•The λ of syn-(decentralized) system is smaller than that of the corresponding anti-(centralized) system.•The Δd of benzene plays an important role in λ both for syn- and anti-systems.•The more vertical the HOMO distribution is, larger the λ.Reorganization energy (λ) is one of the most important parameters regarding the charge transfer in organic semiconductors. In this context, the λ of a series of five-ring-fused benzothiophene derivatives are investigated through density functional theory (DFT). The change of the bond length (Δd), the normal mode (NM) analysis and the highest occupied molecular orbital (HOMO) are employed to shed light on the intricate interplay between reorganization energy and molecular structure (syn-, anti-isomers and the position of thiophene rings). The results show that within our investigated five-ring-fused benzothiophene derivatives, the λ of anti-isomer is larger than its corresponding syn-isomer, and the λ tends to increase as the arrangement of thiophene rings tending to concentrating within molecular backbone. From the syn- to the anti-isomer, the improvement of λ can be mostly ascribed to the benzene ring according to the calculation of Δd. The combined investigation of NM analysis and the distribution of the HOMO shows that the strongly coupled modes are mainly perpendicular to the HOMO distribution. Therefore, the effect of distribution of the HOMO is marked on the hole reorganization energy of these five-ring-fused benzothiophene derivatives. That is, the more similar to anti-S5 the HOMO is, larger the λ; on the contrary, the more similar to PENT the HOMO is, smaller the λ.

In this study, we have designed and synthesized a series of multifunctional cationic iridium(III) complexes with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. The photophysical properties of each complex have been investigated in detail and also ascertained by comprehensive density functional theory calculations. Despite showing negligible influence on their emission spectra and excited-state characteristics in solution, altering the N-alkyl chain length can efficiently modify their photophysical properties in the solid state. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most interestingly, these iridium(III)-based luminophores with longer N-alkyl chains display significant emission colour changes and unique reversible features by mechanical grinding and solvent fuming. The powder X-ray diffraction (PXRD), 1H NMR and MALDI-TOF/TOF mass spectrometry data demonstrate that the phase transitions between crystalline and amorphous states are crucial to the present piezochromism. Moreover, with the merit of high quantum efficiency in aggregate states, the studied iridium(III) complex can serve as an efficient sensor for the sensitive and selective detection of the explosive, 2,4,6-trinitrophenol (TNP).

In this work, a series of luminescent cationic iridium(III) complexes containing 2-phenyl-1H-benzimidazole-type ligands modified with n-alkyl chains of various lengths have been successfully synthesized and characterized. Their photophysical and electrochemical properties have been investigated in detail. Differences in n-alkyl chain length has negligible affect on their respective complex's emission spectra or on their excited-state characteristics in solution, which is supported by density functional theory calculations and cyclic voltammetry. In the solid state, these complexes exhibit piezochromic luminescence (PCL) behaviour which is visible to the naked eye. Their emission colour can be reversibly and quickly switched by grinding–fuming or grinding–heating processes with high contrast. Moreover, the n-alkyl chain lengths can effectively control their PCL and thermodynamic properties, showing chain length dependent emission behaviours: longer alkyl chains were shown to produce more marked mechanochromism. A reproducible and reversible two-colour emission writing/erasing process was achieved by employing the iridium(III) materials as a medium. Powder X-ray diffractometry and differential scanning calorimetric studies suggest that the reversible transformation between crystalline and amorphous states upon application of external stimuli is responsible for the observed piezochromism.

The AIE mechanism for cationic Ir(III) complexes with triazole–pyridine ligands has been determined by the combination of an experimental and a computational study. Larger structural relaxation and weak emissive excited-state intraligand charge transfer (ILCT) characters are responsible for the non-emission in solution, whereas the non-radiative processes are efficiently restricted in the solid state, enhancing the emission.

