Aggregation-induced Emission Mechanism Research Articles

Recent Progress of Advanced AIE Materials for Visualization of 3‐Level Latent Fingerprints

The 3‐level features of Latent fingerprints (LFPs) are pivotal in linking suspects with problematic or incomplete fingerprint data, constituting one of the most robust forms of evidence. Aggregation‐induced emission (AIE) materials, known for their heightened emissive properties in high aggregation states, provide a robust and powerful approach for developing techniques for 3‐level details of LFPs. So far, tremendous efforts have been devoted to solving high background fluorescence interference during the 3‐level LFPs visualization process. This review commences with an overview of 3‐level features and summarizes the interaction and mechanism of visualizing LFPs involving AIE materials and LFPs secretions. Moreover, it encompasses strategies to enhance the contrast and brightness of 3‐level LFPs images. We highlight the mechanism of AIE's physicochemical properties and their effects on the visualization of 3‐level LFPs. Additionally, we provide insights into potential challenges and opportunities in this emerging field.

A Novel Enhanced-Fluorescent Probe Based on DHLA-Stabilized Red-Emitting Copper Nanoclusters for Methimazole Detection Via Aggregation-Induced Emission Effect.

Herein, an aqueous phase synthesis approach was presented for the fabrication of copper nanoclusters (Cu NCs) with aggregation-induced emission (AIE) property, utilizing lipoic acid and NaBH4 as ligands and reducing agent, respectively. The as-synthesized Cu NCs exhibit an average size of 3.0 ± 0.2nm and demonstrate strong solid-state fluorescence upon excitation with UV light. However, when dissolved in water, no observable fluorescent emission is detected in the aqueous solution of Cu NCs. Remarkably, the addition of Methimazole induced a significant red fluorescence from the aqueous solution of Cu NCs. This unexpected phenomenon can be ascribed to the aggregation of negatively charged Cu NCs caused by electrostatic interaction with positively charged imidazole groups in Methimazole, resulting in enhanced fluorescence through AIE mechanism. Therefore, there exists an excellent linear correlation between the fluorescent intensities of Cu NCs aqueous solution and the concentration of Methimazole within a range of 0.1-1.5 mM with a low limit of detection of 82.2 µM. Importantly, the designed enhanced-fluorescent nanoprobe based on Cu NCs exhibits satisfactory performance in assaying commercially available Methimazole tablets, demonstrating its exceptional sensitivity, reliability, and accuracy.

Two series of benzofuran and benzodifuran chelating chromophores with DR/NIR emission and anticancer activity

Benzofuran (BF) and benzodifuran (BDF) based compounds are known for their tunable optical and biological properties. Recently, their potential as chelating and anticancer agents has been examined and exploited. As dyes emissive in a physiological environment are in high demand for integrated therapeutic and imaging tools, BF and BDF scaffolds came up as possible theranostic probes. Here, we examined two series of BF and BDF based compounds with an extended conjugation pattern and a half-salen group producing a copper (II) ion-chelating O,N,O tridentate site. A combined experimental and theoretical approach was employed in the examination of spectroscopic and cytotoxic properties. The novel compounds are emissive according to an aggregation-induced emission mechanism and exhibit the DR/NIR (deep red/near-infrared) fluorescence required for penetration into living tissue. The results collected against HeLa, A431, and MCF7 human cells were discussed and compared, detecting in one case a very good performance against tumor cells as well as selectivity towards healthy HaCaT cells.

