BODIPY Core Research Articles

Substituents modification of meso-aryl BODIPYs for tuning photophysical properties

We successfully synthesized eight meso-aryl BODIPYs with 2,6-diethyl- or 1,2,6,7-tetraethyl substituents and characterized their photophysical properties. The steric hindrance resulting from the phenolic group in the meso-aryl moiety and the ethyl groups on the BODIPY core affected the synthesis of dipyrromethanes as an intermediate as well as the UV–Vis absorption and fluorescence emission of the BODIPYs due to the constrained rotation of the aryl ring. The potential use of the meso-hydroxyphenyl BODIPY as a pH sensor was also shown by the pH-dependent fluorescence emissions.

Meso-Nitromethyl-Substituted BODIPYs – A new type of water switchable fluorogenic dyes useful for further core modifications

A two-step synthetic pathway for direct introduction of the nitromethyl functional group at the meso-position of BODIPY core was explored using a number of meso-unsubstituted BODIPY platforms. Spectral properties of the resulting meso-nitromethyl containing BODIPYs was found to be sensitive to the pH change as well as to the presence of water in organic solvents. Meso-nitromethyl group can be used for further modification of the BODIPY core via reduction or condensation reactions. DFT-PCM and TDDFT-PCM calculations correlate well with the experimental data and explain the observed spectral properties.

Structure and Conformational Studies of Aza‐Crown 8‐Amino‐BODIPY Derivatives: Influence of Steric Hindrance on Their Photophysical Properties

Herein, we report the synthesis, X‐ray crystal structure and photophysical studies of six new 8‐amino‐BODIPY derivatives containing crown or azo‐crown ether moieties. The influence of steric hindrance, caused by the crown ether, on the planarity of the BODIPY core and its relationship with the fluorescent properties has been established. 1H NMR spectroscopic studies were undertaken to clarify the changes in fluorescence observed in the presence of ZnII.

Solvent-Sensitive Emitting Urea-Bridged bis-BODIPYs: Ready Access by a One-Pot Tandem Staudinger/Aza-Wittig Ureation.

Herein we describe the synthesis, and computationally aided photophysical characterization of a new set of urea-bridged bis-BODIPY derivatives. These new dyads are efficiently obtained by a one-pot tandem Staudinger/aza-Wittig ureation protocol, from easily accessible meso-phenyl ortho-azidomethyl BODIPYs. These symmetric bis-BODIPYs outstand by a high absorption probability and excellent fluorescence and laser emission in less polar media. Nevertheless, this emission ability decreases in more polar media, which is ascribed to a light-induced charge-transfer from the urea spacer to the dipyrrin core, a process that can be modulated by appropriate changes in the substitution pattern of the BODIPY core. Furthermore, this ureation protocol can also be employed for the direct conjugation of our BODIPY-azides to amine-containing compounds, thus providing access to fluorescent non-symmetric ureas.

A near-infrared fluorescent probe based on BODIPY derivative with high quantum yield for selective detection of exogenous and endogenous cysteine in biological samples

Cysteine (Cys) is involved in cellular growth and Cys deficiency is related with many diseases. So far, a number of fluorescent probes have been constructed for the detection of Cys successfully. However, the probes are difficult to discriminate Cys from Hcy and the emission wavelength of the probes is in ultraviolet or visible range. Herein, a NIR fluorescent probe named NIR-BODIPY-Ac is synthesized and used to detect Cys. The emission wavelength of the probe is at 708 nm that belongs to near-infrared (NIR) region by attaching indolium to BODIPY core, which is suitable for bioimaging in vivo. Moreover, the probe exhibits high fluorescence quantum yield (Φ = 0.51) after the addition of Cys and high sensitivity toward Cys with 81-fold fluorescence enhancement. The linear range of the probe for Cys covers from 0.2 to 30 μM with a detection limit of 0.05 μM. Furthermore, the probe shows high selectivity towards Cys owing to the fact that there is more fast reaction rate between the probe and Cys than that of Hcy. In particular, the NIR fluorescent probe is applied for the detection of exogenous and endogenous Cys in biological samples such as cell, tissue and mouse with satisfactory results.

