Solvatochromic Fluorescence Research Articles

Novel Solid‐State Fluorophores, a Series of Cyanostilbene‐Based Amorphous Molecular Materials

AbstractWe have designed and synthesized a novel series of cyanostilbene‐based amorphous molecular materials with different methylene chain lengths, BMAC‐n (n=3,4,5,6), and investigated their emitting properties in solution and various solid states. All BMAC‐n molecules showed solvatochromic fluorescence in solutions. Emission spectra of BMAC‐n molecules in the same solution were identical to one another irrespective of the methylene chain length while the spectra in crystalline states depended on their methylene chain length. As in solutions, emission spectra of their spin‐coated amorphous films were identical to each other irrespective of the methylene chain length. Emission spectra of rubbed amorphous films prepared by grinding their crystals were identical to those for spin‐coated amorphous films; however, comparative studies between spin‐coated and rubbed amorphous films indicated that fluorescence quantum yields and photochemical reactivity depend on the preparation method.

Photochemical synthesis and solvatochromic fluorescence behavior of imide-fused phenacenes

Chrysenes, picene, fulminene, modified with imide, bromo, and amino functionalities, were synthesized through Mallory photoreaction as the key step, and their electronic spectra were investigated. Fluorescence spectra of chrysene-diimide CHRDI and bromo-substituted phencanene-imides, BrCHRI, BrPICI, BrFULI were dependent on solvent polarity to display appreciable fluorescence color changes. The solvatofluorochromic behavior was analyzed by conventional relationships between Stokes shift and solvent polarity parameters, such as Lippert-Mataga and Bilot-Kawski equations. The results indicated that the solvatofluorochromism was derived from the intramolecular charge transfer (ICT) nature in the excited state. Theoretical studies using time-dependent density-functional theory revealed that the phenacene-imide molecules in the fluorescent state possessed ICT characters between the strongly electron-withdrawing imide moiety and moderately electron-donating phenacene cores. Amino-substituted chrysene-imide NH2CHRI showed fluorescence band in a red region (λFL = 618 nm) in toluene with a very large Stokes shift (Δν∼ = 7630 cm−1) suggesting that the molecule in the fluorescent state was highly polarized. The present results indicate that phenacenes would provide potential platforms for constructing future functional fluorophores through an appropriate functionalization.

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.

Dansylated Nitrile N-Oxide as the Fluorescent Dye Clickable to Unsaturated Bonds without Catalyst.

Fluorescent polymeric materials have been exploited in the fields of aesthetical purposes, biomedical engineering, and three-dimensional printing applications. While the fluorescent materials are prepared by the polymerization of fluorescent monomer or the blending a fluorescent dye with common polymer, the covalent immobilization of fluorescent dye onto common polymers is not the practical technique. In this paper, dansylated nitrile N-oxide (Dansyl-NO) has been designed and synthesized to be a stable nitrile N-oxide as the derivative of 2-hydroxy-1-naphthaldehyde. While Dansyl-NO shows good reactivity to an alkene and an alkyne to give fluorescent Dansyl-Ene and Dansyl-Yne, respectively, it hardly reacts to a nitrile. The results indicate that Dansyl-NO serves as a fluorescent dye clickable to alkenes and alkynes. To know the effects of solvent on the fluorescent properties, the UV-vis and fluorescence spectra of Dansyl-Ene are measured in three solvents. Dansyl-Ene shows fluorescent solvatochromism, which appears to be red-shifted along with the increase in solvent polarity. Poly(styrene-co-butadiene) directly reacts with Dansyl-NO to give fluorescent modified SB. The emission spectrum of modified SB is blue-shifted compared with that of Dansyl-Ene. The blue-shift could be possibly attributed to the presence of less polar polymer skeleton around the dansyl moieties of modified SB.

Design and synthesis of an environment-sensitive 3-methyleneisoindolin-1-one fluorophore for labeling DNA-interacting proteins.

