Solvatochromic Fluorescence Research Articles

New Poly(Propylene Imine) Dendrimer Modified with Acridine and Its Cu(II) Complex: Synthesis, Characterization and Antimicrobial Activity

A second-generation poly(propylene imine) dendrimer modified with acridine and its Cu(II) complex have been synthesized for the first time. It has been found that two copper ions form complexes with the nitrogen atoms of the dendrimeric core by coordinate bonds. The new compounds have been characterized by nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), fourier-transform infrared spectroscopy (FTIR) and fluorescence spectroscopy. The spectral characteristics of the modified dendrimer have been measured in different organic solvents, and a negative fluorescence solvatochromism has been observed. The antimicrobial activity of the dendrimers has been tested against model pathogenic microorganisms in agar and by broth dilution method. The cotton fabric treated with both dendrimers has been evaluated towards pathogenic microorganisms. The obtained modified cotton fabrics have been shown to hamper bacterial growth and to prevent biofilm formation. Dendrimer cytotoxicity has been investigated in vitro in the model HEp-2 cell line.

Novel aminoquinoline-based solvatochromic fluorescence probe: Interaction with albumin, lysozyme and characterization of amyloid fibrils

A novel aminoquinoline derivative (AQ) was synthesized and applied as a solvatochromic fluorescent probe to study proteins and their alterations. AQ is not fluorescent in aqueous solution but has its fluorescence quantum yield significantly increased upon binding to albumin. The generation of an induced circular dichroism signal in AQ confirmed the complexation. The Job's plot method revealed an 1:1 stoichiometry for the host-guest complex. The binding constant was determined by AQ fluorescence increase (2.7 × 105 mol−1 L) and by protein intrinsic fluorescence quenching (5.1 × 105 mol−1 L). The displacement of AQ from albumin by warfarin and ibuprofen showed that Sudlow's drug site-I is the preferential binding site. By applying the Bilot-Kawski solvatochromic model to the spectral shifts of fifteen solvents, the microenvironment dielectric constant at albumin site-I was estimated (ε = 14.8). In agreement, the average fluorescence lifetime of AQ complexed with albumin (6.11 ns) was close to dichloromethane (6.53 ns) and acetone (6.34 ns), which have dielectric constants of 8.9 and 21.0, respectively. Albumin was thermically treated to formation of amyloid fibril aggregates. AQ was able to differentiate the altered and native protein. Sodium dodecyl sulfate-induced aggregation of lysozyme to amyloid fibril was also efficiently detected by the AQ fluorescence increase. AQ was as efficient as the chromogenic bromocresol purple in the quantitative analysis of albumin. In conclusion, AQ can be considered a new solvatochromic fluorescent probe with several potential applications.

Fabrication of Fluorescent One-dimensional-nanocomposites through One-pot Self-assembling Polymerization on Nano-helical Silica

This paper presents a new class of one-dimensional nanostructures exhibiting remarkable fluorescent solvatochromism. This is fabricated through the self-assembling polymerization of 2,6-dihydroxyan...

Stilbene-Based Two-Photon Thermo- Solvatochromic Fluorescence Probes with Large Two-Photon Absorption Cross Sections and Two- Photon Triple Fluorescence: Detection of Solvent Polarities, Viscosities, and Temperature

The unusually sensitive solvatochromism of 2,5-dicyano-4-methyl-4’-dimethylaminostilbene (P1) whose emission maximum varies from 445 nm in cyclohexane to 641 nm in DMSO are described. 1a with remarkably large two-photon cross sections exhibits very strong polarity-, viscosity-, and temperature-dependence of fluorescence, and can be used to detect polarities, viscosities, and temperature. The successful application of P1 provides a brilliant prototype for the developments of ideal two-photon fluorescence probes.

Synthesis, photophysical and electrochemical properties of pyridine, pyrazine and triazine-based (D-π-)2A fluorescent dyes.

The donor–acceptor–π-conjugated (D–π–)2A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole thiophene units as D (electron-donating group)–π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group, A) have been designed and synthesized. The photophysical and electrochemical properties of the three dyes were investigated by photoabsorption and fluorescence spectroscopy, Lippert–Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λmax,abs) and the fluorescence maximum (λmax,fl) for the intramolecular charge-transfer characteristic band of the (D–π–)2A fluorescent dyes show bathochromic shifts in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption bands of the (D–π–)2A fluorescent dyes are nearly independent of solvent polarity, while the fluorescence bands showed bathochromic shifts with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert–Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δμ (= μe − μg) value, which is the difference in the dipole moment of the dye between the excited (μe) and the ground (μg) states, increases in the order of OUY-2 < OUK-2 < OUJ-2. Therefore, the fact explains our findings that OUJ-2 shows large bathochromic shifts of the fluorescence maxima in polar solvents, as well as the Stokes shift values of OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D–π–)2A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λmax,abs and λmax,fl and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of the azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

Emission-Color Control in Polymer Films by Memorized Fluorescence Solvatochromism in a New Class of Totally Organic Fluorescent Nanogel Particles.

