Synthesis and spectroscopic characterization of fluorescent boron dipyrromethene-derived hydrazones

Functionalization of BODIPY Dyes with Additional C-N Double Bonds and Their Applications

BODIPY (4-bora-3a,4a-diaza-s-indacene) dyes are regarded as highly useful compounds due to their rich photophysical properties, stability, and ease of functionalization. In recent years, hot topics studied with this class of compounds are targeted photodynamic therapy, photothermal therapy, fluorescent bioimaging agents, structural modification of the BODIPY core, synthesis of BODIPY analogs, and BODIPY-based supramolecular constructs. This review covers the advances in BODIPY structures substituted with additional carbon-nitrogen double bonds, namely imines, hydrazones, oximes, and related derivatives for various applications. Works based on fluorescent indicators of anions, cations, and neutral molecules are included in this review. In addition, the use of such structures for pharmaceutical applications, photodynamic therapy, fluorescent switches, and fluorescent building blocks are also investigated. In addition to covering the major literature within the mentioned subclass, design principles, working mechanisms, and outlooks are also provided to enlighten forthcoming promising efforts. With this work, we aim to provide insights about the synthesis, photophysical properties, contribution of C=N bonds to a class of dye, and possible areas of use and stimulate researchers to present new ideas and overcome the current problems using these derivatives.

New Luminescent Pyridine-based Disc type Molecules: Synthesis, Photophysical, Electrochemical, and DFT studies

The design and synthesis of new conjugated luminescent molecules have attracted the attention of researchers because of their various applications, especially in the field of optoelectronic devices. Most of the applications were mainly based on the intramolecular charge transfer (ICT). For this purpose, we designed and synthesized a series of new donor-acceptor based disc type molecules i.e. 2,4,6-tris(4-(alkyloxy)phenyl)pyridines carrying variable alkoxy chains [i.e. n = 2, 4, 6, 8, 10, 12, 14, 16]. Further, the structures of all the synthesized compounds were confirmed by using ATR-IR, ¹H-NMR, ¹³C-NMR, and ESI-MS analysis. Moreover, the photophysical property study indicated that all the molecules are blue light emitting materials, however the change of alkoxy chain length in phenyl arms does not affect their absorption, emission, and energy levels. Besides, the thermal study revealed that core is stable up to 350 °C. Also, the DFT study showed that the photo induced electron transfer caused by HOMO-LUMO excitation in the studied molecules. Therefore, all the molecules have potential applications in optoelectronic applications.

New luminescent ordered liquid crystalline molecules with a 3-cyano-2-pyridone core unit

The self-organized states of liquid crystals (LCs) have recently received a lot of attention because of their various applications, especially in the fields of electron transport materials and optoelectronic devices. In most of these applications, molecules containing a donor-acceptor skeleton have been widely employed and generally these molecular frameworks have been designed mainly on the basis of the donor-acceptor concept. Inspired from this concept, we synthesized a series of new donor-acceptor based luminescent molecules, i.e. 4,6-bis(4-alkyloxyphenyl)-2-oxo-1,2-dihydropyridine-3-carbonitrile (Pn-series), carrying variable alkoxy chains [i.e. n = 8, 10, 12, 14, 16]. The structures of the synthesized molecules were confirmed by various spectral analyses. Further, their absorption and emission studies indicated that these molecules show blue light emitting properties. Moreover, the experimentally obtained optical band gap was analogous to the theoretical band gap calculated from the DFT study. The first two members of the Pn-series (n = 8 and 10) are non-mesogens. As the alkyl chain length increased to n = 12 and n = 14, the smectic C phase appeared along with an additional low temperature ordered lamellar phase. When n = 16, the smectic C phase disappeared and the compound exhibited only an ordered lamellar phase. This ordered lamellar phase is mainly due to the face to face alignment which makes these molecules potential candidates for electron transport materials and optoelectronic devices.

A static and dynamic NMR study of 10-hydrazino-BODIPY

10-Hydrazino-BODIPY, BoNHNH₂ , presents slow rotation about the C10-NH bond that results in anisochronous ¹ H and ¹³ C NMR signals. The assignment of the different signals has been made using traditional two-dimensional methods as well as spin-spin coupling constants and confirmed by DFT calculations (B3LYP) using the 6-311++G(d,p) basis set. The rotational barrier has been determined in three pairs of proton signals and compared with the calculated barrier.

Reducing end modification on cellulose nanocrystals: strategy, characterization, applications and challenges

Different from traditional chemical surface modification, localized modification of the reducing end groups of cellulose nanocrystals (CNCs), i.e. the active aldehyde groups, provides new opportunities for diverse functional applications of this renewable nanomaterial without altering its surface chemistry and properties. Numerous reviews have deeply discussed the surface modification of the hydroxyl groups of CNCs, but no critical comment has been reported on the reducing end modification approach. This review is a comprehensive summary on the modification of the CNC reducing end, presenting the reaction mechanisms and conditions, discussing the different chemical modification strategies and characterization techniques, potential applications and future challenges in this field. In addition, the comparison between surface and end modification strategies of CNCs will highlight the potential of reducing end-functionalized CNCs to be used in various applications as an alternative to traditional surface-modified CNCs, or as additional functional nanoparticles for the design of advanced functional materials.