We designed 6-dimethylamino 3-methyleneisoindolin-1-one as an environment-sensitive fluorophore, examining its applications for protein labeling. Synthesized 3-methyleneisoindolin-1-one exhibits solvatochromic fluorescence (λemmax; 472 nm in 2-PrOH, 512 nm in H2O). A positive linear dependence between λemmax and solvent dielectric constant (DC), as well as between Stokes shift and DC, and a negative correlation between fluorescence quantum yield and DC are observed in protic solvents. These properties are similar to those of the oxygen isosteric fluorophore, 4-dimethylaminophthalimide, a slovatochromic fluorophore utilized for labeling oligodeoxynucleotides (ODNs) and peptides. Notably, fluorescence intensity of 3-methyleneisoindolin-1-one is higher than the phthalimide in protic solvents used in this study. The 3-methyleneisoindolin-1-one demonstrated the higher stability in pH 8 solution than in pH 6 solution in contrast to the stability profile of the phthalimide, which was stable at pH 6 but was hydrolyzed at pH 8. We also synthesized an o-keto benzaldehyde derivative that converts a primary amine to 6-dimethylamino 3-methyleneisoindolin-1-one under biocompatible conditions and introduced it into ODNs for turn-on fluorescent protein labeling. The synthesized ODN with a protein-binding sequence of Escherichia coli DnaA was employed to modify the DNA-binding domain of DnaA, and the fluorescent properties of the modified protein were investigated.

Direct evaluation of polarity of the ligand binding pocket in retinoid X receptor using a fluorescent solvatochromic agonist

High selectivity of small-molecule drug candidates for their target molecule is important to minimize potential side effects. One factor that contributes to the selectivity is the internal polarity of the ligand-binding pocket (LBP) in the target molecule, but this is difficult to measure. Here, we first confirmed that the retinoid X receptor (RXR) agonist 6-(ethyl(1-isobutyl-2-oxo-4-(trifluoromethyl)-1,2-dihydroquinolin-7-yl)amino)nicotinic acid (NEt-iFQ, 1) exhibits fluorescence solvatochromism, i.e., its Stokes shift depends on the polarity of the solvent, and then we utilized this property to directly measure the internal polarity of the RXRα-LBP. The Stokes shift of 1 when bound to the RXRα-LBP corresponded to that of 1 in chloroform solution. This finding is expected to be helpful for designing RXR-selective ligands. A similar approach should be appliable to evaluate the internal polarity of the LBPs of other receptors.

Push-Pull Bridged Distyrylbenzene with Highly Bright Solid-State Red-Orange Aggregation-Induced Emission.

Multiple emission colors in solid-state organic fluorophores with the same main skeleton are essential for improving the performance of light-emitting devices/materials. Specifically, emission in the red/near-infrared region is of great importance in the biological field. Previously, we developed di-bridged-distyrylbenzene DBDB[7] with high-brightness solid-state blue and aggregation-induced emissions (AIE) by introducing the bridging structures of a seven-membered ring into the vinylene groups of distyrylbenzene (DSB). Herein, we synthesize MNDBDMeODB[7] (1), which features substituted methoxy and malononitrile groups as donor and acceptor groups, respectively, in DBDB[7]. In solvents more polar than THF, MNDSDMeOB (3), which has the same main skeleton as 1 but without bridges, shows no emission in the solid state, whereas 1 exhibits highly bright red-orange emission in the solid state owing to the suppression of intermolecular electronic interactions by the bridges and the AIE property. We also synthesize MNDSD(EHO)B (2) in which the methoxy groups of 3 are replaced by ethylhexyloxy groups, thus disrupting the crystallinity of the molecule. 2 exhibits positive fluorescence solvatochromism and has a high fluorescence quantum yield in the solid state as a red-emitting DSB derivative. The solid-state emission properties of 1 and 2 will improve the applicability of DSBs and functionalities of light-emitting devices/materials.

Cover Feature: A Photoswitchable Solvatochromic Dye for Probing Membrane Ordering by RESOLFT Super‐resolution Microscopy (ChemPhysChem 12/2023)

The Cover Feature illustrates how photoswitchable solvatochromic fluorophores can be used to distinguish lipid membrane domains with different lipid ordering using super-resolution microscopy. More ordered membranes have lower water content than disordered membranes so by detecting solvatochromic fluorescence in multiple spectral regions, the lipid ordering in different parts of the membrane can be mapped. Cover design by Kathryn G. Leslie. More information can be found in the Research Article by Andrew T. Frawley, Christian Eggeling, Harry L. Anderson and co-workers.

Ultrafast Photophysics of Regioisomeric Triphenylamine Dyes having Dipolar D-π-A-A and Octupolar D-(π-A-A)3 Character with a Benzothiazole Unit in Matched or Mismatched Orientation.

