Aggregation-induced Phosphorescent Emission Research Articles

Mechano-responsive chiral gold(I) complexes with aggregation- and crystallization-induced circularly polarized luminescence

Chiral materials with circularly polarized luminescence (CPL) have attracted extensive attention due to their unique optical properties. However, it is still a challenge to obtain efficient CPL materials. In this work, noticeable CPL signals were obtained through simple crystallization in the classical aggregation-caused quenching (ACQ)-typed chloro-gold(I) complexes (R)-BINAP-AuCl and (S)-BINAP-AuCl. They exhibited typical crystallization-induced emission (CIE) and crystallization-induced CPL behaviors. Importantly, after rational modification by introducing carbazole units, the resulting carbazole-gold(I) complexes (R)-BINAP-AuCz and (S)-BINAP-AuCz showed remarkable aggregation-induced phosphorescent emission (AIPE) and unique aggregation-induced CPL characteristics due to the formation of highly ordered helical packing modes in the nano-aggregate state. More interestingly, the solids of chiral carbazole-gold(I) complexes (R)-BINAP-AuCz and (S)-BINAP-AuCz could sensitively respond to external stimuli and exhibited mechano-responsive CPL performance, which was tuned by the intermolecular C–H···π interactions. Thus, this work provided a new way for constructing high-performance and smart CPL materials.

Enhanced TNT vapor sensing through a PMMA-mediated AIPE-active monocyclometalated iridium(III) complex: a leap towards real-time monitoring.

Based on the explosive nature and harmful effects of nitro-based explosive materials on living beings and the environment, it is extremely important to develop luminescence-based probe molecules for their detection with excellent selectivity and sensitivity. Two AIPE (aggregation-induced phosphorescence emission)-active iridium(III) complexes (M1 and M2) were developed for the sensitive detection of TNT in both contact and non-contact modes. The aggregate solutions of both complexes (M1 and M2 in THF/H2O, 1/9 by volume) detected TNT at the pico-molar (pM) level. These complexes showed greatly enhanced emission intensity while embedded in a PMMA(polymethyl methacrylate) matrix film. The amplified quantum efficiency, improved phosphorescence lifetime, and enhanced porous network of M2-PMMA composite helps to improve the sesitivity of TNT vapor detection. Interestingly, the sensitivity of the detection of TNT by the M2 complex was significantly improved (5-fold) in a PMMA-incorporated complex (CP) with an observed limit of detection (LOD) of 12.8 ppb. From the BET analysis of CP, it was observed that the mesoporous network of CP has an average pore diameter of 8.52 nm and a surface area of 2.03 m2 g-1. The porous network of CP assists in trapping TNT vapor in a polymeric network containing an electron-rich probe (iridium(III) complex, M2), which helps to effectively trap TNT, thus enhancing electronic communication. As a result, significant emission quenching was observed.

Aggregation-induced phosphorescence emission-active heteroleptic Ir(III) complexes: Synthesis, photophysics, and latent fingerprint detection applications

Three heteroleptic Ir(III) complexes (Ir-TPA, Ir-CzPh and Ir-PhCz) with different electron-donating aryl groups on C^N ligands were synthesized. Their optical properties were systematically investigated by spectroscopic methods and theoretical calculations to elucidate the effects of introducing different electron-donating groups on their triplet excited state properties. All three complexes possess aggregation-induced phosphorescence emission (AIPE) behaviors. Critically, the relationship between the size of organic ligands and AIPE behaviors for these complexes has been characterized. Their significant AIPE properties predicted that they could be used as efficient latent fingerprint (LFP) developers. Rapid (within 10 s) incubation in Ir(III) complex solution produced high-resolution, naked-eye visible and mobile-photographable LFP images, which could provide fast, reproducible and identifiable analyses of level 3 details for LFPs accurately. In addition, complex Ir-PhCz shows the best imaging of the LFP due to its green light emission and high quantum yield (24.5 %) in solid state. This rapid and practical LFP visualization methodology based on AIPE developers could provide a reliable solution for LFPs detection and investigations in actual scenarios.

