Phosphorescence of BODIPY dyes

Photoluminescence mechanisms of BF2-formazanate dye sensitizers: a theoretical study

Photodynamic therapy (PDT) is an alternative, minimally invasive treatment for human diseases such as cancer. PDT uses a photosensitizer to transfer photon energy directly to cellular 3O2 to generate 1O2 (Type II), the toxicity of which leads to cancer cell death. In this work, the photoluminescence mechanisms of a BF2-formazanate dye sensitizer (BF2-FORM) and its iodinated derivative (BF2-FORM-D) were studied using complementary theoretical approaches; the photoluminescence pathways in the S1 and T1 states were studied using density functional theory (DFT) and time-dependent (TD)-DFT methods, the kinetic and thermodynamic properties of the pathways using the transition state theory (TST), and the time evolution and dynamics of key processes using non-adiabatic microcanonical molecular dynamics simulations with surface-hopping dynamics (NVE-MDSH). Evaluation of the potential energy surfaces (PESs) in terms of the rotations of the phenyl rings suggested a pathway for the S1 → S0 transition for the perpendicular structure, whereas two pathways were anticipated for the T1 → S0 transition, namely, [T1 → S0]1 occurring immediately after the S1/T1 intersystem crossing (ISC) and [T1 → S0]2 occurring after the S1/T1 ISC and T1 equilibrium structure relaxation, with the T1 → S0 energy gap being comparable to the energy required for 3O2 → 1O2. The PESs also showed that because of the heavy-atom effect, BF2-FORM-D possessed a significantly smaller S1/T1 energy gap than BF2-FORM. The TST results revealed that at room temperature, BF2-FORM-D was thermodynamically more favorable than the parent molecule. Analysis of the NVE-MDSH results suggested that the librational motions of the phenyl rings play an important role in the internal conversion (IC) and ISC, and the S1/T1 ISC and T1 → S0 transitions could be enhanced by varying the irradiation wavelength and controlling the temperature. These findings can be used as guidelines to improve and/or design photosensitizers for PDT.

Electrochemical Diselenation of BODIPY Fluorophores for Bioimaging Applications and Sensitization of 1 O2

We report a rapid, efficient, and scope-extensive approach for the late-stage electrochemical diselenation of BODIPYs. Photophysical analyses reveal red-shifted absorption - corroborated by TD-DFT and DLPNO-STEOM-CCSD computations - and color-tunable emission with large Stokes shifts in the selenium-containing derivatives compared to their precursors. In addition, due to the presence of the heavy Se atoms, competitive ISC generates triplet states which sensitize 1 O2 and display phosphorescence in PMMA films at RT and in a frozen glass matrix at 77 K. Importantly, the selenium-containing BODIPYs demonstrate the ability to selectively stain lipid droplets, exhibiting distinct fluorescence in both green and red channels. This work highlights the potential of electrochemistry as an efficient method for synthesizing unique emission-tunable fluorophores with broad-ranging applications in bioimaging and related fields.

BINOL blocks as accessible triplet state modulators in BODIPY dyes

BINOL moieties of different electronic demand are useful blocks for enabling the photo-production and modulation of triplet excited states in readily-accesible BINOL-based O-BODIPY dyes from standard F-BODIPY precursors. The rapid and rational development of smarter triplet-enabling BODIPY dyes on the basis of this strategy (e.g., TADF biomarker 4a or room temperature phosphor 4g) paves the way for advancing photonic applications based on organic triplet photosensitizers.

BODIPY-pyrene donor-acceptor sensitizers for triplet-triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency

Triplet-triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin-orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY-pyrene heavy-atom-free donor-acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49-58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor-acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor-acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found.

Does Twisted π-Conjugation Framework Always Induce Efficient Intersystem Crossing? A Case Study with Benzo[b]- and [a]Phenanthrene-Fused BODIPY Derivatives and Identification of a Dark State

The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[b]-fused BODIPY, BDP-B; and [a]phenanthrene-fused BODIPY, BDP-P), are studied with steady-state and time-resolved optical and electron paramagnetic resonance (TREPR) spectroscopic methods as well as with ADC(2) theoretical investigations. Interestingly, BDP-B has a planar π-conjugation framework, but it displays weaker UV-vis absorption (ε = 3.8 × 10⁴ M^-1 cm^-1 at 569 nm) and fluorescence (Φ_F < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τ_F = 0.2 ns), and a long-lived triplet state (τ_T = 132.3 μs). In comparison, the more twisted BDP-P shows stronger UV-vis absorption (ε = 9.8 × 10⁴ M^-1 cm^-1 at 640 nm) and fluorescence (Φ_F = 70%), longer singlet-excited-state lifetime (τ_F = 6.4 ns), and shorter triplet-state lifetime (τ_T = 18.9 μs). In contrast to helicenes (Φ_T = ca. 90%), the ISC of BDP-P and BDP-B is nonefficient (Φ_T < 23%). The electron spin selectivity of the ISC of the derivatives is different, manifested by the phase pattern of the TREPR spectra as AAEAEE and EEEAAA for BDP-B and BDP-P, respectively. The spatially confined T₁ state wave function of the twisted molecule keeps the T₁ state energy high (1.44-1.61 eV). A dark S₁ state was identified for BDP-B. This work demonstrated that the twisted π-conjugated framework does not necessarily induce efficient ISC and we found a dark singlet state for BODIPY, which is rare.

