Rational Design of Push-Pull Fluorene Dyes: Synthesis and Structure-Photophysics Relationship

Fluorescent Solvatochromic Probes for Long-Term Imaging of Lipid Order in Living Cells

High-resolution spatio-temporal monitoring of the cell membrane lipid order provides visual insights into the complex and sophisticated systems that control cellular physiological functions. Solvatochromic fluorescent probes are highly promising noninvasive visualization tools for identifying the ordering of the microenvironment of plasma membrane microdomains. However, conventional probes, although capable of structural analysis, lack the necessary long-term photostability required for live imaging at the cellular level. Here, an ultra-high-light-resistant solvatochromic fluorescence probe, 2-N,N-diethylamino-7-(4-methoxycarbonylphenyl)-9,9-dimethylfluorene (FπCM) is reported, which enables live lipid order imaging of cell division. This probe and its derivatives exhibit sufficient fluorescence wavelengths, brightness, polarity responsiveness, low phototoxicity, and remarkable photostability under physiological conditions compared to conventional solvatochromic probes. Therefore, these probes have the potential to overcome the limitations of fluorescence microscopy, particularly those associated with photobleaching. FπCM probes can serve as valuable tools for elucidating mechanisms of cellular processes at the bio-membrane level.

A new class of teraryl-based AIEgen for highly selective imaging of intracellular lipid droplets and its detection in advanced-stage human cervical cancer tissues

Lipid droplets (LDs) have drawn much attention in recent years. They serve as the energy reservoir of cells and also play an important role in numerous physiological processes. Furthermore, LDs are found to be associated with several pathological conditions, including cancer and diabetes mellitus. Herein, we report a new class of teraryl-based donor-acceptor-appended aggregation-induced emission luminogen (AIEgen), 6a, for selective staining of intracellular LDs in in vitro live 3T3-L1 preadipocytes and the HeLa cancer cell line. In addition, AIEgen 6a was found to be capable of staining and quantifying the LD accumulation in the tissue sections of advanced-stage human cervical cancer patients. Unlike commercial LD staining dyes Nile Red, BODIPY and LipidTOX, AIEgen 6a showed a high Stokes shift (195 nm), a good fluorescence lifetime decay of 12.7 ns, and LD staining persisting for nearly two weeks.

Regioselectivity Control in Spirobifluorene Nitration under Mild Conditions: Explaining the Crivello's Reagent Mechanism

The regioselective nitration of 9,9'-spirobifluorene under mild conditions is reported for the first time by operating under Menke's and Crivello's conditions. The optimized protocol allows obtaining 2-nitro and 2,2'-dinitro-9,9'-spirobifluorene in yields of 79 and 95% and, for the first time, 2,2',7-trinitro-9,9'-spirobifluorene with 66% yield. Besides, the role of dinitrate salt in Crivello's protocol has been now clarified, which opens novel scenarios in the preparation of functional materials.

Synthesis and Optical Properties of a Series of Push-Pull Dyes Based on Pyrene as the Electron Donor

Fifteen push-pull dyes comprising the tetracyclic polyaromatic pyrene have been designed and synthesized. The optical properties of the fifteen dyes have been examined in twenty-two solvents of different polarities. Surprisingly, contrarily to what is classically observed for push-pull dyes of D-π-A structures, a negative solvatochromism could be found for numerous dyes. The photoluminescence and thermal properties of the dyes were also examined. Theoretical calculations were carried out to support the experimental results.

Design Concepts for Solution and Solid-State Emitters - A Modern Viewpoint on Classical and Non-Classical Approaches

For a long time, luminescence phenomena were strictly distinguished between the emission of isolated molecules in dilute solutions or close-packed structures such as in powders or aggregates. This changed with the breakthrough observation of dual-state efficient materials, which led to a rapid boost of publications examining the influence of structural features to achieve balanced emission with disregarded molecular surroundings. Some first general structural design concepts have already been proposed based on reoccurring patterns and pivotal motifs. However, we have found another way to classify these solution and solid-state emitters (SSSEs). Hence, this minireview aims to present an overview of published structural features of SSSEs while shining light on design concepts from a more generalized perspective. Since SSSEs are believed to bridge the gap of hitherto known aggregation-sensitive compound classes, we hope to give future scientists a versatile tool in hand to efficiently design novel luminescent materials.