Enhance the fluorescence and singlet oxygen generation ability of BODIPY: Modification on the meso-phenyl unit with electron withdrawing groups

Abstract Herein, meso -R-carboxyphenyl substituted BODIPY compounds were synthesized and characterized (R = CH 3 , COOH, NO 2 , H and Br 4 ). The fluorescence, excited triplet state and singlet oxygen formation properties for these compounds were measured in nonpolar and polar solvents by UV–vis absorption, steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. These BODIPYs exhibit very different fluorescence quantum yield and lifetime value, as well as singlet oxygen formation capability in polar solvents. In contrast, these compounds show very similar spectral shape and position, emit bright green fluorescence in non polar solvents. The introduction of an ortho -COOH on the benzene unit leads to a 20-fold increase in the fluorescence quantum yield. The further introduction of an electron withdrawing NO 2 or COOH on the phenyl makes the BODIPY dye show much higher capability to generate singlet oxygen (up to 5-fold increase) and causes a sharp decrease in both fluorescence quantum yield and lifetime value. The increase in solvent polarity enhances singlet oxygen generation and intramolecular fluorescence quenching. The results are explained by the presence of intramolecular photoinduced electron/charge transfer from BODIPY core to the carboxyphenyl moiety.

Locally Excited State-Charge Transfer State Coupled Dyes as Optically Responsive Neuron Firing Probes.

A selection of NIR-optically responsive neuron probes was produced comprising of a donor julolidyl group connected to a BODIPY core and several different styryl and vinylpyridinyl derived acceptor moieties. The strength of the donor-acceptor interaction was systematically modulated by altering the electron withdrawing nature of the aryl unit. The fluorescence quantum yield was observed to decrease as the electron withdrawing effect of the aryl subunit increased in line with changes of the Hammett parameter. The effectiveness of these fluorophores as optically responsive dyes for neuronal imaging was assessed by measuring the toxicity and signal-to-noise ratio (SNR) of each dye. A great improvement of SNR was obtained when compared to the first-generation BODIPY-based voltage sensitive dyes with concomitant toxicity decrease. The mechanism for the optical response is disparate from conventional cyanine-based dyes, opening up a new way to produce effective voltage sensitive dyes that respond well into the NIR region.

Effect of methyl substituents on the electronic transitions in simple meso-aniline-BODIPY based dyes: RI-CC2 and TD-CAM-B3LYP computational investigation

The S0→Si, i=1–5 electronic transitions of four 8-(4-aniline)-BODIPY and four 8-(N,N-dimethyl)-BODIPY dyes, differ by number and position of methyl substituents in the BODIPY frame, were investigated theoretically using ab initio the coupled cluster doubles (CC2) and TD-CAM-B3LYP methods. Methyl substituents in the BODIPY frame and the aniline fragment at the meso position disturb energy of local excitations S0→S1, S0→S3, and S0→S4 weakly in comparison with the fully unsubstituted BODIPY molecule. These transitions in experimental spectra form the most long-wave absorption bands at ca. 500nm as well as absorption bands in the region of 300–400nm. At the same time, the presence of aniline fragments leads to the appearance of new S0→S2 transitions of the charge transfer character in electronic spectra of BODIPYs. We also found a linear relationship between vertical energy of these charge transfer transitions and the electron donating power of an aniline fragment and electron accepting power of the BODIPY core depending on the number and position of methyl groups. The CC2 method provides the best overall description of the excitation energies in line with the experimental observations. On average, the quality of TD-CAM-B3LYP is almost equal to that of CC2, however the TD method with the CAM-B3LYP functional slightly underestimates the CT excitation energy.

