Activation and Tuning of Green Fluorescent Protein Chromophore Emission by Alkyl Substituent-Mediated Crystal Packing

Sulphur-atom positional engineering in perylenimide: structure-property relationships and H-aggregation directed type-I photodynamic therapy

An innovative design strategy of placing sulfur (S)-atoms within the pendant functional groups and at carbonyl positions in conventional perylenimide (PNI-O) has been demonstrated to investigate the condensed state structure-property relationship and potential photodynamic therapy (PDT) application. Incorporation of simply S-atoms at the peri-functionalized perylenimide (RPNI-O) leads to an aggregation-induced enhanced emission luminogen (AIEEgen), 2-hexyl-8-(thianthren-1-yl)-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline-1,3(2H)-dione (API), which achieves a remarkable photoluminescence quantum yield (Φ PL) of 0.85 in aqueous environments and established novel AIE mechanisms. Additionally, substitution of the S-atom at the carbonyl position in RPNI-O leads to thioperylenimides (RPNI-S): 2-hexyl-8-phenyl-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline-1,3(2H)-dithione (PPIS), 8-([2,2'-bithiophen]-5-yl)-2-hexyl-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline-1,3(2H)-dithione (THPIS), and 2-hexyl-8-(thianthren-1-yl)-1H-benzo[5,10]anthra[2,1,9-def]isoquinoline-1,3(2H)-dithion (APIS), with distinct photophysical properties (enlarged spin-orbit coupling (SOC) and Φ PL ≈ 0.00), and developed diverse potent photosensitizers (PSs). The present work provides a novel SOC enhancement mechanism via pronounced H-aggregation. Surprisingly, the lowest singlet oxygen quantum yield (Φ Δ) and theoretical calculation suggest the specific type-I PDT for RPNI-S. Interestingly, RPNI-S efficiently produces superoxide (O2˙-) due to its remarkably lower Gibbs free energy (ΔG) values (THPIS: -40.83 kcal mol-1). The non-toxic and heavy-atom free very specific thio-based PPIS and THPIS PSs showed selective and efficient PDT under normoxia, as a rare example.

Self-assembly solid-state enhanced fluorescence emission of GFP chromophore analogues: Formation of microsheets and microtubes oriented by molecular skeleton

The p-, m- and o-N,N-dimethylamino analogs of the green fluorescent protein (GFP) chromophore (denoted as p-DBHI, m-DBHI and o-DBHI) were synthesized by 2,3-cycloaddition. These three compounds were structurally characterized by NMR, HRMS and single crystal X-ray diffraction and were shown to be in the Z-form in both the solid phase and solution. Their fluorescence properties and self-assembly behaviors were investigated by UV-Vis, photoluminescence spectroscopy, fluorescence microscopy and scanning electron microscopy. They exhibited low fluorescence quantum yields in both protic and aprotic solvents, which was consistent with the reported results, and strong emissions in the solid state, thus exhibiting aggregation-induced emission (AIE) behaviors. By a solvent exchange method, the p-DBHI and o-DBHI were assembled into microsheets, while the m-DBHI was assembled into microtubule-like structures. The photoluminescence properties of the assemblies were compared with those of the pristine microcrystalline powders obtained by evaporation from organic solvents. The fluorescence quantum yields of the microcrystals obtained by self-assembly were recorded to 9.86 %, 3.37 % and 31.65 %, respectively, which were much higher than those of the corresponding pristine powders (4.71 %, 2.51 % and 17.03 %). This indicated that the fluorescence properties in the solid state depended on the morphologies of the particles.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Locking the GFP Fluorophore to Enhance Its Emission Intensity

The Green Fluorescent Protein (GFP) and its analogues have been widely used as fluorescent biomarkers in cell biology. Yet, the chromophore responsible for the fluorescence of the GFP is not emissive when isolated in solution, outside the protein environment. The most accepted explanation is that the quenching of the fluorescence results from the rotation of the aryl-alkene bond and from the Z/E isomerization. Over the years, many efforts have been performed to block these torsional rotations, mimicking the environment inside the protein β-barrel, to restore the emission intensity. Molecule rigidification through chemical modifications or complexation, or through crystallization, is one of the strategies used. This review presents an overview of the strategies developed to achieve highly emissive GFP chromophore by hindering the torsional rotations.

