Limited by the energy gap law, purely organic materials with efficient near-infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host-guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital-π bridge-nonbonding orbital (n-π-n) molecular design strategy, we develop a series of heavy atom-free phosphors. Systematic modification of the π-conjugated cores enables the construction of a library with tunable near-infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4-bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2 %. Notably, by eliminating the interference from host phosphorescence, the exciton transition process in hybrid materials can be visualized under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet-triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in-depth insights into constructing novel near-infrared phosphors and exploring emission mechanisms of host-guest materials.
Open AccessCCS ChemistryCOMMUNICATION5 Aug 2022Metallophilicity-Induced Clusterization: Single-Component White-Light Clusteroluminescence with Stimulus Response Xueqian Zhao†, Parvej Alam†, Jianyu Zhang, Shiyun Lin, Qian Peng, Jun Zhang, Guodong Liang, Sijie Chen, Jing Zhang, Herman H. Y. Sung, Jacky W. Y. Lam, Ian D. Williams, Xinggui Gu, Zheng Zhao and Ben Zhong Tang Xueqian Zhao† Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 †X. Zhao and P. Alam contributed equally to this work.Google Scholar More articles by this author , Parvej Alam† Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 †X. Zhao and P. Alam contributed equally to this work.Google Scholar More articles by this author , Jianyu Zhang Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author , Shiyun Lin MOE Key Laboratory of Organic OptoElectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Qian Peng CAS Key Laboratory of Organic Solids, Institute of Chemistry, Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Jun Zhang School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601 Google Scholar More articles by this author , Guodong Liang PCFM and GDHPPC Labs, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275 Google Scholar More articles by this author , Sijie Chen Ming Wai Lau Centre for Reparative Medicine, Karolinska Institutet, Hong Kong 999077 Google Scholar More articles by this author , Jing Zhang Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author , Herman H. Y. Sung Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author , Jacky W. Y. Lam Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author , Ian D. Williams Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Google Scholar More articles by this author , Xinggui Gu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Beijing Advanced Innovation Center for Soft Matter Science and Engineering, College of Materials Science and Engineering, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029 Google Scholar More articles by this author , Zheng Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Shenzhen Institute of Molecular Aggregate Science and Engineering, School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 Google Scholar More articles by this author and Ben Zhong Tang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077 Shenzhen Institute of Molecular Aggregate Science and Engineering, School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 Center for Aggregation-Induced Emission, SCUT-HKUST Joint Research Institute, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 AIE Institute, Guangzhou Development District, Huangpu, Guangzhou 510530 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101392 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Materials showing metallophilic interactions continue to attract considerable theoretical and experimental attention largely because of their unusual and unanticipated photophysical behavior as well as their unique stimuli-responsive behavior in an aggregate or solid state. Metallophilic interactions are mostly found between metals with either identical (d10–d10) or different (s2–d8, d8–d10) configurations. Among various metallophilic interactions, aurophilic interactions (Au⋯Au) are well-known and widely reported. In this study, a new phosphorescent gold(I) complex, [(CF3Ph)3PAuC≡CPh] (TPPGPA) was reported. The crystal structure of TPPGPA demonstrated the formation of trimers caused by intermolecular aurophilic interactions. These trimeric crystals showed an efficient and nearly pure white-light emission with Commission Internationale de L’Eclairage 1931 