Diaroyl(methanato)boron difluoride compounds as medium-sensitive two-photon fluorescent probes

This paper evaluates the use of diaroyl(methanato)boron difluoride compounds for designing efficient fluorescent probes through two-photon absorption. Three different pathways allowing for the syntheses of symmetrical and dissymmetrical molecules are reported. The stable diaroyl(methanato)boron difluoride derivatives can be easily obtained in good yields. They exhibit a large one-photon absorption that is easily tuned in the near-UV range. Their strong fluorescence emission covers the whole visible domain. In addition to these attractive linear properties, several diaroyl(methanato)boron difluoride derivatives possess significant cross sections for two-photon absorption. The derived structure-property relationships are promising for designing new generations of molecules relying on the diaroyl(methanato)boron difluoride backbone.     

Conformationally restricted aza-BODIPY: highly fluorescent, stable near-infrared absorbing dyes

Novel NIR fluorescent, conformational restricted aza-dipyrromethene boron difluoride (aza-BODIPY) dyes were prepared by an efficient process. Such conformational restricted aza-BODIPY dyes possess intense absorption, strong fluorescence, high chemical and photostability. Additionally, the sharp fluorescence of non-amine containing aza-BODIPY dyes is insensitive to solvent polarity.     

Two-photon fluorescent probes for long-term imaging of calcium waves in live tissue

2-Acetyl-6-(dimethylamino)naphthalene-derived two-photon fluorescent Ca2+ probes (ACa1-ACa3) are reported. They can be excited by a 780 nm laser beam, show 23-50-fold enhancement in one- and two-photon excited fluorescence in response to Ca2+, emit fourfold stronger two-photon excited fluorescence than Oregon Green 488 BAPTA-1 upon complexation with Ca2+, and can selectively detect intracellular free Ca2+ ions in live cells and living tissues with minimum interference from other metal ions and membrane-bound probes. Moreover, these probes are capable of monitoring calcium waves at a depth of 120-170 microm in live tissues for 1100-4000 s using two-photon microscopy with no artifacts of photobleaching.     

The chemistry of fluorescent bodipy dyes: versatility unsurpassed

The world of organic luminophores has been confined for a long time to fairly standard biological labeling applications and to certain analytical tests. Recently, however, the field has undergone a major change of direction, driven by the dual needs to develop novel organic electronic materials and to fuel the rapidly emerging nanotechnologies. Among the many diverse fluorescent molecules, the Bodipy family, first developed as luminescent tags and laser dyes, has become a cornerstone for these new applications. The near future looks extremely bright for "porphyrin's little sister".     

Dihydronaphthalene‐Fused Boron–Dipyrromethene (BODIPY) Dyes: Insight into the Electronic and Conformational Tuning Modes of BODIPY Fluorophores

A new series of boron–dipyrromethene (BDP, BODIPY) dyes with dihydronaphthalene units fused to the β‐pyrrole positions ( 1 a – d , 2 ) has been synthesised and spectroscopically investigated. All the dyes, except pH‐responsive 1 d in polar solvents, display intense emission between 550–700 nm. Compounds 1 a and 1 b with a hydrogen atom and a methyl group in the meso position of the BODIPY core show spectroscopic properties that are similar to those of rhodamine 101, thus rendering them potent alternatives to the positively charged rhodamine dyes as stains and labels for less polar environments or for the dyeing of latex beads. Compound 1 d , which carries an electron‐donating 4‐(dimethylamino)phenyl group in the meso position, shows dual fluorescence in solvents more polar than dibutyl ether and can act as a pH‐responsive “light‐up” probe for acidic pH. Correlation of the pK_a data of 1 d and several other meso‐(4‐dimethylanilino)‐substituted BODIPY derivatives allowed us to draw conclusions on the influence of steric crowding at the meso position on the acidity of the aniline nitrogen atom. Preparation and investigation of 2 , which carries a nitrogen instead of a carbon as the meso‐bridgehead atom, suggests that the rules of colour tuning of BODIPYs as established so far have to be reassessed; for all the reported couples of meso‐C‐ and meso‐N‐substituted BODIPYs, the exchange leads to pronounced redshifts of the spectra and reduced fluorescence quantum yields. For 2 , when compared with 1 a , the opposite is found: negligible spectral shifts and enhanced fluorescence. Additional X‐ray crystallographic analysis of 1 a and quantum chemical modelling of the title and related compounds employing density functional theory granted further insight into the features of such sterically crowded chromophores.     

