Aggregation-induced emission enhancement of 2-(2'-hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds

A novel class of 2-(2'-hydroxyphenyl)benzothiazole-based (HBT-based) excited-state intramolecular proton-transfer (ESIPT) compounds, N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]isophthalic amide (DHIA) and N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) has been feasibly synthesized and the properties of their nanoparticles in THF/H2O mixed solvent were investigated. Both compounds were found to exhibit aggregation-induced emission enhancement (AIEE) due to restricted intramolecular motion and easier intramolecular proton transfer in solid state. On identical experimental conditions, the emission of DHBIA aggregates increased more remarkably than that of DHIA. Different aggregation forms of these two organic compounds, due to the steric hindrance of a single tert-butyl group, could be responsible for the notably different degrees of the fluorescence enhancement. Their aggregation modes were investigated on the basis of time-dependent absorption, scanning electron microscope (SEM) images, and molecular modeling with theoretical calculation. The photophysical dynamics were also depicted based on the extremely fast ESIPT four-level cycle.     

Restriction of Photoinduced Twisted Intramolecular Charge Transfer

An intensive investigation of structure–property relationships in the aggregation‐induced enhanced emission (AIEE) of luminescent compounds is essential for the rational design of highly emissive solid‐state materials. In the AIEE‐active compounds N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]isophthalamide and N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]‐5‐tert‐butylisophthalamide, fast photoinduced twisted intramolecular charge transfer (TICT) of the enol excited state is found to be mainly responsible for the weak emission of their dilute solutions. The photoinduced TICT enol excited state is formed with a greatly distorted configuration, due to the large rotation about the C? N single bond. This facilitates nonradiative TICT decay from the normal enol excited state to the highly twisted enol excited state, rather than proton‐transfer decay to the keto excited state. In aggregates, photoinduced nonradiative deactivation of TICT is strongly prohibited, so that excited‐state intramolecular proton transfer (ESIPT) becomes the dominant decay, and hence contributes greatly to the subsequent emission enhancement of the keto form. Molecular design and investigation of analogous single‐armed compounds further verifies this kind of AIEE mechanism.     

A ratiometric fluorescent probe for alkaline phosphatase with high sensitivity

Alkaline phosphatase(ALP)is one of essential biomarkers in mammalian tissue.Here we report a ratiometric probe for ALP,which is rationally designed and synthesized by employing ESIPT fluorophore N-(3-(benzo[d]thiazol-2-yl)-4-hydroxyphenyl)benzamide(BTHPB).The enzymatic dephosphorylation converts the probe to BTHPB,which exhibits a large spectral red-shift(120 nm),allowing extremely high sensitivity of ALP sensing at 0.004 mU/mL.The probe also shows excellent biocompatibility and has been applied for monitoring the endogenic ALP in living cells.     

A small change in molecular structure, a big difference in the AIEE mechanism

An anthracene carboxamide derivative of the excited-state intramolecular proton-transfer compound of 2-(2'-hydroxyphenyl)benzothiazole has been newly developed to produce the prominent characteristics of aggregation-induced enhanced emission (AIEE) with a high solid-state fluorescence quantum efficiency of 78.1%. Compared with our previously reported phenyl carboxamide derivatives, a small tailoring of the molecular structure was found to result in a big difference in the dominant factor of the AIEE mechanism. In the phenyl carboxamide derivatives, the dominant factor of the AIEE mechanism is the restriction of the twisted intramolecular charge transfer (TICT) of the enol excited state, regardless of their different aggregation modes. In the anthracene carboxamide derivative, N-(3-(benzo[d]thiazol-2-yl)-4-hydroxyphenyl) anthracene-9-carboxamide, the AIEE characteristics are not dependent on the restriction of TICT, but mainly attributed to the cooperative effects of J-aggregation and the restriction of the cis-trans tautomerization in the keto excited state. A specific N···π interaction was found to be the main driving force for this J-aggregation, as revealed by the single crystal analysis. The AIEE mechanism of this anthracene carboxamide derivative was studied in detail through photophysical investigations and theoretical calculations. On the basis of its AIEE characteristics, a stable non-doped organic light-emitting diode was achieved, with high color purity and a remarkably low efficiency roll-off.     