•The cis configuration of cyanoacrylic acid is more stable than the trans one.•CN group has more positive effect on the process of electron injection.•The bidentate bridging configuration is the most stable mode.•The absorption spectra are red-shifted after binding to the semiconductor.The structural and electronic properties of four phenothiazine-based sensitizers (TMH–CNcis, TMH–CNtrans, TMH–COOH, and TMH–2COOH) have been examined by means of density functional theory (DFT) and time-dependent DFT calculations. And the periodic DFT method is used to investigate the adsorption of dyes on the TiO2 anatase (1 0 1) surface within the DMol3 code. The results promise that anchor dyes with strong withdrawing CN group can effectively injected electrons to the conduction band of semiconductor TiO2 surface implying that TMH–CNcis and TMH–CNtrans will show better performance among four dyes. Particularly, the adsorption energies and charge density difference maps are calculated to investigate the possible adsorption modes of the dye on the TiO2 surface. After establishing the preferred anchoring configurations, we performed a detailed analysis on the electronic structures of the dye-TiO2 complexes to explore the absorption spectra, charge distribution and the composition of the density of states (DOS), providing a quantitative description of the variation in electronic coupling induced by the different anchoring group. The results show that after binding to the semiconductor, the absorption spectra of four dyes-TiO2 complexes are all red-shifted significantly as compared to that of the isolated dyes, and the TMH–CNtrans–TiO2 will transfer more electrons during the photoexcitation, showing an obvious charge transfer characteristic.Graphical abstract

The bridging effect on the charge transport properties of cyclooctatetrathiophene and its derivatives (systems 1–4) was investigated at the level of density functional theory (DFT). Insights into their geometries, frontier molecular orbitals, reorganization energies, transfer integrals and band structures were provided in detail. Increasing charge mobilities for both holes and electrons were predicted in cyclooctatetrathiophene derivatives as the number of bridging sulfur atoms increased. The improved charge transport from system 1 to 4 can be interpreted from two contributions: (i) decreased reorganization energy with improved molecular planarity under the consideration of intermolecular interactions; and (ii) enhanced transfer integral derived from the π-stacking arrangement for 2 and 3, and multidimensional S⋯S interactions are found to contribute to charge transport in system 4 besides π⋯π interactions. The charge transport properties were also analyzed with a band-like model and the results were in agreement with those gained from the hopping model in the fact that the paths with large transfer integrals are all along the directions with large dispersions in the valence band or conduction band.

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.

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.

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.

We report the synthesis and characterization of two cationic iridium(III) complexes with dendritic carbazole ligands as ancillary ligands, namely, [Ir(ppy)2L3]PF6 (1) and [Ir(ppy)2L4]PF6 (2), where L3 and L4 represent 3,8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,10-phenanthroline and 3,8-bis(3′,6′-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9′-bi(9H-carbazol)-9-yl)-1,10-phenanthroline, respectively. Their photophysical properties have been investigated and compared. The results have shown that complex 2 is aggregation-induced phosphorescent emission (AIPE) active and exhibits the highest photoluminescent quantum yield (PLQY) of 16.2% in neat film among the reported cationic Ir(III) complexes with AIPE activity. In addition, it also enjoys redox reversibility, good film-forming ability, excellent thermal stability as well as off/on luminescence switching properties, revealing its potential application as a candidate for light-emitting electrochemical cells and organic vapor sensing. To explore applications in biology, 2 was used to image cells.

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.

