Near infrared two-photon-excited and -emissive dyes based on a strapped excited-state intramolecular proton-transfer (ESIPT) scaffold

Tail-Assisted Excited-State Intramolecular Proton Transfer (ta-ESIPT) Fluorophores: A Universal Ratiometric Platform for Hydration-Sensitive Biomolecular Imaging and Sensing

Excited-state intramolecular proton transfer (ESIPT) fluorophores are valuable for ratiometric bioimaging due to their microenvironmental sensitivity, but traditional enol-keto systems suffer from poor biocompatibility and reduced efficiency in polar, protic environments. Here, we introduce a tail-assisted ESIPT (ta-ESIPT) strategy in which proton transfer occurs from an amide donor to an amino nitrogen acceptor. This mechanism applies to biocompatible charge-transfer fluorophores, such as naphthalimide, coumarin, NBD, and acedan. ta-ESIPT fluorophores exhibit broad environmental stability and a hydration-gated response─proton transfer is inhibited in aqueous environments but restored in nonaqueous microenvironments, yielding ratiometric red-shifted emission. This property enables the precise visualization of biomolecular interactions. By conjugating ta-ESIPT fluorophores with protein ligands, we achieve precise, ratiometric imaging of targets like SNAP-tag, hCA, avidin, and HaloTag in live cells, with fluorescence signals that directly correlate with binding affinities. This correlation enables real-time monitoring of protein interactions and evaluation of inhibitors while minimizing nonspecific interference in the complex cellular environment, ensuring dynamic and accurate protein recognition.

Theoretical insights into fluorescent properties and ESIPT behavior of novel flavone-based fluorophore and its thiol and thione derivatives

For the typical ESIPT process, the proton transfer process is often completed via the intramolecular hydrogen bond (IHB) with oxygen or nitrogen as proton donor or proton acceptor. In recent years, the ESIPT process for sulfur-containing hydrogen bonds has received more and more attention, but it has been rarely reported. We systematically studied the ESIPT processes and photophysical properties of 2-(benzothiophene-2-yl)-3-hydroxy-4H-chromen-4-one (BTOH), 2-(benzothiophene-2-yl)-3-mercapto-4H-chromen-4-one (BTSH) and 2-(benzothiophen-2-yl)-3-hydroxy-4H-chromene-4-thione (BTS) at the HISSbPBE/6-31+G(d,p) and TD-HISSbPBE/6-31+G(d,p) computational level. The IHBs were investigated by analyzing structural parameters, infrared (IR) spectra and electron densities. All the results showed that the IHBs become stronger in the excited state. Among these three molecules, the ESIPT energy barrier of BTSH is the lowest, while that of BTS is the highest. By calculating the natural population analysis (NPA) charge, we found that SH group as a proton donor is easier to provide protons than OH group, and the S group as a proton acceptor is more difficult to obtain protons than O group. The simulated electronic spectra showed that the absorption and fluorescence wavelengths of BTSH and BTS have more or less red-shift compared with BTOH.

Triazine-Modified Color-Responsive Triarylboron/Acridine Fluorescent Probe with Multi-Channel Charge Transfer for Highly Sensitive Fluoride Ion Detection

A novel fluoride ion fluorescent probe is designed by introducing the strong electron-withdrawing triazine groups into the triarylboron/acridine conjugation system. The A-D-A' molecular configuration endows this molecule with multiple charge-transfer channels; upon reaction with F-, the triazine groups act as primary acceptors within the molecule, facilitating charge transfer between the acridine units and the triazine groups. During fluoride ion detection, changes in the triarylboron moiety lead to a significant bathochromic-shift in fluorescence emission from green to yellow. Theoretical calculations attribute this phenomenon to a reduction in the molecular S1 state energy level upon fluorination, resulting in a pronounced visible color change and chromogenic response during detection. Based on fluorescence intensity changes with varying degrees of F- coordination, a detection limit as low as 10-7 M was determined for TB-1DMAc-2TRZ, demonstrating the high sensitivity of this probe.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Conceptually innovative fluorophores for functional bioimaging

