Conjugation Extension and Halochromic Behaviors of S-Fused Polycyclic Aromatic Hydrocarbons Bearing Cyclopenta[b]thiopyran Moieties

Three S-fused polycyclic aromatic hydrocarbons (PAHs) bearing cyclopenta[b]thiopyran moieties have been designed and successfully synthesized. With the conjugation extension, the absorption onset of the longest PAH reaches 1110 nm. All the three S-fused PAHs exhibit significant halochromic properties in both solution and solid states. Upon protonation, the proton is incorporated on the cyclopentadiene ring while the positive charge is localized on the thiopyrylium ring. Moreover, no significant difference can be found for the two shorter PAHs upon the protonation by different organic acids, such as trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH), while the longest PAH can be only mono-protonated by TFA but di-protonated by stronger TfOH. Furthermore, after protonation, the non-emissive S-fused PAHs exhibit strong fluorescence and can be regenerated by simply neutralization with triethylamine. The enhanced emission of mono-protonated products stem from S₂ →S₀ transitions, which disobey the Kasha's rule.

Synthesis, Characterization, and Antiproliferative Properties of New Bio-Inspired Xanthylium Derivatives

Xanthylium derivatives are curcumin analogs showing photochromic properties. Similarly, to anthocyanins, they follow the same multistate network of chemical species that are reversibly interconverted by external stimuli. In the present work, two new asymmetric monocarbonyl analogues of curcumin, 4-(4-hydroxy-3-metoxybenzylidene)-1,2,3,4-tetrahydroxanthylium chloride (compound 3) and 4-(4-hydroxybenzylidene)-6-methoxy-1,2,3,4-tetrahydroxanthylium chloride (compound 4) were synthesized, and their photochromic and biological properties were investigated. The UV-Vis spectroscopy and the direct and reverse pH-jumps studies confirmed the halochromic properties and the existence of different molecular species. A network of chemical reactions of these species was proposed. Furthermore, the antiproliferative properties of both compounds were evaluated using P19 murine embryocarcinoma cells and compared with each other. The results demonstrate that both new xanthylium derivatives modify the progression through the cell cycle of P19 cells, which translates into a significant antiproliferative effect. The effect of the methoxy group position is discussed and several checkpoint proteins are advanced as putative targets.

Fabrication of Polysaccharide-Based Halochromic Nanofibers via Needle-Less Electrospinning and Their Characterization: A Study of the Leaching Effect

Responsive materials, i.e., smart materials, have the ability to change their physical or chemical properties upon certain external signals. The development of nanofibrous halochromic materials, specifically combining the pH-sensitive functionality and unique nanofiber properties, could yield interesting new applications, especially when the common problem of dye leaching is successfully tackled. Therefore, in this article, we studied the fabrication process of polysaccharide-based halochromic nanofibrous materials by using a combination of various halochromic dyes (bromothymol blue, bromocresol green, and thymol blue) and cellulose acetate in a spinning solution using a one-pot strategy. The inhibition of leaching was addressed by using a complexing agent: poly-diallyl-dimethylammonium chloride (PDADMAC). The preparation of hybrid spinning solutions, their characterization, and ability to form continuous nanofibers were studied using a high production needle-less electrospinning system. The produced hybrid solutions and nanofibers were characterized, in terms of their rheological properties, chemical structure, morphology, and functionality. Fabricated nanofibrous halochromic structures show a clear color change upon exposure to different pH values, as well as the reduced leaching of dyes, upon the addition of a complexing agent. The leaching decreased by 61% in the case of bromocresol green, while, in the case of bromothymol blue and thymol blue, the leaching was reduced by 95 and 99%, respectively.

