Halogenated tetraphenylethene with enhanced aggregation-induced emission: an anomalous anti-heavy-atom effect and self-reversible mechanochromism

Halogen-bonded charge-transfer co-crystal scintillators for high-resolution X-ray imaging

The development of high-quality organic scintillators encounters challenges primarily associated with the weak X-ray absorption ability resulting from the presence of low atomic number elements. An effective strategy involves the incorporation of halogen-containing molecules into the system through co-crystal engineering. Herein, we synthesized a highly fluorescent dye, 2,5-di(4-pyridyl)thiazolo[5,4-d]thiazole (Py2TTz), with a fluorescence quantum yield of 12.09%. Subsequently, Py2TTz was co-crystallized with 1,4-diiodotetrafluorobenzene (I2F4B) and 1,3,5-trifluoro-2,4,6-triiodobenzene (I3F3B) obtaining Py2TTz-I2F4 and Py2TTz-I3F3. Among them, Py2TTz-I2F4 exhibited exceptional scintillation properties, including an ultrafast decay time (1.426 ns), a significant radiation luminescence intensity (146% higher than Bi3Ge4O12), and a low detection limit (70.49 nGy s-1), equivalent to 1/78th of the detection limit for medical applications (5.5 μGy s-1). This outstanding scintillation performance can be attributed to the formation of halogen-bonding between I2F4B and Py2TTz. Theoretical calculations and single-crystal structures demonstrate the formation of halogen-bond-induced rather than π-π-induced charge-transfer cocrystals, which not only enhances the X-ray absorption ability and material conductivity under X-ray exposure, but also constrains molecular vibration and rotation, and thereby reducing non-radiative transition rate and sharply increasing its fluorescence quantum yields. Based on this, the flexible X-ray film prepared based on Py2TTz-I2F4 achieved an ultrahigh spatial resolution of 26.8 lp per mm, underscoring the superiority of this strategy in developing high-performance organic scintillators.

Chiral Canoe-Like Pd0 or Pt0 Alloyed Copper Alkynyl Nanoclusters Display Near-Infrared Luminescence

Alloying nanoclusters (NCs) has emerged as a widely explored and versatile strategy for tailoring tunable properties, facilitating in-depth atomic-level investigations of structure-property correlations. In this study, we have successfully synthesized six atomically precise copper NCs alloyed with Group 10 metals (Pd or Pt). Notably, the Pd0 or Pt0 atom situated at the center of the distorted hexagonal antiprism Pd0/Pt0@Cu12 cage, coordinated with twelve Cu+ and two tBuC≡C- ligands. Moreover, ligand exchange strategies demonstrated the potential for Cl- and Br- to replace one or two alkynyl ligands positioned at the top or side of the NCs. The chirality exhibited by these racemic NCs is primarily attributed to the involvement of halogens and a chiral (Pd/Pt)@Cu18 skeleton. Furthermore, all the NCs exhibit near-infrared (NIR) luminescence, characterized by emission peaks at 705-755 nm, lifetimes ranging from 6.630 to 9.662 μs, and absolute photoluminescence quantum yields (PLQYs) of 1.75 %-2.52 % in their crystalline state. The experimental optical properties of these NCs are found to be in excellent agreement with the results of theoretical calculations. These alloy NCs not only offer valuable insights into the synthesis of Pd0/Pt0-Cu alloy NCs, but also bridge the gap in understanding the structure-luminescence relationships of Pd0/Pt0-Cu molecules.

Bromine atom introduction improves the F- sensing ability of an indolo[3,2-b]carbazole-salicylaldehyde-based fluorescence turn-on sensor

The heavy-atom effect usually quenches fluorescence, but scarcely enhances it. Herein, fluorescence turn-on sensors without or with a bromine atom for F- detection are presented, achieving fast response time within 1 min, and the LODs of 1.9 × 10-7 and 8.5 × 10-8 M, reflecting that halogen atom introduction is beneficial for F- detection ability improvement. The sensing mechanism of -OH unit deprotonation is confirmed based on the results of a 1 : 2 stoichiometric ratio, 1H NMR titration and TD-DFT calculation. The water environment F- detection and spiked recovery experiments demonstrate their potential for real sample detection.

