Highly Solvatochromic 7-Aryl-3-hydroxychromones

Merocyanines: Electronic Structure and Spectroscopy in Solutions, Solid State, and Gas Phase

Merocyanines, owing to their readily tunable electronic structure, are arguably the most versatile functional dyes, with ample opportunities for tailored design via variations of both the donor/acceptor (D/A) end groups and π-conjugated polymethine chain. A plethora of spectral properties, such as strong solvatochromism, high polarizability and hyperpolarizabilities, and sensitizing capacity, motivates extensive studies for their applications in light-converting materials for optoelectronics, nonlinear optics, optical storage, fluorescent probes, etc. Evidently, an understanding of the intrinsic structure-property relationships is a prerequisite for the successful design of functional dyes. For merocyanines, these regularities have been explored for over 70 years, but only in the past three decades have these studies expanded beyond the theory of their color and solvatochromism toward their electronic structure in the ground and excited states. This Review outlines the fundamental principles, essential for comprehension of the variable nature of merocyanines, with the main emphasis on understanding the impact of internal (chemical structure) and external (intermolecular interactions) factors on the electronic symmetry of the D-π-A chromophore. The research on the structure and properties of merocyanines in different media is reviewed in the context of interplay of the three virtual states: nonpolar polyene, ideal polymethine, and zwitterionic polyene.

Synthesis and study of antibiofilm and antivirulence properties of flavonol analogues generated by palladium catalyzed ligand free Suzuki-Miyaura coupling against Pseudomonas aeruginosa PAO1

The Suzuki-Miyaura coupling is one of the ubiquitous method for the carbon-carbon bond-forming reactions in organic chemistry. Its popularity is due to its ability to undergo extensive coupling reactions to generate a broad range of biaryl motifs in a straightforward manner displaying a high level of functional group tolerance. A convenient and efficient synthetic route to arylate different substituted flavonols through the Suzuki-Miyaura cross-coupling reaction has been explained in this study. The arylated products were acquired by the coupling of a variety of aryl boronic acids with flavonols under Pd(OAc)2 catalyzed reaction conditions in a ligand-free reaction strategy. Subsequently, the antibiofilm and antivirulence properties of the arylated flavonols against Pseudomonas aeruginosa PAO1 were studied thoroughly. The best ligands for quorum sensing proteins LasR, RhlR, and PqsR were identified using molecular docking study. These best fitting ligands were then studied for their impact on gene expression level of P. aeruginosa by RT-PCR towards quorum sensing genes lasB, rhlA, and pqsE. The downregulation in the gene expression with the effect of synthesized flavonols endorse the antibiofilm efficiency of the compounds.

Leveraging Coupled Solvatofluorochromism and Fluorescence Quenching in Nitrophenyl-Containing Thiazolothiazoles for Efficient Organic Vapor Sensing

Solvatofluorochromic molecules provide strikingly high fluorescent outputs to monitor a wide range of biological, environmental, or materials-related sensing processes. Here, thiazolo[5,4-d]thiazole (TTz) fluorophores equipped with simple alkylamino and nitrophenyl substituents for solid-state, high-performance chemo-responsive sensing applications are reported. Nitroaromatic substituents are known to strongly quench dye fluorescence, however, the TTz core subtly modulates intramolecular charge transfer (ICT) enabling strong, locally excited-state fluorescence in non-polar conditions. In polar media, a planar ICT excited-state shows near complete quenching, enabling a twisted excited-state emission to be observed. These unique fluorescent properties (spectral shifts of 0.13 - 0.87 eV and large transition dipole moments Δµ = 20.4 - 21.3 D) are leveraged to develop highly sought-after chemo-responsive, organic vapor optical sensors. The sensors are developed by embedding the TTz fluorophores within a poly(styrene-isoprene-styrene) block copolymer to form fluorescent dye/polymer composites (Φ_F = 70 - 97%). The composites respond reversibly to a comprehensive list of organic solvents and show low vapor concentration sensing (e.g., 0.04% solvent saturation vapor pressure of THF - 66 ppm). The composite films can distinguish between solvent vapors with near complete fluorescent quenching observed when exposed to their saturated solvent vapor pressures, making this an extremely promising material for optical chemo-responsive sensing.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T₁ =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S₁ →S₀ CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m^-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (³ LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Synthesis and crystal structure of 2-(2-(2-fluorophenyl)-2-oxoethyl)-4H-chromen-4-one, C17H11FO3