In this work, a new class of totally organic fluorescent nanogel particles and their exceptionally specific behaviors based on their unique structures are introduced, which draws a sharp line from conventional fluorophore-doped and fluorophore-branched-type particles. The nanogel particles, the diameter of which could be controlled by adjusting reaction conditions, such as the solvent system, were spontaneously fabricated with a spherical shape by direct polymerization of non-heterocyclic aromatic compounds, such as 2,6-dihydroxyanthracene, 2,6-dihydroxynaphthalene, and 9,9-bis(4-hydroxyphenyl)fluorene with triazinane as the cross-linker. A fluorophoric moiety formed from a polymer main chain was realized in the particle, and consequently, the resultant content of the fluorophoric moiety was around 70-80 wt % per particle. The uniqueness and versatility of the particles can be emphasized by their good compatibility with various solvents due to their amphiphilic and ampholytic swelling properties, but also by their remarkable fluorescent solvatochromism in the dispersion state. Furthermore, these behaviors were preserved even in their polymer composite system. This study also demonstrates that various fluorescent polymer films can be fabricated with emission color control due to memorization of the solvatochromism phenomenon of the dispersed fluorescent nanoparticles.

Oxophosphoryl Complexes of Dipyrrin: Spectral and Aggregation Characteristics of Solutions and Thin Films

The spectral characteristics of a compound representing a new class of organic luminescent materials—dipyrrin oxophosphoryl complex (PODIPY)—in organic solvents and thin films have been investigated. The compound under study exhibits high-intensity fluorescence in solutions, which is preserved when passing to thin films obtained by layer-by-layer deposition onto a substrate using the Langmuir–Schaeffer technique. PODIPY is characterized by has bright solvatochromic fluorescence, which is related to the redistribution of the intensities of absorption and fluorescence bands and their shift. This effect originates from the specific solvation of PODIPY with partition of the oxygen atoms of oxophosphoryl group. A transition from solutions to films and an increase in the number of transfers lead to the fluorescence quenching without the formation of G aggregates in the film.

Highly soluble polyimide bearing bulky pendant diphenylamine-pyrene for fast-response electrochromic and electrofluorochromic applications

Synchronously switchable electrochromic/electrofluorochromic polymers have captured intense attention in the field of displays and sensors. However, most of the emerged materials could not exhibit satisfactory switching performances, especially the slow response speed. In this work, a novel fluorescent/electroactive polyimide was rationally designed and prepared by introduction of the diphenylamine-pyrene unit. The obtained polyimide showed high solubility, excellent thermostability, fluorescence solvatochromism, and interesting electrochromic (in the vis-NIR region)/electrofluorochromic dual-switching properties. Moreover, due to the loose interchain packing benefiting from bulky pendant diphenylamine-pyrene unit, an impressive switching speed was realized (1.7/0.9 s for EC and 2.8/0.9 s for EFC processes), which will greatly promote the real-life applications in the future.

Synthesis of quinolyl-pyrrole derivatives as novel environment-sensitive fluorescent probes

We synthesized novel environment-sensitive fluorophores using a unique fluorogenic core quinolyl-pyrrole (QP, 2-(1H-pyrrol-2-yl)quinoline). QP was obtained from isatins and 2-acetylpyrrole by Pfitzinger quinoline synthesis. Derivatization at the quinoline and pyrrole rings was carried out using the Sonogashira–Hagiwara cross coupling and Vilsmeier–Haack reactions, respectively. The obtained dyes showed strong fluorescence solvatochromism, but their absorption spectra remained nearly unchanged. The introduction of a dimethylaminophenylethynyl moiety at the quinoline side of the QP resulted in a change in the fluorescence color from blue to red within a narrow relative permittivity range of roughly 2–10, demonstrating that the substitution strengthened the sensitivity of the dye to the low polarity environment. The dye also reflected voltage changes across liposomal membranes, showing a 3.7-fold enhanced fluorescence intensity when tested under the same conditions as di-4-ANEPPS, a popular voltage sensitive dye, which only changed its fluorescence intensity by a factor of 1.2. The voltage sensitivity of the dye was calculated to be 2.3%/mV, much larger than that of di-4-ANEPPS (0.13 – 0.18%/mV). These dyes may be utilized for the studies on the activities of neurons or chloroplasts.