Beyond "Mega": Origin of the "Giga" Stokes Shift for Triazolopyridiniums

Over the past decade, fluorophores that exhibit "mega" Stokes shifts, defined to be Stokes shifts of greater than 100 nm, have gained considerable attention due to their potential technological applications. A subset of these fluorophores have Stokes shifts of at least 10,000 cm^-1, for whom we suggest the moniker "giga" Stokes shift. The majority of "giga" Stokes shifts reported in the literature arise from the twisted intramolecular charge transfer mechanism, but this mechanism does not fit empirical characterization of triazolopyridinium (TOP). This observation inspired a density functional theory (DFT) and time-dependent DFT study of TOP, and several related fluorophores, to elucidate the novel photophysical origin for the "giga" Stokes shift of TOP. The resulting computational models revealed that photoexcitation of TOP yields a zwitterionic excited state that undergoes significant structural relaxation prior to emission. Most notably, TOP has two orthogonal moieties in the ground state that adopt a coplanar geometry in the excited state. According to Hückel's rule, both the heterocycle and phenyl moieties of TOP should be aromatic in an orthogonal ground state. However, according to Baird's rule, these individual moieties should be anti-aromatic in the excited state. By relaxing to a coplanar conformation in the excited state, TOP likely forms a single aromatic system consisting of both the heterocycle and phenyl moieties.

Solvatochromic Cellular Stress Sensors Reveal the Compactness Heterogeneity and Dynamics of Aggregated Proteome

Deterioration of protein homeostasis (proteostasis) often induces aberrant proteome aggregation. Visualization and dissection of the stressed proteome are of particular interest given their association with numerous degenerative diseases. Recent progress in chemical cellular stress sensors allows for direct visualization of aggregated proteome. Beyond its localization and morphology, the physicochemical nature and the dynamics of the aggregated proteome have been challenging to explore. Herein, we developed a series of solvatochromic fluorene-based D-π-A probes that can selectively and noncovalently bind to a misfolded and aggregated proteome and report on their compactness heterogeneity upon cellular stresses. We achieved this goal by variation of the heterocyclic acceptors to modulate their solvatochromism and binding affinity to amorphous aggregated proteins. The optimized sensor P6 was capable of sensing the polarity differences among different aggregated proteins via its fluorescence emission wavelength. In live cells, P6 revealed the cellular compactness heterogeneity in the aggregated proteome upon cellular stresses. Given the combinative solvatochromic and noncovalent properties, our probe can reversibly monitor the dynamic changes in the aggregated proteome compactness upon stress and after stress recovery, suggesting its potential applications in search of therapeutics to counteract disease-causing proteome stresses.

Synthesis of Fluorenes and Dibenzo[g,p]chrysenes through an Oxidative Cascade

We have developed robust, operationally simple syntheses of fluorenes and of dibenzo[g,p]chrysenes through oxidative cascade processes. These structures that are commonly encountered in optoelectronic materials, dyes, and pharmaceutical products are accessible from 1,4-dioxaspiro[4.5]decan-8-one. The reactions are conducted open to air with inexpensive, safe CuBr₂ or CuCl₂.

Synthesis and Structure-Photophysics Evaluation of 2-N-Amino-quinazolines: Small Molecule Fluorophores for Solution and Solid State

2-N-aminoquinazolines were prepared by consecutive S_N Ar functionalization. X-ray structures display the nitrogen lone pair of the 2-N-morpholino group in conjugation with the electron deficient quinazoline core and thus representing electronic push-pull systems. 2-N-aminoquinazolines show a positive solvatochromism and are fluorescent in solution and in solid state with quantum yields up to 0.73. Increase in electron donor strength of the 2-amino substituent causes a red-shift of the intramolecular charge transfer (ICT) band (300-400 nm); whereas the photoluminescence emission maxima (350-450 nm) is also red-shifted significantly along with an enhancement in photoluminescence efficiency. HOMO-LUMO energies were estimated by a combination of electrochemical and photophysical methods and correlate well to those obtained by computational methods. ICT properties are theoretically attributed to an excitation to Rydberg-MO in SAC-CI method, which can be interpreted as n-π* excitation. 7-Amino-2-N-morpholino-4-methoxyquinazoline responds to acidic conditions with significant increases in photoluminescence intensity revealing a new turn-on/off fluorescence probe.