Photoinduced Electron Transfer-based Halogen-free Photosensitizers: Covalent meso-Aryl (Phenyl, Naphthyl, Anthryl, and Pyrenyl) as Electron Donors to Effectively Induce the Formation of the Excited Triplet State and Singlet Oxygen for BODIPY Compounds.

Pristine BODIPY compounds have negligible efficiency to generate the excited triplet state and singlet oxygen. In this report, we show that attaching a good electron donor to the BODIPY core can lead to singlet oxygen formation with up to 58 % quantum efficiency. For this purpose, BODIPYs with meso-aryl groups (phenyl, naphthyl, anthryl, and pyrenyl) were synthesized and characterized. The fluorescence, excited triplet state, and singlet oxygen formation properties for these compounds were measured in various solvents by UV/Vis absorption, steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. In particular, the presence of anthryl and pyrenyl showed substantial enhancement on the singlet oxygen formation ability of BODIPY with up to 58 % and 34 % quantum efficiency, respectively, owing to their stronger electron-donating ability. Upon the increase in singlet oxygen formation, the fluorescence quantum yield and lifetime values of the aryl-BODIPY showed a concomitant decrease. The increase in solvent polarity enhances the singlet oxygen generation but decreases the fluorescence quantum yield. The results are explained by the presence of intramolecular photoinduced electron transfer from the aryl moiety to BODIPY core. This method of promoting T1 formation is very different from the traditional heavy atom effect by I, Br, or transition metal atoms. This type of novel photosensitizers may find important applications in organic oxygenation reactions and photodynamic therapy of tumors.

N-Bridged Annulated BODIPYs: Synthesis of Highly Fluorescent Blueshifted Dyes.

A series of novel BODIPY dyes has been prepared through the introduction of an N-bridged annulated meso-phenyl ring at one of the β-positions of the BODIPY core. An unusual blueshift of the main spectral bands is observed, since the fusion of a meso-substituent results in a marked relative destabilization of the LUMO. The greater rigidity of the ring-fused structure leads to very high fluorescence quantum yields. The position of the main spectral bands can be fine-tuned by introducing electron withdrawing and donating groups onto the meso-phenyl ring.

Design, synthesis, cell imaging, kinetics and thermodynamics of reaction-based turn-on fluorescent probes for the detection of biothiols

Two highly selective red-emitting fluorescent “OFF-ON” probes (3a and 3b) with a BODIPY core were designed and synthesized, based on the recovery of fluorescence upon the cleavage of the fluorescence quenching unit of 2, 4-dinitrobenzenesulfonyl (DNBS) by biothiols. The probes showed 43-fold (3a, λem = 610 nm) and 33-fold (3b, λem = 582 nm) fluorescence enhancement respectively in the presence of biothiols. Compared with 3b, the introduction of five strong electron-withdrawing fluorine atoms in 3a, not only caused a red shift of the emission maximum for 28 nm, but also increased the fluorescence enhancement. Since they had stable fluorescence emission within physiological pH range, the two probes can be applied in imaging of living cells. In contrast, no such fluorescence response was observed in the cells pre-treated with N-ethylmaleimide (NEM), a well-known thiol scavenger. A non-linear curve-fitting of high quality was used to fit the spectroscopic data to determine the pseudo-first-order rate constants and the chemical equilibrium constants at different given temperatures (e.g. 310 K, 298 K and 288 K). Activation energy (Ea) and thermodynamic parameters, including Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), could be calculated. To the best of our knowledge, this was the first report for the chemical kinetics and thermodynamics of the reaction between the fluorescent probes and thiols. This work would greatly inspire the future design of even better fluorescent probes.