Rapid formation of a stable boron-nitrogen heterocycle in dilute, neutral aqueous solution for bioorthogonal coupling reactions

Combining 2-formylphenylboronic acid with 4-hydrazinylbenzoic acid in neutral aqueous solution at low, equimolar concentrations of the reagents results in a single, stable product, a 1,2-dihydro-1-hydroxy-2,3,1-benzodiazaborine, in a matter of minutes with no side products. Application of this reaction to protein conjugation demonstrates that the reaction is orthogonal to protein functional groups, and the resulting conjugate withstands SDS-PAGE analysis. This reaction should be particularly useful for couplings that must be performed with low concentrations of reagents under physiologically compatible conditions.

Formation of 8-RS-BODIPYs via direct substitution of 8-MeS-BODIPY by RSH (R = Et, Pr, Bu, tBu, n-C12H25, C6H5, p-MeC6H4, p-MeOC6H4, and 2,6-Me2C6H3)

Reactions between 8-RS-BODIPY (R = Me, 1) and alkyl and aryl thiols were readily accomplished in dichloromethane to provide a synthetic pathway to a range of new 8-organothio-BODIPYs in good yield. The new alkyl 8-RS-BODIPYs (R = Et, 2; Pr, 3; Bu, 4; tBu, 5; n-dodecyl, 6) exhibit absorption and emission properties essentially unchanged from those of 1 whereas the arylthio analogs (R = Ph, 7; 2,6-Me2C6H3, 8; p-MeC6H4, 9; p-MeOC6H4, 10) exhibit no fluorescence with the exception of 7, and then only in hexane. In common with other related 8-substituted BODIPYs, the new 8-alkylthio-BODIPY dyes show decreasing fluorescence intensity as solvent polarity increases. Compounds 2, 3, 7, and 8 were characterized via single-crystal X-ray analysis; the alkyl derivatives 2 and 3 exhibited planar BODIPY cores with co-planar organothio- substituents whereas the aryl derivatives exhibited both BODIPY core deformation and significant twisting about the S–C (8) bond removing co-planarity between the aryl group and the distorted BODIPY core. These deformations coincide with the significantly reduced emission properties.

A multi-stimuli-responsive fluorescence switch based on E–Z isomerization of hydrazone

We report a new strategy to construct fluorescence switches by taking advantage of the E–Z isomerization of the hydrazone group.

Solvatochromism of BODIPY-Schiff dye

A boron-dipyrrin chromophore connected with an o-hydroxyaryl aldimine by a diazo bridge (BODIPY-Schiff dye) has been developed. The photophysical properties of the BODIPY-Schiff dye have been investigated with UV, steady-state, and time-resolved fluorimetry. The spectral features have been characterized with respect to density functional theory and time-dependent density functional theory. The conformational analysis of the studied compound has been accomplished both in the ground and excited states. A scheme of the processes occurring in the BODIPY-Schiff dye has been proposed.

Design strategies for bioorthogonal smart probes

Bioorthogonal chemistry has enabled the selective labeling and detection of biomolecules in living systems. Bioorthogonal smart probes, which become fluorescent or deliver imaging or therapeutic agents upon reaction, allow for the visualization of biomolecules or targeted delivery even in the presence of excess unreacted probe. This review discusses the strategies used in the development of bioorthogonal smart probes and highlights the potential of these probes to further our understanding of biology.

A BODIPY-based reactive probe for the detection of Au(iii) species and its application to cell imaging

A BODIPY-based chemodosimeter for turn-on detection of gold ions which relies on a novel CN bond hydrolysis mechanism.

Blue-to-orange color-tunable laser emission from tailored boron-dipyrromethene dyes

A series of meso-substituted boron-bipyrromethene (BODIPY) dyes are synthesized and their laser and photophysical properties systematically studied. Laser emission covering a wide visible spectral region (from blue to orange) is obtained by just changing the electron donor character of the heteroatom at position 8. The additional presence of methyl groups at positions 3 and 5 results in dyes with a photostability similar to that of the unsubstituted dye but with much improved efficiency. Correlation of the lasing properties of the different dyes to their photophysical properties provides inklings to define synthetic strategies of new BODIPY dyes with enhanced efficiency and modulated wavelength emission over the visible spectral region.

Fluorescence quenchers for hydrazone and oxime orthogonal bioconjugation

We describe the synthesis and properties of new fluorescence quenchers containing aldehyde, hydrazine, and aminooxy groups, allowing convenient bioconjugation as oximes or hydrazones. Conjugation to oligonucleotides proceeded in high yield with aniline as catalyst. Kinetics studies of conjugation show that, under optimal conditions, a hydrazine or aminooxy quencher can react with aldehyde-modified DNA to form a stable hydrazone or oxime adduct in as little as five minutes. The resulting quencher-containing DNAs were assessed for their ability to quench the emission of fluorescein in labeled complements and compared to the commercially available dabcyl and Black Hole Quencher 2 (BHQ2), which were conjugated as phosphoramidites. Results show that the new quenchers possess slightly different absorbance properties compared to dabcyl and are as efficient as the commercial quenchers in quenching fluorescein emission. Hydrazone-based quenchers were further successfully incorporated into molecular beacons and shown to give high signal to background ratios in single nucleotide polymorphism detection in vitro. Finally, aminooxy and hydrazine quenchers were applied to quenching of an aldehyde-containing fluorophore associated with living cells, demonstrating cellular quenching within one hour.