BODIPYs to the rescue: Potential applications in photodynamic inactivation

Tuning the Polarity of Fullerene C60 Derivatives for Enhanced Photodynamic Inactivation†

In this article, four novel fulleropyrrolidines derivatives were synthesized to study how the effect of polarity and positive charge distribution can influence the efficacy of photodynamic inactivation treatments to kill bacteria. The design of the photosensitizers was based on DFT calculations that allowed us to estimate the dipolar moment of the molecules. Neutral compounds bearing N-methyl bis-acetoxy-ethyl (1) and bis-hydroxyethyl (2) amine were the starting material to obtain the dicationic analogs N,N-dimethyl bis-methoxyethyl (3), and bis-acetoxy-ethyl) (4) methylammonio. As expected from fullerene C₆₀ derivatives, compounds 1-4 absorb in the UV region, with a peak at 430 nm, a broader range of absorption up to 710 nm, and exhibit weak fluorescence emission in toluene and reverse micelles. In the biomimetic AOT micellar system, the highest singlet oxygen photosensitization was found for compounds 1, followed by 3, 2, and 4. Whereas 4 was the most effective reducing nitro blue tetrazolium in the presence of β-NADH. The influence of type I and type II mechanism on the photodynamic activity of compounds 3 and 4 was further examined in the presence of L-tryptophan and two reactive oxygen species scavengers. In vitro experiments indicated that the compounds with the highest dipolar moments, 3 (37.19 D) and 4 (38.46 D), inactivated methicillin-resistant Staphylococcus aureus and Escherichia coli bacteria using an energy dose <2.4 J cm^-2 . No inactivation was observed for the neutral analogs with the lowest dipolar moments. These findings help to optimize sensitizer structures to improve photodynamic inactivation.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

Conjugation-regulating synthesis of high photosensitizing activity porphyrin-based covalent organic frameworks for photodynamic inactivation of bacteria

Preparation of porphyrin-based covalent organic frameworks (Por-COFs) with high photosensitizing activity for photodynamic inactivation of bacteria is of great challenge, but significant for economy and human health. Herein, we show a conjugation-regulating strategy to design and synthesize Por-COFs with high photosensitizing activity for the photodynamic inactivation of bacteria. Terephthalaldehyde (Da), 2,5-Dihydroxyterephthalaldehyde (Dha), and 2,5-Diethoxyterephthalaldehyde (Deta) with different conjugation degrees are selected to condense with 5,10,15,20-Tetrakis(4-aminophenyl)porphyrin (Tph) to synthesize COF-366, DhaTph, and JNU-2, respectively. The higher conjugation of Dha and Deta than Da leads to the higher conjugation of DhaTph and JNU-2, respectively. Moreover, the hydroxyl group in Dha and the ethoxy group in Deta further expand the conjugation of DhaTph and JNU-2 via the formation of intralayer extended π-cloud delocalization and p-π conjunction, respectively. The extension of conjugation for DhaTph and JNU-2 results in the increase of intersystem crossing process and significantly improves their photosensitizing activity. Furthermore, JNU-2 with the highest photosensitizing activity exhibits superior antibacterial effects toward Staphylococcus aureus (99.1%) and Escherichia coli (96.8%). This study offers a new conjugation-regulating strategy for designing high photosensitizing activity of Por-COFs for the inactivation of bacteria.

Facile mitochondria localized fluorescent probe for viscosity detection in living cells

Fluorescent probes act as a powerful tool to understand the function of intracellular viscosity, which are closely associated with many functional disorders and diseases. Herein we report a boron-dipyrromethene (4,4-difluoro-4-borata-3a,4a-diaza-s-indacene, BODIPY) group based new fluorescent probe (BV-1), which was synthesized facilely by a one-step Knoevenagel-type condensation reaction, to detect viscosity in living cells with high selectivity and sensitivity. DFT calculation demonstrated that the unsaturated moiety at the meso-position of BODIPY suppressed the fluorescence via twisted intramolecular charge transfer (TICT) mechanism in low viscosity media. By restricting the rotation of the molecular rotor, the fluorescence would be enhanced significantly with redshift in emission wavelength in high viscosity conditions. The fluorescence intensity ratio (log (I/I₀)) at 570 nm showed a good linearity (R² = 0.991) with the viscosity (log η) in the range of 2-868 cP. And the limit of detection (LOD) and limit of quantification (LOQ) for viscosity were calculated to be 0.16 cP and 0.54 cP, respectively. BV-1 was demonstrated to be mitochondria localized with low cytotoxicity. Utilizing the new probe BV-1, the changes in mitochondrial viscosity caused by monensin or nystatin have been monitored successfully in real time. This work will provide new efficient ways for the development of viscosity probes, which are expected to be used for the study of intracellular viscosity properties and functions.