chromaticity coordinates of (0.33, 0.34) at ambient conditions. Moreover, TPPGPA displayed fascinating mechanochromic and thermochromic luminescence properties in the crystalline state. The application of TPPGPA in temperature-responsive white-light illumination was also successfully demonstrated. The molecular packing and temperature-dependent modulation of the aurophilic interactions were subtly taken as a functional relationship of the experimental correlation with emission wavelength and intensity. It is anticipated that the present results will provide new insights into aurophilic interactions and provide a molecular design strategy for the construction of intelligent and stimuli-responsive gold(I) complex-based luminescent materials. Download figure Download PowerPoint Introduction Interactions between atoms and molecules contribute to the cohesion in chemical and biological systems and play a fundamentally important role in molecular assembly and recognition processes.1 For example, molecular recognition in biological systems is widely observed between receptor–ligand, DNA–protein, antigen–antibody, sugar–lectin, RNA–ribosome, and so on.2 Thus, the study of non-covalent interactions is of profound significance. Unlike other extensively studied non-covalent interactions in nonmetallic systems such as hydrogen bonding,3 dipole–dipole interactions,4 π⋯π interactions,5 or hydrophobic effects,6 metallophilic interactions are generally induced by orbital hybridization, dispersion, or relativistic effects in metal-containing systems.7 These metallophilic interactions are suggested to be critical for structural assembly,8,9 catalysis,10 luminescence,11–13 and sensing applications.14 However, the role of metal atoms in metallophilic interactions and the nature of metallophilic interactions are poorly understood as the overall intermolecular interaction is not equivalent to the metallophilic interaction.15 In the last decade, research on metallophilic interactions, especially in metal clusters16,17 and complexes,18,19 has attracted substantial interest because of the precise alteration of these metallophilic interactions. Therefore, the study on metallophilic interactions is a very attractive topic to comprehensively understand the structure–activity relationship. On the other hand, in some cases, cluster formation in the aggregate state brings new properties that are not found in isolated molecules. For example, many non-conjugated organic systems such as proteins, enzymes, starch, and cellulose are not emissive in their solution state but become emissive after cluster formation. Recently, the turn-on emission of these systems has been attributed to a unique photophysical phenomenon termed clusteroluminescence.20 Similarly, many metal clusters and complexes also exhibit no emission in solution but emit strongly in the aggregate or solid state.19,21 The turn-on emission of metallophilic complexes in aggregate or the solid state may contribute to cluster formation in which metallophilic interactions play a crucial role. Among all metallophilic interactions, aurophilicity exists widely in the field of gold(I) complexes as gold(I) centers have a [5d10] closed-shell electronic configuration to introduce only weak van der Waals forces.22–24 However, aurophilic interactions (7–12 kcal/mol) endow gold with force comparable or even stronger than hydrogen bonds and can be readily regulated even in the aggregate state to tune the molecular packing and photophysical properties.25,26 For example, many interesting luminescence phenomena triggered by aurophilicity were discovered in the aggregate state, such as the transition from fluorescence to phosphorescence,27 the conversion of dark to bright emission,28 or the extension of emission color from the visible to the infrared region,29 and so on.30,31 Therefore, it is foreseeable that intelligent metallophilic-induced clusterization (MIC) molecules with specific functionalities will find wide applications in various research fields and play an increasingly important role in anti-counterfeiting,32 stimuli-responsive materials,33,34 and high-tech devices.35–38 Correspondingly, the study on the photophysical mechanisms of MIC molecules will also be significant and instructive to guide the future development of more efficient luminogens.22,24,39 However, very few systematic studies on the relationship between aurophilicity and photophysical properties have been conducted so far, where specifically temperature-dependent crystallographic evidence