Charge-transfer Dyes Research Articles

Versatile iodinated styryl dyes: Promising probes for viscosity detection with dual anti-cancer and anti-bacterial properties

We have demonstrated how intramolecular charge transfer (ICT) dyes can be combined with the molecular rotor property to create fluorescent viscosity sensors. Cyanine styryl dyes with various substituents were synthesized to alter their photophysical properties in intracellular viscosity detection. The synthesized dyes had strong absorbance and emission properties that are suited for cellular imaging. Fluorescence intensity was observed to increase with the environment's viscosity, which was supported by theoretical calculations. I-Cy-Styryl-OMe outperformed the other Cy-Styryl dyes in terms of viscosity sensing and brightness. I-Cy-Styryl-OMe was also utilized to detect alterations in intracellular viscosity and fatty liver in a mouse model. Furthermore, experiments were performed on various cell lines to determine anti-cancer efficacy and selectivity. Finally, the antibacterial abilities of the dyes were then assessed against gram-positive and gram-negative bacteria. Therefore, our work shows that organic fluorescent viscosity sensors are feasible for a range of biological uses.

Tuning sensing efficacy of anthraimidazoledione-based charge transfer dyes: nitro group positioning impact.

Anthraimidazoledione-based optical sensors have been designed by varying the position of the nitro functional group. All three positional isomers showed highly colored, photostable optical signals owing to intramolecular charge transfer interactions. Despite having the same anion-binding site (imidazole unit), the selectivity and sensitivity of the compounds depend on the positioning of the nitro group. The selectivity was fairly good for the meta isomer, followed by the ortho and para isomers, respectively. In contrast, the sensitivity towards anions followed a completely opposite trend, with the para isomer being the most sensitive one towards anions. Interestingly, the color changing response along the turn-on fluorescence signal was observed only with CN- ions in a semi-aqueous environment. Though the introduction of water as a co-solvent could improve the selectivity, the sensitivity was found to be slightly less than that observed in pure organic medium. Mechanistic studies indicated hydrogen bonding interactions between the imidazole -NH proton and cyanide, which further facilitated the extent of intramolecular charge transfer.

Tunable full-color emission of stilbazoles containing a 2-halo-3,4-dicyanopyridine acceptor.

Synthesis of a series of novel push-pull stilbazole-based chromophores containing a strong 2-halocinchomeronic dinitrile acceptor is reported. The photophysical properties of the compounds are described. Strong positive solvatofluorochromism typical of intramolecular charge transfer (ICT) dyes is observed for the synthesized stilbazoles. Their tunable multicolor emission ranges from 442 nm to 710 nm and covers the whole visible spectrum.

A pillar[5]arene-based planar chiral charge-transfer dye with enhanced circularly polarized luminescence and multiple responsive chiroptical changes.

The fabrication of circularly polarized luminescent (CPL) organic dyes based on macrocyclic architecture has become an importantly studied topic in recent years because it is of great importance to both chiral science and supramolecular chemistry, where pillar[n]arenes are emerging as a promising class of planar chiral macrocyclic hosts for CPL. We herein synthesized an unusual planar chiral charge-transfer dye (P5BB) by covalent coupling of triarylborane (Ar3B) as an electron acceptor to parent pillar[5]arene as an electron donor. The intramolecular charge transfer (ICT) nature of P5BB not only caused a thermally responsive emission but also boosted the luminescence dissymmetry factor (g lum). Interestingly, the specific binding of fluoride ions changed the photophysical properties of P5BB, including absorption, fluorescence, circular dichroism (CD), and CPL, which could be exploited as an optical probe for multi-channel detection of fluoride ions. Furthermore, the chiroptical changes were observed upon addition of 1,4-dibromobutane as an achiral guest.

Bifacial Dye Membranes: Ultrathin and Free-Standing although not Being Covalently Bound.

Layers of aligned dyes are key to photo-driven charge separation in dye sensitized solar cells, but cannot be exploited as rectifying membranes in photocatalysis to separate half-cells because they are not sufficiently stable. While impressive work on the fabrication of stable noncovalent membranes has been recently demonstrated, these membranes are inherently suffering from non-uniform orientation of the constituting dyes. To stabilize layers made from uniformly assembled and aligned dyes, they can be covalently cross-linked via functional groups or via chromophores at the expense of their optical properties. Here stable membranes from established dyes are reported that do not need to be elaborately functionalized nor do their chromophores need to be destroyed. These membranes are free-standing, although being only non-covalently linked. To enable uniform dye-alignment, Langmuir layers made from linear, water-insoluble dyes are used. That water-soluble charge transfer dyes adsorb onto and intercalate into the Langmuir layer from the aqueous subphase, thus yielding free-standing, molecularly thin membranes are demonstrated. The developed bifacial layers consist almost entirely of π-conjugated units and thus can conduct charges and can be further engineered for optoelectronic and photocatalytic applications.

