Recent strategies to improve boron dipyrromethene (BODIPY) for photodynamic cancer therapy: an updated review

Oxoplatin-B, a cisplatin-based platinum(IV) complex with photoactive BODIPY for mitochondria specific "chemo-PDT" activity

Oxoplatin-B, a platinum(IV) complex [Pt(NH₃)₂Cl₂(L¹)(OH)] (1) of 4-methylbenzoic acid (HL¹) functionalized with 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) was prepared, characterized and its antitumor activity studied. [Pt(NH₃)₂Cl₂(L²)(OH)] (2) of 4-methylbenzoic acid (HL²) was studied as a control. Complex 1 showed an absorption band at 500 nm (ɛ = 4.34 × 10⁴ M^-1 cm^-1) and an emission band at 515 nm (λ_ex = 488 nm, Φ_F = 0.64) in 1% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (pH = 7.2). Visible light-induced (400-700 nm) generation of singlet oxygen was evidenced from 1,3-diphenylisobenzofuran titration study. Complex 1 showed photo-induced cytotoxicity in visible light (400-700 nm, 10 J cm^-2) against human breast cancer (MCF-7), cervical cancer (HeLa) and lung cancer (A549) cells (IC₅₀: 1.1-3.8 μM) while being less toxic in normal cells. Confocal imaging showed mitochondrial localization with additional evidence from platinum content from isolated mitochondria and 5,5,6,6'-tetrachloro-1,1',3,3' tetraethylbenzimi-dazoylcarbocyanine iodide (JC-1) assay. Cellular apoptosis was observed from Annexin-V-FITC (fluorescein isothiocyanate)/propidium iodide assay.

Photosensitizing Antivirals

Antiviral action of various photosensitizers is already summarized in several comprehensive reviews, and various mechanisms have been proposed for it. However, a critical consideration of the matter of the area is complicated, since the exact mechanisms are very difficult to explore and clarify, and most publications are of an empirical and "phenomenological" nature, reporting a dependence of the antiviral action on illumination, or a correlation of activity with the photophysical properties of the substances. Of particular interest is substance-assisted photogeneration of highly reactive singlet oxygen (1O2). The damaging action of 1O2 on the lipids of the viral envelope can probably lead to a loss of the ability of the lipid bilayer of enveloped viruses to fuse with the lipid membrane of the host cell. Thus, lipid bilayer-affine 1O2 photosensitizers have prospects as broad-spectrum antivirals against enveloped viruses. In this short review, we want to point out the main types of antiviral photosensitizers with potential affinity to the lipid bilayer and summarize the data on new compounds over the past three years. Further understanding of the data in the field will spur a targeted search for substances with antiviral activity against enveloped viruses among photosensitizers able to bind to the lipid membranes.

BODIPY-vinyl dibromides as triplet sensitisers for photodynamic therapy and triplet-triplet annihilation upconversion

We devised a new generation of halogen-based triplet sensitisers comprising geminal dibromides at the vinyl backbone of a BODIPY fluorophore. Incorporating geminal dibromides into the π-conjugation of BODIPY enhanced intersystem crossing due to the heavy atom effect, which in turn improved the extent of excited triplet states.

Substituent effects on opticalproperties of pyrrolizine-fused BOPYIN

Three new pyrrolizine-fused BOPYINs (DAB-H, DAB-OMe, DAB-ester) have been reported in 26-35% yield. The relationship between structures and optical spectra was investigated, which all the compounds show large Stokes Shift (3146-3884 cm^-1) and high quantum yield (up to 99%) in solvents. Among these dyes, the decoration of electron donating/withdrawing groups on indole, pyrrole and pyrrolizine units has a significant impact on optical properties, especially emission spectra. The results suggested that electron withdrawing group on pyrrole and pyrrolizine units has hypsochromic shift on emission spectra (DAB-H, DAB-OMe, DAB-ester versus DAB-1,4,5). The optimized structure, electron distribution on frontier molecular orbital, energy gap and simulated stick spectra of DABs are discussed by Density Functional Theory (DFT) calculation. We claim the agreement between the experimental and theoretical absorption spectra.

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.