Fluorophore chemistry is at the forefront of bioimaging, revolutionizing the visualization of biological processes with unparalleled precision. From the serendipitous discovery of mauveine in 1856 to cutting-edge fluorophore engineering, this field has undergone transformative evolution. Today, the synergy of chemistry, biology, and imaging technologies has produced diverse, specialized fluorophores that enhance brightness, photostability, and targeting capabilities. This review delves into the history and innovation of fluorescent probes, showcasing their pivotal role in advancing our understanding of cellular dynamics and disease mechanisms. We highlight groundbreaking molecules and their applications, envisioning future breakthroughs that promise to redefine biomedical research and diagnostics.

Effects of substitution and conjugation on photophysical properties of ESIPT-based fluorophores with the core of 4-aminophthalimide

4-Aminophthalimide is a highly fluorescent signaling unit with excellent photophysical properties and wide application foregrounds. Based on this, a range of theoretical investigations are conducted on the fluorescent probe (E)-5-((2-hydroxybenzylidene) amino) isoindoline-1, 3-dione (HID) with the core of 4-aminophthalimide using density functional theory (DFT) and time-containing density functional theory (TD-DFT) methods in this paper. The optimized configurations, vertical excitation and emission energies, electronic characteristics and excited-state intramolecular proton transfer (ESIPT) behaviors of the probe HID are discussed in detail. Furthermore, to enhance the luminescent properties of HID, five novel compounds have been designed based on the structure of HID by introducing amino, methoxy and naphthalene groups (-NH2, -OMe and C10H8). Our work thoroughly explores how the property and position of substituents and conjugation affect photophysical characteristics and ESIPT processes. We find that the ESIPT dynamics can be modulated by the substitution and conjugation effects. Specifically, the introduction of amino and methoxy groups at the ortho-position and the introduction of the naphthalene group promote the ESIPT behavior of HID1, whereas the introduction of amino and methoxy groups at the meta-position exhibits the contrary impact. Therefore, we boldly infer that the introduction of electron-donating groups in the ortho-position and the introduction of the conjugated group make the ESIPT process more effortless to occur, whereas the introduction of substituents with opposing natures in the meta-position makes the ESIPT process more difficult to occur. In addition, the ionization potentials (IP), electron affinities (EA) and reorganization energies (λh and λe) of molecules are calculated to assess their potential as luminescent materials. Our work not only reveals the luminescence and ESIPT mechanism of the probe HID1, but also proposes to modulate the ESIPT process through the substitution and conjugation effects. In particular, the designed molecules have better photoelectric properties as a result of their red-shifted absorption and fluorescence spectra, smaller energy gaps, larger transferred charges and greater charge transferred distances, which offers some valuable ideas for the experimental development of more efficient organic luminescent materials with ESIPT properties.

A chalcone-based ESIPT and AIE fluorophore for β-gal imaging in living cells

β-Galactosidase (β-gal), which is responsible for the hydrolysis of the glycosidic bond of lactose to galactose, has been recognized as an important biomarker of cell or organism status, especially cell senescence and primary ovarian cancer. Extensive efforts have been devoted to develop probes for detecting and visualizing β-gal in cells. Herein, a fluorescent probe gal-HCA which possesses both excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) properties was prepared to monitor β-gal in living cells. The probe consists of 2-hydroxy-4'-dimethylamino-chalcone (HCA) capped with a D-galactose group. The cleavage of the glycosidic bond in gal-HCA triggered by β-gal releases HCA, which results in a significant bathochromic shift in fluorescence from 532 to 615 nm. The probe exhibited high selectivity and sensitivity toward β-gal with a detection limit as low as 0.0122 U mL-1. The confocal imaging investigation demonstrated the potential of gal-HCA in monitoring the endocellular overexpressed β-gal in senescent cells and ovarian cancer cells. This study provides a straightforward approach for the development of fluorescent probes to monitor β-gal and detection of β-gal-associated diseases.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