Benzothiazole is among prominent electron-withdrawing heteroarene moieties used in a variety of π-conjugated molecules. Its relative orientation with respect to the principal dipole vector(s) of chromophores derived thereof is crucial, affecting photophysical and nonlinear optical properties. Here we compare the photophysics and ultrafast dynamics of dipolar and octupolar molecules comprising a triphenylamine electron-donating core, ethynylene π-conjugated linker(s) and benzothiazole acceptor(s) having the matched or mismatched orientation (with respect to the direction of intramolecular charge transfer), while a carbaldehyde group is attached as an auxiliary acceptor. Among chromophores without the auxiliary acceptor, stronger fluorescence solvatochromism and faster excited state dynamics are exhibited for the derivatives with the mismatched geometry. On the contrary, introduction of the auxiliary acceptor to the benzothiazole unit enhances the intramolecular charge transfer ICT (featuring ultrafast dynamics of the excited state) for the matched geometry. The data confirm the crucial role of the relative orientation of asymmetric heteroaromatic unit (regioisomeric effect) in dipolar as well as in multipolar molecules in tuning linear and nonlinear optical properties as well as excited state dynamics.

(Invited, Digital Presentation) Surface Engineering of Fluorescent CsPbBr3 and Carbon Quantum Dots By Perfluorocarboxylic Acid

CsPbX3 (X = Cl, Br, I) perovskite quantum dots (QDs) are an extremely attractive material with a narrow fluorescence peak and tunable emission color by halogen composition. Surface ligands have a variety of effects, including control of size and shape, uniform dispersion through good affinity with solvents and matrices, elimination of quenching sites due to surface defect capping effects and subsequent improvement of luminescent properties, and improvement of stability through surface protection. CsPbX3 QDs can be synthesized by hot injection and room temperature precipitation methods, using oleic acid (OA) and oleylamine (OLA) as surface ligands. One of the problems with these QDs is their low stability; the mechanism of degradation of QDs is the easy proton transfer between the carboxylate groups of OA and the ammonium groups of OLA on the surface of the QDs, resulting in the desorption of OA and OLA from the QD surface. Ligand desorption creates halogen vacancies, which act as trap levels that cause nonradiative recombination; therefore, it is important to design ligands that hardly desorb from the QD surface to suppress degradation of the QDs. In this talk, we will introduce our results on surface ligand of perfluorocarboxylic acid that improves thermal stability of CsPbBr3 QDs [1].Fluorescent carbon quantum dots (CQDs) are attracting attention as an alternative material to toxic QDs. In order to synthesize CQDs with a variety of fluorescent colors, it is effective to select aromatic compounds as carbon sources. The fluorescence color of CQDs changes depending on the polarity of the dispersant. This fluorescence solvatochromism is attributed to the change in the HOMO-LUMO energy gap of CQDs through the dipolar and hydrogen bonding interactions between CQDs and the dispersant molecules. In this talk, we will show that fluorescence solvatochromism of CQDs synthesized from p-phenylenediamine can be modulated by surface modification with perfluorocarboxylic acid [2].[1] D. Sato, Y. Iso, T. Isobe, ACS Omega, 5, 1178-1187 (2020).[2] K. Sato, R. Sato, Y. Iso, T. Isobe, Chem. Commun., 56, 2174-2177 (2020).

Orthogonal‐ and Path‐Dependent Photo/Acidoswitching in an Eight‐State Dihydroazulene‐Spiropyran Dyad

AbstractThe front cover artwork is provided by M. Dowds, S. G. Stenspil, J. H. de Souza, B. W. Laursen, M. Cacciarini and M. Brøndsted Nielsen. The image illustrates switching between the eight logical states of a dihydroazulene–spiropyran dyad, as well as its solvatochromic fluorescence. Cover design by Mathias Dowds. Read the full text of the Research Article at 10.1002/cptc.202200152.

Facile synthesis of red-emissive carbon dots with theoretical understanding for cellular imaging

Recently, red emissive carbon dots (R-CDs) have drawn widespread attention on account of their desirable fluorescence properties and good biocompatibility. Despite great efforts, facile synthesis of R-CDs for cellular imaging remains challenging and the fluorescence mechanism of R-CDs is still elusive. Herein, p-phenylenediamine-derived R-CDs with excitation-independency were successfully obtained through a facile solvothermal approach together with proportional precipitation. The fluorescent solvatochromism of R-CDs is realized, while high polarity leads to higher degree of dipole interaction between R-CDs and different solvents, favoring for emissive red-shift. Furthermore, density functional theory is adopted to explore the optical and electronic characteristics of some polycyclic aromatic molecules. Among different configurations, pyridine nitrogen and carbonyl bonds could relatively increase the charge density and significantly narrow the band gap, which can provide a crucial theoretical basis for the precise preparation of R-CDs. Moreover, R-CDs possess favorable cellular imaging ability, which indicates their potential for a promising candidate as fluorescence probes in bioimaging.