BODIPY dimers: structure, interaction, and absorption spectrum

The object of the present study are BODIPY molecules obtained previously by Piskorz et al. (Dyes Pigm. 178:108322, 2020) for their antimicrobial activity. Structural analysis of the BODIPY dimers is presented in context of the aggregation influence on the photophysical properties. The thorough investigation of the nature of intermolecular interaction in the representative BODIPY dimers is provided together with the decomposition of the interaction energy into the components of well-defined origin according to SAPT procedure. For the model BODIPY systems the careful examination of the interaction nature for the dimer structure based on experimental crystal study as well as fully optimized is given. The tendencies observed in the model dimers are further on investigated for two pairs of BODIPY systems designed for biomedical application. The analyzed molecules are shown to maximize the mutual interaction by the optimization of the stacking dispersion contacts between the aromatic rings of the molecules, therefore producing stable dimers. The estimation of SAPT0 interaction energy components confirms the dominating dispersion character arising from mutual BODIPY core contacts. The influence of the dimerization process on the photophysical properties of the systems studied theoretically depends to the high extend on the dimerization mode and is significant for parallel and antiparallel dispersion-governed dimers.

Boron dipyrromethene (BODIPY) in polymer chemistry

The present review provides both a summary and outlook on the exciting field of BODIPYs in polymer chemistry.

Long-Lived Local Triplet Excited State and Charge Transfer State of 4,4'-Dimethoxy Triphenylamine-BODIPY Compact Electron Donor/Acceptor Dyads

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) and the formation of a long-lived charge transfer (CT) state were studied with a series of 4,4'-dimethoxy triphenylamine-BODIPY compact electron donor/acceptor dyads. Different torsion freedoms were applied in the dyads to tune the electronic coupling between the donor and acceptor, and a red-shifted CT absorption band was observed for one dyad. The dyads show solvent polarity-dependent singlet oxygen photosensitizing ability (quantum yields 3%-79%). Nanosecond transient absorption spectra of the dyad in nonpolar solvent confirm the formation of triplet states. The intrinsic triplet state lifetime is up to 383 μs (in fluid solution), which is much longer than that accessed with the heavy atom effect (276 μs). Intermolecular triplet photosensitizing of the dyads in a polar solvent produces a long-lived ³CT state (lifetime, τ_CT = 8.0 μs supported by the electron spin density surface analysis). The triplet state lifetime of the dyads doped in a Clear Flex 50 polymer film is exceptionally long (7.6-11.4 ms), and formation of a long-lived CT state (37 μs) was observed. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads used as the triplet photosensitizer and perylene used as the triplet acceptor; the upconversion quantum yield is up to 15.8%.

Determination of Upconversion Quantum Yields Using Charge-Transfer State Fluorescence of Heavy-Atom-Free Sensitizer as a Self-Reference

The efficiency of photon upconversion via triplet-triplet annihilation is characterized by an upconversion quantum yield (Φ_UC); however, uncertainties remain for its determination. Here, we present a new approach for the relative measurement of Φ_UC for green-to-blue upconversion using BODIPY-pyrene donor-acceptor dyad (BD1) as a heavy-atom-free triplet sensitizer. This new approach exploits broad fluorescence from a charge-transfer (CT) state of BD1, which possesses (i) a significant Stokes shift of 181 nm in dichloromethane and (ii) a comparably high CT-fluorescence quantum yield (Φ_ref = 7.0 ± 0.2%), which is independent from oxygen presence and emitter (perylene) concentration while also exhibiting a linear intensity dependence. On the basis of this, we developed an upconversion reference using the BD1 sensitizer mixed with perylene (1 × 10^-5 M/1 × 10^-4 M) in dichloromethane. With this reference system, we investigated the performance of three BODIPY donor-acceptor dyads in the upconversion process and achieved one of the highest Φ_UC of 6.9 ± 0.2% observed for heavy-atom-free sensitizers to date.