Dual-Emission GFP Chromophore-Based Derivative for Imaging and Discriminating Aβ Oligomers and Aggregates

β-Amyloid deposition is one of the main pathological features of Alzheimer's disease (AD). The development of fluorescent probes targeting specific β-amyloid species has recently become an attractive strategy to achieve the early diagnosis of AD. In this work, a dual-channel fluorescent protein chromophore derivative C17 was rationally designed and synthesized for the detection and discrimination of Aβ₄₂ aggregates and oligomers. C17 exhibits a specific turn-on emission peak for Aβ₄₂ oligomers at ∼470 nm (peak A) and a peak at ∼600 nm (peak B) for both Aβ₄₂ oligomers and Aβ₄₂ aggregates. Taking advantage of the dual emission of the probe, the dynamic aggregation process of the Aβ₄₂ peptide was monitored in solution. Moreover, double staining of brain sections from transgenic AD mice revealed that peak A of C17 preferentially detected Aβ₄₂ oligomers, whereas peak B was more sensitive to Aβ₄₂ aggregates. The fact that probe C17 can be used for dissecting these two Aβ₄₂ species makes C17 a comprehensive tool for β-amyloid aggregation studies in AD research.

Optimizing Comprehensive Performance of Aggregation-Induced Emission Nanoparticles through Molecular Packing Modulation for Multimodal Image-Guided Synergistic Phototherapy

Fluorescent nanoparticles (NPs) with aggregation-induced emission (AIE) characteristics hold remarkable potential for image-guided phototherapy. The molecular packing is the key point for optimizing the performance of AIE luminogens (AIEgens) in the aggregated or solid state. However, so far, the packing mode of AIEgens in NPs is still vague, causing some challenges for understanding the relationship between the photophysical property and packing mode, as well as further optimizing the performance of NPs for biomedical applications. In this contribution, by simply controlling the length of alkoxy chains in the donor-acceptor conjugated OPTPA, a packing balance between the twisted molecular structure and effective π-conjugation is actualized. Subsequently, the possibility of amorphous-crystalline transition of AIEgens in the polymer-encapsulated NPs is presented for the first time, and the comprehensive performance of NPs is further optimized. Both in vitro and in vivo experiments indicate that crystalline AIEgen-based NPs are remarkably effective in trimodal imaging-guided synergistic phototherapy.

Envisioning Quantum Electrodynamic Frameworks Based on Bio-Photonic Cavities

A bio-photonic cavity quantum electrodynamic (C-QED) framework could be imagined as a system in which both the “cavity” and the “atom” participating in the light-matter interaction scenario are bio-inspired. Can a cavity be made of a bio-polymer? If so, how should such a cavity appear and what are the best polymers to fabricate it? Can a bioluminescent material stand the comparison with new-fashion semiconductors? In this review we answer these fundamental questions to pave the way toward an eco-friendly paradigm, in which the ever-increasing demand for more performing quantum photonics technologies meets the ever-increasing yet silent demand of our planet to reduce our environmental footprint.

Structural Water Molecules Confined in Soft and Hard Nanocavities as Bright Color Emitters

Molecules confined in the nanocavity and nanointerface exhibit rich, unique physicochemical properties, e.g., the chromophore in the β-barrel can of green fluorescent protein (GFP) exhibits tunable bright colors. However, the physical origin of their photoluminescence (PL) emission remains elusive. To mimic the microenvironment of the GFP protein scaffold at the molecule level, two groups of nanocavities were created by molecule self-assembly using organic chromophores and by organic functionalization of mesoporous silica, respectively. We provide strong evidence that structural water molecules confined in these nanocavities are color emitters with a universal formula of {X⁺·(OH^-·H₂O)·(H₂O) _n-1}, in which X is hydrated protons (H₃O⁺) or protonated amino (NH₃ ⁺) groups as an anchoring point, and that the efficiency of PL is strongly dependent on the stability of the main emitter centers of the structural hydrated hydroxide complex (OH^-·H₂O), which is a key intermediate to mediate electron transfer dominated by proton transfer at confined nanospace. Further controlled experiments and combined characterizations by time-resolved steady-state and ultrafast transient optical spectroscopy unveil an unusual multichannel radiative and/or nonradiative mechanism dominated by quantum transient states with a distinctive character of topological excitation. The finding of this work underscores the pivotal role of structurally bound H₂O in regulating the PL efficiency of aggregation-induced emission luminogens and GFP.