Intermolecular dark resonance energy transfer (DRET): upgrading fluorogenic DNA sensing

The sensitivity of FRET-based sensing is usually limited by the spectral overlaps of the FRET donor and acceptor, which generate a poor signal-to-noise ratio. To overcome this limitation, a quenched donor presenting a large Stokes shift can be combined with a bright acceptor to perform Dark Resonance Energy Transfer (DRET). The consequent fluorogenic response from the acceptor considerably improves the signal-to-noise ratio. To date, DRET has mainly relied on a donor that is covalently bound to the acceptor. In this context, our aim was to develop the first intermolecular DRET pair for specific sensing of nucleic acid sequences. To this end, we designed DFK, a push-pull probe based on a fluorenyl π-platform that is strongly quenched in water. DFK was incorporated into a series of oligonucleotides and used as a DRET donor with Cy5-labeled complementary sequences. In line with our expectations, excitation of the dark donor in the double-labeled duplex switched on the far-red Cy5 emission and remained free of cross-excitation. The DRET mechanism was supported by time-resolved fluorescence measurements. This concept was then applied with binary probes, which confirmed the distance dependence of DRET as well as its potency in detecting sequences of interest with low background noise.

Syntheses of Acyclic and Macrocyclic Compounds Derived from 9,9-Diethylfluorene (Part I)

A series of new 9,9-diethylfluorenes consisting of three side-arms each bearing a heterocyclic, bis(carboxymethyl)amino, bis(carbamoylmethyl)amino, bis(ethoxycarbonylmethyl)amino or an amino group were prepared on the basis of 2,4,7-tris(bromomethyl)-9,9-diethylfluorene. Imidazolyl, benzimidazolyl, pyrazolyl, pyrrolyl, 1,3-dioxoisoindolyl and pyridinium groups were taken into account as heterocyclic units, attached to the aromatic skeleton via -CH2-, -CH2NHCH2- or -CH2N=CH- linkers. In addition to the seventeen 2,4,7-trisubstituted 9,9-diethylfluorenes, two macrocyclic compounds were prepared on the basis of 2,7-bis(aminomethyl)-9,9-diethylfluorene. The excellent yield of the macrocyclization reaction is worth a special mention. Both the acyclic and the macrocyclic fluorene-based compounds have, among other things, the potential to act as artificial receptors for different substrates in analogy to the known receptors consisting of a benzene or biphenyl core.

Rational design of small molecule fluorescent probes for biological applications

Fluorescent small molecules are powerful tools for visualizing biological events, embodying an essential facet of chemical biology. Since the discovery of the first organic fluorophore, quinine, in 1845, both synthetic and theoretical efforts have endeavored to "modulate" fluorescent compounds. An advantage of synthetic dyes is the ability to employ modern organic chemistry strategies to tailor chemical structures and thereby rationally tune photophysical properties and functionality of the fluorophore. This review explores general factors affecting fluorophore excitation and emission spectra, molar absorption, Stokes shift, and quantum efficiency; and provides guidelines for chemist to create novel probes. Structure-property relationships concerning the substituents are discussed in detail with examples for several dye families. We also present a survey of functional probes based on PeT, FRET, and environmental or photo-sensitivity, focusing on representative recent work in each category. We believe that a full understanding of dyes with diverse chemical moieties enables the rational design of probes for the precise interrogation of biochemical and biological phenomena.

Hydrogen-bonding-induced bathochromic effect of Si-coumarin and its use in monitoring adipogenic differentiation

Introduction of a heteroatom into a fluorophore was carried out for coumarin through a replacement of its bridging oxygen atom with a silicon atom. The maximum-emission wavelength of Si-coumarin (SiC B) bathochromically shifted from 426 nm in cyclohexane to 626 nm in water. The adipogenic differentiation processes in mesenchymal stem cells were monitored using SiC B.

Electron transfer across a spiro link: extreme solvatofluorochromism of a compact spiro-bridged N,N-dimethylaniline-phthalide dyad

Ultrafast intramolecular electron transfer over a spiro link leads to record-breaking solvatofluorochromism [11 600 cm⁻¹, from 357 (hexane) to 595 nm (acetonitrile)].