A photo-triggered self-accelerated nanoplatform for multifunctional image-guided combination cancer immunotherapy

Precise and efficient image-guided immunotherapy holds great promise for cancer treatment. Here, we report a self-accelerated nanoplatform combining an aggregation-induced emission luminogen (AIEgen) and a hypoxia-responsive prodrug for multifunctional image-guided combination immunotherapy. The near-infrared AIEgen with methoxy substitution simultaneously possesses boosted fluorescence and photoacoustic (PA) brightness for the strong light absorption ability, as well as amplified type I and type II photodynamic therapy (PDT) properties via enhanced intersystem crossing process. By formulating the high-performance AIEgen with a hypoxia-responsive paclitaxel (PTX) prodrug into nanoparticles, and further camouflaging with macrophage cell membrane, a tumor-targeting theranostic agent is built. The integration of fluorescence and PA imaging helps to delineate tumor site sensitively, providing accurate guidance for tumor treatment. The light-induced PDT effect could consume the local oxygen and lead to severer hypoxia, accelerating the release of PTX drug. As a result, the combination of PDT and PTX chemotherapy induces immunogenic cancer cell death, which could not only elicit strong antitumor immunity to suppress the primary tumor, but also inhibit the growth of distant tumor in 4T1 tumor-bearing female mice. Here, we report a strategy to develop theranostic agents via rational molecular design for boosting antitumor immunotherapy.

Acidic tumor microenvironment-activatable fluorescent diagnostic probe for the rapid identification and resection of human tumors via spraying

A fluorescent diagnostic probe for real-time intraoperative image-guided tumor resection can significantly improve the efficiency and quality of oncological therapy, but their development is challenging. Herein, a novel fluorescent diagnostic probe called HLTC based on β-carboline was designed and synthesized. HLTC was found to show a ∼10-fold enhancement of fluorescence quantum field with pH from 7.4 to 4.0, indicating its imaging potential in acid environment which is a typical hallmark of the tumor microenvironment (TME). Following fluorescence microscopy imaging showed HLTC could emit specific signals in cancer cells and sections, by both one-photon excitation and two-photon excitation. Importantly, HLTC enabled the precise and rapid delineation of both transplanted tumor and clinical tumor tissues within several minutes of simple topical spray. The tumor-to-background ratio (TBR) was up to 10.2 ± 1.0 at clinical liver cancer tissues and 9.9 ± 0.3 at clinical colon cancer tissues, allowing precise tumor margin identification and the effective guidance of surgical tumor resection. Furthermore, CCK8 assay, pharmacokinetic evaluation, blood analysis and H&E staining were performed, which verified high biocompatibility and biosafety of HLTC at working concentration. These results reveal the exciting potential of this small-molecule fluorescent diagnostic probe for real-time fluorescence-based navigation during surgical tumor resection.

Bioluminescence, photophysical, computational and molecular docking studies of fully conformationally restricted enamine infraluciferin

A new rationally designed fully rotationally restricted luciferin has been synthesised. This synthetic luciferin, based upon the structure of infraluciferin, has two intramolecular H-bonds to reduce degrees of freedom, an amine group to enhance ICT process, and an alkenyl group to increase π-conjugation. In the spectroscopic measurements and computational calculations, enamine luciferin showed more red-shifted absorption and fluorescence emission than LH₂ and iLH₂. With PpyWT luciferase enamine luciferin gave bioluminescence at 564 nm which is similar to LH₂ at 561 nm. Further investigation by docking studies revealed that the emission wavelength of enamine luciferin might be attributed to the unwanted twisted structure caused by Asp531 within the enzyme. With mutant luciferase FlucRed, the major emission peak was shifted to 606 nm, a distinct shoulder above 700 nm, and 21% of its spectrum located in the nIR range.

