Highly Efficient and Photostable Photosensitizer Based on BODIPY Chromophore

Shell Thickness and Heterogeneity Dependence of Triplet Energy Transfer between Core-Shell Quantum Dots and Adsorbed Molecules

Quantum dot (QD)-sensitized triplet energy transfer (TET) has found promising applications in photon upconversion and photocatalysis. However, the underlying mechanism of TET in the QD-acceptor complex remains unclear despite the well-developed TET theory for the molecular donor-acceptor systems. Herein, the coupling strength of TET from CdSe/CdS core-shell QDs to 9-anthracene carboxylic acid (ACA) was studied by measuring the TET rate as a function of shell thickness with time-resolved photoluminescence. The change of TET-coupling strength with increasing shell thickness was further compared to those of electron and hole transfers from QDs so that we could test whether QD-sensitized TET is mediated by the charge transfer virtual state and can be considered as simultaneous electron and hole transfers as in molecular donor-acceptor systems. The measured coupling strength of TET from the CdSe/CdS QD decreases exponentially with the CdS shell thickness r: |V|(r) = |V|(0)e-βr, with an exponential decay factor β of 0.19 Å-1, which is smaller than the sum of the measured decay factors for electron transfer to methyl viologen (0.18 Å-1) and hole transfer to phenothiazine (0.29 Å-1) from the same QD. This inconsistency is explained by the broadening of QD shell thicknesses in the distance dependence study, which significantly modifies the TET-coupling strength and driving force, resulting in a shallower distance dependence of the TET rate constants. This study sheds light on the fundamental mechanisms of QD-sensitized TET reactions.

Synthesis, structure, spectral and redox properties of 3-pyrrolyl BODIPYs containing one to eight bromides at the pyrrole carbons

We report the regioselective bromination of meso-aryl 3-pyrrolyl BODIPY by introducing one to eight bromides at the pyrrole carbons of three pyrrole rings by treating meso-aryl 3-pyrrolyl BODIPY with N-bromosuccinimide/Br2 in CH2Cl2 under mild reaction conditions. The crystal structures of five of these pyrrole-brominated meso-aryl 3-pyrrolyl BODIPYs indicated that the appended pyrrolyl group underwent distortion from the 12-atom mean plane of the BODIPY, and a maximum distortion of 40° was observed for octabrominated meso-aryl 3-pyrrolyl BODIPY. Upon increasing the number of bromides from one to eight at the pyrrole carbons of meso-aryl 3-pyrrolyl BODIPY, the following key observations were made: (i) the absorption bands experienced a bathochromic shift up to the introduction of five bromides, followed by a hypsochromic shift from six to eight bromides, indicating that the magnitude of the absorption band shifts was non-additive; (ii) the fluorescence band experienced bathochromic shifts along with a decrease in quantum yield and singlet state lifetime; (iii) the electrochemical studies indicated that brominated 3-pyrrolyl BODIPYs were electron deficient and underwent easier reductions with an increase in the number of bromides at the pyrrole carbons of 3-pyrrolyl BODIPY. A linear additive relationship was noted between E1/2red and the number of bromide groups at the pyrrole carbons of 3-pyrrolyl BODIPY and (iv) the halogenated 3-pyrrolyl BODIPYs were highly efficient in generating singlet oxygen. A linear relationship was observed between the singlet oxygen quantum yields and the number of bromides at the pyrrole carbons of 3-pyrrolyl BODIPY. Furthermore, DFT and TD-DFT studies supported these experimental observations.

Effects of Nitro-Substitution on the Spectroscopic and Self-Assembly Properties of BODIPY Dyes

A series of boron dipyrromethene (BODIPY) dyes were nitrated in high yields using nitronium tetrafluoroborate at positions 2, 3, and 2,6 of the BODIPY core. This method allows for the regioselective nitration of the pyrrolic positions under milder conditions than previously reported methods. The photophysical properties and electronic transitions of these BODIPYs were investigated by using UV-vis spectroscopy, fluorescence spectroscopy, and density-functional theory (DFT) calculations. The introduction of one nitro group dramatically increases the dipole moment of the molecule, induces marked blue shifts in the absorption and emission bands, decreases the molar absorptivity, and increases the Stokes shifts of the BODIPYs. When a second nitro group is symmetrically introduced, the calculated dipole moments of the BODIPYs decrease in both the ground and excited states. Our studies show that the spectroscopic and self-assembly properties of nitro-substituted BODIPYs are highly dependent on solvent polarity and polarizability. In a polar organic solvent, nitro-substitution tends to quench the characteristic fluorescence of BODIPYs, while in a nonpolar solvent, significantly higher absolute fluorescence quantum yields are observed. On the other hand, aggregates are formed in aqueous solution, as observed by atomic force microscopy (AFM). Our results suggest a potential application of nitro-BODIPYs as polarity sensors.

Dual emissive ytterbium(III) complexes with π-conjugated BODIPY-bipyridine ligands

Four BODIPY-functionalized bipyridine ligands (B1, B2, B3 and B4) were synthesized for sensitizing the near-infrared emission of Yb(III) ions. In these ligands, a BODIPY moiety was conjugated to 2,2'-bipyridine through an acetylene spacer at its C2 position, whereas its C6 position was substituted by H (B1), iodine (B2), 4-carboxylphenylacetylene (B3) or 4-thiocyanophenylacetylene (B4). The ligands exhibit strong absorption in the visible region and readily form stable complexes with ytterbium(III) trishexafluoroacetylacetonate (hfac-) hydrate in dichloromethane. Single-crystal X-ray diffraction analysis for Yb-B2 showed that the BODIPY unit almost falls into the bipyridine plane with Yb(III) being coordinated by six O from hfac- and two N atoms from bipyridine. All complexes exhibit strong absorption between 530 and 570 nm and can sensitize the ytterbium(III) for its emission at 980 nm under the UV-VIS light illumination. Interestingly, the visible emission from the complex increased when excited at 375 nm, making the complexes dual emissive, which is due to the increased absorption of the complex at 375 nm and inefficient energy transfer from the BODIPY moiety to the Yb(III) ion.

Thionated Coumarins: Study of the Intersystem Crossing and the Zero-field Splitting of the Triplet State Using Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectroscopies

To study the effect of thionation of the carbonyl groups in a chromophore, i. e. replacing the O atom with S atom, on the photophysics, we studied two thionated coumarin derivatives (Cou-S and Cou-6-S) with various steady state and transient spectroscopic methods. Both compounds exhibit red-shifted absorption (up to 4900 cm-1) and strong fluorescence quenching as compared to the unthionated analogues. Femtosecond transient absorption spectra show fast ISC (ca. 10 ps) in the thionated coumarin derivatives, while negligible ISC was observed in the unthionated coumarin. Interestingly, triplet excited state lifetimes of the thionated coumarin (0.14 μs) is much shorter than the unthionated analogues (53.4 μs). Time-resolved electron paramagnetic resonance (TREPR) spectra indicate much larger zero field splitting (ZFS) D parameters (up to 0.287 cm-1) for the T1 state of the thionated coumarins than the unthionated analogues (D=0.1001 cm-1). This large D value is attributed to the strong spin orbital coupling effect. These results demonstrate the advantage and the drawback of thionation-enhanced ISC, i. e. the ISC is efficient, but triplet state lifetimes become substantially shorter. This information is useful for the future design of heavy atom-free triplet photosensitizers for photodynamic therapy, photon upconversion, photocatalytic organic synthesis and photopolymerization, etc.