Halogen-bonding boosting the high performance X-ray imaging of organic scintillators

Organic scintillators with efficient X-ray excited luminescence are essential for medical diagnostics and security screening. However, achieving excellent organic scintillation materials is challenging due to low X-ray absorption coefficients and inferior radioluminescence (RL) intensity. Herein, supramolecular interactions are incorporated, particularly halogen bonding, into organic scintillators to enhance their radioluminescence properties. By introducing heavy atoms (X = Cl, Br, I) into 9,10-bis(4-pyridyl)anthracene (BPA), the formation of halogen bonding (BPA-X) enhances their X-ray absorption coefficient and restricts the molecular vibration and rotation, which boosts their RL intensity. The RL intensity of BPA-Cl and BPA-Br fluorochromes increased by over 2 and 6.3 times compared to BPA, respectively. Especially, BPA-Br exhibits an ultrafast decay time of 8.25 ns and low detection limits of 25.95 ± 2.49 nGy s-1. The flexible film constructed with BPA-Br exhibited excellent X-ray imaging capabilities. Furthermore, this approach is also applicable to organic phosphors. The formation of halogen bonding in bromophenyl-methylpyridinium iodide (PYI) led to a fourfold increase in RL intensity compared to bromophenyl-methyl-pyridinium (PY). It suggests that halogen bonding serves as a promising and effective molecular design strategy for the development of high-performance organic scintillator materials, presenting new opportunities for their applications in radiology and security screening.

Effects of halogen atom substitution on luminescent radicals: a case study on tris(2,4,6-trichlorophenyl)methyl radical-carbazole dyads

A series of halogen-substitute carbazole TTM radicals was synthesized. The effect of halogen substituents on radical luminescence was systematically evaluated. It was found that the well-known heavy atom effect does not work in the emission of radicals and that halogen substitution of the donor carbazole can change the HOMO and alter the absorption and emission wavelengths. In addition, the photostability was found to be improved with respect to TTM but not significantly different from that of closed-shell fluorescent molecules.

Fluorescent properties in solid-state and solution of novel tricyclic derivatives of chloro/bromophenylchromanones and 2-methylpyrazoline

In this work, we reported the synthesis and spectroscopic characterization of seven novel tricyclic compounds resulting from the reaction of 3-benzylidenechromanone with Cl or Br substituent in different positions and without halogen with methylhydrazine. The structural characterization of compounds was done through different techniques i.e., FTIR,1HNMR,a single and powder X-Ray diffraction. Moreover, fluorescence quantum yield and lifetime assessed their fluorescent properties in the solid state and various solvents. Derivatives with Cl or Br substituent in positions 2 and 4 are isostructural. 4-Cl, 4-Br and 3-Cl compounds exhibit fluorescence with moderate efficiency (quantum yield 0.11-0.26) in solid state due to specific arrangements, so-called π-stack brick stone with head-to-tail self-assembly. Other crystalline compounds (2-Cl, 2-Br and 3-Br) that exhibit negligible fluorescence quantum yield have crossed V-type arrangement. In the solution, the nonhalogenated compound shows the best fluorescence efficiency. In turn, the presence of halogen atoms results in fluorescence decreasing. TD-DFT study revealed that unsubstituted compound higher emissive in solution has a different electron density distribution at HOMO and LUMO levels than less emissive substituted compounds (A3 and A3).

Tetraphenylethylene-based AIE nanoprobes for labeling lysosome by two-photon imaging in living cells

Lysosomes are essential cellular organelles, serving vital functions in cellular metabolism and degradation. The design of specifically targeting lysosomes probes with aggregation-induced emission (AIE) characteristics using two-photon excitation techniques is significance and challenging work. Here we designed and synthesized two tetraphenylethylene (TPE)-based AIE fluorescence probes, naming TPE-Ma and TPE-Py, with TPE as the matrix and morpholine (Ma) or pyrrolidone (Py) as the targeting group. These probes exhibit a significant Stokes shift, low cytotoxicity, two-photo fluorescence imaging and lysosome-specific targeting capability ensuring their suitability for fluorescence imaging applications. To enhance the water solubility and cellular accumulation of TPE-Ma and TPE-Py in tumor cells, we employed a biocompatible polymer 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-mPEG2000) as a nanocarrier. By encapsulating TPE-Ma and TPE-Py within DSPE-mPEG2000, we successfully developed two AIE fluorescent nanoprobes known as DSPE@ TPE-Ma and DSPE@ TPE-Py. The results demonstrated that fluorescent nanoprobes DSPE@ TPE-Ma and DSPE@ TPE-Py possess excellent cell permeability, biocompatibility, superior photostability and specific targeting towards lysosomes in MCF-7 cells. Our findings highlight the potential of these fluorescent nanoprobes as effective tools for two-photon fluorescence imaging and targeted detection of lysosomes in cancer cells.