Fluorophenyl-modified AIPE-active cationic Pt(II) complexes for detecting picric acid in aqueous media

Two new fluorophenyl-modified cationic cyclometalated Pt(II) complexes have been synthesized. The effects of the fluorophenyl substituents on the properties of the Pt(II) complexes were investigated systematically in comparison with a phenyl-modified complex. The absolute quantum yields of the complexes are significantly increased due to the presence of fluorophenyl groups. All complexes exhibit evident aggregation-induced phosphorescent emission activities in a CH3CN/H2O system. Furthermore, they can effectively detect picric acid (PA) in CH3CN/H2O. Among them, the complex with a 2,4-difluorophenyl group exhibits the lowest limit of detection at 0.09 μM. These complexes exhibit excellent selectivity for detecting PA in complicated environments and work well in real water samples. The luminescence quenching of the complexes is caused by photo-induced electron transfer.

Efficient detection of picric acid with aggregation-induced phosphorescent emission neutral cyclometalated platinum(II) complexes in aqueous media

Three neutral cyclometalated platinum(II) complexes have been synthesized and fully characterized. All complexes show aggregation-induced phosphorescent emission properties in THF/H2O. These complexes can detect picric acid efficiently in aqueous media with limits of detection of 0.17, 0.03 and 0.15 μM, respectively. Moreover, the complexes exhibit high selectivity for picric acid in the presence of other competitive compounds and perform well in real water samples. The results show that photo-induced electron transfer is responsible for the luminescence quenching of the complexes.

An AIPE-active fluorinated cationic Pt(II) complex for efficient detection of picric acid in aqueous media

A novel cationic Pt(II) complex 2 with 2-(2,4-difluorophenyl)pyridine as the cyclometalating ligand and 1,10-phenanthroline as the auxiliary ligand has been synthesized and fully characterized. This complex exhibits much higher aggregation-induced phosphorescent emission activity than that of a non-fluorinated complex 1 in CH3CN/H2O. The complex 2 demonstrates efficient detection on picric acid (PA) in CH3CN/H2O, providing a high quenching constant (KSV = 2.3 × 104 L/mol) and a low limit of detection (LOD = 0.26 µmol/L). In addition, complex 2 shows high selectivity for detection of PA in real water samples. Density functional theory calculations and proton nuclear magnetic resonance spectra suggest that the detection mechanism is attributed to the photo-induced electron transfer.

Alkyl-substituted bipyridyl platinum (II) complexes bearing alkynyl-naphthalimide ligands: Synthesis, photophysical properties, and tunable aggregation-induced emission activity

Different alkyl chains were introduced into alkynyl-naphthalimide ligands and bipyridine ligands to synthesize five platinum complexes, their structures were characterized and the photophysical properties were systematically investigated. It was found that the introduction of alkyl chains had no obvious effect on the absorption and emission of these complexes in dilute solution, while influence aggregation-induced emission activity in aggregated state. In the mixed solvent system of tetrahydrofuran and ethylene glycol, the shorter alkyl chains on the bipyridine ligands strengthen the aggregation-induced phosphorescence emission (AIPE) properties of complexes, however, the introduction of different alkyl chains on the alkynyl-naphthalimide ligands makes no difference on their AIPE properties. Combining the single crystal results and Hirshfled surface analysis, the photophysical properties of these complexes in aggregated state were affected by the stacking modes, which is due to the different interactions. This research may broaden the scope for the design of multi-functional materials adopting different aggregation states.

Controlling the sensitivity and selectivity for the detection of nitro-based explosives by modulating the electronic substituents on the ligand of AIPE-active cyclometalated iridium(III) complexes.