Theoretical Investigation of Iridium Complex with Aggregation-Induced Emission Properties

The mechanism of aggregation-induced emission (AIE) for the bis(1-(2,4-difluorophenyl)-1H-pyrazole)(2-(20-hydroxyphenyl)-2-oxazoline)iridium(III) complex, denoted as Ir(dfppz)2(oz), was investigated with use DFT and the TD-DFT level of theory. The mechanism of radiationless deactivation of the triplet state was elucidated. Such a mechanism requires an additional, photophysical triplet channel of the internal conversion (IC) type, which is activated as a result of intramolecular motion deforming the structure of the oz ligand and distorting the iridium coordination sphere. Formally, the rotational movement of the oxazoline relative to the C–C bond in the oz ligand is the main active coordinate that leads to the opening of the triplet channel. The rotation of the oxazoline group and the elongation of the Ir-Nox bond cause a transition between the luminescent, low-lying triplet state with a d/π→π* characteristic (T1(eq)), and the radiationless d→d triplet state (T1(Ir)). This transition is made possible by the low energy barrier, which, based on calculations, was estimated at approximately 8.5 kcal/mol. Dimerization, or generally aggregation of the complex molecules, blocks the intramolecular movement in the ligand and is responsible for a strong increase in the energy barrier for the T1(eq)⇝T1(Ir) conversion of triplet states. Thus, the aggregation phenomenon blocks the nonradiative deactivation channel of the excited states and, consequently, contributes to directing the photophysical process toward phosphorescence. The mechanism involved in locking the nonradiative triplet path can be called restricted access to singlet–triplet crossing (RASTC).

Aggregation‐Induced Emission (AIE) of Au(I)‐GSH Complexes is Activated by Optical Manipulation and Exhibits Surprising 780 nm Emission

AbstractAn optical manipulation method to activate the aggregation‐induced emission (AIE) of Au(I)‐glutathione (Au(I)‐GSH) complexes is presented. First, the well‐known AIE‐type Au(I)‐GSH complexes with a solid emission wavelength of 570 nm are synthesized. At the beginning of the focusing a 1064 nm continuous‐wave (CW) laser at the interface of Au(I)‐GSH complexes solution, its luminescence is hardly detectable. However, with the extension of the laser irradiation time, aggregate appears and rapidly grows, and strong luminescence is shown, indicating the activation of AIE of Au(I)‐GSH complexes through optical manipulation. Surprisingly, the emission peak is at 780 nm and no blue‐shift is observed with the emission intensity increases. The activated AIE emission wavelength by optical manipulation is longer than reported previously, which means that optical manipulation activates AIE in a unique manner. The switching on/off of the activation of AIE is arbitrarily controlled by changing the laser power. In this paper, the luminescence phenomenon and mechanism of AIE are discussed from the perspective of the movement limitation of Au(I)‐GSH complexes under optical manipulation. It is of significant importance for further insights into the luminescence of Au(I)‐GSH complexes and their future development.

Platinum complexes with aggregation-induced emission.

Transition metal-containing materials with aggregation-induced emission (AIE) have brought new opportunities for the development of biological probes, optoelectronic materials, stimuli-responsive materials, sensors, and detectors. Coordination compounds containing the platinum metal have emerged as a promising option for constructing effective AIE platinum complexes. In this review, we classified AIE platinum complexes based on the number of ligands. We focused on the development and performance of AIE platinum complexes with different numbers of ligands and discussed the impact of platinum ion coordination and ligand structure variation on the optoelectronic properties. Furthermore, this review analyzes and summarizes the influence of molecular geometries, stacking models, and aggregation environments on the optoelectronic performance of these complexes. We provided a comprehensive overview of the AIE mechanisms exhibited by various AIE platinum complexes. Based on the unique properties of AIE platinum complexes with different numbers of ligands, we systematically summarized their applications in electronics, biological fields, etc. Finally, we illustrated the challenges and opportunities for future research on AIE platinum complexes, aiming at giving a comprehensive summary and outlook on the latest developments of functional AIE platinum complexes and also encouraging more researchers to contribute to this promising field.