Photoactivation of Solid-State Fluorescence through Controllable Intermolecular [2+2] Photodimerization

Intermolecular [2+2] photodimerization provides a distinctive approach to construct photoresponsive fluorescent materials in a manner of switching on solid-state fluorescence. Herein, we report efficient photoactivation of bright solid-state fluorescence based on controllable intermolecular [2+2] photodimerization reaction of benzo[b]thiophene 1,1-dioxide (BTO) derivatives, which provides a simple and effective way to construct smart photoresponsive solid-state fluorescent materials. Rational choice of substituents in BTO molecular skeleton enables them to efficiently undergo photodimerization through regulating molecular stacking in crystal, and also leads to photoactivation of solid-state fluorescence due to the generation of brightly fluorescent photodimers. This intermolecular photodimerization reaction also offers an effective method to synthesize photostable AIEgens with purely through-space conjugation.

Synthesis, structure and mechanofluorochromic properties of phenothiazine-S-oxide and phenothiazine-S,S-dioxide derivatives

In this work, two novel luminescent molecules containing distorted phenothiazine-S-oxide and phenothiazine-S,S dioxide skeletons were synthesized by oxidation reactions using different oxidants (m-chloroperoxybenzoic acid, acetic acid /hydrogen peroxide). The target compounds were all confirmed by ¹H NMR, ¹³C NMR and EI-MS. Combined with the results of UV-vis absorption spectra and fluorescence emission spectra, we found that the different oxidation states of S-atom, from sulfide (+2) to sulfoxide (+4) and sulfone (+6), led to the blue, yellow-green and yellowish fluorescence of these compounds in the solid states. Subsequent studies showed that the molecule containing the phenothiazine-S-oxide skeleton exhibited obvious solvatochromism, and the increase of solvent polarity induced a red-shift in the emission wavelength. Moreover, this molecule also exhibited a rare self-recovery mechanochromatic behavior. In addition, these properties were further confirmed by theoretical calculations and X-ray single-crystal diffraction analysis.

Luminescent Behavior of Liquid–Crystalline Gold(I) Complexes Bearing a Carbazole Moiety: Effects of Substituent Bulkiness

Organometallic materials that exhibit white luminescence in condensed phases are of considerable interest for lighting and display applications. Herein, new carbazole-based Au(I) complexes containing an isocyanide group and a long pentyl chain were synthesized. The complex with an unsubstituted carbazole moiety exhibited a white emission at room temperature as well as nematic liquid crystalline behavior. Color tunability from white to blue was achieved when bulkier substituents were introduced at the 3 and 6 positions of the carbazole moiety. Furthermore, all complexes possessed long phosphorescence lifetimes in the crystal state. The proposed design framework provides new opportunities for practical applications using luminescent organometallic molecules.

Halogen atom effect of fluorinated tolanes on their luminescence characteristics

Modification of the halogen constituent in the fluorinated tolane moiety is an effective method for tailoring the functionality of the material.

Dramatic Emission Enhancement of Aggregation-Induced Emission Luminogens by Dynamic Metal Coordination Bonds and Anti-Heavy-Atom Effect

Restriction of intramolecular motions of AIEgens is greatly intensified by introducing dynamic metal coordination bonds to achieve dramatic fluorescence enhancement, which provides a simple and effective way to dramatically improve...

Fluorescent and stimuli-responsive performance of polymer composites filled with tetraphenylethene derivatives

In this study, a series of tetraphenylethene (TPE) derivatives with 3-butenloxy moieties were synthesized. The developed TPE with different number of substituent groups showed controlled aggregation-induced emission performance and variable...