C17H11FO3, monoclinic, P21/c (no. 14), a = 9.1987(2) Å, b = 17.5458(3) Å, c = 8.5802(2) Å, β = 108.193(3)°, V = 1315.60(5) Å3, Z = 4, R gt (F) = 0.0346, wR ref (F 2) = 0.1196, T = 293(2) K.

Highly Efficient Red/NIR-Emissive Fluorescent Probe with Polarity-Sensitive Character for Visualizing Cellular Lipid Droplets and Determining Their Polarity

Lipid droplets (LDs), which are ubiquitous organelles existing in almost all eukaryotic cells, have attracted a lot of attention in the field of cell biology over the last decade. For the biological study of LDs via fluorescence imaging, the superior LD fluorescent probes with environmental polarity-sensitive character are highly desired and powerful but are very scarce. Herein, we have newly developed such a kind of fluorescent probe named LDs-Red which enables us to visualize LDs and to further reveal their polarity information. This fluorescent probe displays the advantages of intense red/near-infrared emission, high LD staining specificity, and good photostability; thus, it would be very useful for LD fluorescence imaging application. As a result, the three-dimensional confocal imaging to visualize spatial distribution of LDs and the multicolor confocal imaging to simultaneously observe LDs and other cellular organelles have been realized using this new LD fluorescent probe. Furthermore, the polarity-sensitive emission character of this probe enables us to quantitatively determine the LD polarity via spectral scan imaging. Consequently, the cancer cells (HepG2, HeLa, and Panc02) displaying lower polarity of LDs than the normal cells (L929, U251, and HT22) have been systematically demonstrated. In addition, this polarity-sensitive probe displaying shorter fluorescence wavelengths in cancer cells than in normal cells has an important and potential ability to distinguish them.

Synthesis and photophysical properties of a new push-pull pyrene dye with green-to-far-red emission and its application to human cellular and skin tissue imaging

Herein, we discuss a new pyrene-based push-pull dye (PC) and our investigation of its photophysical properties and applicability to biological studies. The newly synthesized dye exhibits highly polarity-sensitive fluorescence over a significantly wide range (i.e., the green to far-red region), accompanied by high fluorescence quantum yields (ΦFL > 0.70 in most organic solvents) and superior photostability to that of the commonly used Nile Red (NR) dye, which also fluoresces in the green to red region. When human prostate cancer cells stained with PC were imaged using a confocal laser scanning fluorescence microscope, PC was found to selectively stain the lipid droplets. Under the cell conditions where the formation of droplets was inhibited, PC could be distributed to both the remaining droplets and the intercellular membranes, which could be distinguished based on the fluorescence solvatochromic function of PC. Furthermore, PC efficiently stained normal human skin tissue blocks treated with a transparency-enhancing agent and enabled clear visualization of individual cells in each tissue architecture by means of two-photon fluorescence microscopy (2PM). Interestingly, PC provides bright 2PM images under tissue-penetrative 960 nm excitation, realizing much clearer and deeper tissue imaging than conventional pyrene dyes and NR. These results suggest that PC could replace several commonly used dyes in various biological applications, particularly the rapid and accurate diagnosis of tissue diseases, typified by biopsy.