Synthesis of BODIPY-Peptide Conjugates for Fluorescence Labeling of EGFR Overexpressing Cells

Regioselective functionalization of 2,3,5,6,8-pentachloro-BODIPY 1 produced unsymmetric BODIPY 5, bearing an isothiocyanate group suitable for conjugation, in only four steps. The X-ray structure of 5 reveals a nearly planar BODIPY core with aryl dihedral angles in the range 47.4-62.9°. Conjugation of 5 to two EGFR-targeting pegylated peptides, 3PEG-LARLLT (6) and 3PEG-GYHWYGYTPQNVI (7), under mild conditions (30 min at room temperature), afforded BODIPY conjugates 8 and 9 in 50-80% isolated yields. These conjugates showed red-shifted absorption and emission spectra compared with 5, in the near-IR region, and were evaluated as potential fluorescence imaging agents for EGFR overexpressing cells. SPR and docking investigations suggested that conjugate 8 bearing the LARLLT sequence binds to EGFR more effectively than 9 bearing the GYHWYGYTPQNVI peptide, in part due to the lower solubility of 9, and its tendency for aggregation at concentrations above 10 μM. Studies in human carcinoma HEp2 cells overexpressing EGFR demonstrated low dark and photo cytotoxicities for BODIPY 5 and the two peptide conjugates, and remarkably high cellular uptake for both conjugates 8 and 9, up to 90-fold compared with BODIPY 5 after 1 h. Fluorescence imaging studies in HEp2 cells revealed subcellular localization of the BODIPY-peptide conjugates mainly in the Golgi apparatus and the cell lysosomes. The low cytotoxicity of the new conjugates and their remarkably high uptake into EGFR overexpressing cells renders them promising imaging agents for cancers overexpressing EGFR.

A BODIPY-Based Fluorescent Probe for Detection of Subnanomolar Phosgene with Rapid Response and High Selectivity.

A new type of phosgene probe with a limit of detection down to 0.12 nM, response time of less than 1.5 s, and high selectivity over other similarly reactive toxic chemicals was developed using ethylenediamine as the recognition moiety and 8-substituted BODIPY unit as the fluorescence signaling component. The probe undergoes sequential phosgene-mediated nucleophilic substitution reaction and intramolecular cyclization reaction with high rate, yielding a product with the intramolecular charge transfer (ICT) process from amine to the BODIPY core significantly inhibited. Owing to the emission feature of 8-substituted BODIPY that is highly sensitive to the substituent's electronic nature, such inhibition on the ICT process strikingly generates strong fluorescence contrast by a factor of more than 23 300, and therefore creates the superhigh sensitivity of the probe for phosgene. Owing to the high reactivity of ethylenediamine of the probe in nucleophilic substitution reactions, the probe displays a very fast response rate to phosgene.

Decorated BODIPY Fluorophores and Thiol-Reactive Fluorescence Probes by an Aldol Addition.

A novel entry to meso-decorated BODIPY motifs on the basis of an unusual aldol-type addition with diethyl ketomalonate is reported. The evolving β-hydroxyl group can be optionally eliminated, which suppresses the fluorescence of the BODIPY core by attachment of a π-electronically coupled methylidene malonate unit. This unit serves as a versatile, highly electrophilic acceptor platform for various nucleophilic additions. Corresponding products benefit from a fully restored fluorescence.

Acid-Sensitive BODIPY Dyes: Synthesis through Pd-Catalyzed Direct C(sp3 )-H Arylation and Photophysics.

A novel palladium-catalyzed direct C(sp3 )-H arylation of the methyl group at the 8-position of BODIPY by bromoarenes was established. A deprotonative cross-coupling process was supposed to be involved in the reaction. This approach allowed us to attach electron-donating/withdrawing, halogen substituted aryls and a heteroaryl with a yield running from 55 to 99 %. Novel pH sensors, which in the absence of acid showed the occurrence of photoinduced electron transfer, were synthesized by attaching dimethylaniline to the methyl at the C8-position of BODIPY. The reference compounds with dimethylaniline directly attached to the C8-position were also synthesized and besides photoinduced electron transfer also showed a charge-transfer emission. Their photophysical properties were investigated by steady-state fluorescence, time-correlated single-photon counting and femtosecond fluorescence up-conversion. Time-dependent density functional (TD-DFT) electronic-structure calculations on the properties of the excited states corresponding to local excitation of the BODIPY core and to charge transfer were conducted. Upon addition of trifluoroacetic acid in toluene and ethanol, the partial fluorescence intensity recovery was at least an order of magnitude more efficient with the newly synthesized sensors compared to the traditional reference sensors. The improved sensitivity of these novel BODIPY-based pH sensors was attributed to less efficient proton-coupled electron transfer of the protonated species.