A Novel BODIPY-Zn Complex as Innovative Sonosensitizer for Enhanced Sonodynamic Therapy

Ultrasound (US)-based sonodynamic therapy (SDT) presents a promising and secure approach to treating cancer with the advantage of enhanced tissue penetration, making it a favorable option compared with traditional photodynamic therapy. However, the search for innovative sonosensitizers that exhibit both high sonosensitizing efficacy and good biocompatibility poses a formidable challenge. In this research, we prepared a novel BODIPY-Zn complex (BSS-Zn) incorporating a hydrophilic short polyethylene glycol unit and explored its feasibility as a sonosensitizer. BSS-Zn exhibited enhanced reactive oxygen species (ROS) generation behavior upon US irradiation, outperforming a control sensitizer, BSS (an analog lacking the Zn complex), and a commercial sonosensitizer, ZnPc (currently undergoing clinical testing), with regard to sonosensitizing properties. The enhanced effect of BSS-Zn was attributed to increased levels of ROS, such as hydroxyl radicals, singlet oxygen, and superoxide, mediated by US exposure in aqueous media. The SDT effect of BSS-Zn on MDA-MB-231 cells was verified by confirming the intracellular types of generated ROS and evaluating the cytotoxicity to MDA-MB-231 cancer cells. This pioneering study highlights the potential of BSS-Zn as an innovative sonosensitizer for SDT. Our findings provide valuable guidance for the design of efficient sonosensitizers.

Ring-fused BODlPY derived heavy-atom-free triplet photosensitizers

Triplet photosensitizers are compounds that demonstrate strong absorption of the excitation light, high intersystem crossing (ISC) efficiency for efficient triplet state generation, and long triplet lifetimes to facilitate subsequent photochemical reactions. Among these, heavy-atom-free triplet photosensitizers have attracted particular attention due to their advantages of long-lived triplet states and low dark toxicity in comparison with heavy-atom-containing photosensitizers. Owing to the superior photophysical and chemical characteristics, boron dipyrromethene (BODIPY) dyes have been developed as promising heavy-atom-free triplet photosensitizers through specific molecular design strategies. However, many heavy-atom-free BODIPY-based photosensitizers exhibit relatively short excitation wavelengths in the visible-light region, and their ISC efficiencies dropped significantly with the extension of π-conjugation via the Knoevenagel condensation reaction. Recently, the ring-fused BODIPY skeleton has provided a feasible approach for the design of long-wavelength NIR photosensitizers. This review provides a comprehensive summary of the strategies utilized for the construction of ring-fused BODIPY-based photosensitizers including the installation of a twisted π-conjugation framework, the fusion of thiophene moieties and the formulation of an aggregation-induced ISC process. Meanwhile, some important spectroscopic and photophysical properties of these photosensitizers, along with their related applications, are also described.

pH-Sensitive and Lysosome Targetable Photosensitizers Based on BODIPYs

Photodynamic therapy (PDT) is an effective and U.S. Food and Drug Administration (FDA) approved treatment for cancer and other diseases. Photosensitizer is one of the three key components that harvest the energy of light at a certain wavelength. Compared to the conventional fluorophores used as photosensitizers, boron dipyrromethene (BODIPY) derivatives have grown fast in recent years due to their low dark toxicity, versatile tunable sites, and easiness of being paired with other treatments. In this paper, two pH-sensitive BODIPY-based photosensitizers (BDC and BDBrC) were synthesized by adding carbazole moieties onto the BODIPY cores (BD and BDBr) through condensation reactions. BDBrC has two Br atoms at the BODIPY core that promote singlet oxygen generation and further red-shift the absorption maximum peak. Both compounds showed sensitivity toward pH change and generated more singlet oxygen under acidic conditions. The cellular uptake and cell imaging experiments showed that BDBrC can selectively target the lysosome organelle. The further dark cell viability and light cytotoxicity indicate the light triggered PDT treatment can be accomplished with BDBrC.

Unveiling the Centrosymmetric Effect in the Design of Narrowband Fluorescent Emitters: From Single to Double Difluoroboron Cores

Narrowband fluorescent emitters are receiving significant attention due to the great potential for creating ultrahigh-definition organic light-emitting diode displays (UHD-OLED). Unveiling innovative mechanisms to design new high-performance narrowband fluorescent emitters is a concerted endeavor in both academic and industrial circles. Theoretical calculations reveal that the centrosymmetric dianilido-bipyridine boron difluoride framework (cs-DAPBF2) exhibits significantly reduced structural relaxation compared to previously reported asymmetric structures with monofluoroboron cores, creating new opportunities for the development of narrowband fluorescent emitters. In this work, we present a dual chelation-assisted C-H/C-H homocoupling strategy to efficiently synthesize the 3,3'-amino-2,2'-bipyridine skeleton, enabling the straightforward construction of a series of symmetric cs-DAPBF2-based fluorescent emitters. Through molecular optimization, we have developed a high-performance narrowband green fluorescent emitter, cs-DMeAPBF2-MP, which demonstrates a narrow full width at half-maximum (fwhm) of 20 nm, a high photoluminescence quantum yield (ΦPL) of 98%, a large molar absorptivity (ε) of 2.10 × 104 M-1 cm-1, and a high horizontal dipole ratio (Θ//) of 77%. These properties make cs-DMeAPBF2-MP a promising candidate for fabricating high-efficiency, narrowband green organic light-emitting diodes (OLEDs) with minimal efficiency roll-off. This study represents the first successful application of the DAPBF2 architecture in the design of narrowband fluorescent emitters for high-performance OLEDs.

Glycosylated AIE-active Red Light-triggered Photocage with Precisely Tumor Targeting Capability for Synergistic Type I Photodynamic Therapy and CPT Chemotherapy

Photocaging is an emerging protocol for precisely manipulating spatial and temporal behaviors over biological activity. However, the red/near-infrared light-triggered photolysis process of current photocage is largely singlet oxygen (1O2)-dependent and lack of compatibility with other reactive oxygen species (ROS)-activated techniques, which has proven to be the major bottleneck in achieving efficient and precise treatment. Herein, we reported a lactosylated photocage BT-LRC by covalently incorporating camptothecin (CPT) into hybrid BODIPY-TPE fluorophore via the superoxide anion radical (O2 -⋅)-cleavable thioketal bond for type I photodynamic therapy (PDT) and anticancer drug release. Amphiphilic BT-LRC could be self-assembled into aggregation-induced emission (AIE)-active nanoparticles (BT-LRCs) owing to the regulation of carbohydrate-carbohydrate interactions (CCIs) among neighboring lactose units in the nanoaggregates. BT-LRCs could simultaneously generate abundant O2 -⋅ through the aggregation modulated by lactose interactions, and DNA-damaging agent CPT was subsequently and effectively released. Notably, the type I PDT and CPT chemotherapy collaboratively amplified the therapeutic efficacy in HepG2 cells and tumor-bearing mice. Furthermore, the inherent AIE property of BT-LRCs endowed the photocaged prodrug with superior bioimaging capability, which provided a powerful tool for real-time tracking and finely tuning the PDT and photoactivated drug release behavior in tumor therapy.

