BODIPY Chromophore Research Articles

Steric hindrances and spectral distributions affecting energy transfer rate: A comparative study on specifically designed donor-acceptor pairs

In this work the photophysics of four bichromophoric units was studied by means of static and time resolved spectroscopy, with the aim of disentangling the contribution of steric and electronic factors in regulating the efficiency of electronic energy transfer (EET). The newly synthesized dyads share the same acceptor moiety, a substituted BODIPY chromophore, and differ either in the donor or in the molecular bridge connecting the two units. The use of different linkers allows for tuning the conformational flexibility of the dyad, while changing the donor has an influence on the electronic coupling and spectral overlap between the two chromophores. The efficiency of energy transfer is extremely high in all the four dyads and can be modelled within the frame of the Förster equation. In the special case of a dimeric donor, a theoretical analysis was performed to further support the experimental findings. Geometry optimization at DFT level indicated that different conformations with similar energy can exist in solution, explaining the observed multi-exponential EET. Furthermore, energy transfer rates, computed at DFT level, resulted in optimal agreement with the experimental ones. Our analysis allowed to conclude that, in case of the studied systems, steric hindrance and donor-acceptor relative orientations plays a prominent role in regulating the EET dynamics, even overcoming electronic effects.

Tuning the SOCT-ISC of bodipy based photosentizers by introducing different electron donating groups and its application in triplet-triplet-annihilation upconversion

To study the effect of electron-donating ability on spin orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, a series of bodipy derivatives (BDP-O, BDP-2-MN, BDP-2-EN) linked with electron-donating groups with different electron-donor abilities were synthesized. For BDP-2-MN and BDP-2-EN, both compounds show a sharp charge transfer emission band at 630 nm, and the local excited (LE) emission in both compounds were completely quenched due to photo-induced electron transfer (PET). By femtosecond transient absorption measurement, the triplet state population in both compounds was confirmed to occur through charge recombination (1996 ps ~2307 ps) process after the photo-induced electron transfer upon photoexcitation of bodipy moiety. A long-lived triplet excited state was observed for both compounds (lifetimes: 37 ~ 42 μs) via nanosecond transient absorption. The BDP-2-MN and BDP-2-EN can act as the triplet photosensitizers without heavy elements for the application of triplet-triplet-annihilation upconversion (TTA-UC, ΦUC can reach 4.88%). In case of BDP-O molecule, no triplet signal was observed for this compound because of poor electronic coupling between donor and acceptor. Therefore, the triplet excited state properties of bodipy chromophore was successfully regulated by adjusting electronic strength of donor moiety and implement these photosensitizers in the application of TTA-UC.

Green to red emitting BODIPY dyes for fluorescent sensing and imaging of carbon dioxide

Several new BODIPY pH indicators are prepared and tested as transducers in polymer-based optical carbon dioxide sensors. Apart from classical BODIPY chromophore absorbing at around 505 nm, π-extended analogs showing absorption maxima at around 635 and 665 nm are prepared. All the dyes possess excellent brightness due to combination of high molar absorption coefficients and fluorescence quantum yields. Additionally, the π-extended BODIPYs show unmatched photostability whereas the classical 1,3,5,7-tetramethyl-8-aryl-4,4-difluoroboradiazaindacene is found to be photolabile. The phenol receptors render BODIPY dyes pH sensitive inducing fluorescence quenching in the phenolate form. Carbon dioxide sensors are prepared via immobilization of the dyes into ethyl cellulose along with tetraoctylammonium hydrogencarbonate. The sensitivity of these sensors is mainly guided by the nature of the receptor and to a smaller extent by the nature of the chromophore, and can be tuned over a very wide range covering all important applications. Introduction of the receptor bearing carboxylic group in the o-position towards OH group of the receptor enables highly sensitive sensors resolving ambient CO2 levels (LOD 0.009 hPa). Sensors based on other indicators show optimal response from 0.2 to 60 hPa and from 20 to 400 hPa, making them attractive for medical and food packaging applications, respectively. Materials for referenced sensing with phase fluorometry and ratiometric imaging with RGB cameras have also been prepared.