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer (ESIPT) Emitters with Sulfobetaine Fragments

This article describes the synthetic efforts towards the solubilization of organic fluorescent emitters based on a 2-(2'-hydroxybenzofuranyl)benzazole (HBBX) scaffold in aqueous media under physiological conditions (PBS, pH 7.4). These dyes are well-known to display the excited-state intramolecular proton transfer (ESIPT) process which leads to a Stokes-shifted fluorescence with enhanced photostability and strong environment dependent features. Organic dyes are hydrophobic by nature and their vectorization into aqueous media usually necessitates amphiphilic polymers. In this study, we show that the incorporation of one or two sulfobetaine fragments, a highly biocompatible zwitterionic unit leads to the vectorization in buffer solution at pH 7.4 while keeping a reasonable ESIPT fluorescence emission. The photophysical properties of all dyes were studied in multiple solvents and showed that, depending on structure and environment, different excited-state species are observed: normal or tautomeric species, as well as a competitive anionic fluorescent derivative. This study shows that it is not only possible to solubilize fluorescent ESIPT dyes in water using sulfobetaine(s) but also that the optical properties can be finely tuned depending on small structural inputs.

Dual Solution-/Solid-State Emissive Excited-State Intramolecular Proton Transfer (ESIPT) Dyes: A Combined Experimental and Theoretical Approach

Excited-state intramolecular proton transfer (ESIPT) dyes typically show strong solid-state emission, but faint fluorescence intensity is observed in the solution state owing to detrimental molecular motions. This article investigates the influence of direct (hetero)arylation on the optical properties of 2-(2'-hydroxyphenyl)benzoxazole ESIPT emitters. The synthesis of two series of ESIPT emitters bearing substituted neutral or charged aryl, thiophene, or pyridine rings is reported herein along with full photophysical studies in solution and solid states, demonstrating the dual solution-/solid-state emission behavior. Depending on the nature of substitution, several excited-state dynamics are observed: quantitative or partially frustrated ESIPT process or deprotonation of the excited species. Protonation studies revealed that pyridine substitution triggered a strong increase of quantum yield in the solution state for the protonated species owing to favorable quinoidal stabilization. These attractive features led to the development of a second series of dyes with alkyl or aryl pyridinium moieties showing strong tunable solution/solid fluorescence intensity. For each series, ab initio calculations helped rationalize and ascertain their behavior in the excited state and the nature of the emission observed by the experimental results.

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine "strap". In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.

2-Hydroxybenzophenone Derivatives: ESIPT Fluorophores Based on Switchable Intramolecular Hydrogen Bonds and Excitation Energy-Dependent Emission

In this work, a new series of 2-hydroxybenzophenone (BPOH) derivatives, BPOH-TPA, BPOH-PhCz, and BPOH-SF substituting with different electron-donating groups are designed and synthesized. Dual-emission spectra are observed in solutions indicating their excited-state intramolecular proton transfer (ESIPT) character. In solid states, all compounds exhibit a broad emission spectrum when excited at low excitation energy, deriving from the enol-type form stabilized by intramolecular hydrogen bonds. Compound BPOH-TPA shows a clear excitation wavelength dependence. However, such behavior is absent in BPOH-PhCz and BPOH-SF, as the rigid and weaker donor moieties may restrict this process. Furthermore, by increasing the excitation energy, dual emission with a high-energy band ranging from 550 to 582 nm and a low-energy band ranging from 625 to 638 nm is obtained in all three molecules. The photophysical studies and single-crystal analyses are performed to further illustrate the excitation-dependent emission. Higher excitation energies can promote more excitons to keto forms via ESIPT, giving a stronger redshifted emission. BPOH-TPA with a stronger donor strength exhibits an obvious color change gradually from yellow to orange-red with the increasing excitation power from 1 to 15 mW/cm². This study provides a novel example of ESIPT materials with tunable emission colors.