Effect of Conjugation Mode on Intramolecular Charge Transfer in Fabricating Acid‐Responsive Fluorophores

Solid‐state acid‐responsive materials are promising for the tunability of their intrinsic properties. However, the relationship between molecular structure and emission shift as a response to acid stimuli has not been systematically studied. Herein, we report the effect of protonation and subsequent intramolecular hydrogen bonding on the photophysical properties of compounds (MPP‐s, MPP‐d, and MPP‐d‐CN) with different conjugation modes between the electron‐donating dimethoxyl phenyl and the electron‐withdrawing benzothiazole ring. The results established that the stronger the intramolecular charge transfer feature of the compound, the smaller is the emission shift after acid stimuli. Our studies also indicated that the conjugation mode significantly affected the solid‐state packing mode: MPP‐s and MPP‐d tended to form dimers, while MPP‐d‐CN exhibited the strongest aggregation‐induced emission enhancement (AIEE). The exploration of structure‐property relationship would provide experimental and theoretical guidance in designing acid‐responsive molecular switches and developing high‐performance AIEE‐active luminogens.     

New cyano functionalized silanes: aggregation‐induced emission enhancement properties and detection of 2,4,6‐trinitrotoluene

Two novel tetrahedral silicon‐centered cyano functionalized silanes, namely bis(4‐cyanophenylethynyl)dimethylsilane ( CN-1 ) and bis(4‐cyanophenylethynyl) diphenylsilane ( CN-2 ), have been synthesized and well characterized. They demonstrated unusual aggregation‐induced emission enhancement (AIEE) properties in the H‐type aggregation state with nanoparticle aggregate formation. Further study shows that the 4‐cyanophenylethynyl unit is the structure base to induce the AIEE phenomenon and the silicon core may enhance the AIEE effect. By single‐crystal analysis, the twisted tetrahedral conformation of silicon core and restricted intramolecular motions are speculated as the AIEE mechanism for these unusual H‐type aggregates of CN-1 and CN-2 . Moreover, both CN-1 and CN-2 show an obvious fluorescent quenching response to 2,4,6‐trinitrotoluene (TNT) in THF solution, making them promising candidates in the application of explosive detection. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and Fluorescent Properties of Tetra‐Biphenyl N‐Substituted Phthalimides

A series of tetra(biphenyl‐4‐yl)phthalimide (TBPPI ) derivatives with different N‐substituents (n‐butyl, phenyl, p‐methyl phenyl, and p‐acetyl phenyl moieties for compounds 7 – 10 , respectively) were prepared to examine their fluorescent behavior under various conditions. The chemical structure of compound 7 has been successfully confirmed by single crystal X‐ray diffraction analysis. The photoluminescence (PL ) spectra in different ratios of CH ₂Cl ₂/EtOH mixture solutions revealed that compounds 7 and 8 exhibited both aggregation‐induced emission (AIE ) and aggregation‐caused quenching (ACQ ) behaviors, while compounds 9 and 10 displayed AIE and aggregation‐induced emission enhancement (AIEE ) properties, respectively.     

Insights into the AIEE of 1,8‐Naphthalimides (NPIs): Inverse Effects of Intermolecular Interactions in Solution and Aggregates

Systematic structural perturbation has been used to fine‐tune and understand the luminescence properties of three new 1,8‐naphthalimides (NPIs) in solution and aggregates. The NPIs show blue emission in the solution state and their fluorescence quantum yields are dependent upon their molecular rigidity. In concentrated solutions of the NPIs, intermolecular interactions were found to quench the fluorescence due to the formation of excimers. In contrast, upon aggregation (in THF / H₂O mixtures), the NPIs show aggregation‐induced emission enhancement (AIEE). The NPIs also show moderately high solid‐state emission quantum yields (ca. 10–12.7 %). The AIEE behaviour of the NPIs depends on their molecular rigidity and the nature of their intermolecular interactions. The NPIs 1 – 3 show different extents of intermolecular (π–π and C?H???O) interactions in their solid‐state crystal structures depending on their substituents. Detailed photophysical, computational and structural investigations suggest that an optimal balance of structural flexibility and intermolecular communication is necessary for achieving AIEE characteristics in these NPIs.     