A New BODIPY Material for Pure Color and Long Lifetime Red Hyperfluorescence Organic Light-Emitting Diode

A red fluorescent material, 1,3,7,9-tetrakis(4-(tert-butyl)phenyl)-5,5-difluoro-10-(2-methoxyphenyl)-5H-4l4,5l4-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinine (4tBuMB), as an emitting dopant in a thermally activated delayed fluorescence (TADF) sensitized hyperfluorescence organic light-emitting diode (HFOLED) is reported. The 4tBuMB shows a high photoluminescence quantum yield (PLQY) of 99% with an emission maximum at 620 nm and a full width at half-maximum (fwhm) of 31 nm in solution. Further, it shows a deep lowest unoccupied molecular orbital (LUMO) of 3.83 eV. Thus, two TADF materials, 4CzIPN and 4CzTPN, as sensitizing hosts, are selected on the basis of a suitable LUMO level and spectrum overlap with 4tBuMB. The fabricated HFOLED device with 4CzTPN as a sensitizing host and 4tBuMB as an emitting dopant shows a maximum external quantum efficiency (EQE), an emission maximum, an fwhm, and CIE coordinates of 19.4%, 617 nm, 44 nm, and (0.64, 0.36), respectively. The electroluminance performances of the 4CzTPN sensitized device are higher than those of the 4CzIPN-based device, which is attributed to a higher Förster resonance energy transfer (FRET) rate and reduced intersystem crossing/reverse intersystem crossing (ISC/RISC) cycles of the former. Also, the 4CzTPN-based HF device shows a longer device lifetime (LT₉₀) of 954 h than the 4CzIPN-baed device (LT₉₀ of 57 h) at 3000 cd m^-2. The higher device stability is due to the higher bond dissociation energies (BDEs) of 4CzTPN and 4tBuMB than that of 4CzIPN.

BODIPY-attached zinc(II) complexes of curcumin drug for visible light assisted photo-sensitization, cellular imaging and targeted PDT

Boron-dipyrromethene (BODIPY) based photosensitizers as porphyrinoids and curcumin as natural product possess exciting photophysical features suitable for theranostic applications, namely, imaging and photodynamic therapy (PDT). Limited aqueous solubility and insufficient physiological stability, however, reduce their efficacy significantly. We have designed a novel strategy to deliver these two unusable cytotoxins simultaneously in cancer cells and herein, report the synthesis, characterization and imaging-assisted photocytotoxicity of three zinc(II) complexes containing N³-donor dipicolylamine (dpa) ligands (L^1-3) and O,O-donor curcumin (Hcur) viz. [Zn(L¹)(cur)]Cl (1), [Zn(L²)(cur)]Cl (2) and [Zn(L³)(cur)]Cl (3), where L² and L³ have pendant fluorescent BODIPY and non-emissive di-iodo-BODIPY moieties. Metal chelation imparted remarkable biological stability (pH ∼7.4) to the respective ligands and induces significant aqueous solubility. These ternary complexes could act as replacements of the existing metalloporphyrin-based PDT photosensitizers as their visible-light photosensitizing ability is reinforced by the dual presence of blue light absorbing curcumin and green light harvesting BODIPY units. Complex 2 having emissive BODIPY unit L² and curcumin, showed mitochondria selective localization in HeLa, MCF-7 cancer cells and complex 3, the di-iodinated analogue of complex 2, exhibited type-I/II PDT activity via inducing apoptosis through mitochondrial membrane disruption in cancer cells while being significantly nontoxic in dark and to the healthy cells.

Design of BODIPY dyes as triplet photosensitizers: electronic properties tailored for solar energy conversion, photoredox catalysis and photodynamic therapy

BODIPYs are renowned fluorescent dyes with strong and tunable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields. Transition metal complexes are the most commonly used triplet photosensitisers, but, recently, the use of organic dyes has emerged as a viable and more sustainable alternative. By proper design, BODIPY dyes have been turned from highly fluorescent labels into efficient triplet photosensitizers with strong absorption in the visible region (from green to orange). In this perspective, we report three design strategies: (i) halogenation of the dye skeleton, (ii) donor-acceptor dyads and (iii) BODIPY dimers. We compare pros and cons of these approaches in terms of optical and electrochemical properties and synthetic viability. The potential applications of these systems span from energy conversion to medicine and key examples are presented.