Nitroaromatic compounds are extremely explosive materials that pose a national security risk and raise environmental concerns. The design and development of sensitive and selective compounds for explosive materials are highly desirable. 'Aggregation-Induced Emission' (AIE) active materials are best suited for sensing purposes because of their sensitivity, fast detection time, and easy operation. By rationally incorporating substituents on the cyclometalated (C^N) ligand, four different AIE active iridium(III) based monocyclometalated complexes with the general formula [Ir(PPh3)2(H)(Cl)(C^N)] were synthesized. The phenyl ring of the phenyl pyridine cyclometalated portion of an iridium(III) complex was substituted with the right substituents to adjust the FMO levels thus, leading to appropriate alignment of the energy levels. Each of the resulting complexes displayed a significant property known as 'Aggregation-Induced Phosphorescent Emission' (AIPE). The complexes were subjected to structural characterization, electrochemical analysis, and photophysical property studies. The synthesized complexes were employed for the detection of aromatic nitro explosive compounds such as trinitrophenol (TNP) and trinitrotoluene (TNT) in the aqueous phase with a high degree of sensitivity. The sensing capabilities of each complex were assessed for these nitro explosive compounds and compared to those of the unsubstituted iridium(III) complex (M). Notably, the best limits of detection for TNP and TNT have been achieved with iridium(III) complexes [M1 (489 pM) and M3 (3.6 nM)] within the literature reported until now. For detecting picric acid with M1, FRET was found to be the potential mechanism, and for TNT, PET was found to be the cause of emission quenching by M3. Furthermore, for low-cost detection, filter paper-based sensing was also found effective for each complex. Real-field sensing of PA in soil samples was also performed.

PtII(C^N)(N-donor ligand)Cl-type complexes showing highly sensitive aggregation-induced phosphorescence emission (AIPE) behavior fulfilled by long-size ligands and a distorted molecular configuration.

A series of four-coordinated PtII(C^N)(N-donor ligand)Cl-type complexes have been synthesized through a combination of long-size C^N-type and N-donor ligands. In addition, by varying the coordinating site in the N-donor ligand, a distorted molecular configuration has been constructed in these complexes. Their photophysical features, aggregation-induced phosphorescence emission (AIPE) behaviors, electrochemical properties and electroluminescence (EL) performance have been investigated in detail. It has been found that their AIE behaviors can be enhanced by both employing long-size ligands, especially the N-donor ligand, and adopting a distorted molecular configuration, furnishing a high AIE factor (αAIE) of ca. 13.8. Critically, benefitting from their long-size C^N-type and N-donor ligands, these PtII(C^N)(N-donor ligand)Cl-type complexes can exhibit very sensitive AIE behaviors in a mixture of THF-H2O, indicated by their noticeable emission increase with a low H2O volumetric fraction (fw) of ca. 0.1 in their THF solution. In solution-processed organic light-emitting diodes (OLEDs), they can achieve a luminance of 6743 cd m-2 at 13.5 V, a maximum external quantum efficiency (ηext) of 13.8%, a maximum current efficiency (ηL) of 42.4 cd A-1 and a maximum power efficiency (ηP) of 34.4 lm W-1, respectively. Hence, this research can provide key information for developing phosphorescent complexes with a highly sensitive AIE response and impressive EL ability.

Improved electroluminescence efficiency derived from functionalized decoration of 1,3,4-oxadiazole (OXD)-based Ir(III) complexes.

The performance of organic light emitting devices (OLEDs) fabricated using Ir(III) complexes bearing 1,3,4-oxadiazole (OXD)-based cyclometallic ligands still needs to be improved. In this work, Ir3+ was coordinated with a 2-(9,9-diethyl-9H-fluoren-2-yl)-1,3,4-oxadiazole (F-OXD) fragment, which was modified with various functionalized substituents, including fluorenyl, OXD and carbazolyl groups. Three complexes, named Ir-Flu, Ir-OXD and Ir-Cz, were synthesized successfully and their photophysical, electrochemical and electroluminescence properties were investigated in detail. All these complexes exhibited yellow-orange emission in solution and a distinct aggregation-induced phosphorescent emission (AIPE) phenomenon was observed. Monochrome OLEDs were fabricated using these phosphorescent dopants, and the turn-on voltage (V), luminance (L) and current efficiency (CE) showed significant improvement compared to analogous OXD-based Ir(III) complexes reported before. In particular, the device with Ir-OXD as the dopant achieved the highest maximum brightness of 25 014 cd m-2 and the lowest efficiency roll-off (42.6%) at the maximum luminance among all the devices. These results provided a proven strategy of functionalized decoration of OXD-based complexes to achieve superior luminous efficiency devices.

Carbazole-based aggregation-induced phosphorescent emission-active gold(I) complexes with various phosphorescent mechanochromisms.

