiro-diarylethene derivative (Spiro-DAE), its photocyclization reaction and multicolour fluorescence modulation performances were investigated based on a series of density functionals calculations in this work. Some elementary properties such as geometric and electronic structures at ground and the first excited state (S1), absorption and emission spectra in respective open-ring and closed-ring structures were compared, which indicate the cyclization reaction may occur at S1. It is further verified by the lower activation barrier (3.4 kcal/mol) of the transition state at S1 potential energy surface. The photochromic reaction rates for both cyclization and cycloreversion processes were quantitatively estimated, which suggests the much smaller radiative and internal conversion rates contribute to the higher photocyclization rate and thus efficiency. Furthermore, we suspect that the dihydroacridine fragment may make important contribution to the cyclization reaction occurring at S1 considering its contribution to electronic structure.

Three new cationic Ir(III) complexes with donor-acceptor (D-A) type ligands, in which a 2-(1H-tetrazol-5-yl)pyridine unit as an electron acceptor and carbazole-based functional moiety as an electron donor, namely Ir-TA-CzDBr, Ir-TA-CzBr and Ir-TA-Cz, are synthesized and characterized. All complexes exhibit obvious aggregation-induced emission (AIE) activities with weak emission in diluted solution but bright light in the aggregation state. The present AIE mechanism was also elucidated by performing the comprehensive theoretical calculation. Moreover, the Ir(III) complexes show excellent reversible mechanochromic luminescent (MCL) behaviors and the emission colors in powder can be switched by grinding and fuming. The reversible, naked-eye visible and efficient mechanochromism of designed Ir(III) complexes paves the way toward the potential application in the field of reversible writing/erasing paper.

An interesting luminescent mechanochromic 9-anthryl gold(I) isocyanide complex has been reported. Its polymorphs perform different luminescent properties and their luminescence spectra are both bathochromically shifted upon grinding. This curious phenomenon arouses our interest to disclose how intermolecular weak interactions tune the luminescent properties from a theoretical perspective. The QM/MM approach was employed through all the calculations to consider the sensitive dependence of geometric and electronic structures on intermolecular interactions. The results show that the monomer is enough to describe the fluorescence of α phase crystal and its almost perpendicular dihedral angle between anthracene unit and phenyl ring leads to the forbidden emission from the first excited state which breaks Kasha’s rule. While the larger spin orbital coupling between excited singlet and triplet states of dimer makes key contribution to the phosphorescence emission of β phase crystal which is related with its near planar structure and stronger intermolecular interactions. Moreover, multiple intermolecular interactions including strong aurophilic interaction were found to be responsible for the red-shifted luminescence of the γ phase crystal. Therefore, the distinct molecular structures and intermolecular interactions codetermine the different photophysical processes in these crystal phases.

To comprehensively understand the structure-property correlations is of fundamental importance for the development of desirable piezochromic luminescence (PCL) materials. Keeping this in mind, two new cationic dinuclear metal Ir(III) complexes containing same acylhydrazone ancillary ligands and easily tunable cyclometalated ligands have been rationally designed and synthesized. Both luminophores show excellent aggregation-induced emission (AIE) behaviours and high-contrast PCL properties. The powder X-ray diffraction (PXRD) studies suggest that the emission changes of complexes before and after grinding derive from effective transformation between crystalline and amorphous phase. Fourier Transform Infrared (FT-IR) spectra further indicate that PCL effect of complexes could be ascribed to the disorder of hydrogen bonds between N–H and C O groups. Importantly, the principle of introducing hydrogen-bonding sites into Ir(III) complexes provides a feasible method for designing smart PCL materials. Based on the fascinating PCL properties of complexes 1 and 2, the efficient proof-of-concept data encryption and re-writable devices are successfully obtained, respectively.

The interface characteristic is a crucial factor determining the power conversion efficiency of organic solar cells (OSCs). In this work, our aim is to conduct a comparative study on the interface characteristics between the very famous non-fullerene acceptor, ITIC, and a fullerene acceptor, PC71BM by combining molecular dynamics simulations with density functional theory. Based on some typical interface models of the acceptor ITIC or PC71BM and the donor PBDB-T selected from MD simulation, besides the evaluation of charge separation/recombination rates, the relative positions of Frenkel exciton (FE) states and the charge transfer states along with their oscillator strengths are also employed to estimate the charge separation abilities. The results show that, when compared with those for the PBDB-T/PC71BM interface, the CT states are more easily formed for the PBDB-T/ITIC interface by either the electron transfer from the FE state or direct excitation, indicating the better charge separation ability of the former. Moreover, the estimation of the charge separation efficiency manifests that although these two types of interfaces have similar charge recombination rates, the PBDB-T/ITIC interface possesses the larger charge separation rates than those of the PBDB-T/PC71BM interface. Therefore, the better match between PBDB-T and ITIC together with a larger charge separation efficiency at the interface are considered to be the reasons for the prominent performance of ITIC in OSCs.