Maloplatin-B, a Cisplatin-Based BODIPY-Tagged Mito-Specific "Chemo-PDT" Agent Active in Red Light

Maloplatin-B, a cisplatin-based complex, namely [Pt(A-BOD)(NH₃)₂](NO₃) (Pt-A-BOD) with a pendant boron-dipyrromethene (BODIPY) moiety, where HA-BOD is a methyl malonyl chloride derived monostyryl BODIPY ligand, was designed and developed as near-IR light (600-720 nm) organelle-targeting photodynamic therapy agent. The complex [Pt(acac)(NH₃)₂](NO₃) (Pt-Ac) was used as a control. Pt-A-BOD displayed an absorption band at 616 nm (ε = 2.9 × 10⁴ M^-1 cm^-1) in 10% dimethyl sulfoxide/Dulbecco's Modified Eagle's Medium (DMSO/DMEM, pH 7.2). This complex displayed a broad emission band within 650-850 nm with a λ_em value of 720 nm in 10% DMSO-DMEM (pH 7.2) upon excitation (λ_ex) at 615 nm with a large Stokes shift. The fluorescence quantum yield (Φ_F) value for Pt-A-BOD is 0.032 and for the ligand HA-BOD is 0.24. The BODIPY complex and ligand showed the formation of singlet oxygen as the ROS (reactive oxygen species) on irradiation with near-IR red light of 660 nm, as evidenced from a 1,3-diphenylisobenzofuran (DPBF) assay. The complex displayed remarkable apoptotic NIR light-induced PDT activity with half-maximum inhibitory concentration values (IC₅₀) of 1.6-2.4 μM in A549 lung and HeLa cervical cancer cells, while it was less active in the dark. The cellular ROS generation by the complex in red light was ascertained by a DCFDA (2',7'-dichlorofluorescein diacetate) assay. Cellular imaging showed its localization primarily in the mitochondria of A549 cancer cells. The JC1 and Annexin-V FITC/PI assays carried out for A549 cancer cells treated with the BODIPY complex showed the alteration of mitochondrial membrane potential and apoptotic cell death on near-IR red light (600-720 nm) irradiation, respectively.

Molecular Design toward Heavy-Atom-free Photosensitizers Based on the C═S Bond and their Dual Functions in Hypoxia Photodynamic Cancer Therapy and ClO- Detection

In this article, we designed and synthesized the thionated NpImidazole derivatives BS and NS, new heavy-atom-free photosensitizers, which efficiently generate a triplet excited state with high singlet oxygen quantum yield. The introduction of the C═S bond to the NpImidazole core is essential for increasing spin-orbit coupling (SOC). The fluorescence emission of BS and NS was quenched at standard ambient temperature, accompanied with the increase in the ISC process from the singlet states to triplet excited states via thionation. BS and NS showed negligible dark cytotoxicity against HeLa cells in working concentration. In contrast, BS and NS rapidly induced cell death under blue light irradiation both under normoxia and hypoxia conditions. Our current study demonstrates that the C═S group can play an important role in type I ROS generation of PSs, which are unprecedented in the previous reports. Finally, the photophysical changes were assigned to the oxidative desulfurization of the C═S group of BS and NS to the C═O group of the corresponding BO and NO via hypochlorite. The combined results demonstrated the dual function of BS and NS as a fluorescent imaging agent for ClO^- and an anti-cancer therapeutic by PDT that showed the potential strategy for "one-for-all" and multifunctional agents.

Twisted BODIPY derivative: intersystem crossing, electron spin polarization and application as a novel photodynamic therapy reagent

The photophysical properties of a heavy atom-free BODIPY derivative with a twisted π-conjugated framework were studied. Efficient intersystem crossing (ISC quantum yield: 56%) and an exceptionally long-lived triplet state were observed (4.5 ms in solid polymer film matrix and 197.5 μs in solution). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of the triplet state on the whole twisted π-conjugated framework and the zero-field-splitting (ZFS) D parameter of D = -69.5 mT, which is smaller than that of 2,6-diiodoBODIPY (D = -104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted BODIPY is (a, a, e, a, e, e), which is different from that of 2,6-diiodo BODIPY (e, e, e, a, a, a), indicating that the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. According to the computed spin-orbit coupling matrix elements (0.154-1.964 cm^-1), together with the matched energy of the S₁/T_n states, ISC was proposed to occur via S₁→T₂/T₃. The computational results were consistent with TREPR results on the electron spin selectivity (the overpopulation of the T_Y sublevel of the T₁ state). The advantage of the long-lived triplet state of the twisted BODIPY was demonstrated by its efficient singlet oxygen (¹O₂) photosensitizing (Φ_Δ = 50.0%) even under a severe hypoxia atmosphere (pO₂ = 0.2%, v/v). A high light toxicity (EC₅₀ = 1.0 μM) and low dark toxicity (EC₅₀ = 78.5 μM) were observed for the twisted BODIPY, and thus the cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) agent.