The Energy Gap Law at Work: Emission Yield and Rate Fluctuations of Single NIR Emitters.

Internal conversion (IC) often is the dominating relaxation pathway in NIR emitters, lowering their fluorescence quantum yield. Here, we investigate dibenzoterrylene (DBT) by bulk and single molecule spectroscopy. With increasing solvent polarity, the S1-S0 energy gap decreases leading to a decrease of the fluorescence quantum yield and an increase of the IC rate in full accordance with the energy gap law. Making use of the unexpectedly strong fluorescence solvatochromism of this aromatic hydrocarbon, the validity of the energy gap law could also be demonstrated at the single molecule level. The S1-S0 energy gap not only controls the fluorescence lifetime and quantum yield of single molecules but also dictates how these quantities develop during spectral fluctuations. Our results open new avenues into unexplored single molecule photophysics and appear as a promising tool for nanoscale probing of dynamic heterogeneities.

Triphenylamine, Carbazole or Tetraphenylethylene-Functionalized Benzothiadiazole Derivatives: Aggregation-Induced Emission (AIE), Solvatochromic and Different Mechanoresponsive Fluorescence Characteristics.

The development of mechanochromic fluorophors with high-brightness, solid-state fluorescence is very significant and challenging. Herein, highly solid-state emissive triphenylamine, carbazole and tetraphenylethylene-functionalized benzothiadiazole derivatives were developed. These compounds showed remarkable aggregation-induced emission and solvatochromic fluorescence characteristics. Furthermore, these fluorogenic compounds also displayed different mechanically triggering fluorescence responses.

Amino acid functionalized benzanthrone dyes: Synthesis and photophysical study

The substitution reaction of 3-N-(2-chloroacetamido)benzanthrone with a number of the amino acids is reported. The possibility of using this halogenated benzanthrone for fluorescent labelling of amino acids has been studied. Arginine is the most reactive amino acid in the reaction, as it was found in the study. The spectral behavior of new benzanthrone derivatives depends on the polarity of the medium, exhibiting fluorescent solvatochromism due to intramolecular charge transfer from the substituted amino group to the carbonyl group. The results obtained indicate that the synthesized compounds are promising for studying peptides and proteins, as well as for visualizing biological objects.

Novel NBN-Embedded Polymers and Their Application as Fluorescent Probes in Fe3+ and Cr3+ Detection.

The isosteric replacement of C═C by B–N units in conjugated organic systems has recently attracted tremendous interest due to its desirable optical, electronic and sensory properties. Compared with BN-, NBN- and BNB-doped polycyclic aromatic hydrocarbons, NBN-embedded polymers are poised to expand the diversity and functionality of olefin polymers, but this new class of materials remain underexplored. Herein, a series of polymers with BNB-doped π-system as a pendant group were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization from NBN-containing vinyl monomers, which was prepared via intermolecular dehydration reaction between boronic acid and diamine moieties in one pot. Poly{2-(4-Vinylphenyl)-2,3-dihydro-1H-naphtho[1,8-de][1,3,2]diazaborinine} (P1), poly{N-(4-(1H-naphtho[1,8-de][1,3,2]diazaborinin-2(3H)-yl)phenyl)acrylamide} (P2) and poly{N-(4-(1H-benzo[d][1,3,2]diazaborol-2(3H)-yl)phenyl)acrylamide} (P3) were successfully synthesized. Their structure, photophysical properties and application in metal ion detection were investigated. Three polymers exhibit obvious solvatochromic fluorescence. As fluorescent sensors for the detection of Fe3+ and Cr3+, P1 and P2 show excellent selectivity and sensitivity. The limit of detection (LOD) achieved by Fe3+ is 7.30 nM, and the LOD achieved by Cr3+ is 14.69 nM, which indicates the great potential of these NBN-embedded polymers as metal fluorescence sensors.

Facile green synthesis of samarium sesquioxide nanoparticle as a quencher for biologically active imidazole analogues: Computational and experimental insights