A rapid and specific fluorescent probe based on aggregation-induced emission enhancement for mercury ion detection in living systems

Mercury is a harmful heavy metal that seriously threatens the environment and organisms. In this study, we combined the aggregation-induced emission mechanism and the advantages of peptides to design a novel tetraphenylene (TPE)-based peptide fluorescent probe, TPE-Cys-Pro-Gly-His (TPE-CPGH), in which the sulfhydryl group of Cys in the peptide chain and the imidazolium nitrogen provided by His were used to mimic the Hg2+ binding site of metalloproteins. The β-fold formed by Pro-Gly was used to promote the spatial coordination of the probe with Hg2+ and the formation of the coordination complex aggregates, these changes led to the “turn on” response to Hg2+. The detection of Hg2+ by TPE-CPGH not only showed high specificity and sensitivity (LOD=46.2 nM), but also had the advantages of fast response and applicability for detection over a wide pH range. Additionally, TPE-CPGH effectively detected Hg2+ in environmental samples, living cells and organisms due to its low cytotoxicity, high water solubility and cell membrane permeability. More interestingly, TPE-CPGH was also mitigated Hg2+ exposure-induced oxidative stress toxicity in vitro and in vivo.

Synthesis of hydrogen-bond-enriched, fluorescent and triboelectric hyperbranched polymers via Michael addition reaction

It is very meaningful to realize color recognition and mechanical-electrical energy conversion in one material simultaneously, which is very essential in improving the equipment of border guards. However, simultaneously endowing one material with color regonition and mechanical-electrical energy conversion presents challenges because they result from different mechanism. Herein, a series of hyperbranched polymers, HP9, HP10, and HP11, were synthesized via the Michael addition reaction to solve the dilemma. Extensive hydrogen bonding was embedded into these hyperbranched polymers, so that the polymers exhibit good triboelectrification ability. The voltage output is about 347 V, the current output is about 2.8 μA, and the charge quantity is about 150 nC, respectively. Moreover, the polymers display excellent fluorescence under 365 nm ultraviolet light based on the mechanism of aggregation-induced emission (AIE). Furthermore, the thermodynamic behavior and mechanical properties of the hyperbranched polymers can be regulated by adjusting the content of the hard segments. The Young's modulus and strength gradually increase with the increase of the hard segment unit. The Young’s modulus, strength and toughness of HP10 are 3.72 MPa, 2.69 MPa and 19.55 MJ/m3, respectively. The combination of fluorescence and frictional electrification will have great application potential in the equipment of border guards.

Theoretical Insights into Aggregation-Induced Emission with the Ionic π Fluorophore: The Importance of Choosing the Dimer QM Model in the ONIOM Study.

In understanding the mechanism of aggregation-induced emission (AIE), the multilevel ONIOM framework has been demonstrated as one of the efficient tools that can capture the essential mechanistic information by choosing a single fluorophore as the quantum mechanics (QM) model and putting all surrounding molecules in the low-level region. Recently, the ionic styryl-pyridine salt (namely, SPH) has been reported as a new class of AIEgen with a high fluorescence yield. In the SPH crystal, a pair of ionic SPH molecules are closely stacked with each other in an antiparallel, head-to-tail pattern, thus the choice of QM models (an individual or dimeric structure) becomes critical in the ONIOM study. Herein we report the AIE mechanism of the ionic SPH at the QM ((TD)-CAM-B3LYP) and ONIOM(QM:MM) levels. As usual, the fluorescence quenching of SPH in tetrahydrofuran (THF) solution is attributed to a nonradiative relaxation via the central C═C bond rotation, with a rather low barrier of 2.7 kcal/mol. In crystals, either with a monomer or dimer model, the fluorescence quenching channel is found to be restricted due to the obvious C═C rotation barriers. Compared with the monomer model, the dimer model, by treating the orbital interaction of the two SPH molecules at the QM level, provides significantly increased barriers and a red-shifted emission wavelength that better matches the experimental value. In addition, the calculated exciton coupling in the fluorescence emission state can be discovered only by a dimer model. The findings here emphasize not only the importance of choosing a proper model in the ONIOM study of AIE but also expanding our understanding of novel AIE systems.

Theoretical study on the mechanism of aggregation-induced emission in red thermally activated delayed fluorescence molecules: trans/cis-arrangement effect

Theoretical study on the mechanism of aggregation-induced emission in red thermally activated delayed fluorescence molecules: trans/cis-arrangement effect

Unraveling the Mechanism of ACQ-to-AIE Transformation of Fluorescein Derivatives.