Investigation and design of efficient intramolecular charge transfer dyes with DBTP-based dual-electron-donor structure

In this work, three recently synthesized D1-D2-π-A type dyes (DB-1/2/3) were regarded as the primary dyes and were theoretically investigated in the context of dye-sensitized solar cells (DSSCs). For each dye, a di (1-benzothieno)[3,2-b:2′,3′-d]pyrrole (DBTP) group (electron donor D2) functions as a dual-electron-donor together with another electron donor (D1). Through density functional theory and time-dependent density functional theory calculations, the ground-state information and the excited-state information was obtained to build structure-property connection for the dyes. In particular, the intramolecular charge transfer capacity was emphatically studied from multiple angles (via absorption spectra, frontier molecular orbitals, electron density difference, etc.). Some key parameters that affect the short-circuit current and the open-circuit voltage of DSSCs were calculated and compared. Finally, based on the primary dyes, four new D1-DBTP-π-A type dyes (D1/2/3/4) were designed with the aim of improving the performances of these type dyes. Our research theoretically explains why DB-1 characterized by a triphenylamine unit as D1 preforms better than the other two primary dyes experimentally. Furthermore, it is predicted that due to the best light-harvesting ability, the designed dye D4 with a coumarin unit as D1 is expected to surpass DB-1 and become the most promising and potential for DSSCs among all the dyes in this work.

Optical Study of Solvatochromic Isocyanoaminoanthracene Dyes and 1,5-Diaminoanthracene.

Isocyanoaminoarenes (ICAAr-s) are a novel and versatile group of solvatochromic fluorophores. Despite their versatile applicability, such as antifungals, cancer drugs and analytical probes, they still represent a mostly unchartered territory among intramolecular charge-transfer (ICT) dyes. The current paper describes the preparation and detailed optical study of novel 1-isocyano-5-aminoanthrace (ICAA) and its N-methylated derivatives along with the starting 1,5-diaminoanthracene. The conversion of one of the amino groups of the diamine into an isocyano group significantly increased the polar character of the dyes, which resulted in a significant 50–70 nm (2077–2609 cm−1) redshift of the emission maximum and a broadened solvatochromic range. The fluorescence quantum yield of ICAAs is strongly influenced by the polarity of the solvent. The starting anthracene-diamine is highly fluorescent in every solvent (√f = 12–53%), while the isocyano derivatives are practically nonfluorescent in solvents more polar than dioxane. This phenomenon implies the potential application of ICAAs to probe the polarity of the medium and is favorable in practical applications, such as cell-staining, resulting in a reduced background fluorescence. The ICT character of the emission states of ICAAs are in good agreement with the computational findings presented in TD-DFT calculations and molecular electrostatic potential (MESP) isosurfaces.

Spiroconjugated Donor-σ-Acceptor Charge-Transfer Dyes: Effect of the π-Subsystems on the Optoelectronic Properties.

Charge-transfer-based materials with intramolecular donor-acceptor structures are attractive for technological applications. Herein, a series of donor-σ-acceptor dyes has been prepared in a modular approach. The design of these intramolecular charge-transfer dyes is based on the concept of spiroconjugation, which leads to unique materials with special optical properties. The optical transitions are based on intramolecular charge transfer, as shown by solvatochromic measurements and density functional theory (DFT) calculations. Crystallographic, computational, electrochemical, and optical studies were performed to clarify the effect of different perpendicular π-moieties on the optoelectronic properties. Our molecular tuning allowed for the synthesis of molecules exhibiting strong visible-range absorption. The compounds are not fluorescent due to structural changes in the excited state, as revealed by DFT calculations. Finally, our study describes enantiomerically pure spiroconjugated absorber molecules using 1,1'-binaphthyl-2,2'-diol (BINOL) units on the donor part.

Optical spectra of organic dyes in condensed phases: the role of the medium polarizability.

When designing molecular functional materials, the properties of the active specie, the dye, must be optimized fully accounting for the presence of a surrounding medium (a solvent, a polymeric matrix, etc.) that may largely alter the dye behavior. Here we present an effective model to account for the effects of the medium electronic polarizability on the spectral properties of charge-transfer dyes. Different classes of molecules are considered and the proposed antiadiabatic approach to solvation is contrasted with the adiabatic approach, currently adopted in all quantum chemical approaches to solvation. Transition frequencies and band-shapes are addressed, and the role of the medium polarizability on symmetry-breaking phenomena is also discussed.