Naphthalimide-Fused Dipyrrins: Tunable Halochromic Switches and Photothermal NIR-II Dyes

A family of tunable halochromic switches is developed using a naphthalimide-fused dipyrrin as the core π-conjugated motif. Electronic properties of these dipyrrins are tuned by substitution of their alpha and meso positions with aryl groups of variable donor-acceptor strength. The first protonation results in a conformational change that enhances electronic coupling between the dipyrrin chromophore and the meso substituent, leading to halochromic effects that occasionally exceed 200 nm and switch the absorption between the near-infrared (NIR)-I and NIR-II ranges. A NIR-II photothermal effect, switchable by acid-base chemistry is demonstrated for selected dipyrrins. Further protonation is possible for derivatives bearing additional amino groups, leading to up to four halochromic switching step. The most electron-rich dipyrrins are also susceptible to chemical oxidation, yielding NIR-absorbing radical cations and closed-shell dications.

Highly Tunable Multicolor Water-Jet Rewritable Paper Based on Simple New-Type Dual-Addressable Oxazolidines

Rewritable paper based on switchable molecules has attracted great attention in both academic research and marketplace. However, most available switchable dyes have single switchable color state only, which cannot meet the long-awaited multicolor reversible displays. Herein, through simple introduction of phenolic hydroxyl group, we develop a series of new oxazolidines with one switch unit, which could reversibly display two different as well as their mix-gradient colors by treating with water and mild acid, respectively, both in solution and solid substrate. The structures and mechanism for the formation of two colors had been studied in detail via UV-vis/NMR spectroscopy, skillfully designing contrast molecules, and kinetics experiments. This multiple switchable colors of the dyes have been further applied to construct a rewritable paper for ink-free printing with multi/gradient-color display.

Organic Salts Suppress Aggregation and Enhance the Hyperpolarizability of a π-Twisted Chromophore

Twisted intramolecular charge-transfer (TICT) chromophores exhibit extraordinary hyperpolarizabilities, β, and are therefore promising for electro-optic technologies. Nevertheless, centrosymmetric aggregate formation severely diminishes β in concentrated solutions or in polymer matrices. Herein, the remarkable effects of organic salts on the linear and nonlinear optical response of a high β benzimidazolium-based TICT chromophore, B2TMC-2, are reported. Addition of Bu₄ P⁺ Br^- to B2TMC-2 solution in CHCl₃ induces a halochromic blueshift, primarily reflecting interactions between Bu₄ P⁺ Br^- and the B2TMC-2 cationic portion. DC electric-field induced second-harmonic generation (EFISH) measurements on B2TMC-2+Bu₄ P⁺ Br^- solution reveal a large μβ=-22,160×10^-48  esu, unprecedented for any chromophore with such a broad optical transparency window. Moreover, Bu₄ P⁺ Br^- is shown to suppress B2TMC-2 aggregation, thereby preserving high μβ in concentrated solutions. This phenomenon should be applicable to many other NLO chromophores.

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore

Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) by structural modification. N-Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole-type binding sites of OxP and serial variation of the chromic properties of the products, double N-benzylated Bz₂OxP and tetra N-benzylated Bz₄OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz₂OxP, and Bz₄OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding-site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

Multi-stimuli responsive luminescent azepane-substituted β-diketones and difluoroboron complexes

Difluoroboron β-diketonate (BF2bdk) compounds show environment-sensitive optical properties in solution, aggregation-induced emission (AIE) and multi-stimuli responsive fluorescence switching in the solid state. Here, a series of 4-azepane-substituted β-diketone (bdk) ligands (L-H, L-OMe, L-Br) and their corresponding difluoroboron dyes (D-H, D-OMe, D-Br) were synthesized, and various responsive fluorescence properties of the compounds were studied, including solvatochromism, viscochromism, AIE, mechanochromic luminescence (ML) and halochromism. Compared to the β-diketones, the boron complexes exhibited higher extinction coefficients but lower quantum yields, and red-shifted absorption and emission in CH2Cl2. Computational studies showed that intramolecular charge transfer (ICT) dominated rather than π-π* transitions in all the compounds regardless of boron coordination. In solution, all the bdk ligands and boron dyes showed red-shifted emission in more polar solvents and increased fluorescence intensity in more viscous media. Upon aggregation, the emission of the β-diketones was quenched, however, the boronated dyes showed increased emission, indicative of AIE. Solid-state emission properties, ML and halochromism, were investigated on spin cast films. For ML, smearing caused a bathochromic emission shift for L-Br, and powder X-ray diffraction (XRD) patterns showed that the "as spun" and thermally annealed states were more crystalline and the smeared state was amorphous. No obvious ML emission shift was observed for L-H or L-OMe, and the boronated dyes were not mechano-active. Trifluoroacetic acid (TFA) and triethylamine (TEA) vapors were used to study halochromism. Large hypsochromic emission shifts were observed for all the compounds after TFA vapor was applied, and reversible fluorescence switching was achieved using the acid/base pair.