Although fluorescein derivatives have excellent properties and strong practicability, they are typical aggregation-induced quenching (ACQ) molecules, which are not conducive to working in the solid state. Recently, the fluorescein derivative Fl-Me with aggregation-induced emission (AIE) property was synthesized, which brought a new dawn for the research and development of fluorescein-based materials. In this study, the AIE mechanism of Fl-Me was investigated based on time-dependent density functional theory and the ONIOM method. The results revealed that an effective dark-state deactivation pathway leads to the fluorescence quenching of Fl-Me in a solution environment. Accordingly, the AIE phenomenon originates from the closure of the dark-state quenching channel. It is worth emphasizing that we found that the carbonyl group of molecular Fl-Me has intermolecular hydrogen bonding interaction with the adjacent molecules, which caused the increase of the dark-state energy in the crystalline state. Moreover, the restriction of the rotational motion and the nonexistence of the π-π stacking interaction are beneficial to the enhancement of fluorescence upon aggregation. Finally, the ACQ-to-AIE transformation mechanisms of fluorescein derivatives have been discussed. This work provides deeper insight into the photophysical mechanism for the fluorescein derivatives Fl-Me with AIE feature and eventually is expected to help researchers to develop more fluorescein-based AIE materials with remarkable properties for various fields.

Improving the sensitivity of protein fluorescent probes by molecular structural fine-tuning based on aggregation-induced emission mechanism

Three 1,3-butadiene derivatives were synthesized to investigate the effect of small structural changes on the detection sensitivity with bovine serum albumin (BSA). The compound 2-MFDNa with methyl group added to the 5-furan ring showed a 132-fold fluorescent enhancement response to BSA, while 3-FDNa with no methyl group on the 5-furan ring showed a 52-fold fluorescent enhancement response to BSA. Compared with the detection limits of 3-FDNa 0.19 μg/mL and 2-FDNa 0.17 μg/mL, that of 2-MFDNa was reduced to 0.01 μg/mL. 2-MFDNa achieved a higher selective and sensitive detection to BSA. Molecular docking analysis revealed that the improved detection sensitivity was due to the enhanced restriction of intramolecular motion, with more C–H···π interactions limiting the rotation of the rotor of the probe molecule, leading to enhanced fluorescence. These results provide a simple and effective strategy for the molecular design of aggregation-induced emission probes for the detection of proteins with higher sensitivity.

Theoretical study on the aggregation‐induced emission mechanism of anthryl‐tetraphenylethene

AbstractSince the concept of aggregation‐induced emission (AIE) was proposed by Benzhong Tang's research group in 2001, the exploration of the mechanism of AIE and the development of new high‐performance AIE materials have been the focus and goal of this field. On the basis of a large number of experiment results, AIE mechanism has been well explained by lots of works, such as restricted intramolecular motion (RIM), J‐aggregate et al. As tetraphenylethlene (TPE) molecules are stacked, the rotation of the benzene ring rotor is blocked, and the energy attenuation is released in the form of radiation, showing the AIE effect. In order to further explore the AIE effect of TPE, we performed electronic structure, spectrum simulation, and AIE mechanism calculations of the anthryl‐tetraphenylethene (TPE‐an) monomer and dimer in the gas phase, tetrahydrofuran (THF), and aqueous solutions at the B3LYP/6‐31G** level. The calculation results show that TPE‐an molecule is in a propeller‐like configuration, and its fluorescence intensity is weak; compared with the monomer, the fluorescence intensity of the dimer increases by 87% in aqueous solution; the fluorescence intensity in the gas phase, THF solution, and aqueous solution gradually enhances with the increase of the degree of aggregation, which are consistent with the experimental results. The enhancement of fluorescence intensity is caused by the change of molecular structure caused by aggregation. This detailed AIE luminescence mechanism will provide theoretical guidance for AIE material design.