Enhancing Phototoxicity in BODIPY-Perylene Charge Transfer Dyads by Combined Iodination and Mesylation

The uptake and phototoxicity of a family of BODIPY-perylene charge transfer dyads are compared in live cancer and non-cancer cell lines to evaluate their performance in imaging and photodynamic therapy (PDT). The impact of iodination and mesylation of the meso position of the compounds on their optical properties, cell uptake and toxicity are compared. Notably, across all derivatives the probes were minimally dark toxic up to 50 μM, (the maximum concentration tested), but exhibited outstanding phototoxicity with nanomolar IC50 values and impressive phototoxic indices (PI, ratio of dark IC50 to light IC50), with best performance for the mesylated iodinated derivative MB2PI, which had a PI of >218 and >8.9 in MCF-7 cells and tumour spheroids respectively. This is significantly higher than non-iodinated analogue MB2P in MCF-7 cells with an observed PI of >109 and slightly higher than MB2PI in spheroids with a PI of >8. This compound also showed interesting emission spectral variation with localisation that responded to stimulation of inflammation. Additional studies confirmed efficient singlet oxygen generation by the BODIPYs, suggesting a Type II mechanism of phototoxicity. Overall, the data indicates that combining charge transfer and iodination is an effective strategy for enhancing phototherapeutic capacity of BODIPY PS.

Moderately Heavy Atom-Substituted BODIPY Photosensitizer with Mitochondrial Targeting Ability for Imaging-Guided Photodynamic Therapy

Advanced photodynamic therapy requires photosensitizers with targeting, diagnostic, and therapeutic properties. To fulfill this multifunctionality, we report the synthesis of two triphenylphosphonium (TPP)-functionalized boron-dipyrromethene (BODIPY) dyes, TPPB-H and TPPB-Br, which incorporate a hydrogen atom and dibrominated vinyl moiety at the 6-position of the BODIPY core, respectively. The heavy-atom effect of the moderately heavy bromine atoms allowed TPPB-Br to achieve a proper balance between the toxic singlet oxygen (1O2) production and fluorescence efficiencies. In this dye, the bromine atom-induced stimulation of the singlet-to-triplet intersystem crossing dynamics resulted in an approximately 45-fold increase in the 1O2 quantum yield with respect to that of the nonbrominated counterpart (0.0059 and 0.28 for TPPB-H and TPPB-Br, respectively). This increase was accompanied only a 2-fold reduction in the fluorescence quantum yield (0.54 and 0.22 for TPPB-H and TPPB-Br, respectively). During multicolor confocal laser scanning microscopy observations conducted using two carcinomas, MCF-7 and HeLa, both BODIPY dyes exhibited high targeting specificity toward cancer cell mitochondria owing to the TPP cation functionalization. The two dyes also showed the feasibility of fluorescence cell imaging; however, only the dibrominated BODIPY TPPB-Br manifested pronounced photocytotoxicity with half-maximal inhibitory concentrations of 0.12 and 0.77 μM obtained for MCF-7 and HeLa cells, respectively. These findings demonstrate the potential applicability of TPPB-Br as an imaging-guided photodynamic therapy agent with mitochondrial specificity.

Efficient light-induced reactive oxygen species production from a far-red ER-targeting BODIPY dye

Generation of reactive oxygen species (ROS) within the ER evokes stress leading to immunogenic cell death. A red light activated BODIPY dye capable of subcellular localization within the ER producing high quantum yields of ROS is reported. The ability of this dye to act as a photodynamic therapy (PDT) agent in breast cancer cells suggests promising organelle-targeted therapeutics.

Enhanced Singlet Oxygen Generation in Aggregates of Naphthalene-Fused BODIPY and Its Application in Photodynamic Therapy

Several reports are available on aggregation-induced emission and its applications in biomedical imaging and other material sciences. However, enhancement of singlet oxygen generation in nanoaggregates is rarely reported. Here, we report the synthesis of Naph-BODIPY Br2, which absorbs at 661 nm (monomer) with a high molar absorption coefficient. The presence of bromine promotes intersystem crossing, thereby enhancing the singlet oxygen quantum yield (ΦΔ ∼ 0.50 in methanol). In order to increase hydrophilicity, we developed Naph-BODIPY Br2 nanoaggregates (∼100 nm), which demonstrated aggregation-induced properties and exhibited a bathochromic shift with an absorption maximum at 757 nm. The bathochromic shift in the UV-vis spectra due to aggregation is corroborated by TD-DFT analysis. The computational data also confirm the presence of a low-lying triplet state, which enhances the generation of singlet oxygen, making it effective for photodynamic therapy. These aggregates showed excellent singlet oxygen generation in aqueous media, compared to their monomeric form and standard methylene blue. Their hydrophilic nature and high singlet oxygen generation enabled significant phototoxicity against human carcinoma cells with IC50 values of 4.06 ± 0.01 and 4.09 ± 0.1 μM, respectively, for MCF-7 and A549 cells upon 5 min exposure to light. Moreover, their phototoxicity further increases with an increasing exposure time of light for both cell lines. Notably, Naph-BODIPY Br2 nanoaggregates exhibited nearly zero dark cell toxicity and effectively induced apoptosis in cancer cells upon light activation, highlighting their potential as powerful photosensitizers for photodynamic cancer therapy.

Design and Synthesis of BODIPY and Its Application in Inhibiting Intestinal Flora

BODIPY-based photosensitizers were synthesized and tested for antimicrobial photodynamic therapy, revealing structural modifications enhancing the photodynamic therapy (PDT) effects. This research may lead to new PDT strategies for treating bacterial infections, including those resistant to traditional antibiotics, and offers insights into the prevention and treatment of Alzheimer's disease through gut microbiota regulation.

Versatile Metal-Free Arylation of BODIPY and Bis(BF2) Chromophores by Using Arylazosulfones in a Sunflow System

BODIPYs have a well-established role in biological sciences as chemosensors and versatile biological markers due to their chemical reactivity, which allows for fine-tuning of their photophysical characteristics. In this work, we combined the unique reactivity of arylazo sulfones with the advantages of a "sunflow" reactor to develop a fast, efficient, and versatile method for the photochemical arylation of BODIPYs and other chromophores. This approach resulted in red-shifted emitting fluorophores due to extended electronic delocalization at the 3- and 5-positions of the BODIPY core. This method represents an advantageous approach for BODIPY functionalization compared to existing strategies.