Synthesis of New AIEE-Active Chalcones for Imaging of Mitochondria in Living Cells and Zebrafish In Vivo

Fluorophores with aggregation-induced emission enhancement (AIEE) properties have attracted increasing interest in recent years. On the basis of our previous research, we successfully designed and synthesized eleven chalcones. Through an optical performance experiment, we confirmed that compounds 1-6 had obvious AIEE properties. As these AIEE molecules had excellent fluorescence properties and a large Stokes shift, we studied their application in living cell imaging, and the results showed that these compounds had low cytotoxicity and good biocompatibility at the experimental concentrations. More importantly, they could specifically label mitochondria. Subsequently, we selected zebrafish as experimental animals to explore the possibilities of these compounds in animal imaging. The fluorescence imaging of zebrafish showed that these AIEE molecules can enter the embryo and can be targeted to aggregate in the digestive tract, which provides a strong foundation for their practical application in the field of biological imaging. Compared with traditional fluorophores, these AIEE molecules have the advantages of possessing a small molecular weight and high flexibility. Therefore, they have excellent application prospects in the field of biological imaging. In addition, the findings of this study have very positive practical significance for the discovery of more AIEE molecules.

Unraveling the Effect on Luminescent Properties by Postsynthetic Covalent and Noncovalent Grafting of gfp Chromophore Analogues in Nanoscale MOF-808

Here, we demonstrate mimicking of photophysical properties of native green fluorescent protein (gfp) by immobilizing the gfp chromophore analogues in nanoscale MOF-808 and further exploring the bioimaging applications. The two virtually nonfluorescent gfp chromophore analogues carrying different functionalities, BDI-AE (COOH/COOMe) and BDI-EE (COOMe/COOMe) were immobilized in nanosized MOF-808 via postsynthetic modification. An ¹H NMR and IR study confirms that BDI-AE was coordinated in NMOF-808, whereas BDI-EE was just noncovalently encapsulated. Interestingly, the extremely weakly fluorescent monomers BDI-AE and BDI-EE (QY = 0.01-0.03%, lifetime = 0.01-0.03 ns) showed a 10²-fold increase in quantum efficiency with a significantly longer excited-state lifetime (QY = 1.8-5.6%, lifetime 0.89-1.49 ns) after immobilization in the NMOF-808 scaffold. Moreover, BDI-AE@MOF-808 has 4 times higher quantum efficiency as well as longer excited-state lifetime in comparison to BDI-EE@NMOF-808 due to the rigidity imposed in the chromophore upon coordination with Zr⁴⁺ in the former case. Further, a cell viability test performed for BDI-AE@NMOF-808 in HeLa cells confirmed the nontoxic nature of the material and, more importantly, bioimaging applications have also been explored successfully.

In situ self-assembled biosupramolecular porphyrin nanofibers for enhancing photodynamic therapy in tumors

Due to the complicated environment and high tissue hydraulic pressure in tumors that easily pumps the nanomedicines back to the systemic circulation, the concentration of released photosensitizers (PSs) retained in a tumor by a traditional nano-delivery system is very low, causing an unsatisfactory photodynamic therapy (PDT) effect. Therefore, we prepared a pH/H₂O₂-responsive nano-system (ZnP-OC-M) through modified porphyrin PS units with a long-unsaturated oleoyl chloride chain, and by the further introduction of hydrophilic hydroxyl groups and MnO₂ through a cis-addition reaction between the unsaturated double bonds of oleoyl chloride and dilute KMnO₄ solution. Making use of the sensitivity of MnO₂ to the H₂O₂ in the acid environment of tumor cells, ZnP-OC-M selectively realized responsive disintegration and O₂ generation. More importantly, the rich amphiphilic PS units were shedded simultaneously and spontaneously completed the self-assembly into nanofibers in situ by helical stacking, which displayed a 1.85-fold higher retention effect of PSs in vivo compared with free PS groups and showed a great tumor inhibition effect in enhancing PDT. This nanosystem effectively solves the problem of the low retention abilities leading to a poor PS concentration in a tumor, prolonging the treatment time window efficiently after only a single administration and achieving the purpose of PDT enhancement.