Enhanced Hypsochromic Shifts, Quantum Yield, and π-π Interactions in a meso,β-Heteroaryl-Fused BODIPY.

We report the synthesis and investigation of an unprecedented 8-heteroaryl-fused BODIPY 4. This compound exhibits enhanced π-π stacking in the solid state, unusually large blue-shifts in the absorbance and emission spectra, and higher fluorescence quantum yield than its unfused precursor; DFT calculations suggest a small energy gap for 4 and strong electronic communication between the 8-OPh and the BODIPY core.

Synthesis, photophysical and concentration-dependent tunable lasing behavior of 2,6-diacetylenyl-functionalized BODIPY dyes

Extended π-conjugation through the 2- and 6-positions of the BODIPY core induces tunable yellow-red lasing with excellent efficiency of up to 41% in toluene.

Bu4NI/tBuOOH catalyzed, α-regioselective cross-dehydrogenative coupling of BODIPY with allylic alkenes and ethers.

A Bu4NI/tBuOOH-catalyzed, highly regioselective cross-dehydrogenative coupling (CDC) of the α-C-H bond(s) of the BODIPY core has been developed. The α-regioselective alkylation reaction utilizes easily accessible coupling partners, namely commercial allylic alkenes and ethers - even common, inert organic solvents, such as tetrahydrofuran, diethyl ether and 1,4-dioxane. The high α-regioselectivity of this CDC reaction is attributable to the radical process involved and provides a facile access to a variety of α-functionalized BODIPYs, which are hard to access through current synthetic methods.

Achieving the highest efficiency using a BODIPY core decorated with dithiafulvalene wings for small molecule based solution-processed organic solar cells.

A novel boron dipyrromethene based dye, coded as BODIPY-DTF, decorated with dithiafulvalene wings has been developed for solar cell application. A very high efficiency of 7.2% has been achieved, which is the highest reported value so far for BODIPY based donors. A remarkable value of about 88.1% of external quantum efficiency has also been observed at 371 nm.

The naphthoate-modifying Cu2 +-detective Bodipy sensors with the fluorescent ON-OFF performance unaffected by molecular configuration

Two new boron-dipyrromethenes decorated with 8-hydroxyquinoline-naphthoate moiety, namely 4,4-difluoro-8-(5-(8-hydroxyquinoline-naphthoate))-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene (8-HQ-N-DMe-Bodipy) (1) and 4,4-difluoro-8-(5-(8-hydroxyquinoline-naphthoate))-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (8-HQ-N-TMe-Bodipy) (2) have been synthesized. Single crystal X-ray diffraction analysis discloses the very much similar steric arrangement of 8-hydroxyquinoline-naphthoate moiety in these two compounds as revealed by the close torsion angle of C-C-O-C bridge, 174.15 and 171.81° for 1 and 2, respectively, despite the different dihedral angle between quinoline moiety and Bodipy fluorophore for 1 (73.46°) and 2 (82.26°) due to the steric hindrance originated from the C-1/C-7 methyl substituents on Bodipy core for the latter species. Systemic optical studies unravel the red-shifted absorption and fluorescence emission together with slightly lower quantum yield for 1 relative to that of 2, indicating the configuration effect on their spectroscopic properties. However, the binding of Cu2+ with hydroxyquinoline-naphthoate receptor in both 1 and 2 leads to similar fluorescent quenching characteristic due to the photo-induced electron transfer process on the basis of density functional theory calculations, suggesting their high sensitively fluorescent ON-OFF sensing potential to Cu2+ almost unaffected by molecular configuration.