Potent BODIPY-based photosensitisers for selective mitochondrial dysfunction and effective photodynamic therapy

The development of new and improved mitochondria-targeting photosensitisers (PSs) for photodynamic therapy (PDT) remains highly desirable, due to the critical role the mitochondria play in maintaining healthy cellular function. Here, we report the design, synthesis, photophysical properties and biological characterisation of a series of di-iodinated BODIPY-based PSs, BODIPY-Mito-I-n, for mitochondria-targeted PDT applications. Six BODIPY-Mito-I-n analogues were synthesised in good yields, with fast reaction times of between 30 and 60 min under mild conditions. The di-iodination of the BODIPY scaffold enabled highly efficient population of the triplet state, leading to high singlet oxygen (1O2) photosensitisation efficiencies (ΦΔ = 0.55-0.65). All BODIPY-Mito-I-n compounds exhibited very high photocytotoxic activity towards HeLa cells, with IC50,light values of between 1.30 and 6.93 nM, due to photoinduced 1O2 generation. Notably, the poly(ethylene glycol) (PEG)-modified BODIPY-Mito-I-6 showed remarkably lower dark cytotoxicity (IC50,dark = 6.68-7.25 μM) than the non-PEGylated analogues BODIPY-Mito-I-1 to BODIPY-Mito-I-5 (IC50,dark = 0.58-1.09 μM), resulting in photocytotoxicity indices up to 2120. Mechanistic studies revealed that BODIPY-Mito-I-6 induced reactive oxygen species overproduction and mitochondrial dysfunction in cells upon irradiation, leading to significant cell death through a combination of apoptosis and necrosis. It is anticipated that our design will contribute to the development of more effective mitochondria-targeting PSs for cancer therapy.

Predicting fluorescence to singlet oxygen generation quantum yield ratio for BODIPY dyes using QSPR and machine learning

Functional dyes that are capable of both bright fluorescence and efficient singlet oxygen generation are crucial for theranostic techniques, which integrate fluorescence imaging and photodynamic therapy (PDT). The development of new functional dyes for theranostics is often costly and time-consuming due to laborious synthesis and post-synthetic screening of large libraries of compounds. In this work, we describe machine learning methods suitable for simultaneous prediction of fluorescence and photosensitizing ability of heavy-atom-free boron dipyrromethene (BODIPY) compounds. We analysed the ratio between fluorescence quantum yield (ΦFl) and singlet oxygen quantum yield (ΦΔ) for over 70 BODIPY structures in polar (acetonitrile) and non-polar (toluene) solvents, which mimic hydrophilic and hydrophobic cell environments, respectively. QSPR models were developed based on more than 5000 calculated molecular descriptors, including quantum chemical and topological descriptors. We applied multiple linear regression (MLR), support vector regression (SVR), and random forest regression (RFR) methods for model building and optimization. The resulting models demonstrated robust statistical parameters (R2 = 0.73-0.91) for both polar and non-polar media. The relative contributions of the descriptors to the models were assessed, identifying Eig03_EA(dm), F01[C-N], and TDB06p as the most influential. These results demonstrate that QSPR machine learning methods are effective in predicting key photochemical parameters of BODIPY photosensitizers, thereby potentially streamlining the development of theranostic agents.

Asymmetric BODIPY Dyes Enabling Triplet-Triplet Annihilation Upconversion

The construction of triplet-triplet annihilation upconversion (TTA-UC) systems with upconversion (UC) emission efficiency at low power densities is still under continuing exploration. From an environmental point of view, the utilization of purely organic pairs is more beneficial than the involvement of transition-metal complexes. In this context, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes, which can be found in a wide range of applications, have been previously used as suitable sensitizers in TTA-UC systems. The versatility of these scaffolds makes them magnificent objectives for designing and synthesizing potential entities with different target abilities. Herein, we prepared several asymmetric BODIPY dyes with excellent optical properties to be applied to a bimolecular TTA-UC system. In the presence of 2,5,8,11-tetra-tert-butylperylene (TBPe) as a suitable annihilator, a green-to-blue light conversion was clearly observed by means of detailed spectroscopic investigations. The results revealed a high UC emission efficiency (ηUC) of ∼8%, together with a low threshold intensity (I th) of ∼40-50 mW/cm2. All data indicated that these asymmetric BODIPY dyes were ideal sensitizers for TTA-UC, providing a particular design for further investigations.

Synthesis and Photodynamic Activities of Pyridine- or Pyridinium-Substituted Aza-BODIPY Photosensitizers

In this work, various novel pyridinyl- and pyridinium-modified Aza-BODIPY PSs were designed and constructed based on monoiodo Aza-BODIPY PSs (BDP-4 and BDP-15) in an attempt to construct "structure-inherent organelles-targeted" PSs to endow potential organelle-targeting ability. Pyridinyl PSs displayed potent photodynamic efficacy, and monorigidified PSs were very effective. The monorigidified PS 20 with meta-pyridinyl moiety displayed the most potent photoactivity and negligible dark toxicity with a favorable dark/phototoxicity ratio (>4800). To our surprise, monorigidified PS with meta-pyridinyl moiety (e.g., 20) was lipid droplet-targeted. 20 showed good cellular uptake and intracellular ROS generation compared with BDP-15. The preliminary cell death process exploration indicated that 20 resulted in lipid peroxidation and induced cell death through an iron-independent ferroptosis-like cell death pathway. In vivo antitumor efficacy experiments manifested that 20 significantly inhibited tumor growth and outperformed BDP-15 and Ce6 even under a single low-dose light irradiation (30 J/cm2).

Polyamine Derived Photosensitizer: A Novel Approach for Photodynamic Therapy of Cancer

Polyamines play a pivotal role in cancer cell proliferation. The excessive polyamine requirement of these malignancies is satisfied through heightened biosynthesis and augmented extracellular uptake via the polyamine transport system (PTS) present on the cell membrane. Meanwhile, photodynamic therapy (PDT) emerges as an effective anti-cancer treatment devoid of drug resistance. Recognizing these intricacies, our study devised a novel polyamine-derived photosensitizer (PS) for targeted photodynamic treatment, focusing predominantly on pancreatic cancer cells. We synthesized and evaluated novel spermine-derived fluorescent probes (N2) and PS (N3), exhibiting selectivity towards pancreatic cancer cells via PTS. N3 showed minimal dark toxicity but significant phototoxicity upon irradiation, effectively causing cell death in vitro. A significant reduction in tumor volume was observed post-treatment with no pronounced dark toxicity using the pancreatic cancer CDX mouse model, affirming the therapeutic potential of N3. Overall, our findings introduce a promising new strategy for cancer treatment, highlighting the potential of polyamine-derived PSs in PDT.

A toolbox for enzymatic modification of nucleic acids with photosensitizers for photodynamic therapy

Photodynamic therapy (PDT) is an approved cancer treatment modality. Despite its high efficiency, PDT is limited in terms of specificity and by the poor solubility of the rather lipophilic photosensitizers (PSs). In order to alleviate these limitations, PSs can be conjugated to oligonucleotides. However, most conjugation methods often involve complex organic synthesis and result in the appendage of single modifications at the 3'/5' termini of oligonucleotides. Here, we have investigated the possibility of bioconjugating a range of known PSs by polymerase-mediated synthesis. We have prepared a range of modified nucleoside triphosphates by different conjugation methods and investigated the substrate tolerance of these nucleotides for template-dependent and -independent DNA polymerases. This method represents a mild and versatile approach for the conjugation of single or multiple PSs onto oligonucleotides and can be useful to further improve the efficiency of the PDT treatment.