Deuterated Indocyanine Green (ICG) with Extended Aqueous Storage Shelf-Life: Chemical and Clinical Implications

Indocyanine Green ( ICG ) is a clinically approved near-infrared fluorescent dye that is used extensively for various imaging and diagnostic procedures. One drawback with ICG is its instability in water, which means that reconstituted clinical doses have to be used very shortly after preparation. Two deuterated versions of ICG were prepared with deuterium atoms on the heptamethine chain, and the spectral, physiochemical, and photostability properties were quantified. A notable mechanistic finding is that self-aggregation of ICG in water strongly favors dye degradation by a photochemical oxidative dimerization reaction that gives a nonfluorescent product. Storage stability studies showed that replacement of C−H with C−D decreased the dimerization rate constant by a factor of 3.1, and it is likely that many medical and preclinical procedures will benefit from the longer shelf-lives of these two deuterated ICG dyes. The discovery that ICG self-aggregation promotes photoinduced electron transfer can be exploited as a new paradigm for next-generation photodynamic therapies.     

Push-pull amino succinimidyl ester thiophene-based fluorescent dyes: synthesis and optical characterization

The design and synthesis of new fluorescent dyes with emission range at 490-650 nm are described. Their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The chromophores are donor-π-bridge-acceptor push-pull compounds with a π bridge of phenyl and thiophene rings and their combination. Compared with previous thiophene fluorophores, these dyes show significant redshift in the absorption and emission spectra and offer compact, red-emitting fluorophores. The dyes have amino succinimidyl active ester and can be readily conjugated to proteins, polymers and other amino-group-containing materials.     

Rational design, synthesis, and spectroscopic and photophysical properties of a visible-light-excitable, ratiometric, fluorescent near-neutral pH indicator based on BODIPY

A visible-light-excitable, ratiometric, brightly fluorescent pH indicator for measurements in the pH range 5-7 has been designed and synthesized by conjugatively linking the BODIPY fluorophore at the 3-position to the pH-sensitive ligand imidazole through an ethenyl bridge. The probe is available as cell membrane permeable methyl ester 8-(4-carbomethoxyphenyl)-4,4-difluoro-3-[2-(1H-imidazol-4-yl)ethenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (I) and corresponding water-soluble sodium carboxylate, sodium 8-(4-carboxylatophenyl)-4,4-difluoro-3-[2-(1H-imidazol-4-yl)ethenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (II). The fluorescence quantum yield Φ(f) of ester I is very high (0.8-1.0) in the organic solvents tested. The fluorescence lifetime (ca. 4 ns) of I in organic solvents with varying polarity/polarizability (from cyclohexane to acetonitrile) is independent of the solvent with a fluorescence rate constant k(f) of 2.4×10(8) s(-1). Probe I is readily loaded in the cytosol of live cells, where its high fluorescence intensity remains nearly constant over an extended time period. Water-soluble indicator II exhibits two acid-base equilibria in aqueous solution, characterized by pK(a) values of 6.0 and 12.6. The Φ(f) value of II in aqueous solution is high: 0.6 for the cationic and anionic forms of the imidazole ligand, and 0.8 for neutral imidazole. On protonation-deprotonation in the near-neutral pH range, UV/Vis absorption and fluorescence spectral shifts along with isosbestic and pseudo-isoemissive points are observed. This dual-excitation and dual-emission pH indicator emits intense green-yellow fluorescence at lower pH and intense orange fluorescence at higher pH. The influence of ionic strength and buffer concentration on the absorbance and steady-state fluorescence of II has also been investigated. The apparent pK(a) of the near-neutral acid-base equilibrium determined by spectrophotometric and fluorometric titration is nearly independent of the added buffer and salt concentration. In aqueous solution in the absence of buffer and in the pH range 5.20-7.45, dual exponential fluorescence decays are obtained with decay time τ(1)=4.3 ns for the cationic and τ(2)=3.3 ns for the neutral form of II. The excited-state proton exchange of II at near-neutral pH becomes reversible on addition of phosphate (H(2)PO(4)(-)/HPO(4)(2-)) buffer, and a pH-dependent change of the fluorescence decay times is induced. Global compartmental analysis of fluorescence decay traces collected as a function of pH and phosphate buffer concentration was used to recover values of the deactivation rate constants of the excited cationic (k(01)=2.4×10(8) s(-1)) and neutral (k(02)=3.0×10(8) s(-1)) forms of II.     