Nucleotides bearing aminophenyl- or aminonaphthyl-3-methoxychromone solvatochromic fluorophores for the enzymatic construction of DNA probes for the detection of protein-DNA binding

We designed and synthesized nucleosides bearing aminophenyl- or aminonaphthyl-3-methoxychromone fluorophores attached at position 5 of cytosine or thymine and converted them to nucleoside triphosphates. The fluorophores showed solvatochromic fluorescence with strong fluorescence at 433-457 nm in non-polar solvents and very weak fluorescence at 567 nm in alcohols. The nucleosides and nucleotides also showed only negligible fluorescence in alcohols or water. The triphosphates were substrates for DNA polymerase in the enzymatic synthesis of modified DNA probes that showed only very weak fluorescence in aqueous buffer but a significant light-up and blue shift were observed when they interacted with proteins (histone H3.1 or p53 for double-stranded DNA probes or single-strand binding protein for single-stranded oligonucleotide probes). Hence, nucleotides have good potential in the construction of DNA sensors for studying protein-DNA interactions. The modified dNTPs were also transported into cells using a cyclodextrin-based transporter but they were not incorporated into the genomic DNA.

A selective fluorescent probe for gadoliniumIII in water based on a PdII-preorganized chromone-receptor

The synthesis, solution studies, photochemical properties and the X-ray structure of a chromone based fluorescent Pd^II complex are reported. The ligand contains two chromone units linked as side arms to an ethylenediamine moiety; in the Pd^II complex the metal ion preorganizes the two hydroxychromone units forming a rigid structure with a negatively charged pocket formed by four oxygen atoms that is able to interact with hard metal cations, such as ions, giving rise to stable bimetallic complexes. Upon interaction with La^III and Gd^III, in particular, the emission intensity at 423 nm increases by a factor of 2 and 8, respectively, while the other rare earth ions quench the fluorescence. Spectrofluorimetric studies on real matrices showed the possibility to use this system as a selective fluorescence probe to detect and trace the presence of Gadolinium in environmental water acting as an OFF-ON chemosensor, with a LOD of 0.4 ppm and a LOQ of 1.2 ppm.

Determining Essential Requirements for Fluorophore Selection in Various Fluorescence Applications Taking Advantage of Diverse Structure-Fluorescence Information of Chromone Derivatives

Herein, we report our work exploring the essential requirements for fluorophore selection during the development of various fluorescence applications. We assembled a library of chromone-derived fluorophores with diverse structure-fluorescence properties, which allowed us to choose the fluorophore pairs with similar structures but differing fluorescence properties and compared the performance of the selected fluorophore pairs in three types of commonly used fluorescence applications. We found that the selection standard of a suitable fluorophore is variable depending on the application. (1) In fluorescence imaging, fluorophores with strong and constant fluorescence under various conditions, such as a large pH range, are preferred. Notably, (2) in the detection of bioactive species, fluorophores with relatively lower fluorescence quantum yield favor the detection sensitivity. Furthermore, (3) in enzymatic assays employing fluorescence, the key parameter is the binding affinity between the fluorophore and the enzyme.

Photophysical properties and fluorosolvatochromism of D-π-A thiophene based derivatives

Solvation-dependent photophysical properties of two push-pull thiophene-based compounds with donor-π-acceptor (D-π-A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute-solvent interactions, namely, Kamlet-Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet-Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán's solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

Novel blue-green emitting NLOphoric triphenylamine-imidazole based donor-π-acceptor compound: Solvatochromism, DFT, TD-DFT and non-linear optical studies

Novel Donor (D)-π-Acceptor (A) NLOphoric triphenylamine-imidazole based dye 9 was designed, synthesized, and confirmed by Mass, ¹³C NMR, and ¹H NMR analysis. Photophysical properties of 9 were studied in solvents of different polarities and compared with analogues compounds 7 and 8. Phenonthroline acceptor based dye 9 shows highly bathochromic shifted absorption and emission compared to dyes 7 and 8. Positive solvatochromism was noticed in 7, 8, and 9 which was supported by the linear (i.e. Lippert-Mataga and Mac-Rae polarity functions) and multi-linear (i.e. Kamlet-Taft and Catalan parameters) analysis. Moreover, solvent polarizability (d_SP) and solvent dipolarity (C_SdP) are the major factors responsible for red shift in absorption as well as in emission spectra. Charge transfer descriptors as well as the polarity graphs are in good relation with Generalized Mulliken-Hush (GMH) parameters. NLO properties of 7, 8, and 9 were studied by using solvatochromic and computational methods. The static first hyperpolarizability (β₀) and relevant microscopic parameters (μ,α₀,α,β,γ) were determined using DFT with B3LYP, BHHLYP, and CAM-B3LYP functionals. Third-order NLO properties of nitrogen containing phenanthroline based compound 9 were observed to be several times higher than those of the compounds 7 and 8, justify the design approach.