A BODIPY-Naphtholimine-BF2 Dyad for Precision Photodynamic Therapy, Targeting, and Dual Imaging of Endoplasmic Reticulum and Lipid Droplets in Cancer

Currently, effective therapeutic modalities for pancreatic ductal adenocarcinoma (PDAC) are quite limited, leading to gloomy prognosis and ∼6-months median patient survival. Recent advances showed the promise of photodynamic therapy (PDT) for PDAC patients. Next generation photosensitizers (PS) are based on "organelle-targeted-PDT" and provide new paradigm in the field of precision medicines to address the current challenge for treating PDAC. In this investigation, we have constructed a novel PS, named as N b B, for precise and simultaneous targeting of endoplasmic reticulum (ER) and lipid droplets (LDs) in PDAC, based on the fact that malignant PDAC cells are heavily relying on ER for hormone synthesis. Our live cell imaging and fluorescence recovery after photobleaching (FRAP) experiments revealed that N b B is quickly targeted to ER and subsequently to LDs and shows simultaneous dual fluorescence color due to polar and nonpolar milieu of ER and LDs. Interestingly, the same molecule generates triplet state and singlet oxygen efficiently and causes robust ER stress and cellular lipid peroxidation, leading to apoptosis in two different PDAC cells in the presence of light. Together, we present, for the first time, a potential next generation precision medicine for ER-LD organelle specific imaging and PDT of pancreatic cancer.

Tuning the Flavin Core via Donor Appendage for Selective Subcellular Bioimaging and PDT Application

Herein, we report a novel flavin analogue as singular chemical component for lysosome bioimaging, and inherited photosensitizer capability of the flavin core was demonstrated as a promising candidate for photodynamic therapy (PDT) application. Fine-tuning the flavin core with the incorporation of methoxy naphthyl appendage provides an appropriate chemical design, thereby offering photostability, selectivity, and lysosomal colocalization, along with the aggregation-induced emissive nature, making it suitable for lysosomal bioimaging applications. Additionally, photosensitization capability of the flavin core with photostable nature of the synthesized analogue has shown remarkable capacity for generating reactive oxygen species (ROS) within cells, making it a promising candidate for photodynamic therapy (PDT) application.

Naphtho[1,8-ef]isoindole-7,8,10(9H)-trione as Novel Theranostic Agents for Photodynamic Therapy and Multi-Subcellular Organelles Localization

A series of naphtho[1,8-ef]isoindole-7,8,10(9H)-trione derivatives as novel theranostic agents for photodynamic therapy and multi-subcellular organelles localization were designed and synthesized. Most of them possess moderate fluorescence quantum yield and long wavelength absorption simultaneously, which made them possible for dual effects of imaging and therapy. Notably, compounds 7 b and 7 d exhibited significant light-toxicity but slight dark-toxicity. Confocal fluorescence microscopy experiments demonstrated that compound 7 b can locate and image in special multi-subcellular organelles. All the research results implied that naphtho[1,8-ef] isoindole-7,8,10(9H)-trione derivatives can be applied as a new series of theranostic agents with the characteristics of photodynamic therapy and multi-subcellular organelles imaging.

Enzyme-Activatable Near-Infrared Hemicyanines as Modular Scaffolds for in vivo Photodynamic Therapy

Photodynamic therapy is an anti-cancer treatment that requires illumination of photosensitizers to induce local cell death. Current near-infrared organic photosensitizers are built from large and non-modular structures that cannot be tuned to improve safety and minimize off-target toxicity. This work describes a novel chemical platform to generate enzyme-activatable near-infrared photosensitizers. We optimized the Se-bridged hemicyanine scaffold to include caging groups and biocompatible moieties, and generated cathepsin-triggered photosensitizers for effective ablation of human glioblastoma cells. Furthermore, we demonstrated that enzyme-activatable Se-bridged hemicyanines are effective photosensitizers for the safe ablation of microtumors in vivo, creating new avenues in the chemical design of targeted anti-cancer photodynamic therapy agents.

Boron Dipyrromethene-Based Nanotheranostic System for Sonophotoassisted Therapy and Simultaneous Monitoring of Tumor Immune Microenvironment Reprogramming

Therapy-induced modulation of the tumor microenvironment (TME) to overcome the immunosuppressive TME is considered to be an opportunity for cancer treatment. However, monitoring of TME modulation during the therapeutic process to accurately determine immune responses and adjust treatment plans in a timely manner remains to be challenging. Herein, we report a carrier-free nanotheranostic system (CANPs) assembled by two boron dipyrromethene (BODIPY) dyes, a sonophotosensitizer C-BDP, and a nitric oxide (NO) probe amino-BODIPY (A-BDP). CANPs can exert combined sonophototherapeutic effects of C-BDP under ultrasound and light irradiation and simultaneously induce inflammatory TME, as well as emit bright fluorescence via A-BDP by monitoring tumor-associated macrophages (TAMs) repolarization through the released NO in vitro and in vivo. Of note, transforming growth factor-β (TGF-β) could be the key cytokine involved in the sonophototherapy-induced TME reprogramming. By virtue of high physiological stability, good biocompatibility, and effective tumor targetability, CANPs could be a potential nanotheranostic system for the simultaneous induction and detection of TME reprogramming triggered by sonophototherapy.

Radical enhanced intersystem crossing mechanism, electron spin dynamics of high spin states and their applications in the design of heavy atom-free triplet photosensitizers

Heavy atom-free triplet photosensitizers (PSs) can overcome the high cost and biological toxicity of traditional molecular systems containing heavy atoms (such as Pt(II), Ir(III), Ru(II), Pd(II), Lu(III), I, or Br atoms) and, therefore, are developing rapidly. Connecting a stable free radical to the chromophore can promote the intersystem crossing (ISC) process through electron spin exchange interaction to produce the triplet state of the chromophore or the doublet (D) and quartet (Q) states when taking the whole spin system into account. These molecular systems based on the radical enhanced ISC (REISC) mechanism are important in the field of heavy atom-free triplet PSs. The REISC system has a simple molecular structure and good biocompatibility, and it is especially helpful for building high-spin quantum states (D and Q states) that have the potential to be developed as qubits in quantum information science. This review introduces the molecular structure design for the purpose of high-spin states. Time-resolved electron paramagnetic resonance (TREPR) is the most important characterization method to reveal the properties of these molecular systems, generation mechanism and electron spin polarization (ESP) of the high spin states. The spin polarization manipulation of high spin states and potential application in the field of quantum information engineering are also summarized. Moreover, molecular design principles of the REISC system to obtain long absorption wavelength, high triplet state quantum yield and long triplet state lifetime are introduced, as well as applications of the compounds in triplet-triplet annihilation upconversion, photodynamic therapy and bioimaging. This review is useful for the design of heavy atom-free triplet PSs based on the radical-chromophore molecular structure motif and the study of the photophysics of the compounds, as well as the electron spin dynamics of the multi electron system upon photoexcitation.