Restriction of crossing conical intersections: the intrinsic mechanism of aggregation-induced emission.

Elucidating the mechanism of aggregation-induced emission (AIE) is a prerequisite for designing more AIE-gens. The diphenylethylene (DPE) featured molecules are one of the most important AIE-gens due to their propeller structure. Three representative DPE-featured AIE-gens, triphenylethylene, cis-stilbene, and trans-stilbene, are explored via ultrafast ultraviolet/infrared (UV/IR) spectroscopy and theoretical calculations. Both experimental and computational results suggest that readily crossing conical intersections (CIs) with flexible structural evolutions in solutions significantly reduces fluorescence, whereas crossing CIs is restricted because of high energy cost, and therefore no fast nonradiative decay can compete with spontaneous emission in solids. The mechanism also well explains the different emission quantum yields and interconversion ratios between cis-stilbene and trans-stilbene after photoexcitation.

Structure-property relationship on aggregation-induced emission properties of simple azine-based AIEgens and its application in metal ions detection

In recent twenty years, aggregation-induced emission (AIE), due to its excellent application prospect, has aroused widespread interests. The development of novel and easy to make AIE luminogens (AIEgens) is an attractive subject. For this purpose, it is very important to study the structure-property relationship of AIEgens. Because azine derivatives are easy to synthesis and some of them have nice AIE properties, herein, a series of azine derivatives (ADs) were employed as models to study the influence of different functional groups, electronic effects and structures on the AIE properties of azine derivatives. The AIE mechanism were studied by single crystal analysis, density functional theory (DFT) calculations and so on. The results indicated that the o-hydroxyl aryl substituted azine compounds could show good AIE properties. Meanwhile, the AIE properties of o-hydroxyl aryl substituted azine compounds were also influenced by the electronic effects of the aryl groups in the azine compounds. The o-hydroxyl groups could form intramolecular hydrogen bond with imine group, which play key role to restrict the intramolecular rotation of the aryl groups and act as base stone for the AIE process of this kind compounds. The HOMO-LUMO energy gaps of o-hydroxyl substituted azine are smaller than other homologous compounds, which is agree with the proposed AIE mechanism. Finally, thanks to the AIE properties, the o-hydroxy-substituted azines could be used as efficient Al3+ and Cu2+ fluorescent chemosensors in different conditions. In addition, test strips based on AD10 has been prepared, which can conveniently detect Cu2+ in industrial wastewater. This research supplied a way for the design of novel easy to make AIEgens through simple azine derivatives.

Aggregation-Induced Emission of Gold Nanoclusters by Ionic Liquids for White Light-Emitting Diode and Multiple-Ion Probe Applications.

White light-emitting diodes (WLEDs) require stable luminophores with bright emission, a large Stokes shift, and good processability. Herein, we fabricate WLEDs on the basis of synergetic aggregation-induced emission (AIE) of Au nanoclusters (NCs) induced by ionic liquids. Au NCs with orange emission at 615 nm are electrostatically bonded to 1-ethyl-3-methyl-imidazolium acetate (EmimOAc). The electrostatic bonding not only introduces complementary blue emission (from EmimOAc) into Au NCs but also enhances cluster emission via an intercluster AIE mechanism. Overall white emission of Au NC-EmimOAc composites can be further improved by their encapsulation into nanogels templated by cationic chitosan. Color conversion films constructed from these nanogels manifest ultrabright white emission on a commercial GaN ultraviolet LED chip, featuring a CIE 1931 color coordinate of (0.366, 0.368) and greatly enhanced mechanical strength. By properly selecting ionic liquids, we are also able to achieve multiple-ion detection of Cu2+, Sn2+, and Fe3+.