Self-Sterilizing 3D-Printed Polylactic Acid Surfaces Coated with a BODIPY Photosensitizer

Herein, we report the use of polylactic acid coated with a halogenated BODIPY photosensitizer (PS) as a novel self-sterilizing, low-cost, and eco-friendly material activated with visible light. In this article, polymeric surfaces were 3D-printed and treated with the PS using three simple methodologies: spin coating, aerosolization, and brush dispersion. Our studies showed that the polymeric matrix remains unaffected upon addition of the PS, as observed by dynamic mechanical analysis, Fourier transform infrared, scanning electron microscopy (SEM), and fluorescence microscopy. Furthermore, the photophysical and photodynamic properties of the dye remained intact after being adsorbed on the polymer. This photoactive material can be reused and was successfully inactivating methicillin-resistant Staphylococcus aureus and Escherichia coli in planktonic media for at least three inactivation cycles after short-time light exposure. A real-time experiment using a fluorescence microscope showed how bacteria anchored to the antimicrobial surface were inactivated within 30 min using visible light and low energy. Moreover, the material effectively eradicated these two bacterial strains on the first stage of biofilm formation, as elucidated by SEM. Unlike other antimicrobial approaches that implement a dissolved PS or non-sustainable materials, we offer an accessible green and economic alternative to acquire self-sterilizing surfaces with any desired shape.

A novel tricationic fullerene C60 as broad-spectrum antimicrobial photosensitizer: mechanisms of action and potentiation with potassium iodide

A novel amphiphilic photosensitizing agent based on a tricationic fullerene C₆₀ (DMC₆₀³⁺) was efficiently synthesized from its non-charged analogue MMC₆₀. These fullerenes presented strong UV absorptions, with a broad range of less intense absorption up to 710 nm. Both compounds showed low fluorescence emission and were able to photosensitize the production of reactive oxygen species. Furthermore, photodecomposition of L-tryptophan sensitized by both fullerenes indicated an involvement of type II pathway. DMC₆₀³⁺ was an effective agent to produce the photodynamic inactivation (PDI) of Staphylococcus aureus, Escherichia coli and Candida albicans. Mechanistic insight indicated that the photodynamic action sensitized by DMC₆₀³⁺ was mainly mediated by both photoprocesses in bacteria, while a greater preponderance of the type II pathway was found in C. albicans. In presence of potassium iodide, a potentiation of PDI was observed due to the formation of reactive iodine species. Therefore, the amphiphilic DMC₆₀³⁺ can be used as an effective potential broad-spectrum antimicrobial photosensitizer.

Exploiting a Neutral BODIPY Copolymer as an Effective Agent for Photodynamic Antimicrobial Inactivation

We report the synthesis and photophysical properties of a neutral BODIPY photosensitizing copolymer (poly-8-(4-hydroxymethylphenyl)-4,4-difluoro-2,6-diethynyl-4-bora-3a,4a-diaza-s-indacene) containing ethynylbenzene links between the BODIPY units. The copolymer absorbs further towards the red in the UV-vis spectrum compared to the BODIPY precursor. Photolysis of the polymer produces a singlet excited state which crosses to the triplet surface in less than 300 ps. This triplet state was used to form singlet oxygen with a quantum yield of 0.34. The steps leading to population of the triplet state were studied using time-resolved spectroscopic techniques spanning the pico- to nanosecond timescales. The ability of the BODIPY polymer to generate a biocidal species for bactericidal activity in both solution- and coating-based studies was assessed. When the BODIPY copolymer was dropcast onto a surface, 4 log and 6 log reductions in colony forming units/ml representative of Gram-positive and Gram-negative bacteria, respectively, under illumination at 525 nm were observed. The potent broad-spectrum antimicrobial activity of a neutral metal-free copolymer when exposed to visible light conditions may have potential clinical applications in infection management.

BODIPY-based monofunctional Pt (II) complexes for specific photocytotoxicity against cancer cells

Photodynamic therapy (PDT) has attracted extensive attention in cancer treatment because of its minimum trauma, less side effects, and so on. Photosensitizers, as one of the core elements of PDT, usually have to face problems such as poor water solubility and light stability, lack of targeting, and other problems, which seriously affect the therapeutic effect. In this work, two BODIPY (boron-dipyrromethene)-based monofunctional Pt (II) complexes, 1a and 2a, were designed and synthesized, and their PDT effect was studied. The Pt atom improved the singlet oxygen quantum yield (0.19 for 1a and 0.14 for 2a, respectively), which effectively improves the efficiency of PDT. MTT assay confirmed that the short time photo-irradiation distinctly promoted antitumor cytotoxicity of Pt (II) compounds against different cell lines. For 1a under irradiation, the IC₅₀ value of cancer cell lines were 13.1 μM for HeLa cells and 7.6 μM for MCF-7 cells, while those of normal cell lines were 32.4 μM for HBL-100 cells and 48.6 μM for L02 cells. The results demonstrated that 1a showed specific phototoxicity to cancer cells. This specific selectivity could be attributed to the synergistic effect of increased cellular uptake (determined by ICP-MS) and higher ROS generation (detected by Cell ROX Deep Red) in cancer cells after irradiation. This study laid the foundation for the future design and synthesis of effective PDT photosensitizers.

18F-labelled BODIPY dye as a dual imaging agent: Radiofluorination and applications in PET and optical imaging

Dual Positron emission tomography (PET)/optical imaging techniques have captured scientific interest for clinical applications due to their potential as an effective tool for visualizing in vivo information such as disease processes. 4,4'-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye has been considered an ideal platform strategy to achieve dual PET/optical imaging due to its photochemical nature and chemical structure. Various radiofluorination methods to prepare [18F]BODIPY dye have been developed and established, ranging from nucleophilic substitution reactions to isotope exchange reactions. In addition, 18F-labelled BODIPY dyes for biologically important targets have been used for in vivo and ex vivo studies. These studies proved the practicality of [18F]BODIPY dyes as a hybrid PET/optical imaging probe. In this review, recent advances in the synthesis and biological evaluation of 18F-labelled BODIPY dyes are described.