A facile way to obtain near-infrared room-temperature phosphorescent soft materials based on Bodipy dyes

Research on pure organic room-temperature phosphorescent (RTP) materials has made great advances but near-infrared (NIR) RTP emitting materials are still rare. Novel amorphous acrylamide copolymers containing iodine substituted borondipyrromethene (Bodipy) were prepared to obtain strong absorption in the visible region and moderate RTP in the NIR region with much larger Stokes shift than the fluorescence emission of traditional Bodipy dyes. Expensive metals and crystallization were left out to avoid biotoxicity and strict preparation conditions. Monoiodo and diiodo-Bodipy derivatives were both designed to study the substitution effect of iodine atoms. Photophysical properties, phosphorescence quantum yield and lifetime were characterized. Gels with NIR RTP emission were facilely prepared with the incorporation of ureidopyrimidone (UPy) and N,N'-methylenebisacrylamide (MBAA). The mechanical properties of the gels were measured using a rheometer and the results showed that the gels displayed fast self-healing ability due to the strong quadruple hydrogen bonding between UPy moieties.

Methylthio BODIPY as a standard triplet photosensitizer for singlet oxygen production: a photophysical study

A complete photophysical study on the iodinated-BODIPY, 3,5-dimethyl-2,6-diiodo-8-thiomethyl-pyrromethene (MeSBDP), demonstrated that it is an excellent triplet photosensitizer for singlet oxygen production in a broad range of apolar and polar solvents. Besides its absorption and fluorescence emission spectra, the dynamics of its excited states including its intersystem crossing rate was characterized by femtosecond transient experiments. The photophysical study of its triplet state by nanosecond transient absorption spectroscopy and phosphorescence emission concluded to a diffusion-controlled quenching of 3MeSBDP by O2 and to a fraction of triplet state quenching by O2 close to unity. The high (>0.87) and solvent-insensitive singlet oxygen quantum yield φΔ measured by singlet oxygen phosphorescence emission, together with the noticeable photostability of MeSBSP, as well as the absence of quenching of singlet oxygen by MeSBDP itself, allows claiming it as an alternative standard photosensitizer for singlet oxygen production, under excitation either in the UV or in the visible range.

gem-Dibromovinyl boron dipyrrins: synthesis, spectral properties and crystal structures

A family of new asymmetric and symmetric 1,3,7,9-tetramethyl-4,4-bora difluoro-diaza-s-indacene (BODIPY) derivatives, bearing gem-dibromovinyl substituents, was synthesized by the Corey-Fuchs olefination method. One or two gem-dibromovinyl moieties were attached at either the p-position of 5-phenyl, or the β-position of the pyrrole ring, directly or, through phenyl spacers. The assigned structures were supported by MS, NMR (¹H, ¹³C, ¹⁹F), X-ray diffraction analysis and for some compounds 2D HSQC and ¹¹B NMR as well as optical spectroscopy. Their absorption and fluorescence properties and solvatochromism in different solvents were investigated. The highest absorption and emission maxima were obtained for compounds having two gem-dibromovinyl groups attached directly or through the phenyl spacer. The best correlation (R-coefficient) between the solvent and spectral properties of the BODIPYs were obtained using the refractive index of the solvent. Although these compounds are structurally quite similar, their solid states show remarkable differences in the crystal system, clearly revealing two distinct patterns of gem-dibromovinyl orientation and torsion angles of the 5-phenyl ring and the indacene plane. Hirshfeld surface analysis data were used to visualize various intermolecular interactions.

Spectroscopic and 1O2 Sensitization Characteristics of a Series of Isomeric Re(bpy)(CO)3Cl Complexes Bearing Pendant BODIPY Chromophores

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)₃Cl and Re(BB4)(CO)₃Cl, featuring BODIPY groups appended to the 5,5'- or 6,6'-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4'-substituted homologue, Re(BB2)(CO)₃Cl. X-ray crystal structures of the compounds show that the 4,4'-, 5,5'-, and 6,6'-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 Å, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)₃Cl and Re(BB3)(CO)₃Cl and by more than two orders of magnitude in Re(BB4)(CO)₃Cl. Furthermore, phosphorescence from Re(BB4)(CO)₃Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and ¹O₂ sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.

PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: tuning photosensitizing ability by dihedral angles

Herein, four covalent BODIPY heterodimers that differ by dihedral angles were shown to be highly efficient excited triplet state (T₁) photosensitizers (PSs) for singlet oxygen formation with a quantum yield (Φ_Δ) of up to 0.94 as compared to their respective monomers, which had only negligible Φ_Δ of ca. 0.060. More interestingly, these PSs generate T₁via charge recombination mechanism rather than traditional inter-system crossing. The photosensitizing ability of dimers is easily tuned by either the dihedral angle (between the two linked BODIPYs) or solvent polarity. Laser flash photolysis, time-resolved and steady state fluorescence, quantum chemical calculation, as well as thermodynamic analysis were employed to study the associated photophysical process to reveal the T₁ formation mechanism: photo-induced electron transfer (PET) followed by charge recombination. Due to its heavy-atom-free nature, polarity selectivity, high efficiency, and easy tunability, this PET-based PS and its mechanism are very useful in developing new PS for photodynamic therapy of tumors, photobiology, and organic photochemistry.