Exploring Differences in Excited-State Properties of Styryl-BODIPY Chromophores upon Change from α- to β-Substitution

Abstract We present a femtosecond pump-probe UV-Vis broadband transient absorption spectroscopy study of two styryl-substituted BODIPY chromophores with different position of the substituent. The α-substituted isomer shows typical BODIPY-type spectral features, such as sharp absorption and emission bands, a small Stokes shift and an excited-state lifetime in the 4 ns range, which only weakly depends on the solvent. In contrast, β-styryl-BODIPY features much broader steady-state absorption and emission spectra and a larger Stokes shift, particularly in polar solvents. Transient absorption spectroscopy including a complete global kinetic analysis reports a substantial decrease in S1 lifetime to 300 ps for polar solvents upon change from α- to β-substitution. In the case of the α-isomer, TD-DFT calculations identify a typical “cyanine-type” electron rearrangement upon S0 → S1 excitation accompanied by a slight reduction in dipole moment. In contrast, the same transition in the β-isomer shows strong intramolecular charge transfer (ICT) character involving a substantial increase in dipole moment. Assuming a simple energy-gap-law argument, the accelerated nonradiative decay of the β-isomer in polar solvents may be linked to the decrease of the S1(ICT)-S0 energy difference. BODIPY dyes with a conjugated substituent in β-position therefore suffer a substantial loss in fluorescence brightness in polar environments compared with their α-substituted counterparts. This might limit their applicability in fluorescence imaging.

Photoactivated Lysosomal Escape of a Monofunctional PtII Complex Pt‐BDPA for Nucleus Access

AbstractA photosensitizing monofunctional Pt complex, Pt‐BDPA, was prepared with a BODIPY chromophore. Apart from its DNA binding ability, this complex displays emission at ca. 578 nm and a singlet oxygen quantum yield of 0.133. Confocal imaging revealed that this complex was sequestered in lysosomes via endocytosis in the dark, preventing its access to the nucleus. Profiting from its photoinduced ROS generation ability, this complex undergoes lysosomal escape to access the nucleus upon photoirradiation. The photoinduced ROS still cause a drop in intracellular GSH, favoring the stability of Pt‐BDPA and contributing to its nuclear DNA accessibility. This complex displayed distinct cytotoxicity to all tested tumor cell lines upon photoirradiation, and the IC50 values were ca. 3–6 μm, which are distinctly lower than those found with only dark incubation (IC50>50 μm). These results are consistent with photoactivated lysosomal escape of this photosensitizing Pt complex to access the nucleus.

Photoactivated Lysosomal Escape of a Monofunctional PtII Complex Pt-BDPA for Nucleus Access.

A photosensitizing monofunctional Pt complex, Pt-BDPA, was prepared with a BODIPY chromophore. Apart from its DNA binding ability, this complex displays emission at ca. 578 nm and a singlet oxygen quantum yield of 0.133. Confocal imaging revealed that this complex was sequestered in lysosomes via endocytosis in the dark, preventing its access to the nucleus. Profiting from its photoinduced ROS generation ability, this complex undergoes lysosomal escape to access the nucleus upon photoirradiation. The photoinduced ROS still cause a drop in intracellular GSH, favoring the stability of Pt-BDPA and contributing to its nuclear DNA accessibility. This complex displayed distinct cytotoxicity to all tested tumor cell lines upon photoirradiation, and the IC50 values were ca. 3-6 μm, which are distinctly lower than those found with only dark incubation (IC50 >50 μm). These results are consistent with photoactivated lysosomal escape of this photosensitizing Pt complex to access the nucleus.