Shedding light on imaging safety: Decoding the origin of photocytotoxicity in RhB-assisted fluorescence imaging

Photocytotoxicity represents a significant limitation in the application of dye-assisted fluorescence imaging (FI), often resulting in undesirable cellular damage or even cell death, thereby restricting their practical utility. The prevalence of Rhodamine B (RhB) in FI underscores the importance of elucidating its photocytotoxicity effects to minimize photodamage. This study identifies the primary cause of photocytotoxicity stems from the generation of cytotoxic singlet oxygen in RhB, utilizing femtosecond transient absorption spectroscopy coupled with quantum chemical calculations. The Laser power-dependent cellular viability reveals a threshold at about 50 mW cm-2, surpassing which produces pronounced photocytotoxicity in vitro and in vivo. Notably, this threshold significantly falls below the safety limits (<200 mW cm-2) for laser use in health care, implying a huge risk of photodamage. This study provides valuable insights into the photocytotoxicity and offers essential guidelines for developing safer imaging protocols.

Charge Transfer-Promoted Excited State of a Heavy-Atom-Free Photosensitizer for Efficient Application of Mitochondria-Targeted Fluorescence Imaging and Hypoxia Photodynamic Therapy

Conventional photosensitizers (PSs) used in photodynamic therapy (PDT) have shown preliminary success; however, they are often associated with several limitations including potential dark toxicity in healthy tissues, limited efficacy under acidic and hypoxic conditions, suboptimal fluorescence imaging capabilities, and nonspecific targeting during treatment. In response to these challenges, we developed a heavy-atom-free PS, denoted as Cz-SB, by incorporating ethyl carbazole into a thiophene-fused BODIPY core. A comprehensive investigation into the photophysical properties of Cz-SB was conducted through a synergistic approach involving experimental and computational investigations. The enhancement of intersystem crossing (kISC) and fluorescence emission (kfl) rate constants was achieved through a donor-acceptor pair-mediated charge transfer mechanism. Consequently, Cz-SB demonstrated remarkable efficiency in generating reactive oxygen species (ROS) under acidic and low-oxygen conditions, making it particularly effective for hypoxic cancer PDT. Furthermore, Cz-SB exhibited good biocompatibility, fluorescence imaging capabilities, and a high degree of localization within the mitochondria of living cells. We posit that Cz-SB holds substantial prospects as a versatile PS with innovative molecular design, representing a potential "one-for-all" solution in the realm of cancer phototheranostics.

Synthesis and optical properties of antimony(V) complexes of a trianionic N2O2-type tetradentate dipyrrin ligand

LSbCl2, an air- and moisture-stable antimony(V)-dipyrrin complex at room temperature, was obtained by treating an excess amount (20 eq.) of SbCl3 with a trianionic N2O2-type tetradentate dipyrrin ligand (L) under visible-light irradiation and O2 atmosphere. The Cl ligands in LSbCl2 were replaced by OH ligands via hydrolysis, yielding LSb(OH)2. Further, the molecular structures and optical properties of the Sb(V)-dippyrin complexes were investigated. While LSbCl2 was non-fluorescent, LSb(OH)2 exhibited an intense red fluorescence with a photoluminescence quantum yield of 68%.

Polymer Dots with Delayed Fluorescence and Tunable Cellular Uptake for Photodynamic Therapy and Time-Gated Imaging

By combining bioimaging and photodynamic therapy (PDT), it is possible to treat cancer through a theranostic approach with targeted action for minimum invasiveness and side effects. Thermally activated delayed fluorescence (TADF) probes have gained recent interest in theranostics due to their ability to generate singlet oxygen (1O2) while providing delayed emission that can be used in time-gated imaging. However, it is still challenging to design systems that simultaneously show (1) high contrast for imaging, (2) low dark toxicity but high phototoxicity and (3) tunable biological uptake. Here, we circumvent shortcomings of TADF systems by designing block copolymers and their corresponding semiconducting polymer dots (Pdots) that encapsulate a TADF dye in the core and expose an additional boron-dipyrromethene (BODIPY) oxygen sensitizer in the corona. This architecture provides orange-red luminescent particles (ΦPL up to 18 %) that can efficiently promote PDT (1O2 QY=42 %) of HeLa cells with very low photosensitizer loading (IC50 ~0.05-0.13 μg/mL after 30 min). Additionally, we design Pdots with tunable cellular uptake but similar PDT efficiencies using either polyethylene glycol or guanidinium-based coronas. Finally, we demonstrate that these Pdots can be used for time-gated imaging to effectively filter out background fluorescence from biological samples and improve image contrast.

Triphenylphosphonium-functionalized dimeric BODIPY-based nanoparticles for mitochondria-targeting photodynamic therapy

The dimerization of boron dipyrromethene (BODIPY) moieties is an appealing molecular design approach for developing heavy-atom-free triplet photosensitizers (PSs). However, BODIPY dimer-based PSs generally lack target specificity, which limits their clinical use for photodynamic therapy. This study reports the synthesis of two mitochondria-targeting triphenylphosphonium (TPP)-functionalized meso-β directly linked BODIPY dimers (BTPP and BeTPP). Both BODIPY dimers exhibited solvent-polarity-dependent singlet oxygen (1O2) quantum yields, with maximum values of 0.84 and 0.55 for BTPP and BeTPP, respectively, in tetrahydrofuran. The compact orthogonal geometry of the BODIPY dimers facilitated the generation of triplet excited states via photoinduced charge separation (CS) and subsequent spin-orbit charge-transfer intersystem crossing (SOCT-ISC) processes and their rates were dependent on the energetic configuration between the frontier molecular orbitals of the two BODIPY subunits. The as-synthesized compounds were amphiphilic and hence formed stable nanoparticles (∼36 nm in diameter) in aqueous solutions, with a zeta potential of ∼33 mV beneficial for mitochondrial targeting. In vitro experiments with MCF-7 and HeLa cancer cells indicated the effective localization of BTPP and BeTPP within cancer-cell mitochondria. Under light irradiation, BTPP and BeTPP exhibited robust photo-induced therapeutic effects in both cell lines, with half-maximal inhibitory concentration (IC50) values of ∼30 and ∼55 nM, respectively.

Photopharmacology of Protease Inhibitors: Current Status and Perspectives

Proteases are involved in many essential biological processes. Dysregulation of their activity underlies a wide variety of human diseases. Photopharmacology, as applied on various classes of proteins, has the potential to assist protease research by enabling spatiotemporal control of protease activity. Moreover, it may be used to decrease side-effects of protease-targeting drugs. In this review, we discuss the current status of the chemical design of photoactivatable proteases inhibitors and their biological application. Additionally, we give insight into future possibilities for further development of this field of research.

Unsymmetrical Benzothieno-Fused BODIPYs as Efficient NIR Heavy-Atom-Free Photosensitizers

Heavy-atom-free photosensitizers are potentially suitable for use in photodynamic therapy (PDT). In this contribution, a new family of unsymmetrical benzothieno-fused BODIPYs with reactive oxygen efficiency up to 50% in air-saturated toluene was reported. Their efficient intersystem crossing (ISC) resulted in the generation of both 1O2 and O2-• under irradiation. More importantly, the PDT efficacy of a respective 4-methoxystyryl-modified benzothieno-fused BODIPY in living cells exhibited an extremely high phototoxicity with an ultralow IC50 value of 2.78 nM. The results revealed that the incorporation of an electron-donating group at the α-position of the unsymmetrical benzothieno-fused BODIPY platform might be an effective approach for developing long-wavelength absorbing heavy-atom-free photosensitizers for precision cancer therapy.