High Photostability in Nonconventional Coumarins with Far-Red/NIR Emission through Azetidinyl Substitution

Replacement of electron-donating N,N-dialkyl groups with three- or four-membered cyclic amines (e.g., aziridine and azetidine, respectively) has been described as a promising approach to improve some of the drawbacks of conventional fluorophores, including low fluorescent quantum yields (Φ_F) in polar solvents. In this work, we have explored the influence of azetidinyl substitution on nonconventional coumarin-based COUPY dyes. Two azetidine-containing scaffolds were first synthesized in four linear synthetic steps and easily transformed into far-red/NIR-emitting fluorophores through N-alkylation of the pyridine moiety. Azetidine introduction in COUPY dyes resulted in enlarged Stokes' shifts with respect to the N,N-dialkylamino-containing parent dyes, but the Φ_F were not significantly modified in aqueous media, which is in contrast with previously reported observations in other fluorophores. However, azetidinyl substitution led to an unprecedented improvement in the photostability of COUPY dyes, and high cell permeability was retained since the fluorophores accumulated selectively in mitochondria and nucleoli of HeLa cells. Overall, our results provide valuable insights for the design and optimization of novel fluorophores operating in the far-red/NIR region, since we have demonstrated that three important parameters (Stokes' shifts, Φ_F, and photostability) cannot be always simultaneously addressed by simply replacing a N,N-dialkylamino group with azetidine, at least in nonconventional coumarin-based fluorophores.

Conjugated Polyimine Dynamers as Phase-Sensitive Membrane Probes

In this report, dynamic polyimines are introduced as multifunctional sensors of lipid bilayer phases. Under mildly acidic conditions, self-condensation of push-pull amino formyl fluorenes into polyimines occurs in solid- or liquid-ordered phases but not in liquid-disordered phases of vesicular membranes. The obtained conjugated polymers are characterized by a progressive red shift of the absorption maxima, the appearance of exciton-coupled circular dichroism (CD) bands, and fluorescence quenching. These characteristics allow multiple modes of detection of membrane phases, which are known to change under membrane tension.

Smartphone-Based VOC Sensor Using Colorimetric Polydiacetylenes

Owing to a unique colorimetric (typically blue-to-red) feature upon environmental stimulation, polydiacetylenes (PDAs) have been actively employed in chemosensor systems. We developed a highly accurate and simple volatile organic compound (VOC) sensor system that can be operated using a conventional smartphone. The procedure begins with forming an array of four different PDAs on conventional paper using inkjet printing of four corresponding diacetylenes followed by photopolymerization. A database of color changes (i.e., red and hue values) is then constructed on the basis of different solvatochromic responses of the 4 PDAs to 11 organic solvents. Exposure of the PDA array to an unknown solvent promotes color changes, which are imaged using a smartphone camera and analyzed using the app. A comparison of the color changes to the database promoted by the 11 solvents enables the smartphone app to identify the unknown solvent with 100% accuracy. Additionally, it was demonstrated that the PDA array sensor was sufficiently sensitive to accurately detect the 11 VOC gases.

Regioselective ortho-functionalization of bromofluorenecarbaldehydes using TMPMgCl·LiCl

A highly regioselective functionalization of 7-bromofluorene-2-carbaldehydes, potent organic chromophores, in position C3 using a mild ortho-metallation strategy (DoM) with TMPMgCl·LiCl has been developed.

Unveiling the photophysical and morphological properties of an acidochromic thiophene flanked dipyrrolopyrazine-based chromophore for optoelectronic application

A series of dipyrrolopyrazine (DPP) based chromophores featuring thiophene and varied donor (N,N-dimethylamine, NH₂, OMe) and acceptor (CF₃, CN, NO₂) appendages have been synthesized. The structures and properties of the chromophores were investigated by absorption spectroscopy, electrochemistry, differential scanning calorimetry, and thermogravimetric analysis. X-ray crystallographic analysis indicates a planar geometry for the molecule 7g. Surface morphological studies reveal the formation of microrods and nanorods. The acidochromic behavior of the chromophore which shows a prominent red-shift in the absorption spectra owing to the protonation of the pyrazine segment provides a valuable opportunity to assess the sensory response. Acid dependent spectral changes could be successfully applied to detect pH in biological fluids and acid impurities in solvents.

A series of dipyrrolopyrazine (DPP) based chromophores featuring thiophene and varied donor (N,N-dimethylamine, NH₂, OMe) and acceptor (CF₃, CN, NO₂) appendages have been synthesized.

Turn-on Fluorene Push-Pull Probes with High Brightness and Photostability for Visualizing Lipid Order in Biomembranes

The rational design of environmentally sensitive dyes with superior properties is critical for elucidating the fundamental biological processes and understanding the biophysical behavior of cell membranes. In this study, a novel group of fluorene-based push-pull probes was developed for imaging membrane lipids. The design of these fluorogenic conjugates is based on a propioloyl linker to preserve the required spectroscopic features of the core dye. This versatile linker allowed the introduction of a polar deoxyribosyl head, a lipophilic chain, and an amphiphilic/anchoring group to tune the cell membrane binding and internalization. It was found that the deoxyribosyl head favored cell internalization and staining of intracellular membranes, whereas an amphiphilic anchor group ensured specific plasma membrane staining. The optimized fluorene probes presented a set of improvements as compared to commonly used environmentally sensitive membrane probe Laurdan such as red-shifted absorption matching the 405 nm diode laser excitation, a blue-green emission range complementary to the red fluorescent proteins, enhanced brightness and photostability, as well as preserved sensitivity to lipid order, as shown in model membranes and living cells.

Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers

The synthesis of a series of push-pull derivatives bearing triphenylamine electron-donating group, cyclopenta[c]thiophen-4,6-dione electron acceptor and various π-linkers including (hetero)aromatic fragments is reported. All target chromophores with systematically varied π-linker structure were further investigated by electrochemistry, absorption measurements, and EFISH experiments in conjunction with DFT calculations. Based on electrochemical and photophysical measurements, when a polarizable 2,5-thienylene moiety is embedded into the chromophore π-backbone the highest intramolecular charge transfer (ICT) is observed. Benzene, pyrimidine, and pyridazine derivatives exhibit lower polarizability and extent of the ICT across these π-linkers. The elongation of the π-conjugated system via additional ethenylene linker results in a significant reduction of the HOMO-LUMO gap and an enhancement of the NLO response. Whereas it does not significantly influence electrochemical and linear optical properties, the orientation of the pyrimidine ring seems to be a key parameter on the μβ value due to significant variation of the dipolar moment (μ) value. In 2a and 2c, pyrimidine is oriented to behave as an acceptor and thus generate dipolar molecule with μ above 5 D, whereas in 2b and 2d ground state dipole moment is significantly reduced. This study seems to indicate a high aromaticity of pyrimidine and pyridazine derivatives, close to the benzene analogues and significantly higher than thiophene analogues.

Development of Green/Red-Absorbing Chromophores Based on a Coumarin Scaffold That Are Useful as Caging Groups

We report the design, synthesis, and spectroscopic characterization of a series of push-pull chromophores based on a novel coumarin scaffold in which the carbonyl of the lactone function of the original coumarin dyes has been replaced by the cyano(4-nitrophenyl)methylene moiety. The skeleton of the compounds was synthesized by condensation of a thiocoumarin precursor with the corresponding arylacetonitrile derivatives, and their photophysical properties were fine-tuned through the incorporation of electron-withdrawing groups (EWGs) like nitro and cyano at the phenyl ring, leading to absorption in the green to red region. Although fluorescence emission was weakened or even canceled upon introduction of two or three strong EWGs, the emission of the mononitro-containing coumarin derivatives in the red region upon excitation with green light is noticeable, as are their significantly large Stokes shifts. The new coumarin derivatives can be useful as photocleavable protecting groups, as demonstrated through the synthesis and characterization of a series of coumarin-based photocages of benzoic acid. Preliminary photolysis studies with green light have demonstrated that the structure of the coumarin chromophore influences the rate of the uncaging process, opening the way to exploiting these new coumarin scaffolds as caging groups that can be removed with visible light.

Synthesis of selectively 4-substituted 9,9′-spirobifluorenes and modulation of their photophysical properties

Synthesis of selectively 4-substituted 9,9′-spirobifluorenes was accomplished by using catalytic [2 + 2 + 2]-cyclotrimerization of specifically substituted diynols with alkynes to the corresponding fluorenols.

Design, synthesis and photophysical studies of styryl-based push–pull fluorophores with remarkable solvatofluorochromism

A library of 20 styryl-based push–pull dyes derived from 6-amino substituted benzothiazoles were prepared by an efficient and practical synthetic route from low-cost starting materials.

Air-Stable Pd Catalytic Systems for Sequential One-Pot Synthesis of Challenging Unsymmetrical Aminoaromatics

The selective functionalization of dibromoaromatic scaffolds using air-stable palladium catalytic systems was carried out. This methodology involved rapid mono and diselective Buchwald-Hartwig aminations via microwave irradiation. The conditions were optimized to couple sequentially different moieties in one pot. Couplings with a wide scope of amines allowed accessing a new library of symmetrical and unsymmetrical derivatives (35 examples). Using this versatile method, a near-IR push-pull sensor was prepared installing the electron-donating and -withdrawing groups through a multicomponent reaction. These conditions revealed to be gram-scalable and adaptable to various groups; hence, promoting facile use in synthetic chemistry.