for aurophilicity-regulated emissions in gold (I) complexes remains largely unexplored. Precise control on the molecular structure and stacking will be one of the vital approaches to access the issue, which heavily relies on an ideal prototype molecule that can achieve precise regulation along with a keen insight into the matters of aurophilicity.40–44 In addition, several reports on luminescent gold(I) complexes have been published so far, covering the entire visible region; however, reports on white-emitting gold(I) complexes are scarce.30,45,46 Herein, a new phosphorescent trinuclear alkynylgold(I) complex, [(CF3Ph)3PAuC≡CPh] (TPPGPA), was synthesized by a one-pot synthetic protocol.47 This simple and easily synthesized complex showed a pure white-light emission (0.33, 0.34) along with interesting aggregation-induced emission (AIE)48–50 characteristics. In addition, TPPGPA exhibited both mechanochromic and thermochromic luminescence behavior due to the compact packing of conglobate trimers triggered by robust aurophilic interactions in the crystalline state. Molecular motions and aurophilic interactions co-governed the thermal equilibrium of triplet charge-transfer nature between the monomer and cluster which explained the distinct dual phosphorescence nature of TPPGPA at room temperature (RT). Because of these distinctive cluster conformations and photophysical properties, we set out to determine the emission changes and investigate a possible correlation with aurophilic interactions. Finally, a fascinating application of TPPGPA as a white-light emissive material in a polymer matrix was studied. Its application in temperature-responsive white-light illumination was further demonstrated by coating a blue-light lamp. Results and Discussion TPPGPA was facilely synthesized by reacting equivalent amounts of phenylacetylene and chloro[tris(4-trifluoromethylphenyl)phosphine]gold(I) complex in a high yield of 75%.51,52 It was characterized by standard spectroscopic techniques such as 1H, 13C, 19F, and 31P NMR spectroscopies ( Supporting Information Figures S1–S4), high-resolution mass spectrometry, and elemental analysis. To obtain precise structural information, X-ray diffraction (XRD) analysis of single crystals of TPPGPA was also performed. The optical properties of TPPGPA were first investigated in degassed dichloromethane (DCM) solution. The UV–vis absorption spectra of TPPGPA exhibited a broad band spanning from 250 to 300 nm, which was tentatively assigned to a mixture of spin-allowed ligand-to-metal charge-transfer (1LMCT) and ligand-centered (1LC) π–π* transition ( Supporting Information Figure S5). Such assignment was in good accordance with the interpretation from time-dependent density functional theory (TDDFT). As shown in Supporting Information Table S1, DFT results indicated that the maximum absorption located at 288 nm is the S0 → S4 transition, which was mainly attributed to the 1LC and 1LMCT states with an oscillator strength of 0.4419. The photoluminescence (PL) spectrum of the complex was recorded at RT (293 K) and frozen state (77 K) at concentrations from 10−5 to 10−3 M. TPPGPA showed a concentration-independent weak emission at 293 K. However, at frozen conditions, three narrow concentration-dependent peaks at 423, 443, and 464 nm were observed in Figures 1a and 1b. The three peaks spread from 400 to 500 nm were assigned to the vibrational emissions of the TPPGPA monomer. According to the mechanism of restriction of intramolecular motion (RIM),53 the non-emissive nature of TPPGPA in DCM solution was attributed to molecular motions, which allowed the excited state to decay non-radiatively. The PL of TPPGPA was further studied in dimethylformamide (DMF)/water mixtures to evaluate the aggregation effect on the light emission process. As shown in Figures 1c and 1d, increasing the water fraction (fw) in DMF/water mixtures from 0% to 40% exerted no visible change in the PL intensity. At fw ≥ 50%, the PL became stronger, and a new broad peak appeared at ∼490 nm, signaling the formation of aggregates. These aggregates could be revealed by dynamic light scattering (DLS) measurements with an average hydrodynamic diameter of 210 nm in 80% water fraction as shown in Supporting Information Figure S6. The maximum PL intensity was observed at fw = 80%, being ∼20 times higher than that in pure DMF. The newly appeared broad emission at ∼490 nm could be assigned as a monomeric emission due to the formation of aggregates. However, the PL quantum yield (QY) was still low (QY < 1%), suggesting the formation of aggregates with no Au⋯Au interactions. Figure 1 | (a and b) PL spectra of TPPGPA in DCM solution with different concentrations at 293 and 77 K. The inset in (a) shows the structure of TPPGPA. (c) PL spectra of TPPGPA in DMF/water mixtures with different water fractions (fw). (d) A plot of PL maximum and relative PL intensity (αAIE = I/I0) versus the composition of the DMF/water mixtures of TPPGPA, where I0 is the PL intensity at fw = 0%, concentration = 10 μM, λex = 365 nm. Inset: photographs of TPPGPA in DMF/water mixtures with 0% or 80% water content were taken in the presence of 365 nm UV irradiation from a hand-held UV lamp. Download figure Download PowerPoint Interestingly, the crystalline TPPGPA showed bright and efficient white-light emission at ambient conditions including emission maxima bands centered at 475 and 578 nm with a relatively high QY of 15.5%. The associated Commission Internationale de L’Eclairage (CIE) chromaticity coordinates were found to be (0.33, 0.34) which is quite close to (0.33, 0.33) of pure white color defined by CIE in 1931 and demonstrates an interesting single component white-light emission in the crystalline state (Figures 2a and 2b).54 To obtain insights into the white-light emission mechanism, we prepared and analyzed the single-crystal of TPPGPA at RT. As shown in Figure 2c and Supporting Information Figure S7, the unit cell of single crystals exhibited unusual molecular packing motifs with a triclinic crystal system. In the unit cell, unusual trimeric structures displayed relatively shorter Au⋯Au interactions of 2.95 and 3.15 Å. These values were found to be shorter than the sum of two Van der Waals’ radii of the gold atoms (3.32 Å), indicating strong aurophilic interactions among gold atoms.55 Additionally, the trimer experienced a staggered conformation with respect to the tris(4-(trifluoromethyl)phenyl)phosphine group which can minimize the molecular hindrance and free energy. Thus, the driving force for such an intriguing assembly of trimers in the crystal structure of TPPGPA could be attributed to the intrinsic and strong intermolecular aurophilic interactions. To reveal the relationship between PL and cluster arrangements in the crystal packing, mechanical grinding of the TPPGPA crystals was performed and the PL change was studied. It was anticipated that the emission of TPPGPA crystals could be precisely tuned by altering the Au–Au interactions or other non-covalent interactions. As shown in Figure 2d, the white-light emissive (λmax = 475 and 578 nm) crystals were converted into greenish-yellow emissive (λmax = 515 nm) powder after grinding. Powder XRD (PXRD) analysis suggested that the change in molecular arrangements, that is, from ordered to disordered or crystalline-to-amorphous, was responsible for the intriguing mechanochromic luminescence phenomenon as shown in Supporting Information Figure S8a. The PL spectra depicted in Supporting Information Figure S8b presented the detailed emission changes. The short-wavelength band (λmax = 475 nm) was red-shifted by 40 nm (λmax = 515 nm), while the long-wavelength one at 578 nm disappeared after grinding. In addition, the QY value decreased from 15.5% to 1%. Presumably, the white-light emission was associated with the formation of the only gold trimer, which could be disassembled easily and increase the possibility of having more Au⋯Au interactions in the presence of external stimuli. The red-shifted emission of TPPGPA crystals suggests that the origin of new emission could have more contributions from Au⋯Au interactions. Figure 2 | (a) PL spectrum of crystalline TPPGPA. Inset: PL image taken at λex = 360 nm. (b) CIE 1931 coordinates of crystalline TPPGPA. (c) The molecular structure of TPPGPA with 50% probability ellipsoids. Hydrogen atoms were omitted for clarity. (d) PXRD patterns of TPPGPA in crystalline and amorphous states, and an image of the solid sample of TPPGPA taken under UV irradiation before and after mechanical grinding. (e) Electron cloud distributions of TPPGPA monomer (gas state) and trimer (solid-state) in the ground state at the TD-DFT B3LYP/(6-31G**+LANL2DZ) level and the schematic illustration of emission from triplet state of the crystalline TPPGPA. Download figure Download PowerPoint The emission change of TPPGPA crystal by anisotropic pressure ignited our interest to further examine its piezochromic effect under high isotropic pressure. Diamond anvil cell (DAC) equipment was taken to perform the high-pressure experiments, which