Functional group engineering in naphthalimides: a conceptual insight to fine-tune the supramolecular self-assembly and condensed state luminescence

Engineering well-defined supramolecular fluorescent nano-architectures based on organic conjugated small molecules has been an essential scientific challenge. Herein, a library of sixteen naphthalimide congeners (1-15 and HNI) has been strategically designed that unveils a conceptual insight into the functional group controlled condensed state emission and aggregation-induced enhanced emission (AIEE) in conventional strong aggregation-caused quenching (ACQ) active fluorophores. Along with the regulation of ACQ-to-AIEE transformation and tailoring of the condensed state emission, a simple yet potential design strategy of functional group engineering has been established for the first time to spontaneously generate and systematically tailor the supramolecular self-assembly of organic small molecules into highly defined nano-architectures. Single-crystal XRD analysis of six congeners revealed that, unlike the well-established electronic contribution of the functional groups in the molecularly dispersed state, the condensed state photophysical and morphological properties are dictated by the distinct intermolecular π-π stacking interaction of the planar aromatic core. This work demonstrates an unconventional influence of the functional motif in the condensed state that could emerge as a promising route to build a fluorescent supramolecular nanoassembly from non-fluorescent conjugated molecules for a variety of future applications.

Crystal structure and spectral studies of green fluorescent protein (GFP) chromophore analogue ethyl 2-[(4Z)-(6-hydroxy naphthalen-2-yl) methylene)-2-methyl-5-oxo-4,5-di hydro-1H-imidazol-1-yl] acetate

Synthetically modified green fluorescent protein chromophore derivative was prepared, its crystal structure and spectral properties were studied. Crystal data for C19H18N2O4: triclinic, space group P-1 (no. 2), a = 8.2506(17) Å, b = 11.934(2) Å, c = 17.461(4) Å, α = 102.89(3)°,   β = 94.62(3)°, γ = 96.68(3)°, V = 1654.5(6) Å3, Z = 4, T = 173(2) K, μ(MoKα) = 0.096 mm-1, Dcalc = 1.358 g/cm3, 7227 reflections measured (4.722° ≤ 2Θ ≤ 53.996°), 7227 unique (Rint = 0.0453, Rsigma = 0.0662) which were used in all calculations. The final R1 was 0.0561 (I > 2σ(I)) and wR2 was 0.1658 (all data). The single crystal structure showed, the benzylidine moiety adopts Z-conformation in solid state and the molecules were associated by various O−H···O and C−H···O non-covalent interactions. The UV absorption-emission spectral analysis indicated that a significant red shift of emission observed at alkaline pH indicating its utility for live cell imaging applications.

S1/S0 Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/ E Photoisomerization and E/ Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group?

Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/ E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C═C double bond in a S₁ excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/ E photoisomerization and E/ Z thermoisomerization phenomena. We found that the S₁ and S₀ potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S₁ PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S₁ excited state of 2b that makes it have a higher barrier for the Z/ E photoisomerization, a smaller Z/ E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S₀ PES, it is not the reduced bond order of the I-bond in the S₀ ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/ Z thermoisomerization and a much smaller E/ Z thermoisomerization rate constant.

Luminescence Tuning of Layered Rare-Earth Hydroxides (LRHs, R = Tb, Y) Composites with 3-Hydroxy-2-naphthoic Acid and Application to the Fluorescent Detection of Al3

Tunable luminescence (quenching or blue shift) of HNA/OS-LRH composites (HNA is 3-hydroxy-2-naphthoic acid; OS is the anionic surfactant of 1-octanesulfonic acid sodium; LRHs are layered rare-earth hydroxides, R = Tb³⁺, Y³⁺) in the solid state and delaminated state is reported, which is utilized as an effective fluorescent probe for detecting metal ions. HNA/OS species are intercalated into LRH layers to generate composites of HNA _xOS_{1- x}-LTbH ( x = 0.10, 0.15, 0.20 , 0.25) and HNA _yOS_{1- y}-LYH ( y = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30). In the solid state, LYH composites exhibit green emissions (from 493 to 504 nm) with a large blue shift in comparison to the 542 nm emission of free HNA^- anions, while in the delaminated state in formamide (FM), the composites display blue emission (480 nm) relative to the green emission (512 nm) of an HNA soltuion in FM. However, LTbH composites display coquenched luminescence in both the solid state and delaminated state. Also, HNA_0.25OS_0.75-1:1-LYH, HNA_0.25OS_0.75-1:2-LYH, and HNA_0.05OS_0.95-1:1-LYH (1:1 and 1:2 are HNA:NaOH molar ratios) show significantly elongated fluorescence lifetimes of 15.35, 14.37, and 12.72 ns, respectively, in comparison with free HNA-Na (6.44 ns), and their quantum yields of 23.40%, 21.97%, and 22.31%, respectively, are much larger than that of free HNA-Na (4.86%). The LTbH composite (HNA_0.25OS_0.75-1:1-LTbH) has also a relatively higher quantum yield of 12.46%. The HNA_0.25OS_0.75-1:1-LYH colloid exhibits excellent recognition selectivity for Al³⁺ over other metal ions (Mg²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, and Hg²⁺) with distinct fluorescence sensitization. It shows an intense change in its fluorescence emission when it is bound to Al³⁺ ions, giving a lower detection limit of 6.32 × 10^-6 M. This is novel research on the fluorescence chemosensing of LRH composites.