The synthesis and characterization of the octahedral CoIII complex of a pyrrolopyrrolizine derivative formed with dicyanovinylene-bis-(meso-aryl)dipyrrin

A low spin state and diamagnetic CoIII complex 1 possessing pyrrolopyrrolizine ligands formed with dicyanovinylene-bis-(meso-aryl)dipyrrin was synthesized via the thermally activated metalation with CoCl2 and isolated via column chromatography. The nuclear magnetic resonance of complex 1 revealed diamagnetism, thereby confirming the structure of the octahedral CoIII-complex of strong-field ligands. The resulting molecular structure of 1 was elucidated by the X-ray diffraction analysis. An arrangement of two pyrrolizine-ligands for the metal chelation was found in the AB-BA order, which was distinct from the case observed during the formation of bis-NiII-expanded porphyrinoids.

Heavy-Atom-Modulated Supramolecular Assembly Increases Antitumor Potency against Malignant Breast Tumors via Tunable Cooperativity

Triple-negative breast cancer (TNBC) remains with highest incidence and mortality rates among females, and a critical bottleneck lies in rationally establishing potent therapeutics against TNBC. Here, the self-assembled micellar nanoarchitecture of heavy-atom-modulated supramolecules with efficient cytoplasmic translocation and tunable photoconversion is shown, for potent suppression against primary, metastatic, and recurrent TNBC. Multi-iodinated boron dipyrromethene micelles yield tunable photoconversion into singlet oxygen and a thermal effect, together with deep penetration and subsequent cytoplasmic translocation at the tumor. Tetra-iodinated boron dipyrromethene micelles (4-IBMs) particularly show a distinctly enhanced cooperativity of antitumor efficiency through considerable expressions of apoptotic proteins, potently suppressing subcutaneous, and orthotopic TNBC models, together with reduced oxygen dependence. Furthermore, 4-IBMs yield preferable anti-metastatic and anti-recurrent efficacies through the inhibition of metastasis-relevant proteins, distinct immunogenic cell death, and re-education of M2 macrophages into tumoricidal M1 phenotype as compared to chemotherapy and surgical resection. These results offer insights into the cooperativity of supramolecular nanoarchitectures for potent phototherapy against TNBC.

Single component photoresponsive fluorescent organic nanoparticles: a smart platform for improved biomedical and agrochemical applications

Single component photoresponsive fluorescent organic nanoparticles for the regulated release of anticancer drugs, antibacterial agents, gasotransmitters, and agrochemicals and as effective PDT agents.

Boron dipyrromethene (BODIPY) in polymer chemistry

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

Photodynamic activity of 2,6-dibrominated dimethylaminophenylbuta-1,3-dienylBODIPY dyes

Mono- and disubstituted 2,6-dibromo-dimethylaminophenylbuta-1,3-dienylBODIPY dyes were successfully prepared, and their in vitro photodynamic activities against MCF-7 breast cancer cells were evaluated with a Thorlabs M660L4 660 nm LED (336 J · cm[Formula: see text]. The IC[Formula: see text] value of the monophenylbuta-1,3-dienylBODIPY was ca. 2.1 [Formula: see text]M, while that of the diphenylbuta-1,3-dienylBODIPY was > 50 [Formula: see text]M. Both dyes exhibited minimal dark toxicity. The results demonstrate that monosubstituted 2,6-dibromo-dimethylaminophenylbuta-1,3-dienylBODIPY dyes merit further in-depth study for use as photosensitizer dyes in photodynamic therapy.

Highly Phototoxic Transplatin-Modified Distyryl-BODIPY Photosensitizers for Photodynamic Therapy

We report the synthesis of the first transplatin-BODIPY conjugates for application in photodynamic therapy (PDT). The distyryl BODIPYs containing two iodine atoms were designed to absorb in the red region, easily undergo intersystem crossing for efficient singlet oxygen generation, and additionally offer the possibility for coordination with mono-activated transplatin. We were able to demonstrate that coordination of the BODIPYs with a mono-activated transplatin increases the phototoxic index of the photosensitizers significantly, giving rise to highly phototoxic distyryl BODIPY derivatives, of which one was shown to have the highest ever reported phototoxic index against any cell line. Furthermore, the photophysical mechanism of singlet oxygen generation in distyryl BODIPYs undergoing intramolecular charge transfer was studied experimentally and using time-dependent density functional theory.