Single-Benzene-Based Solvatochromic Chromophores: Color-Tunable and Bright Fluorescence in the Solid and Solution States

The search for structurally simple chromophores with superior fluorescence brightness and a wide range of solvent compatibility is highly desirable. Herein, a new type of single-benzene-based solvatochromic chromophore with a symmetric bifunctional structure, in which azetidine and ethoxycarbonyl moieties serve as the electron-donating and -withdrawing groups, respectively, is reported. This chromophore exhibits an extraordinary wide range of solvent compatibility and preserves excellent fluorescence quantum yields from nonpolar n-hexane to polar methanol and even in water. Unusually, the symmetric structure of the chromophore shows a distinct color change from bright green to red with increasing solvent polarity and possesses large Stokes shifts (λ=132-207 nm) in the tested solvents. Moreover, this single-benzene-based chromophore displays good photochemical stability in both solution and solid states, and even exhibits reversible mechanochromic luminescence.

Photostable Voltage-Sensitive Dyes Based on Simple, Solvatofluorochromic, Asymmetric Thiazolothiazoles

A family of asymmetric thiazolo[5,4-d]thiazole (TTz) fluorescent dye sensors has been developed, and their photophysical sensing properties are reported. The π-conjugated, TTz-bridged compounds are synthesized via a single-step, double condensation/oxidation of dithiooxamide and two different aromatic aldehydes: one with strong electron-donating characteristics and one with strong electron-accepting characteristics. The four reported dyes include electron-donating moieties (N,N-dibutylaniline and N,N-diphenylaniline) matched with three different electron-accepting moieties (pyridine, benzoic acid, and carboxaldehyde). The asymmetric TTz derivatives exhibit strong solvatofluorochromism with Stokes shifts between 0.269 and 0.750 eV (2270 and 6050 cm^-1) and transition dipole moments (Δμ = 13-18 D) that are among the highest reported for push-pull dyes. Fluorescence quantum yields are as high as 0.93 in nonpolar solvents, and the fluorescence lifetimes (τ_F) vary from 1.50 to 3.01 ns depending on the solvent polarity. In addition, thermofluorochromic studies and spectrophotometric acid titrations were performed and indicate the possibility of using these dyes as temperature and/or acid sensors. In vitro cell studies indicate good cell membrane localization, negligible cytotoxicity, promising voltage sensitivities, and photostabilities that are 4 times higher than comparable dyes. Their ease of synthesis and purification, remarkable photophysical properties, and chemically sensitive TTz π-bridge make these asymmetric dye derivatives attractive for environmental and biological sensing or similar molecular optoelectronic applications.

Integrated pyrazolo[1,5-a]pyrimidine-hemicyanine system as a colorimetric and fluorometric chemosensor for cyanide recognition in water

A new probe for cyanide detection based on the integrated pyrazolo[1,5-a]pyrimidine-hemicyanine (PpHe) system was synthesized in an efficient and straightforward manner using microwave-assisted heating. Photophysical studies in a 100% aqueous solution demonstrated high cyanide selectivity and detection limits as low as 600 and 86 nmol L^-1 for UV-vis absorption and fluorescence emission, respectively. Both values are well below 1900 nmol L^-1, which is the maximum concentration permitted for drinking water by the World Health Organization (WHO). HRMS analysis and NMR experiments were performed to confirm the mechanism of detection based on blocking the ICT phenomenon via nucleophilic addition of CN^- on the C˭N⁺ bond (iminium salt moiety) of the probe.