Natural products with aggregation-induced emission properties: from discovery to their multifunctional applications

<p indent="0mm">Aggregation-induced emission (AIE) materials derived from natural products have the advantages of non-synthetic, good biocompatibility, natural degradation and pharmacological activity, and are gradually becoming a powerful tool in environmental monitoring, biological imaging and disease diagnosis. Here, we review AIE materials discovered from natural products, including alkaloids such as berberine, flavonoid polyphenols such as quercetin, non-flavonoid polyphenols such as tannic acid, and other natural compounds such as saponins and vitamins. AIE mechanisms of these materials include restriction of intramolecular/intermolecular motion (RIM), torsional intramolecular charge transfer (TICT), excited state intramolecular proton transfer (ESIPT), J-aggregate and clusterization-triggered emission (CTE). Based on remarkable AIE properties, these natural products show great potential in the fields of pH, ion and biomarker sensing, cell imaging and biological diagnosis and treatment. This paper is expected to deepen the understanding of aggregation-induced luminescence, inspire the exploration of AIE molecules from natural products and develop new applications.

Construction of turn-on mode fluorescent sensor for Pb&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; and Al&amp;lt;sup&amp;gt;3+&amp;lt;/sup&amp;gt; with structure of phthalic acid and tetraphenylethylene core

<p indent="0mm">Lead is one of the most hazardous metals to humans. It accumulates in liver, kidney and central nervous system, and interferes with a variety of physiological processes such as biosynthesis of hemoglobin. Lead is especially dangerous for children, causing mental retardation. Aluminum has an important role in Alzheimer’s disease, Parkinson’s disease, bone softening, chronic renal failure. However, both lead and aluminum are widely used in industrial applications and daily life. The limits of Pb²⁺ and Al³⁺ for drinking water according to the World Health Organization (WHO) standard are 0.5 and <sc>7.41 µmol/L</sc> respectively. Thus, it is necessary to develop real-time and rapid detection of Pb²⁺ and Al³⁺. Fluorescent probes are favored by researchers because of their high sensitivity and selectivity, rapid real-time monitoring and potential application in biological systems. The emission of the traditional panel-like chromophore is weakened or even quenched at high concentration or aggregation, while at low concentration, it exhibits low sensitivity and is easy to photobleaching. Tang and coworkers noticed that some luminogens with twisted conformations are nearly nonemssive in solution but exhibit boosted emission in aggregate states, thus, Tang coined this phenomenon as aggregation induced emission (AIE). With the disclosure of the AIE mechanism of restriction of motion (RIM), many AIE active luminogens have been developed. Some turn-on mode fluorescent sensors for Pb²⁺ and Al³⁺ have also been reported based on AIE mechanism, but few of them worked in pure water. Two water-soluble fluorescent sensors are constructed using phthalic acid structure as electron acceptor and tetraphenylethylene as core: 4,4′-(2,2-diphenylethene-1,1-diyl)diphthalic acid (<bold>1</bold>) and 4,4′-(2,2-bis(4-(dimethylamino)phenyl)ethene-1,1-diyl)diphthalic acid (<bold>2</bold>). Both <bold>1</bold> and <bold>2</bold> are nearly nonemissive in solution, but their solids are highly luminescent with quantum yield of 17.1% and 7.3%. Emission of <bold>1</bold> and <bold>2</bold> in water in the presence of KOH is turned on upon addition of Pb²⁺ or Al³⁺, while exhibit nearly no response to the addition of other metal ions, indicating the high selectivity for Pb²⁺ or Al³⁺. The detection limit of <bold>1</bold> and <bold>2</bold> towards Al³⁺ was estimated as 2 and <sc>0.17 µmol/L,</sc> which meets the limit for drinking water according to the WHO standard <sc>(7.41 µmol/L).</sc> Fluorescent intensities of <bold>1</bold> and <bold>2</bold> enhanced by Pb²⁺ and Al³⁺ are not much affected by the existence of the interfering ions. The exception is that the emission intensity of <bold>1</bold> can be greatly enhanced by Zn²⁺ when detecting Pb²⁺, which may afford the the construction for highly sensitive sensors for Pb²⁺ through combination of <bold>1</bold> and Zn²⁺. The average particles size of <bold>1</bold> and <bold>2</bold> in water solution are 36 and <sc>98 nm</sc> determined by dynamic light scattering (DLS) analysis, respectively. The average particles size of <bold>1</bold> increased to 707 and <sc>292 nm</sc> upon addition of Pb²⁺ and Al³⁺, respectively. And average particles size of <bold>2</bold> increased to 986 and <sc>906 nm</sc> upon addition of Pb²⁺ and Al³⁺. Thus the emission enhancement of <bold>1</bold> and <bold>2</bold> upon addition of Pb²⁺ and Al³⁺ should be caused by the aggregate formation. The emissions of the water solutions of <bold>1</bold> or 2 in the presence of Pb²⁺ or Al³⁺ were quenched upon addition of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) due to the competing coordination of EDTA-2Na with metal ions.