Bithiophene-Cored, mono-, bis-, and tris-(Trimethylammonium)-Substituted, bis-Triarylborane Chromophores: Effect of the Number and Position of Charges on Cell Imaging and DNA/RNA Sensing

The synthesis, photophysical, and electrochemical properties of selectively mono-, bis- and tris-dimethylamino- and trimethylammonium-substituted bis-triarylborane bithiophene chromophores are presented along with the water solubility and singlet oxygen sensitizing efficiency of the cationic compounds Cat1+ , Cat2+ , Cat(i)2+ , and Cat3+ . Comparison with the mono-triarylboranes reveals the large influence of the bridging unit on the properties of the bis-triarylboranes, especially those of the cationic compounds. Based on these preliminary investigations, the interactions of Cat1+ , Cat2+ , Cat(i)2+ , and Cat3+ with DNA, RNA, and DNApore were investigated in buffered solutions. The same compounds were investigated for their ability to enter and localize within organelles of human lung carcinoma (A549) and normal lung (WI38) cells showing that not only the number of charges but also their distribution over the chromophore influences interactions and staining properties.

Synthesis of Highly Functionalizable Symmetrically and Unsymmetrically Substituted Triarylboranes from Bench-Stable Boron Precursors

A novel and convenient methodology for the one-pot synthesis of sterically congested triarylboranes by using bench-stable aryltrifluoroborates as the boron source is reported. This procedure gives systematic access to symmetrically and unsymmetrically substituted triarylboranes of the types BAr2 Ar' and BArAr'Ar'', respectively. Three unsymmetrically substituted triarylboranes as well as their iridium-catalyzed C-H borylation products are reported. These borylated triarylboranes contain one to three positions that can subsequently be orthogonally functionalized in follow-up reactions, such as Suzuki-Miyaura cross-couplings or Sonogashira couplings.

Impact of Heteroatom Substitution on Dual-State Emissive Rigidified 2-(2'-hydroxyphenyl)benzazole Dyes: Towards Ultra-Bright ESIPT Fluorophores*

2-(2'-Hydroxyphenyl)benzazole (HBX) fluorophores are well-known excited-state intramolecular proton transfer (ESIPT) emitters largely studied for their synthetic versatility, photostability, strong solid-state fluorescence and ability to engineer dual emission, thus paving the way to applications as white emitters, ratiometric sensors, and cryptographic dyes. However, they are heavily quenched in solution, due to efficient non-radiative pathways taking place as a consequence of the proton transfer in the excited-state. In this contribution, the nature of the heteroring constitutive of these rigidified HBX dyes was modified and we demonstrate that this simple structural modification triggers major optical changes in terms of emission color, dual emission engineering, and importantly, fluorescent quantum yield. Investigation of the photophysical properties in solution and in the solid state of a series of ethynyl-TIPS extended HBX fluorophores, along with ab initio calculations demonstrate the very promising abilities of these dyes to act as bright dual-state emitters, in both solution (even in protic environments) and solid state.

ESIPT-Related Origin of Dual Fluorescence in the Selected Model 1,3,4-Thiadiazole Derivatives

In our previous work, we discussed the emergence of the dual fluorescence phenomenon in selected compounds from the group of 1,3,4-thiadiazoles. The results obtained in a number of experimental studies, supported by [TD]DFT calculations, clearly indicated that the phenomenon of dual fluorescence stemmed from an overlap of several factors, including the correct conformation of the analyzed molecule and, very significantly in this context, aggregation effects. Where those two conditions were met, we could observe the phenomenon of intermolecular charge transfer (CT) and the emergence of electronic states responsible for long wave emissions. However, in light of the new studies presented in this paper, we were able, for the first time, to provide a specific theory for the effect of dual fluorescence observed in the analyzed group of 1,3,4-thiadiazoles. We present the results of spectroscopic measurements conducted for two selected analogues from the 1,3,4-thiadiazole group, both in polar and non-polar solvents, which clearly evidence (as we have already suspected in the past, albeit have not shown in publications to date) the possibility of processes related to emission from the tautomer formed in the process of excited state intramolecular proton transfer, which is responsible for the long-wavelength emissions observed in the selected analogues. The presented results obtained with the use of UV-Vis, fluorescence (stationary and time-resolved), FTIR, and Raman spectroscopy, as well as from calculations of dipole moment changes between the ground and excited state with the use of two derivatives with different structures of the resorcylic system, corroborated our standing hypothesis. At the same time, they excluded the presence of ground state keto forms of the analyzed analogues unless necessitated by the structure of the molecule itself. In this case, aggregation factors enhance the observed effects related to the dual fluorescence of the analyzed compounds (by way of AIE-aggregated induced emissions).