A multi-chromic boron trifluoride-pyridyl Lewis adduct

A boron trifluoride-pyridyl Lewis adduct is reported, which exhibits various types of chromism and high solid-state photoluminescence quantum yields, as well as excitation-dependent emission in the mechanically ground form. The facile synthetic approach offers a simple and potentially versatile strategy for inducing chromism in pyridyl ligands with donor moieties. We envisage this approach as having a dual benefit: simplicity and extensive applicability.

Combining Multiple Photosensitizer Modules into One Supramolecular System for Synergetic Enhanced Photodynamic Therapy

Photodynamic therapy (PDT), as an emerging cancer treatment, requires the development of highly desirable photosensitizers (PSs) with integrated functional groups to achieve enhanced therapeutic efficacy. Coordination-driven self-assembly (CDSA) would provide an alternative approach for combining multiple PSs synergistically. Here, we demonstrate a simple yet powerful strategy of combining conventional chromophores (tetraphenylethylene, porphyrin, or Zn-porphyrin) with pyridinium salt PSs together through condensation reactions, followed by CDSA to construct a series of novel metallo-supramolecular PSs (S1-S3). The generation of reactive oxygen species (ROS) is dramatically enhanced by the direct combination of two different PSs, and further reinforced in the subsequent ensembles. Among all the ensembles, S2 with two porphyrin cores shows the highest ROS generation efficiency, specific interactions with lysosome, and strong emission for probing cells. Moreover, the cellular and living experiments confirm that S2 has excellent PDT efficacy, biocompatibility, and biosafety. As such, this study will enable the development of more efficient PSs with potential clinical applications.

Lysosome-targeted Aza-BODIPY photosensitizers for anti-cancer photodynamic therapy

Developing effective near-infrared (NIR) photosensitizers (PSs) has been an attractive goal of photodynamic therapy (PDT) for cancer treatment. In this study, we synthesized N, N-diethylaminomethylphenyl-containing Aza-BODIPY photosensitizers and comprehensively investigated their photophysical/photochemical properties, as well as cell-based and animal-based anti-tumor studies. Among them, BDP 1 has strong NIR absorption at 680 nm and higher singlet oxygen yield in PBS which showed favorable pH-activatable and lysosome-targeting ability. BDP 1 could be easily taken up by tumor cells and showed negligible dark activity (IC50 > 50 μM), however strong phototoxicity upon exposure to light irradiation. The acceptable fluorescence emission from BDP 1 allowed convenient in vivo fluorescence imaging for organ distribution studies in mice. After PDT treatment with upon single time PDT treatment at the beginning using relatively low light dose (54 J/ cm2), BDP 1 (2 mg/kg, 0.1 mL) was found to have strong efficacy to inhibit tumor growth and even to ablate off tumor without causing body weight loss. Therefore, pH-activatable and lysosome-targeted PS may become an effective way to develop potent PDT agent.

Spin-Vibronic Coupling Controls the Intersystem Crossing of Iodine-Substituted BODIPY Triplet Chromophores

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes are extensively used in various applications of their triplet states, ranging from photoredox catalysis, through triplet sensitization to photodynamic therapy. However, the rational design of BODIPY triplet chromophores by ab initio modelling is limited by their strong interactions of spin, electronic and vibrational dynamics. In particular, spin-vibronic coupling is often overlooked when estimating intersystem crossing (ISC) rates. In this study, a combined experimental and theoretical approach using spin-vibronic coupling to correctly describe ISC in BODIPY dyes was developed. For this purpose, seven π-extended BODIPY derivatives with iodine atoms in different positions were examined. It was found that the heavy-atom effect of iodine atoms is site specific, causing high triplet yields in only some positions. This site-specific ISC was explained by El-Sayed rules, so both the contribution and character of the molecular orbitals involved in the excitation must be considered when predicting the ISC rates. Overall, the rational design of BODIPY triplet chromophores requires using (i) the high-quality electronic structure theory, including both static and dynamical correlations; and (ii) the two-component wave function Hamiltonian, and rationalizing; and (iii) ISC based on the character of the molecular orbitals of heavy atoms involved in the excitation, expanding El-Sayed rules beyond their traditional applications.

Exploring BODIPY derivatives as sonosensitizers for anticancer sonodynamic therapy

Sonodynamic therapy (SDT) is an emerging non-invasive and effective therapeutic modality for cancer treatment bearing benefit of deep tissue-penetration in comparison to photo-inspired therapy. However, exploring novel sonosensitizers with high sonosensitivity and desirable biosafety remains a significant challenge. Although boron dipyrromethene (BODIPY) dyes have been widely used in biomedical filed, no BODIPY-based sonosensitizers have been reported yet. Herein, we synthesized four BODIPY dyes (BDP1-BDP4) and investigated their potential applications in SDT. BDP4 exhibited superb sonosensitivity and high SDT efficiency against cancer cells and tumors in tumor-bearing mice. The types of the generated reactive oxygen species, cavitation effect, and cell apoptosis were investigated to figure out the sonodynamic therapeutic mechanisms of BDP4. This work for the first time demonstrates the potential of BODIPY dyes as novel sonosensitizers for SDT, which may pave an avenue for developing more efficient and safer sonosensitizers in future.

Singlet fission initiating organic photosensitizations

The feasibility of singlet fission (SF) in organic photosensitizers is investigated through spin-flip long-range corrected time-dependent density functional theory. This study focuses on four major organic photosensitizer molecules: benzophenone, boron-dipyrromethene, methylene blue, and rose bengal. Calculations demonstrate that all these molecules possess moderate [Formula: see text]-stacking energies and closely-lying singlet (S) and quintet (triplet-triplet, TT) excitations, satisfying the essential conditions for SF: (1) Near-degenerate low-lying S and (TT) excitations with a significant S-T energy gap, and (2) Moderate [Formula: see text]-stacking energy of chromophores, slightly higher than solvation energy, enabling dissociation for triplet-state chromophore generation. Moreover, based on the El-Sayed rule, intersystem crossing is found to simultaneously proceed at very slow rates in all these photosensitizers. This is attributed to the fact that the lowest singlet excitation of the monomers partly involves [Formula: see text] transitions alongside the main [Formula: see text] transitions. The proposed mechanisms are strongly substantiated by comparisons with experimental studies.

Host-Guest Photosensitizer of a Cationic BODIPY Derivative and Cucurbit[7]uril for High-Efficiency Visible Light-Induced Photooxidation Reactions

In recent years, there has been a notable surge of interest in the fields of organic and pharmaceutical research about photocatalysts (PCs) and photosensitizers (PSs). In this study, a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) molecule adorned with quaternary ammonium (TMB) functionality was meticulously designed and synthesized. This compound has remarkable characteristics such as exceptional water solubility, great optical qualities, and commendable photostability. It can form a 1:1 complex (TMB-CB[7]) with cucurbit[7]uril (CB[7]) through host-guest interactions in the aqueous solution and shows obvious fluorescence enhancement. The reactive oxygen species (ROS) including superoxide anion radical (O2·-) and singlet oxygen (1O2) generation ability of TMB-CB[7] were promoted compared with that of TMB in the aqueous solution. More interestingly, the ROS generated from TMB-CB[7] can be used as PCs for aerobic cross dehydrogenation coupling reactions and photooxidation reactions in water with high yields of 89 and 95%, respectively. Therefore, the utilization of a host-guest PS presents a novel and environmentally friendly approach for conducting photocatalyzed organic processes under ambient conditions using visible light.