A series of carbazole-containing gold(I) complexes modified with different substituents were successfully designed and synthesized, and their molecular structures were characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. The aggregation-induced behaviors of these gold(I) complexes were studied by ultraviolet/visible and photoluminescence spectroscopy. Meanwhile, their mechanical force-responsive emissive properties were also investigated via solid-state photoluminescence spectroscopy. Interestingly, all these gold(I)-based luminogenic molecules were capable of exhibiting aggregation-induced phosphorescent emission phenomena. Furthermore, their solids of three gold(I) complexes displayed contrasting mechano-responsive phosphorescence features. More specifically, trifluoromethyl or methoxyl-substituted luminophores 1 and 3 demonstrated mechanochromic behaviors involving blue-shifted phosphorescence changes, and their mechanoluminochromic phenomena were reversible. However, the solid-state phosphorescence of phenyl-substituted luminophor 2 was not sensitive to external mechanical force.

Red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes showing aggregation-induced phosphorescent emission (AIPE) behavior for both red and white OLEDs

Two red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes bearing C^N ligands with carbazole functional group have been successfully prepared with different P-donor ligands of triphenylphosphine and triethylphosphine, respectively. The investigation of their phosphorescent behavior in the mixture of THF and water to reveal their aggregation induced phosphorescent emission (AIPE) ability, which is also indicated by their much higher phosphorescent quantum yield (ΦP) in doped film than those in the dilute solution. Mainly, their AIPE are induced by the blocked stretching motion of aromatic segments in C^N ligand and restrained the deformation of their coordinating skeletons. The AIPE complexes can possess AIE factor (αAIE) of ca. 7.4. In addition, the carbazole group can effectively promote hole transporting ability of the concerned AIPE emitters, which can benefit their electroluminescent ability. Hence, the IrIII(C^N)2(P-donor ligand)Cl-type complexes can show decent EL efficiencies in the solution-processed red-emitting organic Light-emitting diodes (OLEDs) with a maximum external quantum efficiency (ηext) of 8.5%, a maximum current efficiency (ηL) of 22.0 cd A−1 and a maximum power efficiency (ηP) of 15.9 lm W−1. Furthermore, as long-wavelength emitter, solution-processed white OLEDs (WOLEDs) have been constructed based on the red-emitting AIPE IrIII(C^N)2(P-donor ligand)Cl-type complexes, which can play critical role in achieving stable white electroluminescent spectra at high luminescence. The concerned WOLEDs can show attractive EL efficiencies of 6.6%, 23.7 cd A−1 and 16.0 lm W−1. All these results can provide valuable information for developing new AIPE materials with high EL ability.

Deep-Red/Near-Infrared to Blue-Green Phosphorescent Iridium(III) Complexes Featuring Three Differently Charged (0, -1, and -2) Ligands: Structures, Photophysics, and Organic Light-Emitting Diode Application.

We have designed and synthesized a new family of neutral phosphorescent iridium(III) complexes (Ir1-Ir6) featuring three differently charged (0, -1, and -2) ligands, in which biphenyl (bp) is used as a dianionic (-2) ligand, 4,6-difluorophenylpyridine (dfppy) or 1-phenylisoquinoline (piq) is used as a monoanionic (-1) ligand, and 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen), 1,2-bis(diphenylphosphanyl)benzene (dppb), or 1,2-bis(diphenylphosphanyl)ethane (dppe) is used as a neutral (0) ligand. The X-ray structures confirm that three coordination carbon atoms of all complexes assume a facial geometry, which can be beneficial to the stability of the structure. More importantly, the emitting color of the complexes can be tuned from deep red/near-infrared (NIR) (680-710 nm) to blue-green (466-496 nm) with different monoanionic (-1) ligands and neutral (0) ligands. Interestingly, the complex Ir5 shows a significant aggregation-induced phosphorescent emission effect, while Ir6 with a similar structure shows an opposite aggregation-caused quenching effect, mainly due to slight differences in the neutral (0) ligand structure. Notably, all deep red/NIR-emitting complexes (Ir1-Ir4) exhibit a distinct charge transfer (CT) excited state from the dianionic (-2) ligand to the neutral (0) ligand according to density functional theory calculations, whereas the excited state of blue-green-emitting complexes (Ir5-Ir6) displays the CT from the dianionic (-2) ligand to the monoanionic (-1) ligand. Considering better stability and optical performance, the deep red-emitting complexes (Ir2 and Ir4) with a simple structure are used as emitting layers of organic light-emitting diode devices and achieved good maximum external quantum efficiency (4.9 and 5.8%) peaking at 676 and 655 nm, respectively, with a very low turn-on voltage (2.5 V). This research provides a good strategy for the design of phosphorescent iridium complexes based on three differently charged (0, -1, and -2) ligands and their optoelectric applications.