A cationic amphiphilic tetraphenylethylene derivative with hydrochromic sensitive property: Applications in anti-counterfeiting ink and rewritable paper

Since the discovery of aggregation induced emission (AIE) phenomenon, various stimuli-responsive materials have been rapidly developed, but there are still great challenges in the application of ink printing due to the bad water solubility. In this research, a new cationic amphiphilic TPE-functionalized pyridine salt (TPE-OTs) was designed, which shows good water solubility and hydrochromic properties. The optical properties of the compound have been studied, which is equipped with the typical AIEE characteristics and TICT effect. The compound can self-assemble to form aggregates with a particle size of about 30 nm in water. What is more, the compound is responsive to the environmental humidity, whose fluorescent color changes from green to yellow as the humidity gradually increased. Based on this characteristic, we applied it to the fluorescent anti-counterfeiting ink, realizing the protection and encryption of information.     

Aggregation-induced emission enhancement of yellow photoluminescent carbon dots for highly selective detection of environmental and intracellular copper(II) ions

Photoluminescent (PL) carbon nanodots (CDs) are prepared through a simple one-step hydrothermal treatment of o-phenylendiamine (OPD), and the as-prepared OPD-CDs show yellow PL emission under the ultraviolet excitation, which can be further enhanced by Cu²⁺ ions owing to Cu²⁺ ions induced aggregation of OPD-CDs through the coordination of Cu²⁺ with amino groups on the surface of OPD-CDs.     

Highlights on the Road towards Highly Emitting Solid‐State Luminophores: Two Classes of Thiazole‐Based Organoboron Fluorophores with the AIEE/AIE Effect

Developing a novel, small‐sized molecular building block that may be capable of emitting light in the solid state is a challenging task and has rarely been reported in the literature. BF₂‐containing dyes seem to be promising candidates towards this aim. Two series of new N^NBF₂ complexes showing aggregation‐induced emission (AIE) and aggregation‐induced emission enhancement (AIEE) were designed and synthesized by means of a new protocol, which improved on the traditional method by employing microwave irradiation. The optical and photophysical properties of the BF₂ complexes were investigated in depth. The synthesized complexes showed fluorescence in both solution and the solid state and, in a mixture of tetrahydrofuran/water, may aggregate into fluorescent nanoparticles. The experimental investigation was supported by quantum mechanical calculations. Their availability, stability, large Stokes shifts, and aggregation capabilities, along with their solid‐state emission capability, render this new class of BF₂ complexes promising AIEE/AIE fluorophores for further applications in the fields of fluorescence imaging and materials science.     

Spatiotemporal Dynamics of Aggregation‐Induced Emission Enhancement Controlled by Optical Manipulation

We present spatiotemporal control of aggregation‐induced emission enhancement (AIEE) of a protonated tetraphenylethene derivative by optical manipulation. A single submicrometer‐sized aggregate is initially confined by laser irradiation when its fluorescence is hardly detectable. The continuous irradiation of the formed aggregate leads to sudden and rapid growth, resulting in bright yellow fluorescence emission. The fluorescence intensity at the peak wavelength of 540 nm is tremendously enhanced with growth, meaning that AIEE is activated by optical manipulation. Amazingly, the switching on/off of the activation of AIEE is arbitrarily controlled by alternating the laser power. This result means that optical manipulation increases the local concentration, which overcomes the electrostatic repulsion between the protonated molecules, namely, optical manipulation changes the aggregate structure. The dynamics and mechanism in AIEE controlled by optical manipulation will be discussed from the viewpoint of molecular conformation and association depending on the laser power.     