Achieving highly efficient aggregation-induced emission, reversible and irreversible photochromism by heavy halogen-regulated photophysics and D-A molecular pattern-controlled photochemistry of through-space conjugated luminogens

It is extremely challenging but desirable to regulate the photophysical and photochemical processes of aggregation-induced emission luminogens (AIEgens) in distinct states in a controllable manner. Herein, we design two groups of AIEgens based on a triphenylacrylonitrile (TPAN) skeleton with through-space conjugation (TSC) property, demonstrate controlled regulation of photophysical emission efficiency/color and photochemical photochromic and photoactivatable fluorescence behaviours of these compounds, and further validate design principles to achieve highly efficient and emission-tuning AIEgens and to accomplish photo-dependent color switches and fluorescence changes. It is surprisingly found that the introduction of heavy halogens like bromine into a TPAN skeleton dramatically enhances the emission efficiency, and such an abnormal phenomenon against the heavy-atom effect is attributed to the specific through-space conjugation nature of the AIE-active skeleton, effective intermolecular halogen-bond-induced restriction of intramolecular motions, and heavy atom-induced vibration reduction. The incorporation of two electron-donating amino groups into the TPAN skeleton cause the luminogens to undergo a bathochromic shifted emission due to the formation of a D-A pattern. Apart from the regulation of photophysical processes in the solid state, the construction of the D-A pattern in luminogens also results in extremely different photochemical reactions accompanying reversible/irreversible photochromism and photoactivatable fluorescence phenomena in a dispersed state. It is revealed that photo-triggered cyclization and decyclization reactions dominantly contribute to reversible photochromism of the TPAN family, and the photo-induced cyclization-dehydrogenation reaction is responsible for the irreversible color changes and photoactivatable fluorescence behaviours of the NTPAN family. The demonstrations of multiple-mode signaling in photoswitchable patterning and information encryption highlight the importance of controlled regulation of photophysics and photochemistry of fused chromic and AIE-active luminogens in distinct states.

Shedding light on ultrafast ring-twisting pathways of halogenated GFP chromophores from the excited to ground state

Since green fluorescent protein (GFP) has revolutionized molecular and cellular biology for about three decades, there has been a keen interest in understanding, designing, and controlling the fluorescence properties of GFP chromophore (i.e., HBDI) derivatives from the protein matrix to solution. Amongst these cross-disciplinary efforts, the elucidation of excited-state dynamics of HBDI derivatives holds the key to correlating the light-induced processes and fluorescence quantum yield (FQY). Herein, we implement steady-state electronic spectroscopy, femtosecond transient absorption (fs-TA), femtosecond stimulated Raman spectroscopy (FSRS), and quantum calculations to study a series of mono- and dihalogenated HBDI derivatives (X = F, Cl, Br, 2F, 2Cl, and 2Br) in basic aqueous solution, gaining new insights into the photophysical reaction coordinates. In the excited state, the halogenated "floppy" chromophores exhibit an anti-heavy atom effect, reflected by strong correlations between FQY vs. Franck-Condon energy (EFC) or Stokes shift, and knrvs. EFC, as well as a swift bifurcation into the I-ring (major) and P-ring (minor) twisting motions. In the ground state, both ring-twisting motions become more susceptible to sterics and exhibit spectral signatures from the halogen-dependent hot ground-state absorption band decay in TA data. We envision this type of systematic analysis of the halogenated HBDI derivatives to provide guiding principles for the site-specific modification of GFP chromophores, and expand design space for brighter and potentially photoswitchable organic chemical probes in aqueous solution with discernible spectral signatures throughout the photocycle.

Inverse heavy-atom effect in near infrared photoluminescent gold nanoclusters

Fluorophores functionalized with heavy elements show enhanced intersystem crossing due to increased spin-orbit coupling, which in turn shortens the fluorescence decay lifetime (τPL). This phenomenon is known as the heavy-atom effect (HAE). Here, we report the observation of increased τPL upon functionalisation of near-infrared photoluminescent gold nanoclusters with iodine. The heavy atom-mediated increase in τPL is in striking contrast with the HAE and referred to as inverse HAE. Femtosecond and nanosecond transient absorption spectroscopy revealed overcompensation of a slight decrease in lifetime of the transition associated with the Au core (ps) by a large increase in the long-lived triplet state lifetime associated with the Au shell, which contributed to the observed inverse HAE. This unique observation of inverse HAE in gold nanoclusters provides the means to enhance the triplet excited state lifetime.