A green synthetic approach for the synthesis of nanoparticles (NPs) is important to maintain sustainable development. Here we report the synthesis of samarium sesquioxide (Sm2O3) nanoparticles using an eco-friendly and unconventional method that utilizes pineapple peel extract as a reducing agent and stabilizer. The synthesized Sm2O3 NP was characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The density function theory (DFT) calculation was utilized to examine the various molecular properties of the biological potent imidazole analogues, namely 4-(4-(4-nitrophenyl)-5-(4-(trifluoromethyl)phenyl)-1H-imidazol-2-yl)phenol (NTIP) and 4-(4-(4-nitrophenyl)-5-(4-(trifluoromethyl)phenyl)-1H-imidazol-2-yl)aniline (NTIA) at 6–31 G basis set. The predicted HOMO and LUMO energies revealed that the molecules were chemically active and have a tendency for molecular interactions. The solvatochromic fluorescence behavior of NTIP and NTIA has been studied in different solvents ranging from non-polar to polar. Spectroscopic methods such as absorption and fluorescence methods were used to analyze the interaction of Sm2O3 nanoparticles with NTIP and NTIA. Association strength of these compounds with nanoparticle was estimated and observed that the NTIP has a stronger association with Sm2O3 nanoparticle. Stern-Volmer plots were observed to be linear except for the compounds. The estimation of the quenching rate parameter and fluorescence lifetime affirmed the occurrence of a static quenching mechanism. From the fluorescence data, the binding constants and the number of the binding sites were estimated and the quenching mechanism was driven by the electron transfer process.

Tetraphenylethene-Modified Colorimetric and Fluorescent Chemosensor for Hg2+ With Aggregation-Induced Emission Enhancement, Solvatochromic, and Mechanochromic Fluorescence Features.

A tetraphenylethene (TPE)-modified rhodanine derivative was successfully designed and prepared, and this luminophor showed intramolecular charge transfer nature from the TPE unit to the rhodanine-3-acetic acid unit. Interestingly, this luminogen not only exhibited typical aggregation-induced emission enhancement (AIEE) behavior but also showed good cell imaging performance. Remarkably, this AIEE-active TPE-containing rhodanine derivative possessed noticeable solvatochromic fluorescence effect involving multiple fluorescent colors of green, yellow-green, yellow, orange, and red. Meanwhile, this fluorescigenic compound displayed reversible mechanochromic fluorescence behavior based on the mutual transformation of between stable crystalline and metastable amorphous states. On the other hand, this multifunctional fluorophor could selectively and sensitively detect Hg2+ in an acetonitrile solution. Furthermore, this chemosensor could also be used to detect Hg2+ on test paper strips.

Regulation of Fluorescence Solvatochromism To Resolve Cellular Polarity upon Protein Aggregation.

Common solvatochromic fluorophores exhibit a bathochromic fluorescence emission wavelength shift accompanied by intensity attenuation due to the presence of nonradiative decay pathways at the excited state. Such intrinsic but inevitable fluorescence quenching of solvatochromism impedes its applications to faithfully quantify local polarity, especially in a polar environment. Herein, we report a new donor-π-acceptor (D-π-A) type solvatochromic fluorophore scaffold containing a perfluorophenyl group that exhibits both a solvatochromic emission wavelength shift and a controllable emission intensity upon polarity fluctuation. The regulation of fluorescence solvatochromism and colors was achieved by tuning the aryl donors. We exploited such desired solvatochromism of these probes to monitor protein misfolding and aggregation via wavelength shift. Finally, the polarity of pathogenic aggregated proteins was quantified by HaloTag bioorthogonal labeling technology in live cells. While much effort has been devoted to resolving the morphology of pathogenic aggregated proteins, this work provides quantitative hints regarding the chemical information at this disease-related protein interphase.

Rotating the C-N Bond in a Coumarin-Pyridine-Based Sensor for Pattern Recognition of Versatile Metal Ions.

A cross-reactive sensor array is powerful for high-throughput discrimination of various kinds of metal ions. However, the construction of a multicomponent sensor array is always time-consuming and cost-ineffective. Herein, a practical four-component X1-based sensor array (X1SA) was obtained by simply dissolving a single dye molecule X1 in respective solvents such as methanol, ethanol, dimethyl sulfoxide, and acetonitrile. In this design, X1 exhibits strong solvatochromic fluorescence properties via an excited-state intramolecular proton transfer and intramolecular charge transfer combined mechanism. Moreover, rotation of the C-N bond between the pyridine and coumarin units in X1 enabled it to coordinate with metal ions through different binding modes, which acted as an additional dimension of the sensor array. Inspired by this C-N bond rotation strategy, X1SA was determined to be powerful in discriminating 20 kinds of metal ions in both phosphate-buffered saline and 5% serum media in a range of 0.1-100 μM. In addition, the sensor array was also successfully applied in differentiating similar and mixed metal ions such as Fe3+/Fe2+, Cd2+/Hg2+, and Sn2+/Pb2+ in serum samples, which is meaningful for investigating the biological roles of iron and early diagnosis of related metal poisoning accidents.