Layered rare-earth hydroxide (LRH, R = Tb, Y) composites with fluorescein: delamination, tunable luminescence and application in chemosensoring for detecting Fe(iii) ions

We demonstrate a novel example of tunable luminescence and the application of the delaminated FLN/OS-LRH composites (LRHs are layered rare-earth hydroxides, R = Tb, Y; FLN is the fluorescein named 2-(6-hydroxy-3-oxo-(3H)-xanthen-9-yl)benzoic acid; OS is the anionic surfactant 1-octane sulfonic acid sodium) in detecting Fe(iii) ions. The FLNxOS1-x species (x = 0.02, 0.05, 0.10, and 0.20) are intercalated into the LTbyY1-yH layers (y = 1, 0.9, 0.7, 0.5, 0.3, 0.1 and 0) by ion exchange reactions to yield the composites FLNxOS1-x-LTbyY1-yH. In the solid state, the LYH composites display green emission (564 nm) arising from the organic FLN, while in LTbH composites, the luminescence of the Tb3+ in the layers (545 nm) and the FLN in the interlayers is co-quenched. In the delaminated state in formamide (FM), FLNxOS1-x-LTbH composites display green to yellowish-green luminescence (540-574 nm) following the increasing FLN/OS ratio; while the FLN0.02OS0.98-LTbyY1-yH composites show green emission at ∼540 nm. The fluorescence lifetimes of the composites (4.22-4.63 ns) are comparable to the free FLN-Na, and the quantum yields (31.62-78.70%) of the composites especially that (78.70%) of the FLN0.02OS0.98-LYH are much higher than that (28.40%) of free FLN-Na. The recognition ability of the FLN0.02OS0.98-LYH composite for metal cations is researched. The delaminated FLN0.02OS0.98-LYH colloidal suspension exhibits high selectivity for Fe3+ over other ions (Mg2+, Al3+, Ni2+, Co2+, Cu2+, Zn2+, Mn2+, Pb2+, and Cd2+) with fluorescence quenching, which can work as a kind of turn-off fluorescence sensor for the detection of Fe3+. The detection limit of Fe3+ is determined to be 2.58 × 10-8 M and the quenching constant (Ksv) is 1.70 × 103 M-1. This is the first work on LRH materials working as a chemosensor for recognising metal cations. It provides a new approach for the design of LRH materials to be applied in fluorescence chemosensing.

Rational Design for Multicolor Flavone-Based Fluorophores with Aggregation-Induced Emission Enhancement Characteristics and Applications in Mitochondria-Imaging

Fluorophores with aggregation-induced emission enhancement (AIEE) properties have attracted more attention in recent years. In order to realise more valuable applications, the different kinds of AIEE molecules are in serious need of further development. Therefore, a novel flavone-based AIEE system derived from restriction of intramolecular rotation (RIR) was designed and synthesized in this work. The results revealed that six of the compounds showed typical AIEE characteristics, with fluorescence emissions from purple, blue, cyan to green, tunable by changing substituent groups. This flavone-based AIEE system has never been reported before. The AIEE characteristics were investigated by optical spectroscopy, fluorescence photographs, scanning electron microscopy (SEM), fluorescence quantum yields (Ф_F) and fluorescence lifetime in the CH₃OH/H₂O mixed solution. Moreover, benefiting from the simple structures and small molecular weight, they could permeate cells faster than current high-molecular-weight AIEE molecules. Furthermore, to examine possible biomedical applications, fluorescence imaging in living A549 lung cells and cell viabilities were examined, and the results displayed that these fluorophores showed good cellular uptake and low cytotoxicity within the experimental concentration range. In addition, these AIEE compounds possessed excellent specificity for mitochondrial targeting and mitochondrial morphological change tracking, besides, they displayed superior photostability, which indicated they are potential candidates for mitochondrial imaging.