Access to isoindole-derived BODIPYs by an aminopalladation cascade

Here, we present a new route to dyes of the BODIPY family. We first built up a N-Boc-protected dipyrromethene scaffold via an aminopalladation cascade. Subsequentially, the pyrrole moiety was deprotected and the BF2 unit inserted. Depending on the terminating reaction, BODIPYs with either aryl or alkynyl moieties were accessible.

Synthesis and biological evaluation of bi-modal BODIPY-conjugated Hoechst applicable for Auger-electron and photodynamic cancer therapy

Current therapeutic approaches to cancer are not fully effective, and so development of more effective treatment is needed. Auger-electron therapy and photodynamic therapy have attracted marked attentions as a promising strategy in cancer treatment. In this study, we synthesized [125I]BH-2/BH-2, which comprised Hoechst and 2,6-diiodo-substituted BODIPY, and evaluated its usefulness as a bi-modal agent for Auger-electron/photodynamic therapy by comparison with the previously reported compound [125I]BH/BH. [125I]BH-2 was obtained at a 13% radiochemical yield. [125I]BH-2 showed similar uptake into the nucleus to [125I]BH, suggesting that Hoechst can function as a nuclear localization tag. HeLa cell viabilities were reduced in both cells exposed to [125I]BH-2 and [125I]BH. γ-H2AX foci in HeLa cells exposed to [125I]BH-2 or [125I]BH were increased in a dose-dependent manner, indicating that DNA double-strand breaks may have occurred. No significant difference was observed between [125I]BH-2 and [125I]BH at these investigations. For PDT application, BH-2 showed a higher singlet oxygen quantum yield (ΦΔ) and caused superior photo-induced cytotoxicity in HeLa cells compared with BH. These results suggest that bi-modal [125I]BH-2/BH-2 can cause anti-tumor effects with Auger-electron and photodynamic therapy.

Efficient Spin-Orbit Charge-Transfer Intersystem Crossing and Slow Intramolecular Triplet-Triplet Energy Transfer in Bodipy-Perylenebisimide Compact Dyads and Triads

Bodipy (BDP)-perylenebisimide (PBI) donor-acceptor dyads/triad were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). For BDP-PBI-3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ =85 %) was observed than the bay-substituted derivative BDP-PBI-1 (ΦΔ =30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP-PBI-3, while for BDP-PBI-1, CS takes 2.8 ps. For triad BDP-PBI-2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long-lived 3 PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP-PBI-3 show that the CR gives upper triplet excited state (3 BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the 3 PBI* state is finally populated, indicating that upper triplet state is involved in SOCT-ISC. Time-resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT-ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S-T+1 /T0 states mixing.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

Pictet-Spengler synthesis of twisted quinoline-fused BODIPYs as heavy-atom-free photosensitizers

Singly and doubly quinoline-fused BODIPYs were effectively synthesized through a reaction sequence consisting of the reduction of nitrophenyl-substituted BODIPYs and subsequent Pictet-Spengler cyclization. The combination of the BODIPY core and fused quinoline rings imposed significantly twisted conformations in the quinoline-fused BODIPYs (around 20.0° deviation from coplanarity obtained from X-ray crystal structure analysis). These twisted BODIPYs showed significantly reduced LUMO, redshifted absorption/emission bands, high molar extinction coefficients and satisfactory reactive oxygen species generation efficiency up to 0.56, indicating potential use as heavy-atom-free photosensitizers.

Molecular Design of Highly Efficient Heavy-Atom-free NpImidazole Derivatives for Two-Photon Photodynamic Therapy and ClO- Detection

Two-photon photodynamic therapy (TP-PDT), as a treatment technology with deep penetration and less damage, provides a broad prospect for cancer treatment. Nowadays, the development of TP-PDT suffers from the low two-photon absorption (TPA) intensity and short triplet state lifetime of photosensitizers (PSs) used in TP-PDT. Herein, we propose some novel modification strategies based on the thionated NpImidazole (the combination of naphthalimide and imidazole) derivatives to make efforts on those issues and obtain corresponding fluorescent probes for detecting ClO- and excellent PSs for TP-PDT. Density functional theory (DFT) and time-dependent DFT (TD-DFT) are used to help us characterize the photophysical properties and TP-PDT process of the newly designed compounds. Our results show that the introduction of different electron-donating groups at the position 4 of NpImidazole can effectively improve their TPA and emission properties. Specifically, 3s with a N,N-dimethylamino group has a large triplet state lifetime (τ = 699 μs) and TPA cross section value (δTPA = 314 GM), which can effectively achieve TP-PDT; additionally, 4s (with electron-donating group 2-oxa-6-azaspiro[3.3]heptane in NpImidazole) effectively realizes the dual-function of a PS for TP-PDT (τ = 25,122 μs, δTPA = 351 GM) and a fluorescent probe for detecting ClO- (Φf = 29% of the product 4o). Moreover, an important problem is clarified from a microscopic perspective, that is, why the transition property of 3s and 4s (1π-π*) from S1 to S0 is different from that of 1s and 2s (1n-π*). It is hoped that our work can provides valuable theoretical clues for the design and synthesis of heavy-atom-free NpImidazole-based PSs and fluorescent probes for the detection of hypochlorite.

Redox-Triggered Activation of Heavy-Atom-Free Photosensitizer and Implications in Targeted Photodynamic Therapy

A strategy for a redox-activatable heavy-atom-free photosensitizer (PS) based on thiolated naphthalimide has been demonstrated. The PS exhibits excellent reactive oxygen species (ROS) generation in the monomeric state. However, when encapsulated in a disulfide containing bioreducible amphiphilic triblock copolymer aggregate (polymersome), the PS exhibits aggregation in the confined hydrophobic environment, which results in a smaller exciton exchange rate between the singlet and triplet excited states (TDDFT studies), and consequently, the ROS generation ability of the PS was almost fully diminished. Such a PS (in the dormant state)-loaded redox-responsive polymersome showed excellent cellular uptake and intracellular release of the PS in its active form, which enabled cell killing upon light irradiation due to ROS generation. In a control experiment involving aggregates of a similar block copolymer, but lacking the bioreducible disulfide linkage, no intracellular reactivation of the PS was noticed, highlighting the importance of stimuli-responsive polymer assemblies in the area of targeted photodynamic therapy.

Synthesis of N,O-bidentate organic difluoroboron complexes and their photophysical studies

We disclose a novel boron trifluoride induced C-H activation and difluoroboronation at room temperature, thus providing a straightforward gateway to a series of N,O-bidentate organic BF₂ complexes. The scope of the method is demonstrated with 24 examples. All the synthesized compounds exhibit fluorescence and some of them have large Stokes shifts.