could generate hydrostatic pressure with dozens of gigapascals (GPa) at RT. As shown in Supporting Information Figure S9, the short-wavelength band thoroughly showed no shift in the wavelength while the long-wavelength band experienced a red-shift within a pressure of about 2.18 GPa. The variable luminescent chromism could be modeled by gradually varying the intra-/inter-molecular distance between gold–gold atoms at different pressures. It is noticeable that the long-wavelength bands are more easily affected by applied pressure, which indicates the formation of more intra-/inter-molecular Au⋯Au interactions and results in red-shifted emission in the TPPGPA crystals. To further determine the origin of the dual emission of TPPGPA crystals, lifetime measurements at both the emission maxima, λmax = 488 and 580 nm, were carried out at different temperatures ranging from 77 to 297 K. Independent of temperature, a monoexponentially emission decay with a microsecond lifetime was obtained at both wavelengths suggesting their phosphorescence nature ( Supporting Information Figure S10). When the temperature was decreased from 297 to 77 K, the average lifetime at both the emission maxima, λmax = 488 and 580 nm, was gradually increased from 6.9 to 9.5 μs and 10.8 to 17.6 μs, respectively. The increasing lifetime values reveal the decreasing nonradiative rate which usually occurs at lower temperatures. The temperature-dependent lifetime measurement also ruled out the probability of thermally activated delayed fluorescence in the TPPGPA crystals. To understand the nature of the emitting state, excitation spectra of TPPGPA crystals were monitoring at both the emission maxima 488 and 580 nm shown in Supporting Information Figure S11. The excitation maxima at λmax = ∼360 nm indicated that two self-governed emission bands came from two closely spaced excited states. Hence, the lowest emitting state of TPPGPA crystals may ascribe to a mixture of two closely spaced electronic states, that is, monomeric and cluster excited states. To gain further insights into the emission nature and effect of intermolecular interactions, DFT and TDDFT calculations based on B3LYP/(6-31G**+LANL2DZ) were conducted for monomer and trimer clusters. The frontier molecular orbitals of the single complex in the gas state and trimer picked from the crystal structure are depicted in Figure 2e. In the case of monomer, the highest occupied molecular orbital (HOMO) is mainly composed of the delocalized orbitals on the phenylethynyl group with little contributions on the gold atom; however, the lowest unoccupied molecular orbital (LUMO) was mainly located on the tris[para-(trifluoromethyl)phenyl]phosphine group (Figure 2e and Supporting Information Figure S12). In the case of the trimer, the HOMO is mainly spread over the phenylethynyl group along with minor contributions on the two neighboring Au(I) centers. The LUMO showed the electron densities were mainly located on the tris[para-(trifluoromethyl)phenyl]phosphine group. Considering the TD-DFT calculations, the lowest-energy transitions in monomeric TPPGPA could be described as predominantly a LMCT with small contributions of ligand-to-ligand charge-transfer (LLCT) transitions ( Supporting Information Figure S14 and Table S2). However, the ligand-to-metal–metal charge-transfer (LMMCT) could be proposed as the lowest-energy transition in trimeric TPPGPA crystals. Based on the above discussion, a rational mechanism was proposed and shown in Figure 2e. After excitation and relaxation to the S1 state, fast intersystem crossing occurred to the two closely spaced triplet states, T1 and T1′. The T1 state was attributed to the metal-perturbed 3ππ* intracomplex charge-transfer (3LLCT) character. Meanwhile, the metallophilicity-induced clusterization of the trimer endowed a low-lying triplet energy state (T′) due to the intercomplex CT behavior that could be tentatively assigned to 3LMMCT in nature.56 Hence, these results suggest that the dual phosphorescence of TPPGPA originates from closely lying mixed triplet states (3LMMCT)/(3LMCT). This mixed state was found to be very sensitive to aurophilic interactions, which offered an opportunity to achieve stimuli-responsive behavior by regulating the molecular packing mode and Au⋯Au interactions. Furthermore, the energy gap between HOMO and LUMO of monomer and trimer were calculated and found to be 3.77 and 3.31 eV, respectively ( Supporting Information Figure S13). The calculated energy difference (ΔE = 0.46 eV) between monomer and