激发态分子内质子转移(ESIPT)发色团修饰的树枝形聚合物合成及光物理研究

设计合成了0～4代外围修饰激发态分子内质子转移(ESIPT)发色团的聚酰胺-胺树枝形聚合物G0～G4,化合物结构经过IR,1H NMR,13C NMR和MS表征.稳态光谱研究表明,树枝形聚合物在四氢呋喃溶液中形成了聚集体,发色团酮式发光随着化合物代数增大呈先增加后减小的变化.质子化树枝形聚合物G1-H～G4-H能溶于水,并在水中形成20 nm左右的聚集体,发色团在聚集体疏水区中构象受限,仅发射酮式发光,并且发光强度受树枝形聚合物分子大小的影响.     

White‐Light Electroluminescence with Tetraphenylethylene as Emitting Layer of Aggregation‐Induced Emissions Enhancement

Tetraphenylethylene (TPE) based molecules with easy synthesis, good thermal stability, and especially their aggregation‐induced emissions enhancement (AIEE) effect recently become attractive organic emitting materials due to their potentially practical application in OLEDs. Herein, the AIEE behaviors of tetraphenylethylene dyes (TMTPE and TBTPE) were investigated. Fabricated luminesent device using TMTPE dye as emitting layer displays two strong emitting bands: the blue emission coming from the first‐step aggregation and the yellow emission attributed to the second‐step aggregation. Thus, it can be utilized to fabricate the white‐light OLEDs (WOLEDs) of the single‐emitting‐component. A three‐layer device with the brightness of 1200 cd·m^?2 and current ef?ciency of 0.78 cd·A^?1 emits the close to white light with the CIE coordinates of x=0.333 andy=0.358, when applied voltage from 8–13 V, verifying that the TPE‐based dyes of AIEE effect can be effectively applied in single‐emitting‐component WOLEDs fabrication.     

A novel AIEE and EISPT fluorescent probe for selective detection of cysteine

We designed and synthesized a simple and readily available fluorescent probe 3 for cysteine (Cys) based on naphthalene derivative. The probe is composed of a new class of aggregation-induced emission enhancement (AIEE) active dye 2 based on excited-state intramolecular proton transfer (ESIPT) and an acrylate group as the Cys recognition unit as well as the ESIPT blocking agent, which can be cut off by Cys from the probe in aqueous solution with mild conditions. The probe had great sensitivity and selectivity for the detection of Cys over homocysteine (Hcy) and glutathione (GSH) with a detection limit of 0.05 µM. In addition, we have successfully applied the probe for bioimaging studies of Cys in living cells, indicating that the probe holds great potential for biological applications.     

Time‐Dependent Aggregation‐Induced Enhanced Emission,Absorption Spectral Broadening,and Aggregation Morphology of a Novel Perylene Derivative with a Large D–π–A Structure

Strong aggregation‐caused quenching of perylene diimides (PDI) is changed successfully by simple chemical modification with two quinoline moieties through C?C at the bay positions to obtain aggregation‐induced enhanced emission (AIEE) of a perylene derivative ( Cya‐PDI ) with a large π‐conjugation system. Cya‐PDI is weakly luminescent in the well‐dispersed CH₃CN or THF solutions and exhibits an evident time‐dependent AIEE and absorption spectra broadening in the aggregated state. In addition, morphological inspection demonstrates that the morphology of the aggregated form of Cya‐PDI molecules changed from plate‐shaped to rod‐like aggregates under the co‐effects of time and water. An edge‐to‐face arrangement of aggregation was proposed and discussed. The fact that the Cya‐PDI aggregates show a broad absorption covering the whole visible‐light range and strong intermolecular interaction through π–π stacking in the solid state makes them promising materials for optoelectric applications.     