Tuning the Optical Properties of Sulfonylaniline Derivatives: Degeneracy Breaking of Benzene Orbitals and Linkage through Nodal Planes.

The orbital degeneracy of benzene rings is resolved by an asymmetric push-pull system in 2,6-bis(methylsulfonyl)aniline (BMeSA), in which the highest occupied molecular orbital (HOMO) is located at the 4-position, while the lowest unoccupied molecular orbital (LUMO) is located at a different position and has a nodal plane through the carbon atoms at the 1- and 4-positions. Therefore, the π-extension of BMeSA at the 4-position reveals a strong overlap in the HOMO and a minimal overlap in the LUMO. Consequently, π-extended BMeSA derivatives exhibit longer absorbance and emission wavelengths in the order of the electron-donating abilities of their substituents at the 4-position, which is based on a decrease in an absolute HOMO-level-dependent HOMO-LUMO gap in accordance with the nodal arrangement. Positive fluorescent solvatochromism with polarity-dependent decrease in fluorescent intensity was also observed. The biaryls exhibited more planar geometries in the excited state than in the ground state. The charge transfer mechanism, which can be described as node-induced intramolecular charge transfer (NICT), differs from the planar intramolecular charge transfer (PICT) and twisted intramolecular charge transfer (TICT).

Spectroscopic properties of push-pull 2-(4-carboxyphenyl)-6-dimethylaminobenzothiazole derivatives in solution and the solid state

6-Dimethylamino-2-phenylbenzothiazoles with a carboxy, ester and amide substituent on the 2-phenyl group were prepared as a series of the push-pull benzothiazoles, and their spectroscopic and photophysical properties in solutions were investigated. The derivatives showed solvatochromic fluorescence with quantum yields over 0.68 in various organic solvents similar to the core fluorophore structure. The color variation ranges in fluorescence solvatochromism of the derivatives were wider than that of the original compound having the core structure, 6-dimethylamino-2-phenylbenzothiazole. It was also found that the derivatives together with the original compound show solid state fluorescence depending on their crystal structures. In particular, an ester derivative with a protected serine has a reasonable crystal packing leading to increase in fluorescence efficiency. The results of the present study provide a guide to design push-pull 2-phenylbenzothiazoles exhibiting fluorescence in solution and the solid state.

Design of a quinazolinone-based environment-sensitive fluorescent dye: Solvatochromic fluorescence and application for one-photon and two-photon bioimaging

A novel quinazolinone-based fluorescent dye, 6-dimethylamino-2-(quinolin-2-yl)- quinazolin-4(3H)-one (DMAQQ), has been designed and synthesized. This quinazolinone-based dye exhibited good efficient solvatochromic fluorescence with appreciated fluorescence quantum yield in different organic solvents (0.57–0.78) and remarkable two-photon fluorescence. The theoretical calculation showed that it was derived from charge transfer (CT) character and intramolecular hydrogen bond. Further investigation on the interaction with anions also showed that the fluorescence intensity of this dye decreased and quenched through the destruction of intramolecular hydrogen bond by the addition of hydrogen bond strong acceptor. As a good bioimaging candidate, DMAQQ showed excellent biocompatibility. More importantly, DMAQQ exhibited blue and green one-photon fluorescence emission in HeLa cells with the Pearson's coefficient 0.39, which suggested possible different local polarity in HeLa cells. And the DMAQQ also showed bright green fluorescence in HeLa cells, which excited by two-photon, offering a new dye platform for high spatial resolution bio-imaging application.

Fluorescence solvatochromism and modulated anticholinergic activity of novel coumarin compounds sequestered in human serum albumin nanocavities

The antagonistic effect of the HSA matrix towards the acetylcholinesterase inhibition potencies of novel coumarin compounds is probed by multi-spectroscopic techniques.

Probing interactions within liquid media via a model H-bond donor-acceptor mixture.