Complexation of a guanidinium-modified calixarene with diverse dyes and investigation of the corresponding photophysical response.

We herein describe the comprehensive investigation of the complexation behavior of a guanidinium-modified calix[5]arene pentaisohexyl ether (GC5A) with a variety of typical luminescent dyes. Fluorescein, eosin Y, rose bengal, tetraphenylporphine sulfonate and sulfonated aluminum phthalocyanine were employed as classical aggregation-induced quenching dyes. 2-(p-Toluidinyl)naphthalene-6-sulfonic acid and 1-anilinonaphthalene-8-sulfonic acid were selected as representatives of intramolecular charge-transfer dyes. Phosphated tetraphenylethylene was involved as the classical aggregation-induced emission dye. Sulfonated acedan representing one example of two-photon fluorescent probes, was also investigated. A ruthenium(II) complex with carboxylated bipyridyl ligands was included as a representative candidate of luminescent transition-metal complexes. We determined the association constants of the GC5A–dye complexes by fluorescence titration and discuss the complexation-induced photophysical changes. In addition, a comparison of the complexation behavior of GC5A with that of other macrocycles and potential applications according to the diverse photophysical responses are provided.

Synthesis, Solvatochromic Performance, pH Sensing, Dyeing Ability, and Antimicrobial Activity of Novel Hydrazone Dyestuffs

New tricyanofuran intramolecular charge transfer dyes comprising the hydrazone group were prepared and fully characterized in order to study their possible solvatochromism, dyeing ability, and antimicrobial activity. The preparation of the hydrazone dyes was achieved in relatively good yields starting from different aromatic amines. The hydrazone functional group was presented via the azo-coupling reaction of the tricyanofuran compound by the properly substituted diazonium chloride. Chemical structures of the prepared hydrazones were confirmed via nuclear magnetic resonance spectroscopy (1H- and 13C-NMR), Fourier-transform infrared spectroscopy (FT-IR), and elemental analysis (C, H, and N). The UV-visible absorption spectra of the produced sensor colorants displayed interesting solvatochromism in solvents with a different polarity, which was found to be affected by the substituents bonded to the aromatic hydrazone moiety. The pH molecular switching was investigated through tuning the intramolecular charge transfer stimulated by the reversible deprotonation/protonation process in acetonitrile solution showing color change from yellow to purple. The produced disperse dyestuffs were employed for dyeing polyester fibers to introduce acceptable color strength and colorfastness properties. Moreover, the dyes verified a weak to moderate antimicrobial activity against some selected pathogens, including S. aureus, E. coli, and Candida albicans.

Tuning the optical properties of spiro-centered charge-transfer dyes by extending the donor or acceptor part

Spiroconjugated charge-transfer dyes with N–CO-substructure are presented, whose optical properties are tuned by extending the donor or acceptor part.

Salt–cocrystal continuum for photofunction modulation: stimuli-responsive fluorescence color-tuning of pyridine-modified intramolecular charge-transfer dyes and acid complexes

A series of acid–base complexes reveal ΔpKa value and the crystalline environment determine the extent of proton transfer, which governs the intramolecular charge-transfer (ICT) strength of the complexes and tunes the photoluminescence properties.

Metal Complex as an Optical Sensing Platform for Rapid Multimodal Recognition of a Pathogenic Biomarker in Real-Life Samples

Anthraimidazoledione-based charge transfer dyes have been designed for multimodal detection of a pathogenic biomarker, dipicolinic acid (DPA), at physiological pH. A change in visible color from ye...

Charge-transfer dynamics at the dye-semiconductor interface of photocathodes for solar energy applications.

This article describes a comparison between the photophysical properties of two charge-transfer dyes adsorbed onto NiO via two different binding moieties. Transient spectroscopy measurements suggest that the structure of the anchoring group affects both the rate of charge recombination between the dye and NiO surface and the rate of dye regeneration by an iodide/triiodide redox couple. This is consistent with the performance of the dyes in p-type dye sensitised solar cells. A key finding was that the recombination rate differed in the presence of the redox couple. These results have important implications on the study of electron transfer at dye|semiconductor interfaces for solar energy applications.

Vibrational coherences in charge-transfer dyes: a non-adiabatic picture.