Excited state intramolecular proton transfer in 2-(2'-arylsulfonamidophenyl)benzimidazole derivatives: insights into the origin of donor substituent-induced emission energy shifts

Donor-substituted 2-(2'-arylsulfonamidophenyl)benzimidazoles undergo efficient excited-state intramolecular proton transfer (ESIPT) upon photoexcitation. The tautomer emission energy depends strongly on the substituent attachment position on the fluorophore pi-system. While substitution with a donor group in the para-position relative to the sulfonamide moiety yields an emission energy that is red-shifted relative to the unsubstituted fluorophore, fluorescence of the meta-substituted derivative appears blue-shifted. To elucidate the origin of the surprisingly divergent emission shifts, we performed detailed photophysical and quantum chemical studies with a series of methoxy- and pyrrole-substituted derivatives. The nature and contribution of solvent-solute interactions on the emission properties were analyzed on the basis of solvatochromic shift data using Onsager's reaction field model, Reichardt's empirical solvent polarity scale ET(30), as well as Kamlet-Abboud-Taft's empirical solvent index. The studies revealed that all ESIPT tautomers emit from a moderately polarized excited-state whose dipole moment is not strongly influenced by the donor-attachment position. Furthermore, the negative solvatochromic shift behavior was most pronounced in protic solvents presumably due to specific hydrogen-bonding interactions. The extrapolated gas-phase emission energies correlated qualitatively well with the trends in Stokes shifts, suggesting that solute-solvent interactions do not play a significant role in explaining the divergent emission energy shifts. Detailed quantum chemical calculations not only confirmed the moderately polarized nature of the ESIPT tautomers but also provided a rational for the observed emission shifts based on the differential change in the HOMO and LUMO energies. The results gained from this study should provide guidelines for tuning the emission properties of this class of ESIPT fluorophores with potential applications in analytical chemistry, biochemistry, or materials science.     

Aggregation-enhanced emissions of intramolecular excimers in disubstituted polyacetylenes

Whereas chain aggregation commonly quenches light emission of conjugated polymers, we here report a phenomenon of aggregation-induced emission enhancement (AIEE): luminescence of polyacetylenes is dramatically boosted by aggregate formation. Upon photoexcitation, poly(1-phenyl-1-alkyne)s and poly(diphenylacetylene)s emit blue and green lights, respectively, in dilute THF solutions. The polymers become more emissive when their chains are induced to aggregate by adding water into their THF solutions. The polymer emissions are also enhanced by increasing concentration and decreasing temperature. Lifetime measurements reveal that the excited species of the polymers become longer-lived in the aggregates. Conformational simulations suggest that the polymer chains contain n=3 repeat units that facilitate the formation of intramolecular excimers. The AIEE effects of the polymers are rationalized to be caused by the restrictions of their intramolecular rotations by the aggregate formation.     

Conjugated hyperbranched poly(aryleneethynylene)s: synthesis, photophysical properties, superquenching by explosive, photopatternability, and tunable high refractive indices

Triphenylamine (TPA)-based conjugated hyperbranched poly(aryleneethynylene)s (PAEs), hb-P1/2, hb-P1/3, and hb-P1/4, were synthesized with high molecular weights and good solubilities through Sonogashira coupling reactions. These PAEs exhibited outstanding thermal stabilities and different emission behaviors. Tetraphenylethene (TPE)-containing hb-P1/2 fluoresced faintly in THF, although its light emission was enhanced by aggregate formation in aqueous media or in thin films, thereby exhibiting an aggregation-induced emission-enhancement (AIEE) effect. Whereas 1,1,2,3,4,5-hexaphenylsilole (HPS)-bearing hb-P1/3 showed no significant change in emission intensity with increasing water content in aqueous media, hb-P1/4, which consisted of TPA-fluorenone donor-acceptor groups, presented almost identical absorptions, but both positive and negative solvatochromic emissions in various solvents. A superquenching effect was observed in the picric-acid-detection process by using nanosuspensions of hb-P1/2. All of the polymers possessed good film formability. UV irradiation of the thin films induced simultaneous photobleaching and cross-linking, thus making them applicable in the fabrication of 2D and 3D patterns. Furthermore, the polymer films also showed high refractive indices, which were tunable upon exposure to UV light.     

Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

The emission process of the o ‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o ‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o ‐carborane would allow for rotation even in the condensed state.