Investigation of electronic energy transfer in a BODIPY-decorated calix[4]arene

The photophysics of a donor-acceptor system in which efficient electronic energy transfer occurs is analyzed and discussed by the combined use of steady-state and time-resolved spectroscopy and DFT/TD-DFT computations. The donor and acceptor units belong to the class of BODIPY chromophores, and are conveniently linked through a calixarene scaffold, which allows the control of the mutual orientation and distance between chromophores. Our results highlight that the energy transfer process occurs with multiexponential dynamics strongly influenced by the solvent. Although the conformation adopted by the system is very similar in all the analyzed solvents, highly polar media favour fast and efficient energy transfer. On the contrary, in non-polar media, the concomitant occurrence of backward energy transfer causes a significant slowdown of the process. The inverse of the energy transfer rates calculated at the TDDFT level are in very good agreement with the experimental kinetics measured with transient absorption spectroscopy.

A general modular approach for the solubility tagging of BODIPY dyes

We describe a general and practical strategy for the direct one-step incorporation of a tunable solubility module at the boron atom of F-BODIPY dyes. The tethering reaction uses easy-to-handle reagents, has broad functional group compatibility and proceeds under mild conditions without requiring any pre-functionalization of the starting F-BODIPY to yield the solubility-tagged O-BODIPY derivative in 51–86% yield. The module can be introduced at the end of the synthetic route without perturbing the fluorophore scaffold, thus minimizing difficulties in product isolation. Its orthogonal geometrical arrangement with respect to the plane of the BODIPY chromophore hampers intermolecular aggregation processes that quench fluorescence, while the covalent attachment to the boron atom has a minimal effect on the absorption properties. Fully water-soluble and hydrolytically stable BODIPYs were prepared by incorporating either neutral (tetra- and octaethylene glycol chains) or zwitterionic (sulfobetaine) hydrophilic tags in the module. The new dyes are valuable live cell imaging probes that ameliorate the undesired partitioning into lipophilic compartments that is often observed for standard BODIPYs. This strategy can be readily adapted to the general and highly practical post-synthetic introduction of new functionalities into F-BODIPY dyes, including phase-tagging, by appropriately choosing the nature of the chemical tags attached to the module.

Self-assembly of aliphatic dipeptides coupled with porphyrin and BODIPY chromophores.

In this work, the self assembly ability of chromophores covalently linked to aliphatic dipeptides is described. Altering various parameters such as the protecting group, the solvent mixture, the dipeptide and the chromophore resulted in different nanostructures. Interestingly, a peptide-porphyrin hybrid is capable of forming a hydrogel in HFIP-water solvent mixture.

Structural characterization and optical properties of two copper(i)-iodide BODIPY coordination polymers

BODIPY chromophores with pyridyl or pyrimidinyl substituents exhibit strong solid-state fluorescence which is quenched in CuI coordination polymers.

Theoretical insight into joint photodynamic action of a gold(i) complex and a BODIPY chromophore for singlet oxygen generation.

Density functional theory is herein employed to provide theoretical insight into the mechanism involved in 1O2 photosensitization by a gold-BODIPY combined complex proposed as a promising photodynamic therapy agent. The protocol is thus used to compute the non-radiative rate constants for the S1 → Tj intersystem crossing transitions. Calculations show that while the incorporation of an iodine atom into the core skeleton of BODIPY enhances the singlet-triplet intersystem crossing (ISC) efficiency due to the occurrence of the singlet-triplet transition between states with different orbital characters (ππ* → πn*), the presence of a gold atom, even if not directly anchored to the chromophore core but through a triplet bond, equally entails an increase of the spin-orbit coupling constant due to the heavy atom effect. In this way, the system is able to generate singlet molecular oxygen, the key cytotoxic agent in PDT. Our results fit well with the experimental singlet oxygen quantum yield and cytotoxicity determination.

A chemosensor with a paddle structure based on a BODIPY chromophore for sequential recognition of Cu2+ and HSO3.