Anion-Responsive Molecules That Exhibit Switching of Two-Photon Optical Properties

Two-photon-excited fluorescent probes are important for two-photon microscopy for biomedical studies. In contrast to the many examples of probes for cationic species, such as metal ions, there have been fewer reports on the control of two-photon optical properties by anions because in such systems it is difficult to control the associated π-electronic states. This Minireview summarizes anion-responsive molecules that exhibit changes in two-photon optical properties and describes their molecular design and anion-response mechanisms, which are driven by changes in covalent bonds and noncovalent interactions. Results from a recent study of two-photon systems, where geometries and optical properties are modulated by anion binding, are also discussed.

The endeavor of vibration-induced emission (VIE) for dynamic emissions

Organic chromophores with large Stokes shifts and dual emissions are fascinating because of their fundamental and applied interest. Vibration-induced emission (VIE) refers to a tunable multiple fluorescence exhibited by saddle-shaped N,N'-disubstituted-dihydribenzo[a,c]phenazines (DHPs), which involves photo-induced configuration vibrations from bent to planar form along the N-N axis. VIE-active molecules show intrinsic long-wavelength emissions in the unconstrained state (planar state) but bright short-wavelength emissions in the constrained state (bent state). The emission response for VIE-active luminogens is highly sensitive to steric hindrance encountered during the planarization process such that a tiny structural variation can induce an evident change in fluorescence. This can often be achieved by tuning the intensity ratio of short- and long-wavelength bands. In some special cases, the alterations in the emission wavelength of VIE fluorophores can be achieved step by step by harnessing the degree of bending angle motion in the excited state. In this perspective, we summarize the latest progress in the field of VIE research. New bent heterocyclic structures, as novel types of VIE molecules, are being developed, and the general features of the chemical structures are also being proposed. Technologically, novel emission color-tuning approaches and VIE-based probes for visualizing biological activity are presented to demonstrate how the dynamic VIE effect can be exploited for cutting-edge applications.

Simultaneously boosting the conjugation, brightness and solubility of organic fluorophores by using AIEgens

Organic near-infrared (NIR) emitters hold great promise for biomedical applications. Yet, most organic NIR fluorophores face the limitations of short emission wavelengths, low brightness, unsatisfactory processability, and the aggregation-caused quenching effect. Therefore, development of effective molecular design strategies to improve these important properties at the same time is a highly pursued topic, but very challenging. Herein, aggregation-induced emission luminogens (AIEgens) are employed as substituents to simultaneously extend the conjugation length, boost the fluorescence quantum yield, and increase the solubility of organic NIR fluorophores, being favourable for biological applications. A series of donor-acceptor type compounds with different substituent groups (i.e., hydrogen, phenyl, and tetraphenylethene (TPE)) are synthesized and investigated. Compared to the other two analogs, MTPE-TP3 with TPE substituents exhibits the reddest fluorescence, highest brightness, and best solubility. Both the conjugated structure and twisted conformation of TPE groups endow the resulting compounds with improved fluorescence properties and processability for biomedical applications. The in vitro and in vivo applications reveal that the NIR nanoparticles function as a potent probe for tumour imaging. This study would provide new insights into the development of efficient building blocks for improving the performance of organic NIR emitters.