A New «off-on» Fluorescence Zinc Ion Sensors Based on Iodo- and Bromosubstituted Dipyrromethenes

Monoiodo- and dibromsubstituted dipyrromethenes HL¹ - HL³ were described as a highly sensitive and selective «Off-On» fluorescent chemosensor for Zn²⁺ based on the chelation-enhanced fluorescence (CHEF) effect. Сoordination reactions of HL¹ - HL³ with Zn²⁺ cations are accompanied by a significant (124 to 215-fold) increase in fluorescence intensity against the background of other metal ions in the binary propanol-1/cyclohexane mixture (1:30). The fluorometric detection limit of Zn²⁺ ions using HL¹ - HL³ sensors is from 3.0∙10^-8 to 3.3·10^-9 mol/L. The presence of Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Co²⁺, Pb²⁺ cations does not interfere with the detection of Zn²⁺. Complexation reactions are accompanied by a visual change in the color of the solution from yellow-orange to pink-raspberry so that the HL¹ - HL³ ligands can also be used as a «naked-eye» indicators of the presence of Zn²⁺ ions.

Molecular Imaging of Tau Protein: New Insights and Future Directions

Tau is a microtubule-associated protein (MAPT) that is highly expressed in neurons and implicated in several cellular processes. Tau misfolding and self-aggregation give rise to proteinaceous deposits known as neuro-fibrillary tangles. Tau tangles play a key role in the genesis of a group of diseases commonly referred to as tauopathies; notably, these aggregates start to form decades before any clinical symptoms manifest. Advanced imaging methodologies have clarified important structural and functional aspects of tau and could have a role as diagnostic tools in clinical research. In the present review, recent progresses in tau imaging will be discussed. We will focus mainly on super-resolution imaging methods and the development of near-infrared fluorescent probes.

Inhibition of the Citrus Canker Pathogen Using a Photosensitizer Assisted by Sunlight Irradiation

Citrus canker, induced by bacterial infection, seriously affects the growth and productivity of citrus around the world and has attracted strong research interest. The current treatment for this disease uses copper salts to inactivate the pathogenic bacteria: Xanthomonas citri subsp. citri (Xcc) strain. However, copper salts may have a negative impact on the environment or plant. In this work, we identify a chemical compound, 2,6-diiodo-1,3,5,7-tetramethyl-8-(P-benzoic acid)-4,4′-difluoroboradiazaindacene (DIBDP), to inactivate the pathogenic Xcc strain (29-1). DIBDP is activated by sunlight and generates reactive oxygen species to kill the bacteria. In order to overcome the degradation of DIBDP under sunlight, an adjuvant agent was identified to limit the photodegradation of DIBDP by forming a photosensitizer complex (PSC). This complex demonstrated significant antimicrobial activity to Xcc 29-1, which was 64-fold more potent than the copper biocides. The antimicrobial efficacy of PSC on citrus leaves infected by Xcc 29-1 also was much stronger than copper agent and, at the same time, the PSC was safe to the host exposed to sunlight. Thus, this PSC is a promising antibacterial agent to control citrus canker disease.

BODIPY-Based Photosensitizers as Potential Anticancer and Antibacterial Agents: Role of the Positive Charge and the Heavy Atom Effect

Boron-dipyrromethene derivatives, including cationic and iodinated analogs, were obtained and subjected to physicochemical and in vitro photodynamic activity studies. Iodinated derivatives revealed a substantial heavy atom effect manifested by a bathochromic shift of the absorption band by about 30 nm and fluorescence intensity reduced by about 30-35 times, compared to that obtained for non-iodinated ones. In consequence, singlet oxygen generation significantly increased with Φ_Δ values in the range 0.69-0.97. The in vitro photodynamic activity was evaluated on Gram-positive Staphylococcus aureus, Gram-negative Escherichia coli, and on human androgen-sensitive prostate adenocarcinoma cells (LNCaP). The novel cationic, iodinated BODIPY, demonstrated the highest activity toward all studied cells. An excellent cytotoxic effect was found against LNCaP cells with an IC₅₀ value of 19.3 nM, whereas the viability of S. aureus was reduced by >5.6 log₁₀ at 0.25 μM concentration and by >5.3 log₁₀ in the case of E. coli at 5 μM. Thus, this analog seems to be a very promising candidate for the application in both anticancer and antimicrobial photodynamic therapy.

BODIPYs bearing a dimethylaminopropoxy substituent for imaging and photodynamic inactivation of bacteria

A new BODIPY (BDP 1) bearing a dimethylaminopropoxy group attached to a phenylene unit was synthesized. This compound was brominated to obtain the halogenated analog BDP 2, which was designed to enhance the photodynamic effect of BODIPY to kill bacteria without an intrinsic cationic charge. The basic amino group located at the end of the propoxy bridge can acquire a positive charge by protonation in an aqueous medium, increasing the binding to bacterial cells. Interaction and photokilling activity mediated by these compounds was evaluated in Staphylococcus aureus and Escherichia coli. BDP 1 and BDP 2 were rapidly bound to bacterial cells, showing bioimages with green emission. Complete elimination of S. aureus was detected when cells were incubated with 1 μM BDP 2 and irradiated for 5 min. Comparable photoinactivation was obtained with E. coli, after an irradiation of 30 min. Furthermore, BDP 2 was effective to kill bacteria at very low concentration (0.5 μM). Thus, BDP 1 showed mainly interesting properties as a fluorophore, whereas BDP 2 was highly effective photosensitizer as a broad-spectrum antibacterial agent.