AIE mechanism of 2-(2-hydroxyphenyl) benzothiazole derivatives: CASPT2 and spin-flip study

Wang et al. synthesized phenylacetylene modified HBT derivative (HBT-s-Ph) and diphenylacetylene modified HBT derivative (HBT-d-Ph) with aggregation-induced emission (AIE) phenomenon. In this work, we have theoretically compared the photochemical and photophysical processes of HBT-s-Ph and HBT-d-Ph in the gas phase, toluene and solid phase using several theoretical methods. The ground-state, excited-state equilibrium structures, the potential energy curves and the excited-state properties were comparatively calculated for a better understanding of the AIE mechanism. The excited-state intramolecular proton transfer (ESIPT) process within HBT derivatives is very fast in both gas phase, toluene, and solid phase due to the small energy barrier, inducing the formation of Keto tautomer which is responsible for emission peaks observed in experiment in both gas phase, toluene and solid phase. But, the reaction paths are very different from there. In toluene, the non-radiative path via HBT intramolecular rotation along the C–C bond is favorable to occur, leading to low Φf of HBT-s-Ph and HBT-d-Ph in toluene. In contrast, the non-radiative path of HBT intramolecular rotation is strongly suppressed in solid phase due to the high energy barriers, which is responsible for high Φf of HBT-s-Ph and HBT-d-Ph in solid phase. The substitution effect on AIE and the large difference of Φf in HBT-s-Ph and HBT-d-Ph in solid phase have been revealed. The presents results contribute to a better understanding of the different photophysical properties of HBT derivatives in toluene and solid phase, and unveil the AIE mechanism in HBT derivatives, which provides deep insight into molecular design of organic photoelectric materials with high Φf.

ESIPT and AIE characteristics of three Schiff base derivatives and the relevant photophysical mechanism analyses

Recently, three novel Schiff base derivatives (L1-3) were experimentally found to show water-induced aggregation-induced emission (AIE) and ultrafast excited state intramolecular proton transfer (ESIPT) characteristics, which have potential applications in fields of both optical material and fluoride ion detection. However, the corresponding optical properties have not been recognized completely and the relevant theoretical mechanism is lacking in analysis. In this work, based on the constructed relaxed potential energy curves, we investigated the stable configurations and clarified the proton transfer reaction mechanism for L1-3 with different substituents. The results show that the proton transfer associated with the interconversion from enol to keto isomer in S0 state is increasingly activated as substituted from electron-withdrawing group to electron-donating group, while the reverse trend is observed in S1 state. Above phenomenon can be ascribed to the change in strength of intramolecular hydrogen bond contributed to the reaction, which is demonstrated by hydrogen-bond parameters and interaction region indicator (IRI) analyses. Based on the optimized geometries, the single-fluorescence emission is found following the dual-absorption. The calculated fluorescence peaks (545 nm for L1, 536 nm for L2, 704 nm for L3) show little difference from those of experiment, which are reassigned to the ESIPT induced formation of keto isomer instead of enol form. In addition, the mechanism of water-induced AIE is clarified by increasing radiative rate and orbital overlap. To essentially comprehend the photophysical properties of organic luminogens with AIE and ESIPT characteristics is of fundamental interests in molecular electronics and photonics, and is also helpful to design new optical materials.