Investigating the structure–fluorescence properties of tetraphenylethylene fused imidazole AIEgens: reversible mechanofluorochromism and polymer matrix controlled fluorescence tuning

A series of stimuli-responsive AIEgens of tetraphenylethylene (TPE) fused imidazole derivatives (1–7) were synthesized, and their substituent controlled fluorescence properties in the solid state were explored.

Two novel tetraphenylethylene-skeleton salamo-type fluorescent probes: specific recognition of cyanide through different response patterns

The possible sensing mechanism of probes TPES1 and TPES2 towards CN− ions.

The design of dihalogenated TPE monoboronate complexes as mechanofluorochromic crystals

Mechanofluorochromic crystals based on tetraphenylethylene and boronates reversibly change their emission upon grinding, setting the path to develop bistable switches in the future.

Syntheses and structures of three macrocyclic supramolecular complexes and one ZnII-containing coordination polymer generated from a semi-rigid multidentate N-containing ligand

Semirigid organic ligands can adopt different conformations to construct coordination polymers with more diverse structures when compared to those constructed from rigid ligands. A new asymmetric semirigid organic ligand, 4-{2-[(pyridin-3-yl)methyl]-2H-tetrazol-5-yl}pyridine (L), has been prepared and used to synthesize three bimetallic macrocyclic complexes and one coordination polymer, namely, bis(μ-4-{2-[(pyridin-3-yl)methyl]-2H-tetrazol-5-yl}pyridine)bis[dichloridozinc(II)] dichloromethane disolvate, [Zn₂Cl₄(C₁₂H₁₀N₆)₂]·2CH₂Cl₂, (I), the analogous chloroform monosolvate, [Zn₂Cl₄(C₁₂H₁₀N₆)₂]·CHCl₃, (II), bis(μ-4-{2-[(pyridin-3-yl)methyl]-2H-tetrazol-5-yl}pyridine)bis[diiodidozinc(II)] dichloromethane disolvate, [Zn₂I₄(C₁₂H₁₀N₆)₂]·2CH₂Cl₂, (III), and catena-poly[[[diiodidozinc(II)]-μ-4-{2-[(pyridin-3-yl)methyl]-2H-tetrazol-5-yl}pyridine] chloroform monosolvate], {[ZnI₂(C₁₂H₁₀N₆)]·CHCl₃}_n, (IV), by solution reaction with ZnX₂ (X = Cl and I) in a CH₂Cl₂/CH₃OH or CHCl₃/CH₃OH mixed solvent system at room temperature. Complex (I) is isomorphic with complex (III) and has a bimetallic ring possessing similar coordination environments for both of the Zn^II cations. Although complex (II) also contains a bimetallic ring, the two Zn^II cations have different coordination environments. Under the influence of the I^- anion and guest CHCl₃ molecule, complex (IV) displays a significantly different structure with respect to complexes (I)-(III). C-H...Cl and C-H...N hydrogen bonds, and π-π stacking or C-Cl...π interactions exist in complexes (I)-(IV), and these weak interactions play an important role in the three-dimensional structures of (I)-(IV) in the solid state. In addition, the fluorescence properties of L and complexes (I)-(IV) were investigated.

A Pure-Red Doublet Emission with 90 % Quantum Yield: Stable, Colorless, Iodinated Triphenylmethane Solid

Red luminescence is found in off-white tris(iodoperchlorophenyl)methane (3I-PTM^H ) crystals which is characterized by a high photoluminescence quantum yield (PLQY 91 %) and color purity (CIE coordinates 0.66, 0.34). The emission originates from the doublet excited state of the neutral radical 3I-PTM^R , which is spontaneously formed and becomes embedded in the 3I-PTM^H matrix. The radical defect can also be deliberately introduced into 3I-PTM^H crystals which maintain a high PLQY with up to 4 % radical concentration. The immobilized iodinated radical demonstrates excellent photostability (estimated half-life >1 year under continuous irradiation) and intriguing luminescent lifetime (69 ns). TD-DFT calculations demonstrate that electron-donating iodine atoms accelerate the radiative transition while the rigid halogen-bonded matrix suppresses the nonradiative decay.