The art of fluorescence imaging with chemical sensors

Fluorescence imaging techniques involving chemical sensors are essential tools in many fields of science and technology because they enable the visualization of parameters which exhibit no intrinsic color or fluorescence, for example, oxygen, pH value, CO(2), H(2)O(2), Ca(2+), or temperature, to name just a few. This Review aims to highlight the state of the art of fluorescence sensing and imaging, starting from a comprehensive overview of the basic functional principles of fluorescent probes (or indicators) and the design of sensor materials. The focus is directed towards the progress made in the development of multiple sensors and methods for their signal read out. Imaging methods involving optical sensors are applied in quite diverse scientific areas, such as medical research, aerodynamics, and marine research.     

A Simple Central Seven-Membered BOPYIN: Synthesis,Structural, Spectroscopic Properties,and Cellular Imaging Application

A two-step, one-flask synthesis of central seven-membered borondifluoride-3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole (BOPYIN) ligands has been developed by using the unexplored 3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole. The simple synthetic approach has enabled modification of the electronic structure by changing the substituents on the indole unit. X-ray analysis indicated that conformations of the seven-membered BF₂ complex including BOPYIN and diazaborepin differ from that of the five- and six-membered organoboron complexes. Interestingly, the bond angle of the N⋅⋅⋅B−N bond increases with the number of atoms in the core ring, based on Baeyer strain theory. These unsymmetric BOPYIN derivatives have excellent photophysical properties, including high fluorescence quantum yields, except for BOPYIN- 4 in the solution state, large Stokes shifts, and good molar absorptivity. The dipole moment of BOPYIN- 3 in the first excited singlet state and ground state was demonstrated by a linear Lippert–Mataga plot. The absorption and emission spectra were not mirror images for BOPYIN- 1 – 3 and 5 , in contradiction to Kasha's rule, as determined by TDDFT. The synthesized BOPYINs have been shown to be biocompatible fluorophores in cell bioimaging.     

Bis(BF2)-2,2'-bidipyrrins (BisBODIPYs): highly fluorescent BODIPY dimers with large stokes shifts

Four new dimeric bis(BF(2))-2,2'-bidipyrrins (bisBODIPYs), and their corresponding BODIPY monomers, have been prepared and studied with respect to their structural and photophysical properties. The solid-state molecular structure of the dimers and the relative orientation of the subunits have been revealed by an X-ray diffraction study, which showed that the molecules contain two directly linked BODIPY chromophores in a conformationally fixed, almost orthogonal arrangement. Two of the fluorine atoms are in close contact with each other and the (19)F NMR spectra show a characteristic through-space coupling in solution. The new chromophores all exhibit a clear exciton splitting in the absorption spectra with maxima at about 490 and 560 nm, and are highly luminescent with an intense emission band at around 640 nm. The Stokes shift, which is the difference between the maximum of the lowest-energy absorption band and the maximum of the emission band, has a typical value of 5 to 15 nm for simple BODIPYs, whereas this value increases to 80 nm or more for the dimers, along with a slight decrease in fluorescence quantum yields and lifetimes. These properties indicate potential uses of these new fluorophoric materials as functional dyes in biomedical and materials applications and also in model compounds for BODIPY aggregates.