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.

Bimetallic cyclometalated iridium complexes bridged by a BODIPY linker

Presented here is a new class of supramolecular cyclometalated Ir(iii) complexes. The 2 : 1 assemblies include two phosphorescent cyclometalated Ir(iii) centers spanned by a BODIPY bridge with pyridine substituents at the β-pyrrole positions. The three complexes, which vary with respect to the cyclometalating ligand on iridium, are prepared via a simple one-pot procedure, with the target complexes isolated in 31-75% yield. The photophysics of these new compounds are described in detail. All complexes are strongly photoluminescent, with fluorescence from BODIPY being the dominant emission pathway. One member of the series has a near-unity photoluminescence quantum yield, significantly enhanced relative to the free BODIPY. The cyclometalating ligand on iridium controls the energy of the Ir-centered triplet excited state, but in all cases energy transfer from the Ir centers to the BODIPY quenches almost all phosphorescence. This work outlines a new, simple synthetic method for accessing supramolecular complexes.

Discovery of a Monoiodo Aza-BODIPY Near-Infrared Photosensitizer: in vitro and in vivo Evaluation for Photodynamic Therapy

Photodynamic therapy (PDT) as a rising platform of the cancer treatment method is receiving increased attention. Through systematic evaluation of halogen substitution on aza-4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY), we have found that monoiodo-derived aza-BODIPYs provided greater efficacy than other halogenated aza-BODIPY PSs. 4 and 15 as monoiodinated aza-BODIPY dyes containing p-methoxyphenyl moiety were identified to be potent NIR aza-BODIPY-type PSs with IC₅₀ values against HeLa cells at a light dose of 54 J/cm² as low as 76 and 81 nM, respectively. 4 possessed superior phototoxicity, low dark toxicity, and good thermal/photostability and distributed majorly in mitochondria in cells. Apoptosis was verified to be the main cell death pathway, and in vitro reactive oxygen species generation was demonstrated. In vivo whole-body fluorescence imaging and ex vivo organ distribution studies suggested that 4 afforded an excellent PDT effect with a low drug dose under single-time light irradiation and revealed advantages over known PSs of ADPM06 and Ce6.

New BODIPYs for photodynamic therapy (PDT): Synthesis and activity on human cancer cell lines

A new class of compounds based on the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene core, known as BODIPYs, has attracted significant attention as photosensitizers suitable for application in photodynamic therapy (PDT), which is a minimally invasive procedure to treat cancer. In PDT the combination of a photosensitizer (PS), light, and oxygen leads to a series of photochemical reactions generating reactive oxygen species (ROS) exerting cytotoxic action on tumor cells. Here we present the synthesis and the study of the in vitro photodynamic effects of two BODIPYs which differ in the structure of the substituent placed on the meso (or 8) position of the dipyrrolylmethenic nucleus. The two compounds were tested on three human cancer cell lines of different origin and degree of malignancy. Our results indicate that the BODIPYs are very effective in reducing the growth/viability of HCT116, SKOV3 and MCF7 cells when irradiated with a green LED source, whereas they are practically devoid of activity in the dark. Phototoxicity occurs mainly through apoptotic cell death, however necrotic cell death also seems to play a role. Furthermore, singlet oxygen generation and induction of the increase of reactive oxygen species also appear to be involved in the photodynamic effect of the BODIPYs. Finally, it is worth noting that the two BODIPYs are also able to exert anti-migratory activity.

Recent advances in strategies for overcoming hypoxia in photodynamic therapy of cancer

The selectivity of photodynamic therapy (PDT) derived from the tailored accumulation of photosensitizing drug (photosensitizer; PS) in the tumor microenvironment (TME), and from local irradiation, turns it into a "magic bullet" for the treatment of resistant tumors without sparing the healthy tissue and possible adverse effects. However, locally-induced hypoxia is one of the undesirable consequences of PDT, which may contribute to the emergence of resistance and significantly reduce therapeutic outcomes. Therefore, the development of strategies using new approaches in nanotechnology and molecular biology can offer an increased opportunity to eliminate the disadvantages of hypoxia. Emerging evidence indicates that wisely designed phototherapeutic procedures, including: (i) ROS-tunable photosensitizers, (ii) organelle targeting, (iii) nano-based photoactive drugs and/or PS delivery nanosystems, as well as (iv) combining them with other strategies (i.e. PTT, chemotherapy, theranostics or the design of dual anticancer drug and photosensitizers) can significantly improve the PDT efficacy and overcome the resistance. This mini-review addresses the role of hypoxia and hypoxia-related molecular mechanisms of the HIF-1α pathway in the regulation of PDT efficacy. It also discusses the most recent achievements as well as future perspectives and potential challenges of PDT application against hypoxic tumors.