Microcapsule-Containing Self-Reporting Polymers

Self-reporting polymers, which can indicate damage or exposure to excessive stress with a clearly perceptible optical signal, are potentially useful for several technological applications, including stress-sensitive sensors that enable in situ monitoring of mechanical events and structural health monitoring systems. A versatile and simple concept to realize this function is the exploitation of microcapsules that are filled with solutions of dyes that are released and chemically or physically activated when the protective shell is damaged. Such microcapsules can readily be incorporated into polymers and the composites thus made can be processed into films, coatings, or other objects. Mechanochromic effects can be realized with different types of dyes and activation schemes. In this concept article, a selection of recent key studies is presented to provide an overview of the state of the field. Different architectures and operating principles and their advantages and drawbacks are reviewed. The parameters that influence the design of microcapsule-based mechanochromic systems are considered and unexplored chromophore systems that might be useful to design future self-reporting polymers are discussed. Finally, specific aspects of capsule design, fabrication, and integration into polymers are presented. Throughout the article, challenges and opportunities of the concept are highlighted and possible future directions are discussed.

Viscosity-active D-π-A chromophores derived from benzo[b]thiophen-3(2H)-one 1,1-dioxide (BTD): Synthesis, photophysical, and NLO properties

Donor-π-Acceptor (D-π-A) compounds comprising of benzo[b]thiophen-3(2H)-one 1,1-dioxide (BTD) as acceptor with dibenzofuran, carbazole, triphenylamine, and N-methyl diphenylamine moieties as donors were synthesized for aggregation, viscosity induced emission enhancement and nonlinear optical studies. Compounds 3a-3d exhibited solid state emission. The compounds 3a-3d are viscosity sensitive in a solution of MeOH: PEG-400 and showed 10, 23, 14, and 25 fold viscosity induced enhanced emission. The compounds 3a, 3c, and 3d are aggregation-induced emission enhancement (AIEE) active while 3b quenches the fluorescence on aggregation. The quantum yield of 3a, 3c, and 3d in acetonitrile are 0.041, 0.002 and 0.002 which are enhanced in the aggregate state to 0.31, 0.009, and 0.22 respectively. Solvent-dependent parameters like dipole moment (μ), static polarizability (α), and hyperpolarizability (β and γ) were determined spectroscopically and using Density Functional Theory (DFT) calculations. First and second order hyperpolarizability increase as donor strength increases and the trend is found as 3a < 3b < 3c < 3d. Two-photon absorption (2PA) cross sections were calculated by the spectroscopic method, and large 2PA was observed 484.39 GM for compound 3c.

Exploration of the Fluorescent Properties and the Modulated Activities against Sirtuin Fluorogenic Assays of Chromenone-Derived Natural Products

Chromenone-derived natural products include chromones (flavone, isoflavone) and coumarins. Chromenone compounds not only exhibit impressive biological activities, but also are an important resource of experimentally used fluorophores, such as, 7-amino-4-methylcoumarin (AMC). Various chromenone compounds have reported to have weak fluorescence, and this has the potential to interfere with the measurements during AMC fluorogenic assays and result in non-robust assay readouts. Several flavones and isoflavones were found as SIRT1 activators, while fluorogenic sirtuin assays utilized AMC labelled peptides as the substrates. In this study we investigated whether the fluorescent properties of chromenone-derived natural products interrupt the measurement of SIRT1/2 modulated activities. We found that the reported SIRT1 activators: flavones were detected with the SIRT1 activation activity, but isoflavones were not detected with SIRT1 activation activity, and instead that they were found to be fluorogenic compounds. Another chromenone compound, osthole, exhibited a moderate SIRT2 inhibitory activity with an IC₅₀ of 10 μM. In conclusion, the fluorescent properties of these chromenone compounds do affect the measurement of the sirtuin activities of both inhibitors and activators. However, if the possible fluorescence properties are mitigated in the assay readout, these fluorogenic assays enable the screening of activity modulators.