Fine Tuning of Emission Behavior, Self-Assembly, Anion Sensing, and Mitochondria Targeting of Pyridinium-Functionalized Tetraphenylethene by Alkyl Chain Engineering

Compared to the many studies that focus on the development of novel molecular frameworks pertaining to functionalized fluorescent materials, there is lesser emphasis on side chains even though they have a significant impact on the properties and applications of fluorescent materials. In this study, a series of pyridinium-functionalized tetraphenylethene salts (TPEPy-1 to TPEPy-4) possessing different alkyl chains are synthesized, and the influence of chain length on their optical performance and applications is thoroughly investigated. By changing the alkyl chain, the fluorogens exhibit opposite emission behavior in aqueous media because of their distinct hydrophobic nature, and their solid-state emission can be fine-tuned from green to red owing to their distinct molecular configuration. In addition, by increasing the chain length, the microstructure of the self-assembled fluorogens converts from microplates to microrods with various emission colors. Moreover, TPEPy-1 exhibits dual-mode fluorescence "turn-on" response toward NO₃^- and ClO₄^- in aqueous media because the anions induce the self-assembly of fluorogens. Furthermore, the fluorogens display cellular uptake selectivity while the proper alkyl chain impels the fluorogens to penetrate the cell membrane and accumulate in the mitochondria with high specificity.

A new twist in the photophysics of the GFP chromophore: a volume-conserving molecular torsion couple

Dynamics of a nonplanar GFP chromophore are studied experimentally and theoretically. Coupled torsional motion is responsible for the ultrafast decay.

The simple structure of the chromophore of the green fluorescent protein (GFP), a phenol and an imidazolone ring linked by a methyne bridge, supports an exceptionally diverse range of excited state phenomena. Here we describe experimentally and theoretically the photochemistry of a novel sterically crowded nonplanar derivative of the GFP chromophore. It undergoes an excited state isomerization reaction accompanied by an exceptionally fast (sub 100 fs) excited state decay. The decay dynamics are essentially independent of solvent polarity and viscosity. Excited state structural dynamics are probed by high level quantum chemical calculations revealing that the fast decay is due to a conical intersection characterized by a twist of the rings and pyramidalization of the methyne bridge carbon. The intersection can be accessed without a barrier from the pre-twisted Franck–Condon structure, and the lack of viscosity dependence is due to the fact that the rings twist in the same direction, giving rise to a volume-conserving decay coordinate. Moreover, the rotation of the phenyl, methyl and imidazolone groups is coupled in the sterically crowded structure, with the methyl group translating the rotation of one ring to the next. As a consequence, the excited state dynamics can be viewed as a torsional couple, where the absorbed photon energy leads to conversion of the out-of-plane orientation from one ring to the other in a volume conserving fashion. A similar modification of the range of methyne dyes may provide a new family of devices for molecular machines, specifically torsional couples.

Development of Imidazole-Reactive Molecules Leading to a New Aggregation-Induced Emission Fluorophore Based on the Cinnamic Scaffold

In order to obtain new fluorophores potentially useful in imidazole labeling and subsequent conjugation, a small series of Morita-Baylis-Hillman acetates (3a-c) was designed, synthesized, and reacted with imidazole. The optical properties of the corresponding imidazole derivatives 4a-c were analyzed both in solution and in the solid state. Although the solutions display a very weak emission, the powders show a blue emission, particularly enhanced in the case of compound 4c possessing two methoxy groups in the cinnamic scaffold. The photophysical study confirmed the hypothesis that the molecular rigidity of the solid state enhances the emission properties of these compounds by triggering the restriction of intramolecular motions, paving the way for their applications in fluorogenic labeling.

A metal–organic framework as a flask: photophysics of confined chromophores with a benzylidene imidazolinone core

Photophysics and dynamics of chromophores with a benzylidene imidazolinone core, responsible for emission of green fluorescent protein variants, were studied as a function of host topology by three approaches.

Emission enhancement of GFP chromophore in aggregated state via combination of self-restricted effect and supramolecular host–guest complexation

The emission response of GFP chromophore in aggregated state is greatly enhanced more than 100-fold due to the inhibition of conformational motion and the reduction of strong π–π interaction.