AIPE-Active Ir(III) complexes with tuneable photophysical properties and application in mitochondria-targeted dual-mode photodynamic therapy

Aggregation-induced phosphorescence emission (AIPE) materials based on transition metal Ir(III) complexes have significant advantages in bioimaging and photodynamic therapy (PDT) due to the long lifetime, the reduced photobleaching and the good reactive oxygen species (ROS) generation. Herein, four cationic Ir(III) complexes (Ir1–Ir4) have been synthesized and studied. Tunable phosphorescence from green to red with the excellent properties of AIPE and long lifetimes can be achieved by varying the substituents. Moreover, these phosphorescence Ir(III) complexes exhibited dual-mode PDT potential (type I and type II). Complex Ir4 showed great prospect in bioimaging and PDT with the large Stokes shift (259 nm), the long lifetime (9.85 μs) and the high ROS yield (0.73). Confocal microscopy demonstrated that Ir4 accumulated in the mitochondria selectively and possessed remarkable photostability (reduced photobleaching up to 600 s). The results indicate that Ir4 may be used in dual-mode PDT guided by mitochondria-targeted imaging. This work provides an in-depth understanding of the relationship between structure and photophysical properties and facilitates the study in PDT applications.

Pt(II) diimine complexes bearing varied alkyl chains: Synthesis, tunable photophysical properties and aggregation-induced optical power limiting enhancement

A series of Pt(II) diimine complexes with varied alkyl chains on 2,2′- dipyridyl ligands (Pt-C1–Pt-C3) have been synthesized and characterized. The photophysical properties and nonlinear absorption properties were elucidated using UV–vis absorption, emission and transient absorption spectroscopy, density functional theory (DFT) calculations and electrochemical experiments. It was found that increasing the alkyl chain led to regular changes in the photophysical properties of Pt-C1–Pt-C3. The original conjugated skeleton of the Pt(II) complexes were affected when the alkyl chain was introduced and extended. All complexes exhibited an obvious aggregation-induced phosphorescence emission (AIPE) in a mixed solution comprised of tetrahydrofuran/water. The formation of nanoparticles in the aggregated state induced these complexes to exhibit different excited state properties. When the water content increased, the emission intensity increases 3 ∼ 13-flod and the excited state lifetime increased 98-flod due to the formation of Pt(II) complex nanoparticles. As a result, the optical power limiting (OPL) performance of these complexes was greatly improved. Based on the systematical investigation of nonlinear optical complexes in aggregated state, this work provided a theoretical basis for the development of new OPL materials. Furthermore, the Pt(II) complex nanoparticles will be more conducive to the potential application of OPL devices.

Iridium complexes having phosphrous ligands of large steric hindrance: Photophysical comparison between solution state, solid state and electrospun fibers

In this work, two luminescent iridium complexes were synthesized with 2-phenyl pyridine (ppy) and two phosphine-based ligands. There was large steric hindrance in these phosphine-based ligands, so that there would be enough time for the excited state based on highly luminescent 3MLLCT (metal-to-ligand-ligand-charge-transfer) electronic transformation It was found that there was limited aggregation-induced phosphorescence emission (AIPE) effect in these two iridium complexes since their ligands with large steric hindrance minimized the intermolecular (π-π) interactions between molecules. A distorted octahedral coordination field was found in the single crystals of these two iridium complexes. A systematical comparison on the photophysical parameters of these two iridium complexes in solution state, in solid state and in electrospun fiber was carried out, so that the limited aggregation-caused quenching (ACQ) could be confirmed. The potential surface crossing and energy transfer in the excited state of these two iridium complexes were analyzed by means of density functional theory calculation and temperature-dependent emission spectra. The limited ACQ effect in these two iridium complexes was finally confirmed due to the large ligand steric hindrance, which allowed phosphorescent decay of excited state. High emission quantum yield (0.27) and long emissive lifetime (0.43 µs) in solid state were observed.