Magnetic Nanoplatforms for in Situ Modification of Macromolecules: Synthesis, Characterization, and Photoinactivating Power of Cationic Nanoiman-Porphyrin Conjugates

A nanoplatform concept was developed to synthesize accessible photoactive magnetic nanoparticles (MNPs) of Fe₃O₄ coated with silica. This approach was based on the covalent binding of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPPF₂₀) to aminopropyl-grafted MNPs by nucleophilic aromatic substitution reaction (S_NAr) to obtain conjugate MNP-P1. After in situ modification, the remaining pentafluorophenyl groups of TPPF₂₀ attached to MNPs were substituted by dimethylaminoethoxy groups to form MNP-P2. The basic amine group of these conjugates can be protonated in aqueous media. In addition, MNP-P1 and MNP-P2 were intrinsically charged to produce cationic conjugates MNP⁺-P1 and MNP⁺-P2⁺ by methylation. All of them were easily purified by magnetic decantation in high yields. The average size of the MNPs was ∼15 nm, and the main difference between these conjugates was the greater coating with positive charges of MNP⁺-P2⁺, as shown by the zeta potential values. Absorption spectra exhibited the Soret and Q bands characteristic of TPPF₂₀ linked to MNPs. Furthermore, these conjugates showed red fluorescence emission of porphyrin with quantum yields of 0.011-0.036. The photodynamic effect sensitized by the conjugates indicated the efficient formation of singlet molecular oxygen in different media, reaching quantum yield values of 0.17-0.34 in N,N-dimethylformamide. The photodynamic activity of the conjugates was evaluated to inactivate the Gram-positive bacteria Staphylococcus aureus, the Gram-negative bacteria Escherichia coli, and the yeast Candida albicans. The modified cationic MNP⁺-P2⁺ was the most effective conjugate for photodynamic inactivation (PDI) of microorganisms. Binding of this conjugate to bacteria and photoinactivation capability was checked by means of fluorescence microscopy. Also, sustainable use by recycling was determined after three PDI treatments. Therefore, this methodology is a suitable scaffold for the in situ modification of conjugates, and in particular, MNP⁺-P2⁺ represents a useful photodynamic active material to eradicate microorganisms.

Lipid Peroxidation Assay Using BODIPY-Phenylbutadiene Probes: A Methodological Overview

The assessment of reactive oxygen species has increasing importance in biomedical sciences, due to their biological role in signaling pathways and induction of cell damage at low and high concentrations, respectively. Detection of lipid peroxidation with sensing probes such as some BODIPY dyes has now wide application in studies using fluorescent microplate readers, flow cytometry, and fluorescence microscopy. Two phenylbutadiene derivatives of BODIPY are commonly used as peroxidation probes, non-oxidized probes and oxidized products giving red and green fluorescence, respectively. Peculiar features of lipoperoxidation and BODIPY dye properties make this assessment a rather complex process, not exempt of doubts and troubles. Color changes and fluorescence fading that are not due to lipid peroxidation must be taken into account to avoid misleading results. As a characteristic feature of lipoperoxidation is the propagation of peroxyl radicals, pitfalls and advantages of a delayed detection by BODIPY probes should be considered.

Anchoring BODIPY photosensitizers enable pan-microbial photoinactivation

Photodynamic antimicrobial chemotherapy (PACT) is an effective strategy to inactivate pathogenic and resistant microbes. However, pan-microbial photoinactivation has hardly achieved. In this manuscript, we built anti-microbial PSs based on 2,6-diiodo-1,3,5,7-tetramethyl BODIPY (2I-BDP) using anchoring strategy through modifications on boron atom with bis-cationic moieties. With appropriate bis-cationic anchoring, we could achieve effective PACT for pan-microbial photoinactivation via straight forward modifications. Our studies suggested that integration of an efficient photosensitizer, good amphiphilicity, as well as tight interaction with microbial membrane could be essential for effective PACT.

BCl3-Activated Synthesis of COO-BODIPY Laser Dyes: General Scope and High Yields under Mild Conditions

A general and straightforward method for the synthesis of COO-BODIPYs from F-BODIPYs and carboxylic acids is established. The method is based on the use of boron trichloride to activate the involved substitution of fluorine, which leads to high yields through rapid reactions under soft conditions. This mild method opens the way to unprecedented laser dyes with outstanding efficiencies and photostabilities, which are difficult to obtain by the current methods.