trimer further suggests that the emitting states of these two species are closely spaced. The current calculations indicate that the LUMO energy levels are −1.73 and −1.98 eV for monomer and trimer, respectively. In contrast, the energy levels of HOMO are −5.50 and −5.29 eV, respectively. Destabilization of the HOMO orbital and the stabilization of the LUMO orbital are typically observed in gold complexes with the formation of aurophilic interactions ( Supporting Information Figure S14).25,57 To precisely regulate the aurophilic interactions at the molecular level, we performed temperature-variable single-crystal XRD measurements at temperatures from 100 to 297 K. The crystallographic details and refined values are given in Figure 3a and Supporting Information Tables S3–S11, respectively. Results showed that there existed no structural phase transitions. However, the average coefficients of thermal expansion of a, b, and c axes were large and were equal to 53.3 × 10−6 K−1, 121.5 × 10−6 K−1, and 72.1 × 10−6 K−1, respectively, along with the coefficient of cubical expansion of 236.3 × 10−6 K−1. The details are shown in Supporting Information Figure S15. It is worth mentioning that the relative rate (α) at which a material expands at an elevated temperature usually falls within the range of 0 × 10−6 K−1 to 20 × 10−6 K−1. Herein, high positive thermal expansion was observed in the crystal lattice of this system, which was two orders of magnitude higher than those seen in other crystalline materials.58,59 Besides, the thermal expansion coefficient of the average Au⋯Au distance was maintained at the same magnitude (134.6 × 10−6 K−1). Thus, we speculate that the thermal expansion of the lattice parameters is induced by the regulation of aurophilic interactions in the crystal structure. These results showed that the thermal stimulus could induce more subtle but equally intriguing systematic responses of aurophilic interactions in the crystal structure. Figure 3 | (a) Plots of unit cell volume (black line) and Au–Au distances (red line) of TPPGPA crystals at different temperatures with values of thermal expansion coefficient (α) of volume and Au–Au distance (average). (b) PL spectra of TPPGPA during the change of cooling process with an interval time of 10 min and (c) the PL image during a cycle of cooling–heating process. (d) CIE 1931 coordinates at different temperatures (e) relative PL intensity during cooling and heating processes, and (f) position during the cooling process of the short- and long-wavelength bands of crystalline TPPGPA. I0,S = intensity of the short-wavelength band at 77 K, I0,L = intensity of the long-wavelength band at 77 K, λS = maximum wavelength of the short-wavelength band, λL = maximum wavelength of the long-wavelength band. Download figure Download PowerPoint To verify the exact connection between PL and aurophilic interactions, temperature-variable PL analysis of crystalline TPPGPA at a temperature range of 77-297 K was systematically conducted. As shown in Figure 3b, the intensity of both short- and long-wavelength bands increased with decreasing temperature. The long-wavelength band red-shifted from 585 (297 K) to 603 nm (77 K) while the short-wavelength band remained unchanged during the whole process. The redshift in the PL spectrum could be attributed to stronger Au⋯Au interactions at 77 K than at RT. The Stokes shift between the maxima of excitation (λex = 365 nm) and emission (λem = 585 nm) bands was calculated to be 10,303.24 cm−1 at 297 K. The large Stokes shift suggests the possibility of substantial structural distortion in the excited states, which often indicates stronger metal–metal interactions. The calculated Stokes shift between the maxima of excitation (λex = 365 nm) and emission (λem = 603 nm) was found to be 10,813.51 cm−1, which further supports the stronger Au⋯Au interactions at 77 K. It should be mentioned that the above temperature-dependent emission color conversion is reversible as shown in Figure 3c and Sup