Photocatalytic ATRA reaction promoted by iodo-Bodipy and sodium ascorbate

Using ascorbate as a sacrificial reductant, iodo-Bodipy dye 1b is able to promote the ATRA reaction between bromoderivatives and alkenes. This finding expands the possibility of using Bodipy dyes to promote photocatalytic reactions in efficient ways.

BODIPY Dye, the Most Versatile Fluorophore Ever?

BODIPY laser dyes constitute a fascinating topic of research in modern photochemistry due to the large variety of options its chromophore offers, which is ready available for a multitude of synthetic routes. Indeed, in the literature one can find a huge battery of compounds based on the indacene core. The possibility of modulating the spectroscopic properties or inducing new photophysical processes by the substitution pattern of the BODIPY dyes has boosted the number of scientific and technological applications for these fluorophores. Along the following lines, I will overview the main results achieved in our laboratory with BODIPYs oriented to optoelectronic as well to biophotonic applications, stressing the more relevant photophysical issues to be considered in the design of a tailor-made BODIPY for a certain application and pointing out some of the remaining challenges.

An experimental and theoretical study of dimethylaminostyryl BODIPY-perylenetetracarboxylic derivative dyads: synthesis, properties and DFT calculation

Three novel dyads composed of perylene tetracarboxylic unit and BODIPY moiety were synthesized and subjected to detailed spectroscopic examination and DFT calculations. Comparisons about the physical properties with changed acceptors were made.

Construction of the carbon–chalcogen (S, Se, Te) bond at the 2,6-positions of BODIPY via Stille cross-coupling reaction

Novel 2-chalcogen- or 2,6-dichalcogen-BODIPY derivatives were synthesized by a Pd-catalyzed C–heteroatom Stille cross-coupling reaction, overcoming the limitations of S_NAr.

High singlet oxygen production and negative solvatochromism of octabrominated 3-pyrrolyl boron dipyrromethenes

The newly synthesized and structurally characterized octabrominated 3-pyrrolyl boron dipyrromethenes is shown reveal negative solvatochromism, and quantitative generation of singlet oxygen; properties relevant to develop new PDT agents.

Transition-Metal-Free CO-Releasing BODIPY Derivatives Activatable by Visible to NIR Light as Promising Bioactive Molecules

Carbon monoxide-releasing molecules (CORMs) are chemical agents used to administer CO as an endogenous, biologically active molecule. A precise spatial and temporal control over the CO release is the major requirement for their applications. Here, we report the synthesis and properties of a new generation of transition-metal-free carbon monoxide-releasing molecules based on BODIPY chromophores (COR-BDPs) activatable by visible-to-NIR (up to 730 nm) light. We demonstrate their performance for both in vitro and in vivo experimental settings, and we propose the mechanism of the CO release based on steady-state and transient spectroscopy experiments and quantum chemical calculations.

Multistimuli Sensitive Behavior of Novel Bodipy-Involved Pillar[5]arene-Based Fluorescent [2]Rotaxane and Its Supramolecular Gel

Fluorescent [2]rotaxane BC12P5 is successfully constructed with 1,4-diethoxypillar[5]arene as wheel over a long alkyl axle with Bodipy chromophore as one stopper for the first time. NMR spectra clearly reveal its molecular shuttle nature triggered by multiple external stimuli including solvent polarity and temperature. In particular, the fluorescence nature introduced into [2]rotaxane BC12P5 renders it a good sensor for the external stimuli. Nevertheless, the supramolecular gel successfully fabricated from this novel rotaxane system via self-assembly in dimethyl sulfoxide (DMSO) also shows reversible gel-sol phase transition upon multiple external stimuli such as heating/cooling, shaking/resting, or the addition of different anions. Interestingly, exposure of the supramolecular gel film to HCl or ammonia vapor induces the change in the film fluorescence intensity, endowing this system with a potential application in gas detecting.

The triplet excited state of Bodipy: formation, modulation and application

The accessing of the triplet excited state of one of the most popular fluorophores, boron-dipyrromethene (Bodipy), was summarized.

Meso enyne substituted BODIPYs: synthesis, structure and properties

We report the synthesis of meso enyne substituted BODIPYs by the reaction of 8-chloro BODIPY with terminal alkynes under Sonogashira coupling conditions, and by Pd–Cu catalyzed hydroalkynylation reaction of terminal alkynes, across the –CC– bond of meso alkynylated BODIPYs.