Bacteriochlorin-bis(spermine) conjugate affords an effective photodynamic action to eradicate microorganisms

A novel bacteriochlorin bearing two spermine units (BCS) was synthesized from 3,13-dibromo-8,8,18,18-tetramethylbacteriochlorin (BC-Br ^3,13 ). The synthesis involved the Suzuki coupling of BC-Br ^3,13 to obtain a bacteriochlorin-dibenzaldehyde (BCA), which was subjected to reductive amination with spermine. The resulting bacteriochlorin BCS presents a strong near-infrared absorption band at 747 nm, emits at 750 nm with fluorescence quantum yield of 0.14, and generates singlet molecular oxygen, O₂ (¹ Δ_g ), with a quantum yield of 0.27. Photokilling capacities mediated by BCS were evaluated in microbial cells. The viability of Staphylococcus aureus decreased 7 logs when cells were incubated with 1 μM BCS and irradiated for 15 minutes. Comparable photocytotoxic effect was obtained with Escherichia coli, when cells were treated for 30 minutes with visible light. BCS was also an effective photosensitizer to inactivate Candida albicans. In addition, this bacteriochlorin was able to eradicate bacteria at short incubation times. The structure of BCS contains eight basic amino groups that, when protonated in water, increase the binding to the cell envelope. In summary, the readily accessible bacteriochlorin BCS was highly effective at low concentrations as a broad-spectrum antimicrobial photosensitizer.

Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH2 -rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm-1 in BODIPY versus >4170 cm-1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

Tetraphenylporphine-Modified Polymeric Nanoparticles Containing NIR Photosensitizer for Mitochondria-Targeting and Imaging-Guided Photodynamic Therapy

Near-infrared (NIR) photodynamic therapy (PDT) is a promising antitumor strategy under NIR light irradiation to kill cancer cells. Mitochondria has a critical function in sustaining cellular viability and death, which is the ideal organelle for PDT. Here, we reported a tetraphenylporphine (TPP)-conjugated amphiphilic copolymer and an iodinated boron dipyrromethene photosensitizer (BDPI) with high singlet oxygen yield to form nanoparticles (PBDPI-TPP), which could realize mitochondria-targeting and improve the NIR imaging-guided PDT. The as-prepared mitochondria-targeting nanoplatform could show effective subcellular localization and bring about significant irreversible mitochondrial injury for enhanced PDT. Both in vitro and in vivo experiments revealed that the mitochondria-targeting PDT system could achieve a remarkable therapeutic effect, indicating that it is a promising nanoplatform for NIR imaging-guided PDT in cancer therapeutics.

Rigid, yet flexible: formation of unprecedented silver MB-DIPY dimers with orthogonal chromophore geometry

Unprecedented for BODIPY/DIPY and aza-BODIPY/azaDIPY chemistry, MB-DIPY₂Ag₂ dimers with a twisted chromophore geometry were prepared and characterized by spectroscopy, X-ray crystallography, and DFT calculations.

Multichromophoric COO-BODIPYs: an advantageous design for the development of energy transfer and electron transfer systems

Synthesis and photonics avails a new design for multichromophoric arrays.

Synthesis and Photophysical Behavior of a Highly Fluorescent Family of Unsymmetrical Organoboron Complexes Containing 5-(Pyridin-2-ylmethylene)imidazolidine-2,4-dione Moieties

A new and highly fluorescent family of unsymmetrical organoboron complexes containing 5-(pyridin-2-ylmethylene)imidazolidine-2,4-dione moieties has been synthesized in three steps. These compounds show strong absorptions covering a wide range of the UV-vis spectrum and are strongly emissive (ϕ_f of up to 0.92 in CH₃CN). Moreover, two fluorophores that include an alkyne or an azide group at the end of the alkyl chain and with potential utility in bioorthogonal chemistry have been developed. One of these, in which the glycol substituent provides an enhanced water solubility without compromising the fluorescence (ϕ_f = 0.85 in water), may be of particular importance.

Intramolecular Hydrogen Bonds in Selected Aromatic Compounds: Recent Developments

A review of intramolecular hydrogen bonding in ortho-hydroxyaryl Schiff bases, ortho-hydroxyaryl Mannich bases, dipyrrins, ortho-hydroxyaryl ketones, ortho-hydroxyaryl amides, and 4-Bora-3a,4a-diaza-s-indacene (BODIPY) dyes with tautomeric sensors as substituents is presented in this paper. Ortho-hydroxy Schiff and Mannich base derivatives are known as model molecules for analysing the properties of intramolecular hydrogen bonding. The compounds under discussion possess physicochemical features modulated by the presence of strong intramolecular hydrogen bonds. The equilibrium between intra- and inter-molecular hydrogen bonds in BODIPY is discussed. Therefore, the summary can serve as a knowledge compendium of the influence of the hydrogen bond on the molecular properties of aromatic compounds.

Phenotypic Resistance in Photodynamic Inactivation Unravelled at the Single Bacterium Level

Herein we report a simple fluorescence microscopy methodology that, jointly with four photosensitizers (PSs) and a cell viability marker, allows monitoring of phenotypic bacterial resistance to photodynamic inactivation (PDI) treatments. The PSs, composed of BODIPY dyes, were selected according to their ability to interact with the cell wall and the photoinactivating mechanism involved (type I or type II). In a first approach, the phenotypic heterogeneity allowing bacteria to persist during PDI treatment was evaluated in methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli as Gram-positive and Gram-negative models, respectively. By means of propidium iodide (PI), we monitored with spatiotemporal resolution cell viability at the single bacterium level. All the PSs were effective at inactivating pathogens; however, the cationic nonhalogenated PS (compound 1) surpassed the others and was capable of photoinactivating E. coli even under optimal growth conditions. Compound 1 was further tested on two other Gram-negative strains, Pseudomonas aeruginosa and Klebsiella pneumoniae, with outstanding results. All bacterial strains used here are well-known ESKAPE pathogens, which are the leading cause of nosocomial infections worldwide. Thorough data analysis of individual cell survival times revealed clear phenotypic variation expressed in the cell wall that affected PI permeation and thus its intercalation with DNA. For the same bacterial sample, death times may vary from seconds to hours. In addition, the PI incorporation time is also a parameter governed by the phenotypic characteristics of the microbes. Finally, we demonstrate that the results gathered for the bacteria provide direct and unique experimental evidence that supports the time-kill curve profiles.