Understanding interactions within liquids so as to obtain desired physicochemical properties is of utmost importance in chemistry. Solute-solvent and solvent-solvent interactions within mixtures of hexamethylphosphoramide (HMPA) and 2,2,2-trifluoroethanol (TFE) are assessed using various invasive and non-invasive techniques. HMPA and TFE possess similar static dielectric constants. While HMPA has high H-bond accepting basicity (β ∼ 1) and no H-bond donating acidity (α ∼ 0), TFE has high H-bond donating acidity (α ∼ 1) and almost no H-bond accepting basicity (β ∼ 0). Kamlet-Taft parameters obtained using solvatochromic UV-Vis molecular absorbance probes suggest α and β of the (HMPA + TFE) mixture to be higher than those expected from mole-fraction weighted ideal-additive values with α and β exhibiting synergism in TFE-rich and HMPA-rich regions where α > 1 and β > 1, respectively. HMPA has lower dipolarity/polarizability (π*) than TFE, however, the addition of TFE to HMPA results in even further lowering of π* implying significant non-polar nature of the (HMPA + TFE) mixture as compared to its neat constituents. Photoinduced charge-transfer (neutral and anionic) and ratiometric solvatochromic fluorescence polarity probes also confirm the mixture dipolarity to be significantly lower than that expected. FTIR absorbance and Raman spectroscopic measurements hint at the weakening of the strong H-bonds among TFE and/or HMPA as the two solvents are mixed. Positive excess molar volumes (VE) and excess logarithmic viscosities [(ln η)E] indicate weakened interactions in the (HMPA + TFE) mixtures as compared to the interactions within neat TFE and neat HMPA. Microfluidity of the mixture obtained from an intramolecular excimer-forming fluorescence probe further supports these outcomes. Formation of a low polarity liquid milieu upon mixing of two higher polarity solvents is amply highlighted.

Tuning the π-bridge of quadrupolar triarylborane chromophores for one- and two-photon excited fluorescence imaging of lysosomes in live cells.

A series of tetracationic quadrupolar chromophores containing three-coordinate boron π-acceptors linked by different π-bridges, namely 4,4'-biphenyl, 2,7-pyrene, 2,7-fluorene, 3,6-carbazole and 5,5'-di(thien-2-yl)-3,6-diketopyrrolopyrrole, were synthesized. While their neutral precursors 1-5 displayed highly solvatochromic fluorescence, the water-soluble tetracationic target molecules 1M-5M, did not, but their emission colour could be tuned from blue to pink by changing the π-bridge. Compound 5M, containing the diketopyrrolopyrrole bridge, exhibits the most red-shifted absorption and emission maxima and the largest two-photon absorption cross-section (4560 GM at 740 nm in MeCN). Confocal laser scanning fluorescence microscopy studies in live cells confirm localization of the dye at the lysosome. Moreover, the low cytotoxicity, and high photostability of 5M combined with two-photon excited fluorescence imaging studies demonstrate its excellent potential for lysosomal imaging in live cells.

Synthesis of Bis(arylethynyl)pyrrolo[3,2-b]pyrroles and Effect of Intramolecular Charge Transfer on Their Photophysical Behavior.

3,6-Bis(arylethynyl)pyrrolo[3,2-b]pyrroles were synthesized through a two-step procedure involving double direct alkynylation of the electron-rich core followed by Sonogashira coupling. In comparison with the parent tetraarylo-pyrrolo[3,2-b]pyrroles and benzo-fused pyrrolopyrroles, these new dyes showed moderately redshifted absorption. Almost all derivatives showed positive fluorescence solvatochromism and, for the first time, red-emitting pyrrolopyrroles were obtained. Computational studies revealed that, in most cases, there is negligible change in the geometry between ground and excited states. Interestingly, there was a fundamental difference between pyrrolopyrroles possessing electron-withdrawing substituents at positions 2 and 5 and their analogs lacking these substituents. The former dyes behaved like dipolar chromophores with the lowest excited state both one-photon and two-photon allowed, which corresponds to intramolecular charge transfer occurring along the branches perpendicular to the pyrrolopyrrole long axis. In compounds lacking electron-withdrawing substituents at positions 2 and 5, intramolecular charge transfer took place along the long axis of pyrrolopyrrole and consequently the one-photon transitions are not two-photon allowed. Despite displaying quadrupolar core-to-peripheral intramolecular charge transfer, these derivatives showed two-photon absorption cross sections in the NIR1 region comparable to tetraaryl-pyrrolo[3,2-b]pyrroles lacking π-expansion (up to about 500 GM).