Essential-state models efficiently describe linear and nonlinear spectral properties of different families of charge-transfer chromophores. Here, the essential-state machinery is applied to the calculation of the early-stage dynamics after ultrafast (coherent) excitation of polar and quadrupolar chromophores. The fully non-adiabatic treatment of coupled electronic and vibrational motion allows for a reliable description of the dynamics of these intriguing systems. In particular, the proposed approach is reliable even when the adiabatic and harmonic approximations do not apply, such as for quadrupolar dyes that show a multistable, broken-symmetry excited state. Our approach quite naturally leads to a clear picture for a dynamical Jahn-Teller effect in these systems. The recovery of symmetry due to dynamical effects is however disrupted in polar solvents where a static symmetry lowering is observed. More generally, thermal disorder in polar solvents is responsible for dephasing phenomena, damping the coherent oscillations with particularly important effects in the case of polar dyes.

Preparation and pH-Switching of Fluorescent Borylated Arylisoquinolines for Multilevel Molecular Logic

The preparation of pH-switchable fluorescent borylated arylisoquinoline dyes via a flexible iridium-catalyzed route is reported. The obtained dyes feature aromatic amino substitution and lateral aliphatic amino groups as electron donors. The photophysical properties of the internal charge transfer dyes were studied, which was complemented by density functional theory calculations. Appreciable fluorescence quantum yields (Φf up to ca. 0.4) and characteristic spectral fingerprints in the green to red emission range were observed. The fluorescence modulation upon multiple and orthogonal protonation with triflic acid was studied and led to the interpretation of multilevel switching including off-on-off, ternary, and quaternary responses.

Synthesis and Solvatofluorochromism Behaviors on Intramolecular Charge Transfer System of Novel D-π-A Dyes

Solvatochromic donor-π-acceptor (D-π-A) charge transfer dyes have been great attention from the viewpoint of their applicability as probes for the determination of solvent polarity and colorimetric chemosensors. In this work, novel solvatochromic fluorophore dyes based on D-π-A charge transfer system were designed and synthesized containing a strong electron-donating unit as a carbazole moiety and three different strong electron-withdrawing units. The corresponding molecular design and the optical properties of the D-π-A dyes were examined in various solutions with widely different polarity. The prepared dyes were found to exhibit a positive fluorescence solvatochromism. Furthermore, the energy properties of these dyes were also computationally optimized and calculated by computational simulation approaches using Material Studio 4.3 and electrochemical measurement.

Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents

Joint experimental and theoretical work is presented on two quadrupolar D-π-A-π-D chromophores characterized by the same bulky donor (D) group and two different central cores. The first chromophore, a newly synthesized species with a malononitrile-based acceptor (A) group, has a V-shaped structure that makes its absorption spectrum very broad, covering most of the visible region. The second chromophore has a squaraine-based core and therefore a linear structure, as also evinced from its absorption spectra. Both chromophores show an anomalous red shift of the absorption band upon increasing solvent polarity, a feature that is ascribed to the large, bulky structure of the molecules. For these molecules, the basic description of polar solvation in terms of a uniform reaction field fails. Indeed, a simple extension of the model to account for two independent reaction fields associated with the two molecular arms quantitatively reproduces the observed linear absorption and fluorescence as well as fluorescence anisotropy spectra, fully rationalizing their nontrivial dependence on solvent polarity. The model derived from the analysis of linear spectra is adopted to predict nonlinear spectra and specifically hyper-Rayleigh scattering and two-photon absorption spectra. In polar solvents, the V-shaped chromophore is predicted to have a large HRS response in a wide spectral region (approximately 600-1300 nm). Anomalously large and largely solvent-dependent HRS responses for the linear chromophores are ascribed to symmetry lowering induced by polar solvation and amplified in this bulky system by the presence of two reaction fields.

Resonance energy transfer between polar charge-transfer dyes: A focus on the limits of the dipolar approximation

Resonance energy transfer (RET) is investigated in a pair of polar charge-transfer (CT) chromophores, adopting essential-state models and time dependent density functional theory (TDDFT) calculations. Essential-state models describe in an efficient way linear and nonlinear optical properties of CT dyes, and prove very useful to rationalize the effects of electrostatic interchromophoric interactions on optical properties of multichromophoric systems. In this paper we adopt the same strategy developed for multichromophoric systems to investigate interchromophoric interactions responsible for RET. In the late forties, Th. Förster proposed a powerful method, based on the dipolar approximation, that directly relates the rate of the RET process to experimental accessible quantities. Here we discuss the applicability of the dipolar approximation for RET between CT dyes. The results obtained with essential-state models are confirmed by TDDFT calculations.