Trends and targets in antiviral phototherapy

Photodynamic therapy (PDT) is a well-established treatment option in the treatment of certain cancerous and pre-cancerous lesions. Though best-known for its application in tumor therapy, historically the photodynamic effect was first demonstrated against bacteria at the beginning of the 20^th century. Today, in light of spreading antibiotic resistance and the rise of new infections, this photodynamic inactivation (PDI) of microbes, such as bacteria, fungi, and viruses, is gaining considerable attention. This review focuses on the PDI of viruses as an alternative treatment in antiviral therapy, but also as a means of viral decontamination, covering mainly the literature of the last decade. The PDI of viruses shares the general action mechanism of photodynamic applications: the irradiation of a dye with light and the subsequent generation of reactive oxygen species (ROS) which are the effective phototoxic agents damaging virus targets by reacting with viral nucleic acids, lipids and proteins. Interestingly, a light-independent antiviral activity has also been found for some of these dyes. This review covers the compound classes employed in the PDI of viruses and their various areas of use. In the medical area, currently two fields stand out in which the PDI of viruses has found broader application: the purification of blood products and the treatment of human papilloma virus manifestations. However, the PDI of viruses has also found interest in such diverse areas as water and surface decontamination, and biosafety.

Antimicrobial Photodynamic Polymeric Films Bearing Biscarbazol Triphenylamine End-Capped Dendrimeric Zn(II) Porphyrin

A novel biscarbazol triphenylamine end-capped dendrimeric zinc(II) porphyrin (DP 5) was synthesized by click chemistry. This compound is a cruciform dendrimer that bears a nucleus of zinc(II) tetrapyrrolic macrocycle substituted at the meso positions by four identical substituents. These are formed by a tetrafluorophenyl group that possesses a triazole unit in the para position. This nitrogenous heterocyclic is connected to a 4,4'-di(N-carbazolyl)triphenylamine group by means of a phenylenevinylene bridge, which allows the conjugation between the nucleus and this external electropolymerizable carbazoyl group. In this structure, dendrimeric arms act as light-harvesting antennas, increasing the absorption of blue light, and as electroactive moieties. The electrochemical oxidation of the carbazole groups contained in the terminal arms of the DP 5 was used to obtain novel, stable, and reproducible fully π-conjugated photoactive polymeric films (FDP 5). First, the spectroscopic characteristics and photodynamic properties of DP 5 were compared with its constitutional components derived of porphyrin P 6 and carbazole D 7 moieties in solution. The fluorescence emissions of the dendrimeric units in DP 5 were more strongly quenched by the tetrapyrrolic macrocycle, indicating photoinduced energy transfer. In addition, FDP 5 film showed the Soret and Q absorption bands and red fluorescence emission of the corresponding zinc(II) porphyrin. Also, FDP 5 film was highly stable to photobleaching, and it was able to produce singlet molecular oxygen in both N,N-dimethylformamide (DMF) and water. Therefore, the porphyrin units embedded in the polymeric matrix of FDP 5 film mainly retain the photochemical properties. Photodynamic inactivation mediated by FDP 5 film was investigated in Staphylococcus aureus and Escherichia coli. When a cell suspension was deposited on the surface, complete eradication of S. aureus and a 99% reduction in E. coli survival were found after 15 and 30 min of irradiation, respectively. Also, FDP 5 film was highly effective to eliminate individual bacteria attached to the surface. In addition, photodynamic inactivation (PDI) sensitized by FDP 5 film produced >99.99% bacterial killing in biofilms formed on the surface after 60 min irradiation. The results indicate that FDP 5 film represents an interesting and versatile photodynamic active material to eradicate bacteria as planktonic cells, individual attached microbes, or biofilms.

BODIPY analogues: synthesis and photophysical studies of difluoro boron complexes from 2-aminotropone scaffolds through N,O-chelation

Boron-dipyrromethene (BODIPY) fluorophore derivatives were prepared from a Pyrromethene scaffold and di-fluoroboron. To add to the range of biocompatible fluorophores, this report describes the synthesis and photophysical studies of carboxylate-functionalized BODIPY analogues from natural products, Tropolone and α-amino acids, through N,O-chelation. The structure of a few derivatives was confirmed by single crystal X-ray studies. UV/fluorescence studies revealed their fluorescence behaviors in organic solvents with a quantum yield of ∼0-15%, which varied based on the structure of amino acids. Further, electrochemical studies were accomplished by cyclic voltammetry, which confirm only one irreversible reduction reaction with E_pc = -1.3 (V). Finally, their HOMO and LUMO molecular orbitals/energy differences were calculated from a theoretically (DFT) optimized structure, which also supported the role of amino group residues in the enhancement of the fluorescence. The glycinate derivative exhibited the greatest quantum yield compared to the other amino groups. Hence, 2-aminotropone containing BODIPY analogues are potential candidates for the development of various types of functionalized fluorophores as BODIPY analogues.