Experimental and theoretical study of donor-π-acceptor compounds based on malononitrile

A set of different donor-π-acceptor compounds having dicyanovinyl as the acceptor and aryl moieties as donors were synthesized by Knoevenagel condensation. The UV–visible absorption and fluorescence spectra were investigated in different solvents. The optical band gab energy (Eg) was linearly correlated with the Hammett resonance effect of the donor to reveal that the higher the value of Hammett resonance effect of a donor, the lower the Eg of the molecule. The photophysical data revealed that compounds M4–M6 are typical molecular rotors with fluorescence due to twisted intramolecular charge transfer. Compound M5 revealed the largest Stokes shift (11,089 cm⁻¹) making it a useful fluorescent sensor for the changes of the microenvironment. The effect of substituents on the optical properties of donor-π-acceptor compounds having dicyanovinyl as the acceptor are studied using density functional theory and time-dependent density functional theory (DFT/TD-DFT). The optical transitions are thoroughly examined to reveal the impact of subtituents on both absorption and fluorescence, mainly through the modification of the structure in the excited state. The theoretical results have shown that TD-DFT calculations, with a hybrid exchange–correlation and the long-range corrected density functional PBEPBE with a 6–311++G** basis set, was reasonably capable of predicting the excitation energies, the absorption and the emission spectra of these molecules.

Gaining insight into the photophysical properties of a coumarin STP ester with potential for bioconjugation

The photophysical properties of a coumarin 392 4-sulfotetrafluorophenyl ester, C392STP (sodium (E/Z)-4-(4-(2-(6,7-dimethoxycoumarin-3-yl)vinyl)-benzoyl)-2,3,5,6-tetrafluoro-benzenesulfonate), an amine reactive coumarine with potential for bioconjungation, have been studied in different solvents.

Pre-aggregation kinetics and intermediates of α-synuclein monitored by the ESIPT probe 7MFE

The defining feature of the extensive family of amyloid diseases is the formation of networks of entangled elongated protein fibrils and amorphous aggregates exhibiting crossed β-sheet secondary structure. The time course of amyloid conversion has been studied extensively in vitro with the proteins involved in the neurodegenerative pathology of Parkinson's disease (α-synuclein), Alzheimer's disease (Tau) and Huntington's disease (Huntingtin). Although much is known about the thermodynamics and kinetics of the transition from a soluble, intrinsically disordered monomer to the fibrillar end state, the putative oligomeric intermediates, currently considered to be the major initiators of cellular toxicity, are as yet poorly defined. We have detected and characterized amyloid precursors by monitoring AS aggregation with ESIPT (excited state intramolecular protein transfer) probes, one of which, 7MFE [7-(3-maleimido-N-propanamide)-2-(4-diethyaminophenyl)-3-hydroxychromone], is introduced here and compared with a related compound, 6MFC, used previously. A series of 140 spectra for sparsely labeled AS was acquired during the course of aggregation, and resolved into the relative contributions (spectra, intensities) of discrete molecular species including the monomeric, fibrillar, and ensemble of intermediate forms. Based on these findings, a kinetic scheme was devised to simulate progress curves as a function of key parameters. An essential feature of the model, one not previously invoked in schemes of amyloid aggregation, is the catalysis of molecular fuzziness by discrete colloidal nanoparticles arising spontaneously via monomer condensation upon exposure of AS to ≥ 37 °C.

Single-Molecule-based White-Light Emissive Organic Solids with Molecular-Packing-Dependent Thermally Activated Delayed Fluorescence

White-light-emitting single molecules have attracted broad attention because of their great potential for use in flat-panel displays and future light sources. We report a unique molecule of 3-(diphenylamino)-9H-xanthen-9-one (3-DPH-XO), which was found to exhibit bright white-light emission in the solid state caused by the spontaneous formation of a mixture with different polymorphs. Single-crystal analyses demonstrate that noncovalent interactions (such as π···π stacking, hydrogen bonding, and C-H···π interactions) induce different stacking arrangements (polymorphs A, B, and C) with different photophysical properties in a molecular solid. In addition, crystals B and C with the acceptor···acceptor stacking feature show the thermally activated delayed fluorescence (TADF) characteristics, indicating that appropriate noncovalent interactions could enhance the reverse intersystem crossing process and consequently lead to delayed fluorescence. This discovery provides an effective strategy for the design of new white-light-emitting single molecules as well as TADF materials.