In this study, a highly selective chemosensor ML based on a BODIPY fluorescent chromophore was synthesized for sequential recognition of Cu2+ and HSO3− in a CH3OH/H2O (99 : 1 v/v) system, which contained three recognition sites and its structure characterized by 1H NMR, 13C NMR and ESI-HR-MS. The sensor ML showed an obvious “on–off” fluorescence quenching response toward Cu2+ and the ML-Cu2+ complex showed an “off–on” fluorescence enhancement response toward HSO3−. The detection limit of the sensor ML was 0.36 μM to Cu2+ and 1.4 μM to HSO3−. In addition, the sensor ML showed a 1 : 3 binding stoichiometry to Cu2+ and the recovery rate of ML-Cu2+ complex identifying HSO3− could be over 70%. Sensor ML showed remarkable detection ability in a pH range of 4–8.

A Rotaxane‐like Cage‐in‐Ring Structural Motif for a Metallosupramolecular Pd6L12 Aggregate

A BODIPY-based bis(3-pyridyl) ligand undergoes self-assembly upon coordination to tetravalent palladium(II) cations to form a Pd6 L12 metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane-like cage-in-ring arrangement. In this assembly the ligand adopts two different conformations-a C-shaped one to form a Pd2 L4 cage which is located in the center of a Pd4 L8 ring consisting of ligands in a W-shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π-stacking between the peripheral BODIPY chromophores and the ligands' skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co-arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane-like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X-ray crystallography.

Unexpected Nucleophilic Substitution Reaction of BODIPY: Preparation of the BODIPY–TEMPO Triad Showing Radical‐Enhanced Intersystem Crossing

We have discovered a new and unexpected substitution reaction of BODIPY. Through single‐crystal XRD analysis, DFT calculations, and other complimentary spectroscopic methods, we have confirmed that the molecular structural assignment of the previously reported nucleophilic substitution of the halogenated carbon atom at the meso position of BODIPY was inaccurate. Indeed, in these substitution reactions, the amino attaches to the methyl group at the 3‐ or 5‐position and not on the meso‐carbon of the BODIPY core. As a result of this unexpected substitution reaction, we prepared a BODIPY dimer, attached to the stable radical TEMPO (2,2,6,6‐tetramethyl‐1‐piperidinyloxyl). We confirmed the efficient formation of the triplet state (ΦΔ = 59 %, τT = 2.8 µs) of this BODIPY–TEMPO triad through the radical‐enhanced intersystem crossing mechanism by using nanosecond time‐resolved transient absorption spectroscopy. Through time‐resolved EPR spectroscopy, the doublet (spin angular momentum quantum number S = 1/2) and quartet (S = 3/2) states of the triad were observed. Moreover, we observed a strong intramolecular ground‐state interaction between the BODIPY chromophores in the triad, which shows new spectral features different to those of H‐ or J‐aggregates and the exciton coupling effect. These TEMPO‐labeled compounds were used as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion (quantum yield ΦUC = 4.7 %).

Ultrafast dynamics of differently aligned COOH-DTE-BODIPY conjugates linked to the surface of TiO2

The photoinduced dynamics of two DTE-BODIPY conjugates A, B with carboxylic acid anchoring groups coupled to the surface of TiO2 were studied by ultrafast transient absorption spectroscopy. For compound A, with an orthogonal orientation of the BODIPY chromophore and the photoswitchable DTE unit, a charge separated state could not be reliably detected. Nevertheless, besides the energy transfer from the BODIPY to the ring-closed DTE-c, indications for an electron transfer reaction were found by analyzing fluorescence quenching on TiO2 in steady state fluorescence measurements. For compound B with a parallel orientation of chromophore and photoswitch, a charge separated state was conclusively identified for the coupled dyad (TiO2) via the observation of a positive absorption signal (at λpr > 610 nm) at later delay times. An electron transfer rate of 7 × 1010 s−1 can be extracted, indicating slower processes in the dyads in comparison to previously published electron transfer reactions of DTE compounds coupled to TiO2.