Luminescent properties of new 2,2-, 2,3- and 3,3-CH2-bis(BODIPY)s dyes: Structural and solvation effects

In this paper the synthesis and spectral properties of three new dimeric bis(BODIPY)s with two indacene domains connected by a methylene (-CH₂-) spacer at 2,2-, 2,3- or 3,3- positions were reported. It was found bis(BODIPY)s exhibit a high sensitivity of fluorescence to the medium properties. To interpret solvatochromic effects of bis(BODIPY)s, a multilinear correlation analysis of bis(BODIPY)s fluorescence quantum yields with respect to solvent different parameters was carried out. To understand the features of the spectral properties of bis(BODIPY)s, we carried out a thorough quantum-chemical analysis of the structural, conformational, and spectral characteristics of bis(BODIPY)s. The obtained bis(BODIPY)s have a high potential for application as sensors of medium polarity.

BODIPY-Decorated Nanoscale Covalent Organic Frameworks for Photodynamic Therapy

Covalent organic frameworks (COFs), an emerging class of organic porous materials, have attracted intense attention due to their versatile applications. However, the deliberate fabrication of COF-based nanomaterials for nanomedical application remains challenging due to difficulty in their size- and structure-controlled synthesis and poor aqueous dispersibility. Herein, we report two boron-dipyrromethene (BODIPY)-decorated nanoscale COFs (NCOFs), which were prepared by the Schiff-base condensation of the free end -CHO (bonding defects in COFs) on the established imine-based NCOFs with the amino-substituted organic photosensitizer BODIPY via "bonding defects functionalization" approach. Thus BODIPY has been successfully nanocrystallized via the NCOF platform, and can be used for photodynamic therapy (PDT) to treat tumors. These NCOF-based PDT agents featured nanometer size (∼110 nm), low dark toxicity, and high phototoxicity as evidenced by in vitro and in vivo experiments. Moreover, the "bonding defects functionalization" approach might open up new avenues for the fabrication of additional COF-based platforms for biomedical treatment.

Light-Harvesting Antenna and Proton-Activated Photodynamic Effect of a Novel BODIPY-Fullerene C60 Dyad as Potential Antimicrobial Agent

A covalently linked BODIPY-fullerene C₆₀ dyad (BDP-C₆₀ ) was synthesized as a two-segment structure, which consists of a visible light-harvesting antenna attached to an energy or electron acceptor moiety. This structure was designed to improve the photodynamic action of fullerene C₆₀ to inactivate bacteria. The absorption spectrum of BDP-C₆₀ was found to be a superposition of the spectra of its constitutional moieties, whereas the fluorescence emission of the BODIPY unit was strongly quenched by the fullerene C₆₀ . Spectroscopic, calculations, and redox studies indicate a competence between photoinduced energy and electron transfer. Protonating the dimethylaminophenyl substituent through addition of an acidic medium led to a substantial increase in the fluorescence emission, triplet excited state formation, and singlet molecular oxygen production. At physiological pH, photosensitized inactivation of Staphylococcus aureus mediated by 1 μM BDP-C₆₀ exhibited a 4.5 log decrease of cell survival (>99.997 %) after 15 min irradiation. A similar result was obtained with Escherichia coli using 30 min irradiation. Moreover, proton-activated photodynamic action of BDP-C₆₀ turned this dyad into a highly effective photosensitizer to eradicate E. coli. Therefore, BDP-C₆₀ is an interesting photosensitizing structure in which the light-harvesting antenna effect of the BODIPY unit combined with the protonation of dimethylaminophenyl group can be used to improve the photoinactivation of bacteria.

A versatile bacterial membrane-binding chimeric peptide with enhanced photodynamic antimicrobial activity

Photodynamic therapy (PDT) has become an effective antibiosis method for overcoming antibiotic resistance. In this study, we developed a versatile bacterial membrane-binding chimeric peptide PpIX-[PEG₈-(KLAKLAK)₂]₂ (denoted as PPK) by conjugating a photosensitizer protoporphyrin IX (PpIX) with an antimicrobial peptide (KLAKLAK)₂ (KLA) for effective photodynamic inactivation of bacteria. The chimeric peptide PPK with positively charged properties and an α-helical conformation could rapidly bind to microbial cells through electrostatic interactions and membrane insertion. Moreover, PPK could disrupt the bacterial membrane and further elicit lipid bilayer leakage to kill bacteria by toxic reactive oxygen species (ROS) generated by PpIX under 660 nm light. In vitro experiments demonstrated that cationic PPK possessed excellent antimicrobial activity against both Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli). Afterward, PPK also exhibited perfect therapeutic effects on S. aureus-infected mice without any systemic side effects. This chimeric peptide PPK will show great potential for photodynamic antibiosis.