Three cationic iridium complexes containing 4,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,10-phenanthroline (L1) and 4,7-bis(3′,6′-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9′-bi(9H-carbazol)-9-yl)-1,10-phenanthroline (L2) as the ancillary ligands, namely, [Ir(ppy)2(L1)]PF6 (1), [Ir(ppy)2(L2)]PF6 (2) and [Ir(oxd)2(L2)]PF6 (3) (ppy is 2-phenylpyridine, oxd is 2,5-diphenyl-1,3,4-oxadiazole), have been designed and prepared. With more intramolecular rotational units on the ancillary ligand (L2), 2 and 3 possess a unique aggregation-induced phosphorescent emission (AIPE) property. This phenomenon was unprecedentedly observed in the cationic iridium(III) complexes. In order to investigate the underlying mechanism of this AIPE behavior, their photophysical, temperature-dependent aggregation properties as well as theoretical calculations, were performed. The results suggest that restricted intramolecular rotation is responsible for the AIPE of cationic complexes. Moreover, photoluminescent quantum yields in the neat film, thermal stabilities and off/on luminescence switching of 2 were investigated, revealing its potential application as a candidate for LECs and organic vapor sensing.

Fused thiophenes have been an important class of materials due to their intriguing organic optoelectronic application. Here, comparative theoretical investigation on the fluorescence and charge transport properties of dithienothiophene compounds (1 and 2) and their dioxide derivatives (3 and 4) was carried out to shed light on the role of the thienyl-S,S-dioxide unit. The lower HOMO, LUMO energy levels, and red-shift spectra (absorption and emission) of 3 and 4 compared with 1 and 2 were attributed to the electron-withdrawing nature of the thienyl-S,S-dioxide unit. The phenomenon that fluorescence quantum yield of 4 was significantly increased through thienyl-S,S-dioxidation, compared with those of 1 and 2, was analyzed by the evaluations of the radiative decay rates and the radiationless decay rates in theory at the single molecule level and the simulation of absorption spectrum of dimer of 1. For 1, a much higher hole mobility (0.12 cm2/V.s) calculated by carrier hopping model than the experimental value ~10−4 cm2/V.s was also further elucidated by molecular dynamics simulation. Furthermore, a preliminary investigation of the transport property of 3 was performed by combining the molecular dynamics simulation with dispersion-corrected B3LYP functional to provide insight into the effect of thienyl-S,S-dioxidation on the charge transport.

Phosphole-based systems due to the unique electronic and optical properties have recently been paid much attention as optoelectronic materials. In this work, the relationship among the electronic structure, charge injection, and transport was investigated for five derivatives of dithieno[3,2-b:2′,3′-d]phosphole (systems 1–5). The structures of systems 1–5 in the ground (S0) and the lowest singlet excited (S1) states were optimized at the HF/6-31G* and CIS/6-31G* levels of theory, respectively. Based on these structures, electronic spectra were calculated by time-dependent density functional theory. The simulated emission peaks of five phosphole derivatives locating at the blue–green region (448–516 nm), are in good agreement with the experimental data. Compared with tris-(8-quinolinolate) aluminum (III) (Alq3), normally used as an excellent electron transporter, systems 1–5 show a significant improvement in electron affinity (EA) due to σ*–π* hyperconjugation, which can effectively promote ability of electron injection. The small differences between λh and λe for systems 1–5 (0.06–0.14 eV) facilitate charge transfer balance, which suggests systems 1–5 can act as potential ambipolar materials. Owing to good rigidity, low-lying LUMO levels, delocalized frontier molecular orbitals, and the small reorganization energies, the five derivatives of dithieno[3,2-b:2′,3′-d]phosphole are expected to be high-efficiency blue materials in single-layer OLEDs.

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.

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.

Three cationic iridium complexes containing 4,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,10-phenanthroline (L1) and 4,7-bis(3′,6′-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9′-bi(9H-carbazol)-9-yl)-1,10-phenanthroline (L2) as the ancillary ligands, namely, [Ir(ppy)2(L1)]PF6 (1), [Ir(ppy)2(L2)]PF6 (2) and [Ir(oxd)2(L2)]PF6 (3) (ppy is 2-phenylpyridine, oxd is 2,5-diphenyl-1,3,4-oxadiazole), have been designed and prepared. With more intramolecular rotational units on the ancillary ligand (L2), 2 and 3 possess a unique aggregation-induced phosphorescent emission (AIPE) property. This phenomenon was unprecedentedly observed in the cationic iridium(III) complexes. In order to investigate the underlying mechanism of this AIPE behavior, their photophysical, temperature-dependent aggregation properties as well as theoretical calculations, were performed. The results suggest that restricted intramolecular rotation is responsible for the AIPE of cationic complexes. Moreover, photoluminescent quantum yields in the neat film, thermal stabilities and off/on luminescence switching of 2 were investigated, revealing its potential application as a candidate for LECs and organic vapor sensing.