Two Luminescent Iridium Complexes with Phosphorous Ligands and Their Photophysical Comparison in Solution, Solid and Electrospun Fibers: Decreased Aggregation-Caused Emission Quenching by Steric Hindrance

In this paper, we prepared two phosphorescent Ir complexes with ligands of 2-phenyl pyridine (ppy), and two phosphorous ligands with large steric hindrance, hoping to allow enough time for the transformation of the highly phosphorescent 3MLLCT (metal-to-ligand-ligand-charge-transfer) excited state. Their large steric hindrance minimized the π-π interaction between complex molecules, so that the aggregation-induced phosphorescence emission (AIPE) influence could be minimized. Their single crystals indicated that they took a distorted octahedral coordination mode. Photophysical comparison between these Ir complexes in solution, in the solid state and in electrospun fibers was performed to confirm the realization of limited aggregation-caused quenching (ACQ). The potential surface crossing and energy transfer from 3MLBPECT/3MLBPELppyCT to 3MLppyCT in these Ir complexes were revealed by density functional theory calculation and temperature-dependent emission. It was confirmed that these two phosphorous ligands offered large steric hindrance, which decreased the ACQ effect, allowing the efficient emissive decay of the 3MLppyCT excited state. This is one of the several luminescent Ir complexes with a high emission yield (Φ = 0.27) and long emission lifetime (0.43 μs) in the solid state.

A Carbazole-Bridged Biscyclometalated Diplatinum Complex: Synthesis, Characterization, and Dual-Mode Aggregation-Enhanced Phosphorescence.

A cationic carbazole-bridged biscyclometalated diplatinum complex 4 has been synthesized and characterized. Single-crystal X-ray analysis demonstrates that complex 4 displays a dimeric structure with noncovalent π-π stacking and unique double Pt-Pt interactions. In aerated dilute CH3CN, complex 4 is characterized by a very weak monomeric yellow emission (λemi = 547 nm; Φ = 0.51%), which is attributed to the triplet intraligand (3LC) excited state mixing with some charge transfer characters. In contrast, under aerated conditions, the dispersion of 4 in a mixed solvent of CH3CN/Et2O (1/9, v/v) or CH3CN/H2O (1/9, v/v) displays intense yellow (λemi = 550 nm; Φ = 35.5%; τ = 11.10 μs) and red emission (λemi = 635 nm; Φ = 14.1%; τ = 7.00 μs), respectively. These aggregation-induced phosphorescent emission enhancements are considered being caused by the oxygen-shielding effect and the molecular rigidification-induced decrease of nonradiative decays in the aggregate state. The morphology and size of the aggregates under these two conditions are examined by scanning electron microscope and dynamic light scattering analysis. The absorption and emission properties of 4 are further rationalized by time-dependent density functional theory calculations on a model compound.

Luminescence properties of cyclometalated platinum(II) complexes in a dichloromethane/n-hexane system

The luminescence properties of three neutral cyclometalated Pt(II) complexes were investigated in a CH2Cl2/n-hexane system. The diphenylamino and trifluoromethyl modified Pt2 exhibits aggregation-induced phosphorescent emission (AIPE) feature. The crystal stacking of Pt2 shows that the intermolecular hydrogen bonding restricts both intramolecular rotation (RIR) and vibration (RIV), which weakens the non-radiative decay ways and leads to an AIPE performance. TEM images reveal that Pt2 aggregates into nanosheets.

Aggregation-induced phosphorescence emission (AIPE) behaviors in PtII(C^N)(N-donor ligand)Cl-type complexes through restrained D2d deformation of the coordinating skeleton and their optoelectronic properties

A series of four-coordinated PtII(C^N)(N-donor ligand)Cl-type complexes have been synthesized to show aggregation-induced phosphorescence emission (AIPE) behaviors and impressive OLED device efficiencies.