A "reactive" turn-on fluorescence probe for hypochlorous acid and its bioimaging application

An aza-BODIPY-CNOH probe attached aldoxime group demonstrated the specific detection for hypochlorous acid by the turn-on red emission signal. NMR and HRMS experiments confirmed that the fluorescence originated from the oxidation degradation of the non-fluorescence, aldoxime-based aza-BODIPY-CNOH probe into the red-fluorescence, nitrile oxide-based aza-BODIPY compound aza-BODIPY-CNO. The aza-BODIPY-CNOH probe showed good biocompatibility and was low toxic to living cells as shown from MTT experiments. Living RAW264.7 cells imaging indicated the aza-BODIPY-CNOH probe had good permeability and either exogenous or endogenous HClO caused the intracellular bright-red fluorescence, showing its potential hypochlorous acid-specific sensing ability in biological systems.

Multinuclear Ru(ii) and Ir(iii) decorated tetraphenylporphyrins as efficient PDT agents

Two novel multi-metallic porphyrin complexes were synthesised and evaluated as effective PDT agents against human breast epithelial cells (SKBR-3).

The effect of low- and high-penetration light on localized cancer therapy

The design of a delivery system allowing targeted and controlled drug release has been considered one of the main strategies used to provide individualized cancer therapy, to improve survival statistics, and to enhance quality-of-life. External stimuli including low- and high-penetration light have been shown to have the ability to turn drug delivery on and off in a non-invasive remotely-controlled fashion. The success of this approach has been closely related to the development of a variety of drug delivery systems - from photosensitive liposomes to gold nanocages - and relies on multiple mechanisms of drug release activation. In this review, we make reference to the two extremes of the light spectrum and their potential as triggers for the delivery of antitumor drugs, along with the most recent achievements in preclinical trials and the challenges to an efficient translation of this technology to the clinical setting.

Porphyrins containing basic aliphatic amino groups as potential broad-spectrum antimicrobial agents

New porphyrin derivatives bearing basic aliphatic amino groups were synthesized from the condensation of meso-4-[(3-N,N-dimethylaminopropoxy)phenyl]dipyrromethane, pentafluorobenzaldehyde and 4-(3-N,N-dimethylaminopropoxy)benzaldehyde. The reaction was catalyzed by trifluoroacetic acid in acetonitrile. This approach was used to obtain porphyrins with different patterns of substitution, of which three of them were isolated: 5,15-di(4-pentafluorophenyl)-10,20-di[4-(3-N,N-dimethylaminopropoxy)phenyl]porphyrin (F₁₀APP), 5-(4-pentafluorophenyl)-10,15,20-tris[4-(3-N,N-dimethylaminopropoxy)phenyl]porphyrin (F₅APP) and 5,10,15,20-tetrakis[4-(3-N,N-dimethylaminopropoxy)phenyl]porphyrin (TAPP). The UV-vis spectroscopic characterizations and the photodynamic effect of these compounds were compared in N,N-dimethylformamide. These porphyrins showed red fluorescence emission with quantum yields of 0.09-0.15. Moreover, they sensitized the production of singlet molecular oxygen, reaching quantum yields values of 0.33-0.53. Photodynamic inactivation was studied in two bacteria, Staphylococcus aureus and Escherichia coli, and a yeast Candida albicans. High amount of cell-bound porphyrin was obtained at short times (<2 min) of incubation. After 15 min irradiation, a 7 log reduction of S. aureus was found for cells treated with 1 μM F₅APP. Similar photokilling was obtained in E. coli, but using 7.5 μM F₅APP and 30 min irradiation. Under these conditions, a decrease of 5 log was observed in C. albicans cells. An increase in cell survival was observed by addition of sodium azide, whereas a slight protective effect was found in the presence of D-mannitol. Moreover, the photoinactivation mediated by these porphyrins was higher in D₂O than in water. Thus, these porphyrins induced the photodynamic activity mainly through the intermediacy of O₂(¹Δ_g). In particular, F₅APP was a highly effective photosensitizer with application as a broad-spectrum antimicrobial. This porphyrin contains three basic aliphatic amino groups that may be protonated at physiological pH. In addition, it is substituted by a lipophilic pentafluorophenyl group, which confers an amphiphilic character to the tetrapyrrolic macrocycle. This effect can increase the interaction with the cell envelopment, improving the photocytotoxic activity against the microorganisms.

Mitochondria-targeting BODIPY-loaded micelles as novel class of photosensitizer for photodynamic therapy

In this paper, a series of novel BODIPY-based photosensitizers have been designed and synthesized for photodynamic therapy. BODIPY3 was screened out as the most potential photosensitizer due to its excellent optical properties, high singlet oxygen efficiency and good photostability. However, as an organic photosensitizer, BODIPY3 still suffered from the drawbacks of insolubility and instability in aqueous system. In view of these problems, DSPE-PEG2000 was used to trap BODIPY3 into the hydrophobic core of micelles to obtain well-dispersing nano complexes BODIPY3-PEG₃ in aqueous system. More importantly, BODIPY3-PEG₃ not only has better solubility and stability in aqueous media but can generate significant singlet oxygen (¹O₂, one of the reactive oxygen species, the real cytotoxic agent in photodynamic therapy) in living cells and exhibit high light cytotoxicity to three cancer cell lines. The mechanism studies indicated the mitochondrial localization of BODIPY3-PEG₃ was able to generate ROS in mitochondria, which further result in mitochondrial dysfunction and photoinduced apoptosis via caspase-8 and caspase-3 pathway.