Conjugates of Benzoxazole and GFP Chromophore with Aggregation-Induced Enhanced Emission: Influence of the Chain Length on the Formation of Particles and on the Dye Uptake by Living Cells

Six conjugates of benzoxazole and green fluorescent protein chromophore that differ by the length of their alkyl chain (from C1 to C16) are investigated. They exhibit rigidofluorochromism and clear aggregation-induced emission enhancement (AIEE) behavior with emission in the orange-red that is specific to the solid state. A preparation method based on solvent exchange is used to prepare particles. The self-association properties of these molecules depend on the length of the alkyl chain. Microfibers, platelets, and rounded microparticles are successively obtained by increasing the chain length. The same method is used to prepare nanoparticles (NPs) that are fully characterized. In particular, homogeneous populations of stable NPs measuring around 70 nm are obtained with the analogs whose chains contain four to eight carbon atoms. The behavior with respect to living cells is also influenced by the nature of the compounds. Only the dyes with intermediate hydrophobicity are efficiently uptaken by both normal and tumor cells, and fluorescence only originates from dispersed dye molecules. There is no evidence for incorporation of NPs into cells. This work shows that small variations of the chemical structure must be taken into account for making the best use of AIEE compounds in view of precise applications.

Photophysics of GFP-related chromophores imposed by a scaffold design

In this paper, a rigid scaffold imposes the photophysics of chromophores with a benzylidene imidazolidinone core by mimicking the β-barrel structure of the green fluorescent protein (GFP) and its analogs.

Stereochemical modulation of emission behaviour in E/Z isomers of diphenyldipyrroethene from aggregation induced emission to crystallization induced emission

We have successfully separated and characterized E- and Z-isomers of diphenyldipyrroethene molecules and studies show that the E-isomer behaves as AIEgen, whereas the Z-isomer behaves as CIEgen.

An organic luminescent molecule: what will happen when the "butterflies" come together?

Cross dipole stacking based on a novel fluorescent molecule, 9,10-bis (2,2-diphenylvinyl) anthracene (BDPVA), is presented. The butterfly-like structure of BDPVA is the key feature to form the unique aggregation structure and such a stacking mode is highly beneficial for fluorescence emission, resulting in high-performance amplified spontaneous emission and electroluminescence of BDPVA.

Microcrystals with enhanced emission prepared from hydrophobic analogues of the green fluorescent protein chromophore via reprecipitation

Certain synthetic analogues of the green fluorescent protein (GFP) chromophore are almost nonfluorescent in dilute solutions but are strongly light-emissive in the solid state, thus exhibiting aggregation-induced emission (AIE) behavior. In the present work, two such hydrophobic derivatives of the GFP chromophore known to be fluorescent in the crystalline state (p-hexyloxy- and p-dodecyloxybenzylideneimidazolinone) were used to prepare aqueous suspensions of particles via a mild solvent-exchange reprecipitation (RP) method. This evolution was monitored at various experimental conditions by UV-vis absorption and fluorescence spectroscopy, fluorescence microscopy, as well as electron transmission and scanning microscopy. Both compounds spontaneously produced platelet-like microcrystals, the size and shape of which were influenced by the experimental conditions. The dodecyl derivative also led to the concomitant formation of nanofibers, a tendency reinforced by addition of poly(acrylic acid) to the RP medium. The photoluminescence properties of the solids produced by RP were compared to pristine microcrystalline powders obtained by crystallization in an organic solvent. Significant differences in the emission properties were found and are discussed. This study illustrates the fact that AIE fluorescence is strongly dependent on the nature of the particles and hence on the preparation methods. Being aware of these variations is important for the preparation and subsequent use of AIE-active compounds as fluorescent materials.

(Z)-1-(2-Hy-droxy-eth-yl)-4-(2-meth-oxy-benzyl-idene)-2-methyl-1H-imidazol-5(4H)-one

In the title compound, C14H16N2O3, an analog of the chromophore in green fluorescent protein, the meth-oxy-phenyl substituent and the imidazole N adopt a Z conformation with respect to the C=C bond. Aside from the hy-droxy-ethyl group, the mol-ecule is approximately planar, with the five- and six-membered ring planes forming a dihedral angle of 9.3 (1)°. An intra-molecular C-H⋯N contact occurs. In the crystal, O-H⋯N hydrogen bonds link the mol-ecules, forming chains along the b-axis direction. C-H⋯O hydrogen bonds are also observed.