Novel Imidazole Substituted Bodipy-Based Organic Sensitizers in Dye-Sensitized Solar Cells

A comparative study on the photophysical, electrochemical properties and photovoltaic performances of pure imidazole dyes containing varying linker groups is done. Two new organic dyes containing 4,5-bis(4-methoxyphenyl)-1H-imidazole (BPI) unit as an electron donor, boron dipyrromethene (Bodipy) chromophore as a conjugate bridge, cyanoacetic acid as an electron acceptor, and phenylene (BPI-P) or thienyl (BPI-T) as a additional linker have been synthesized for fabricating dye-sensitized solar cells (DSSCs). A reference dye (DPI-T) with 6,9-dimethoxy-1H-phenanthro[9,10-d]imidazole as the donor has also been synthesized for comparison. The overall conversion efficiencies of 0.18%, 0.32%, and 1.28% were obtained for DSSCs based on BPI-P, BPI-T, and DPI-T, respectively. DPI-T was found to be more efficient than BPI-P and BPI-T because of its enhanced light harvesting efficiency and better coplanar geometry of the electronic structure.

Different Quenching Effect of Intramolecular Rotation on the Singlet and Triplet Excited States of Bodipy

It is well-known that the fluorescence of a chromophore can be efficiently quenched by the free rotor effect, sometimes called intramolecular rotation (IMR), i.e. by a large-amplitude torsional motion. Using this effect, aggregation induced enhanced emission (AIE) and fluorescent molecular probes for viscosity measurements have been devised. However, the rotor effect on triplet excited states was rarely studied. Herein, with molecular rotors of Bodipy and diiodoBodipy, and by using steady state and time-resolved transient absorption/emission spectroscopies, we confirmed that the triplet excited state of the Bodipy chromophore is not quenched by IMR. This is in stark contrast to the fluorescence (singlet excited state), which is significantly quenched by IMR. This result is rather interesting since a long-lived excited state (triplet, 276 μs) is not quenched by the IMR, but the short-lived excited state (singlet, 3.8 ns) is quenched by the same IMR. The unquenched triplet excited state of the Bodipy was us...

Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation Upconversion.

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34-40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s-1 and 2 × 1012 s-1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 and 1.5 × 1011 s-1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends on both the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4 and 97.5% and lifetimes are 13 and 116 μs for BDP-1 and BDP-2, respectively. The spin-orbit charge transfer (SO-CT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet-triplet annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%.

Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks.

We have prepared a series of MII4L6 tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C60 or C70 fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.

Sodium-Selective Fluoroionophore-Based Optodes for Seawater Salinity Measurement.

A new fluorescent sensor for Na+ is presented. The sensor relies on a Na+ selective fluoroionophore based on a bright red-emitting BODIPY chromophore. The fluorescence of the fluoroionophore is enhanced upon binding of Na+-ions to the highly selective aza-crown ether receptor due to reduction of the photoinduced electron transfer (PET) quenching. Solid state sensing materials were prepared by physically embedding the fluoroionophore into water-swellable biocompatible polymer matrices (polyurethane hydrogels), thus enabling continuous measurements of aqueous samples. Despite the simple design, the sensor showed no leaching of the indicator and featured fast and reversible response. Among different polyurethane hydrogels investigated, the hydrogel D1 featuring the highest water uptake was found to be the most suitable due to the highest dynamics between "off" and "on" states. Due to little or no cross sensitivity to other ions (e.g., Mg2+, Ca2+, K+) and its insensitivity to potential changes in pH, this sensor is promising for use in clinical diagnostics and for biological and marine applications. Fiber-optic sensors based on referenced read-out with a compact phase fluorimeter were prepared. To demonstrate their practical applicability, the sensors were used to determine the salinity in the seawater and brackish water of the Baltic Sea.