Insights into the solvents effect on spectral and photophysical properties of novel fluorescent heteroaromatic bis-peri-fused azoxonium cations

Perchlorates of novel fluorescent heteroaromatic bis-peri-fused azoxonium cations were prepared and thoroughly investigated with regard to their spectral and photophysical behaviour. Solvatochromic fluorescence properties of the compounds obtained are determined by intermolecular electrophilic-nucleophilic interactions of cations with solvent molecules. Gutmann's donor numbers correctly assess the solvent effect on the cation spectral characteristics. Perchlorates of bis-peri-fused azoxonium cations are effective fluorescent dyes characterized by the quantum yields reaching 0.67. Single-exponential fluorescence decay of the dyes is disturbed in the case of DMSO solvent possessing strong Electron-Pair-Donor properties and is accompanied by a sharp decrease of the fluorescence quantum yields. In this case coordinating interaction of the DMSO molecules with Electron-Pair-Acceptor center of the cations leads to the appearance of an additional efficient non-radiative channel of the energy deactivation of the electronic excitation.

Photophysical and Protonation Time Resolved Studies of Donor-Acceptor Branched Systems With Pyridine Acceptors.

A comparative study of the photophysical properties of octupolar pyridyl-terminated triphenylamine molecule, with its quadrupolar and dipolar analogues, by means of ambient and low temperature steady state spectroscopy and femtosecond to nanosecond time-resolved fluorescence spectroscopy is reported. The push-pull molecules bear triphenylamine electron donating core, pyridine peripheral electron acceptors, and acetylene π-bridge. The samples were studied in solvents of varying polarity and also upon addition of small amounts of acetic acid to induce protonation of the pyridine group. All samples exhibit significant positive fluorescence solvatochromism as well as a relaxation of their excited state to a solvent relaxed intramolecular charge transfer state on the picosecond time scale. For the octupolar compound, excited state relaxation occurs simultaneously with excitation energy hopping among the branches. The hopping time is solvent polarity controlled since it becomes slower as the polarity increases. The experimental hopping times are compared to those predicted by Förster and Fermi formulations. The samples are capable of emitting broadband light covering almost the whole visible spectrum by careful control of protonation. Energy transfer from the neutral toward the protonated species on the 1 ps time scale is revealed.

Solvatochromic fluorescence in an immiscible two-phase system of alcohols and hydrophobic polymers: Distinction between light alcohol and water, ink printing

A solvatochromic dye monomer with an allyl group (ASCD) was polymerized together with methyl methacrylate (MMA) or styrene (St) to obtain hydrophobic fluorescent polymers. The wettability of the polymers to alcohols was examined by monitoring the fluorescence (FL) changes over time. On the one hand, when the ASCD-incorporated poly MMA (PMMA) film was exposed to liquid alcohols, a remarkable red-shift occurred instantly in the FL emission spectra for light alcohols such as methanol, ethanol, and propanol whereas for heavier alcohols, a blue-shift occurred first and subsequently a red-shift occurred gradually. On the other hand, the ASCD-incorporated poly St (PSt) film exhibited less significant FL change upon contact with the light alcohols owing to the relatively higher hydrophobicity. In contrast to the case of alcohols, the polymer films exhibited no FL response to water even after prolonged exposure. Particularly, the ASCD-incorporated PSt film showed a reversible and reproducible FL change during the repeated methanol and water contact cycle, indicating a high potential for use as a sensor to discriminate between light alcohol and water. Moreover, when the ASCD-incorporated PMMA was ground mechanically and added to the alcohols, the nanoparticle powder was dispersed very well in the alcohols. When the colloidal solution was deposited onto a paper as an ink, high-resolution FL images were obtained on the surface of the paper.

Structure-fluorescence relationship of push-pull 2-phenylbenzothiazole derivatives designed based on the firefly light-emitter

6-Dimethylamino-2-phenylbenzothiazole (1a) mimicking the firefly oxyluciferin structure and the derivatives with an electron-withdrawing substituent on the phenyl group were prepared, and their fluorescence properties were investigated in various solvents. 1a showed solvatochromic fluorescence with good fluorescence quantum yields (Φf >0.8). The introduction of an electron-withdrawing group led to a red-shift of the emission maximum. In particular, the derivatives with the 2,2-dicyanoethenyl and (1,3-dihydro-1,3-dioxo-2H-inden-2-ylidene)methyl groups showed near-infrared fluorescence in chloroform. In addition, the derivative with the phenylimine moiety showed efficient solid-state fluorescence, resulted from a molecular arrangement inhibiting intermolecular interactions for quenching the fluorescence state in crystals.