A revolutionary transformation in biomedical imaging is unfolding with the advent of aggregation-induced emission luminogens (AIEgens). These cutting-edge molecules not only overcome the limitations of traditional fluorescent probes but also improve the boundaries of high-contrast imaging. Unlike conventional fluorophores suffering from aggregation-caused quenching, AIEgens exhibit enhanced luminescence when aggregated, enabling superior imaging performance. This review delves into the molecular mechanisms of aggregation-induced emission (AIE), demonstrating how strategic molecular design unlocks exceptional luminescence and superior imaging contrast, which is crucial for distinguishing healthy and diseased tissues. This review also highlights key applications of AIEgens, such as time-resolved imaging, second near-infrared window (NIR-II), and the advancement of AIEgens in sensitivity to physical and biochemical cue-responsive imaging. The development of AIE technology promises to transform healthcare from early disease detection to targeted therapies, potentially reshaping personalized medicine. This paradigm shift in biophotonics offers efficient tools to decode the complexities of biological systems at the molecular level, bringing us closer to a future where the invisible becomes visible and the incurable becomes treatable.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
<p>The single-component white-light-emitting materials play an essential role in the next-generation solid-state lighting technology. Herein, linear gold(I) complex TPPGPA with conglobate trimer configuration trigged by aurophilic interactions in crystalline state was prepared to emit dual phosphorescent white-light emission, which also exhibited multi-stimuli responsive luminescent properties including thermochromism and mechanochromism. Specifically, the molecular packing mode and aurophilic interactions regulation were subtly taken as a functional relationship of the experimental correlation with emission. The results showed that the regulated aurophilic interactions and restriction of molecular motion were determined to be the precipitating factor and as a function of the wavelength and intensity, which is significant for the design guide about intelligent stimuli-responsive white-light emissive luminescent materials. Furthermore, their application in temperature-responsive white-light illumination was successfully demonstrated. </p>
Organic light emitters have shown potential for the application in a variety of optoelectronic devices. However, the quenching of luminescence in the solid state severely restricts the lighting applications. The rational supramolecular design strategy of the luminescent organic emitters in solid state remains challenging. Here, supramolecular assemblies of a series of iridium(III) complexes are designed on a water surface to realize solid state tunable luminescence in the visible region. Large area two-dimensional supramolecular platelets and disks, yet with monomolecular thickness, are formed depending on the cyclometalating ligands. Controlled supramolecular aggregates of iridium(III) complexes enhances the aggregation-induced-emission phenomenon by restricting monodentate triphenylphosphine and cyclometalating ligands at the water surface. As a consequence, a large enhancement of luminescence comparable to the solid powder is obtained from the supramolecular assemblies of iridium(III) complexes. Supramolecular assemblies display fine-tuning of the luminescence color from green to red in solid state depending on the cyclometalating ligand. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) reveal phosphorescence origin of the luminescence. These findings emphasize the importance of controlled organization of organic emitters to explore optimal luminescence properties for efficient lighting applications.
A greenish-blue emissive bis-cyclometalated iridium(III) complex with octahedral geometry was synthesized in a convenient route where a bulky substituted ligand, N1-tritylethane-1,2-diamine ligand (trityl-based rotating unit) (L1), was coordinated to iridium(III) in nonchelating mode, [Ir(F2ppy)2(L1)(Cl)], [F2ppy = 2-(2′,4′-difluoro)phenylpyridine; L1 = N1-tritylethane-1,2-diamine], 1. The purpose of introducing a rotor in 1 was anticipated to initiate aggregation-induced emission (AIE) activity in it. The presence of a secondary amine in L1 has attributed to 1 the ability to sense acids. The mechanism of this change in 1 under acidic medium was explored. A bright yellow emissive complex was formed on exposing 1 to hydroxide ion, which was isolated, characterized, and identified as a new aggregation-induced enhanced emission (AIEE) active complex. The detection limit of hydroxide ion was determined to 126 nM. Ground- and excited-state properties of 1 were investigated using DFT- and TD-DFT-based calculations, and several important aspects of the experimental facts were validated.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
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