Temperature dependent spectroscopic and excited state dynamics of 3-hydroxychromones with electron donor and acceptor substituents

We have studied the photophysical and photochemical behavior of three compounds derived from 3-hydroxychromone (3-HC), capable of undergoing excited state proton transfer (ESIPT). The compounds have two substituents, located in positions 2 and 7, one on each ring of the 3-HC heterocycle. The substituent pattern shows different electron donating and acceptor features. The compounds were studied by absorption and emission spectroscopy, steady state anisotropy, and time resolved emission spectroscopy (TRES) as a function of temperature. Results were interpreted using time dependent density functional theory calculations. Compared to reference compounds of 3-HC substituted only in the 2 position, the compounds show similar absorption and emission spectra, shifted 20-30 nm to higher wavelengths due to extended conjugation. TRES shows the existence of ESIPT in the thermodynamic equilibrium regime. This process is endothermic in all three compounds. The different behavior compared to monosubstituted 3-HC is attributed to the extended conjugation and to the electron donor acceptor character of the substituents, which has a more pronounced effect when the electron acceptor is located in position 2.

Comprehensive DFT and TD-DFT Studies on the Photophysical Properties of 5,6-Dichloro-1,3-Bis(2-Pyridylimino)-4,7-Dihydroxyisoindole: A New Class of ESIPT Fluorophore

Hanson et al. [Org. Lett., 2011] reported the absorption and emission spectrum of 5,6-dichloro-1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole but the excited-state intramolecular proton transfer (ESIPT) process was not investigated. The photo-physical behaviour of 5,6-dichloro-1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole was studied using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The functional used was B3LYP and 6-31G(d) was the basis set for all the atoms. All the ten tautomers were studied for the absorption and emission properties. It is found that the tautomer where hydroxyl groups are syn to nitrogen of isoindoline ring is most stable and thus, responsible for the ESIPT process. The computed absorption and emission values of tautomers using TD-DFT are in good agreement with those obtained experimentally.

Push-pull dioxaborine as fluorescent molecular rotor: far-red fluorogenic probe for ligand-receptor interactions

Fluorescent solvatochromic dyes and molecular rotors increase their popularity as fluorogenic probes for background-free detection of biomolecules in cellulo in no-wash conditions. Here, we introduce a push-pull boron-containing (dioxaborine) dye that presents unique spectroscopic behavior combining solvatochromism and molecular rotor properties. Indeed, in organic solvents, it shows strong red shifts in the absorption and fluorescence spectra upon increase in solvent polarity, typical for push-pull dyes. On the other hand, in polar solvents, where it probably undergoes Twisted Intramolecular Charge Transfer (TICT), the dye displays strong dependence of its quantum yield on solvent viscosity, in accordance to Förster-Hoffmann equation. In comparison to solvatochromic and molecular rotor dyes, dioxaborine derivative shows exceptional extinction coefficient (120,000 M^-1 cm^-1), high fluorescence quantum yields and red/far-red operating spectral range. It also displays much higher photostability in apolar media as compared to Nile Red, a fluorogenic dye of similar color. Its reactive carboxy derivative has been successfully grafted to carbetocin, a ligand of the oxytocin G protein-coupled receptor. This conjugate exhibits >1000-fold turn on between apolar 1,4-dioxane and water. It targets specifically the oxytocin receptor at the cell surface, which enables receptor imaging with excellent signal-to-background ratio (>130). We believe that presented push-pull dioxaborine dye opens a new page in the development of fluorogenic probes for bioimaging applications.

Synthesis of ester-substituted dihydroacridine derivatives and their spectroscopic properties

Ester substituted dihydroacridine derivatives exhibit a noticeable fluorescence solvatochromism.

Development of 3-alkyl-6-methoxy-7-hydroxy-chromones (AMHCs) from natural isoflavones, a new class of fluorescent scaffolds for biological imaging

A new class of fluorophores 3-alkyl-6-methoxy-7-hydroxy-chromones (AMHCs) is developed and is suitable as reagents for biological imaging.