The AIE mechanism for cationic Ir(III) complexes with triazole–pyridine ligands has been determined by the combination of an experimental and a computational study. Larger structural relaxation and weak emissive excited-state intraligand charge transfer (ILCT) characters are responsible for the non-emission in solution, whereas the non-radiative processes are efficiently restricted in the solid state, enhancing the emission.

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.

In this work, a series of luminescent cationic iridium(III) complexes containing 2-phenyl-1H-benzimidazole-type ligands modified with n-alkyl chains of various lengths have been successfully synthesized and characterized. Their photophysical and electrochemical properties have been investigated in detail. Differences in n-alkyl chain length has negligible affect on their respective complex's emission spectra or on their excited-state characteristics in solution, which is supported by density functional theory calculations and cyclic voltammetry. In the solid state, these complexes exhibit piezochromic luminescence (PCL) behaviour which is visible to the naked eye. Their emission colour can be reversibly and quickly switched by grinding–fuming or grinding–heating processes with high contrast. Moreover, the n-alkyl chain lengths can effectively control their PCL and thermodynamic properties, showing chain length dependent emission behaviours: longer alkyl chains were shown to produce more marked mechanochromism. A reproducible and reversible two-colour emission writing/erasing process was achieved by employing the iridium(III) materials as a medium. Powder X-ray diffractometry and differential scanning calorimetric studies suggest that the reversible transformation between crystalline and amorphous states upon application of external stimuli is responsible for the observed piezochromism.

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.

In this study, we have designed and synthesized a series of multifunctional cationic iridium(III) complexes with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. The photophysical properties of each complex have been investigated in detail and also ascertained by comprehensive density functional theory calculations. Despite showing negligible influence on their emission spectra and excited-state characteristics in solution, altering the N-alkyl chain length can efficiently modify their photophysical properties in the solid state. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most interestingly, these iridium(III)-based luminophores with longer N-alkyl chains display significant emission colour changes and unique reversible features by mechanical grinding and solvent fuming. The powder X-ray diffraction (PXRD), 1H NMR and MALDI-TOF/TOF mass spectrometry data demonstrate that the phase transitions between crystalline and amorphous states are crucial to the present piezochromism. Moreover, with the merit of high quantum efficiency in aggregate states, the studied iridium(III) complex can serve as an efficient sensor for the sensitive and selective detection of the explosive, 2,4,6-trinitrophenol (TNP).

We report the synthesis and characterization of two cationic iridium(III) complexes with dendritic carbazole ligands as ancillary ligands, namely, [Ir(ppy)2L3]PF6 (1) and [Ir(ppy)2L4]PF6 (2), where L3 and L4 represent 3,8-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,10-phenanthroline and 3,8-bis(3′,6′-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9′-bi(9H-carbazol)-9-yl)-1,10-phenanthroline, respectively. Their photophysical properties have been investigated and compared. The results have shown that complex 2 is aggregation-induced phosphorescent emission (AIPE) active and exhibits the highest photoluminescent quantum yield (PLQY) of 16.2% in neat film among the reported cationic Ir(III) complexes with AIPE activity. In addition, it also enjoys redox reversibility, good film-forming ability, excellent thermal stability as well as off/on luminescence switching properties, revealing its potential application as a candidate for light-emitting electrochemical cells and organic vapor sensing. To explore applications in biology, 2 was used to image cells.