Chalcone-based molecules: Experimental and theoretical studies on the two-photon absorption and molecular first hyperpolarizability

Five chalcone-based molecules denominated by C-3 ((E)-1-(4-methoxyphenyl)-3-phenylprop-2-en-1-one), C-4 ((E)-1,3-bis(4-methoxyphenyl)prop-2-en-1-one), C-5 ((E)-1-(benzo[d][1,3]dioxol-5-yl)-3-(4-methoxyphenyl)prop-2-en-1-one), C-6 ((E)-3-(naphthalen-1-yl)-1-phenylprop-2-en-1-one) and C-7 ((E)-1-(4-methoxyphenyl)-3-(naphthalen-1-yl)prop-2-en-1-one) were synthesized by Claisen-Schmidt reaction in solution of NaOH in water/ethanol 2:1. The aldehydes used were benzaldehyde, anisaldehyde, and β-naphthaldehyde, while the used ketones were acetophenone, p-methoxyacetophenone, and 3,4-methylenedioxyacetophenone. Z-scan and hyper-Rayleigh scattering techniques were used to study the nonlinear optical properties of these compounds in dichloromethane medium. By using Z-scan technique with femtosecond pulses, two-photon absorption cross-sections (σ^TPA) were determined, while the first molecular electronic hyperpolarizabilities (β^HRS) were evaluated by the hyper-Rayleigh scattering technique, with picosecond pulses. From the recorded two-photon absorption spectra, it was identified that compound C-7 presented the highest σ^TPA, regarding the HOMO-LUMO transition, with a value of 40 GM, while C-6 achieved the lowest value for the same transition with 13 GM. Concerning the values of the first molecular hyperpolarizability, compound C-4 presented the highest value, 38 × 10^-30 cm⁴ statvolt^-1, while C-3 presented the lowest β^HRS value of about 16 × 10^-30 cm⁴ statvolt^-1. Time-dependent density functional theory calculations were used to simulate the one- and two-photon absorption spectra, as well to predict the theoretical value of β^HRS in dichloromethane and vacuum medium.

Dual fluorescence in strap ESIPT systems: a theoretical study

Alkylamine-strapped chromophores based on a dithienylpyrrole core, and in which the Excited State Intramolecular Proton Transfer (ESIPT) process yields a zwitterionic structure rather than a keto tautomer have been reported recently (Suzuki et al., Angew. Chem. Int. Ed., 2014, 53, 8231), and are known to exhibit large Stokes shifts. Using Time-Dependent Density Functional Theory (TD-DFT) we investigate the ESIPT mechanism in this family of chromophores considering various substituents and two solvents (cyclohexane and acetone). In order to model the solvent effects, three polarisation models have been applied: the linear response (LR), the corrected linear-response (cLR), and the combination of these two formalisms (LR + cLR). The selected protocol is shown to be effective for a series of compounds with known experimental behaviors, and is then applied to novel derivatives with various donor and acceptor groups and heteroatoms. We determine the absorption and emission wavelengths as well as the energies of the different states that play a role in the ESIPT process. We show that the introduction of electron-withdrawing and electron-donating groups plays an important role in achieving redshifted emission from the ESIPT state.

Synthesis and characterization of poly(tetraphenylimidazole)s and their application in the detection of fluoride ions

Here, we describe the synthesis and characterization of a silyl protected tetraphenylimidazole monomer and its homo and co-polymer. The requisite monomer was accessed by Suzuki–Miyaura cross-coupling of 2-(1-(4-bromophenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenol and 4-vinylphenylboronic acid followed by protection of the phenolic group by tert-butyl(chloro)diphenylsilane. The desired polymers were readily synthesized by using free radical polymerization. Both the polymers and monomer were characterized using different analytical techniques including multinuclear NMR, GPC (for polymers), and single crystal X-ray crystallography (for the monomer). By utilizing the greater fluorophilicity of the silyl atom, the polymers were studied as probes for the detection of fluoride ions. The selectivity and sensitivity of the synthesized polymers were investigated in detail.