Highlighting Cancer Cells with Halochromic Switches

Halochromic coumarin-oxazine prefluorophores and targeting folate ligands can be connected covalently to the side chains of amphiphilic polymers. The resulting macromolecular constructs assemble into nanoparticles in aqueous environments. The prefluorophores do not produce any detectable fluorescence at neutral pH, but are converted into fluorophores with intense visible emission at acidic pH. Protonation opens the oxazine heterocycle to shift bathochromically the coumarin absorption and activate fluorescence with a brightness per nanoparticle approaching 5 × 10⁵ M^-1 cm^-1. This value translates into a 170-fold enhancement relative to the isolated fluorophores dissolved in organic solvent. The folate ligands direct these multicomponent constructs into acidic intracellular compartments of folate-positive cells, where the prefluorophores switch to the corresponding fluorophores and produce fluorescence. The pH-induced activation of the signaling units ensures negligible background fluorescence from the extracellular matrix, which instead limits considerably the contrast accessible with model systems incorporating conventional nonactivatable fluorophores. Furthermore, no intracellular fluorescence can be detected when the very same measurements are performed with folate-negative cells. Nonetheless, control experiments demonstrate that the covalent connection of the prefluorophores to the polymer backbone of the amphiphilic constructs is essential to ensure selectivity. Model systems with prefluorophores noncovalently encapsulated cannot discriminate folate-positive from -negative cells. Thus, our structural design for the covalent integration of activatable signaling units and targeting ligands within the same nanostructured assembly together with the photophysical properties engineered into the emissive components offer the opportunity to highlight cancer cells selectively with high brightness and optimal contrast.

Halochromic Isoquinoline with Mechanochromic Triphenylamine: Smart Fluorescent Material for Rewritable and Self-Erasable Fluorescent Platform

Halochromic isoquinoline attached mechanochromic triphenylamine, N-phenyl-N-(4-(quinolin-2-yl)phenyl)benzenamine (PQPBA) and tris(4-(quinolin-2-yl)phenyl)amine (TQPA), smart fluorescent materials exhibit thermo/mechanochromism and tunable solid state fluorescence and their unusual halochromic response in PMMA matrix have been used for fabricating rewritable and self-erasable fluorescent platforms. PQPBA and TQPA showed strong fluorescence in solution (Φ_f = 0.9290 (PQPBA) and 0.9160 (TQPA)) and moderate solid state fluorescence (Φ_f = 20 (PQPBA) and 17% (TQPA). Interestingly, they exhibited a rare temperature (0-100 °C) dependent positive fluorescence enhancement via activating radiative vibrational transition. The deaggregation of PQPBA and TQPA in PMMA polymer matrix lead to the enhancement of fluorescence intensity strongly and fabricated strong blue fluorescent thin films (Φ_f = 58% (PQPBA) and 54% (TQPA). The halochromic isoquinoline has been exploited for demonstrating reversible off-on fluorescence switching by acid (TFA (trifluoroacetic acid)/HCl) and base (NH₃) treatment in both solids as well as PMMA thin films. Importantly, rewritable and self-erasable fluorescent platform has been achieved by make use of unusual fluorescence responses of PQPBA/TQPA with TFA/HCl after exposing NH₃. Single crystal and powder X-ray diffraction (PXRD) studies provided the insight on the solid-state fluorescence and external stimuli-induced fluorescence changes.