We describe the synthesis and characterization of a silyl protected tetraphenylimidazole monomer and its polymers. The polymers were studied as probes for the detection of fluoride ions. Both the probes showed high selectivity and sensitivity over other ions tested.

Tetraphenylene-Coated Near-Infrared Benzoselenodiazole Dye: AIE Behavior, Mechanochromism, and Bioimaging

A D-π-A-π-D type of tetraphenylene-coating benzoselenodiazole fluorescence dye with near-infrared emission has been designed and constructed. This dye shows an obvious aggregation-induced-emission behavior. In the solid state, it exhibits a reversible mechanochromism with the changes of near-infrared emission. Furthermore, this dye can be used to track the lysosomes of living cells and images in vivo.

Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies

A series of five excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2′-hydroxyphenyl) benzoxazole (HBO) scaffold, functionalized with a mono-or bis-(trialkylsilyl) acetylene extended spacer are presented. Investigation of their photophysical properties in solution and in the solid-state in different matrix, along with ab initio calculations gave useful insights into their optical behavior. Random lasing studies were conducted on a series of PMMA doped thin films, showing the presence of stimulated emission above the threshold of pumping energy density (ρth ≈ 0.5–2.6 mJ cm⁻²). In this work, the similarity of four level laser systems is discussed in light of the ESIPT photocycle.

Excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2′-hydroxybenzofuranyl)benzoxazole (HBBO) scaffold functionalised with oligo(ethylene glycol) (OEG) chains

This article describes the multi-step synthesis of 2-(2′-hydroxybenzofuran)benzoxazole (HBBO) derivatives functionalised with one to three oligo(ethylene glycol) (OEG) chains with the goal to allow a good vectorization in aqueous media.

Excited-State Intramolecular Proton Transfer-Based Multifunctional Solid-State Emitter: A Fluorescent Platform with "Write-Erase-Write" Function

The excited-state intramolecular proton transfer (ESIPT)-based molecular probes have drawn significant attention owing to their environment-sensitive fluorescence properties, large Stokes shift, and emerged as building blocks for the development of molecular sensors and switches. However, most of the ESIPT-based fluorophores exhibit weak emission in the solid state limiting the scope of real-time applications. Addressing such issues, herein, we presented a C₃ symmetric-like molecular architecture employing a simple one-step Schiff base condensation between triaminoguanidinium chloride and 3,5-di- tert-butyl-2-hydroxybenzaldehyde (TGHB). The temperature-dependent fluorescence studies including at 77 K indicated the strong emission from the keto tautomer compared to that of the enol tautomer. The facile ESIPT in TGHB in the solid-state led to a remarkable enhancement of fluorescence quantum yield of 1600 times compared to that of the solution (λ_em = 545 nm) by restricting the intramolecular rotation and subsequently suppressing the nonradiative deactivation. The excited-state processes were further elucidated through time-resolved fluorescence measurements. TGHB exhibited turn on-off fluorescence upon exposure to acid/base vapor in the form of a powder as well as a transparent, free-standing thin film. A rewritable and erasable fluorescent platform was demonstrated using TGHB as molecular ink, which offers a potential testbed for performing "write-erase-write" cycles multiple times. In addition, TGHB, possessing multiple binding sites (O and N donors) involving the central core of the triaminoguanidinium cation displayed selective turn-on fluorescence with Zn²⁺. The structure-property relationship revealed in the present study provides insight into the development of novel cost-effective multifunctional materials, which are promising for stimuli-responsive molecular switches.

ESIPT-capable 2,6-di(1H-imidazol-2-yl)phenols with very strong fluorescent sensing signals towards Cr(iii), Zn(ii), and Cd(ii): molecular variation effects on turn-on efficiency

The derivatization influence was studied for twelve 2,6-di(1H-imidazol-2-yl)phenols, whereby a strong fluorescent sensitivity for Cr(iii) was reported, while Zn(ii)/Cd(ii) sensing potentials also appeared.