Influence of molecular design on radical spin multiplicity: characterisation of BODIPY dyad and triad radical anions

A strategy to create organic molecules with high degrees of radical spin multiplicity is reported in which molecular design is correlated with the behaviour of radical anions in a series of BODIPY dyads. Upon reduction of each BODIPY moiety radical anions are formed which are shown to have different spin multiplicities by electron paramagnetic resonance (EPR) spectroscopy and distinct profiles in their cyclic voltammograms and UV-visible spectra. The relationship between structure and multiplicity is demonstrated showing that the balance between singlet, biradical or triplet states in the dyads depends on relative orientation and connectivity of the BODIPY groups. The strategy is applied to the synthesis of a BODIPY triad which adopts an unusual quartet state upon reduction to its radical trianion.

Temperature- and Structure-Dependent Optical Properties and Photophysics of BODIPY Dyes

We report on the temperature- and structural-dependent optical properties and photophysics of a set of boron dipyrromethene (BODIPY) dyes with different substitution patterns of their meso-aryl subunit. Single-crystal X-ray diffraction analysis of the compounds enabled a classification of the dyes into a sterically hindered and a unhindered group. The steric hindrance refers to a blocked rotational motion of the aryl subunit around the bond connecting this moiety to the meso-position of the BODIPY core. The energy barriers related to this rotation were simulated by DFT calculations. As follows from the relatively low rotational barrier calculated to about 17 kcal/mol, a free rotation is only possible for sterically unhindered compounds. Rotational barriers of more than 40 kcal/mol determined for the sterically hindered compounds suggest an effective freezing of the rotational motion in these molecules. With the aid of temperature-dependent spectroscopic measurements, we could show that the ability to rotate directly affects the optical properties of our set of BODIPY dyes. This accounts for the strong temperature dependence of the fluorescence of the sterically unhindered compounds which show a drastic decrease in fluorescence quantum yield and a significant shortening in fluorescence lifetime upon heating. The optical properties of the sterically hindered compounds, however, are barely affected by temperature. Our results suggest a nonradiative deactivation of the first excited singlet state of the sterically unhindered compounds caused by a conical intersection of the potential energy surfaces of the ground and first excited state which is accessible by rotation of the meso-subunit. This is in good agreement with previously reported deactivation mechanisms. In addition, our results suggest the presence of a second nonradiative depopulation pathway of the first excited singlet state which is particularly relevant for the sterically hindered compounds.

BODIPYS and aza-BODIPY derivatives as promising fluorophores for in vivo molecular imaging and theranostic applications

Since their discovery in 1968, the BODIPYs dyes (4,4-difluoro-4-bora-3a, 4a diaza-s-indacene) have found an exponentially increasing number of applications in a large variety of scientific fields. In particular, studies reporting bioapplications of BODIPYs have increased dramatically. However, most of the time, only in vitro investigations have been reported. The in vivo potential of BODIPYs and aza-BODIPYs is more recent, but considering the number of in vivo studies with BODIPY and aza-BODIPY which have been reported in the last five years, we can now affirm that this family of fluorophores can be considered important as cyanine dyes for future in vivo and even clinical applications. This review aims to present representative examples of recent in vivo applications of BODIPYs or aza-BODIPYs, and to highlight the potential of these dyes for optical molecular imaging.

Novel BODIPY-subphthalocyanine dyads with reasonable photodynamic therapy behaviours

In this study, a set of bio-compatible and NIR emissive BODIPY-subphthalocyanine dyads (SP–DBD1–3) that contain amphiphilic triethyleneglycol units supporting partial water solubility and red absorbing BODIPY monomers (DBD1–3) were prepared.

Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

Organic photosensitizers possessing efficient intersystem crossing (ISC) and forming long-living triplet excited states, play a crucial role in a number of applications. A common approach in the design of such dyes relies on the introduction of heavy atoms (e.g. transition metals or halogens) into the structure, which promote ISC via spin-orbit coupling interaction. In recent years, alternative methods to enhance ISC have been actively studied. Among those, the generation of triplet excited states through photoinduced electron transfer (PET) in heavy-atom-free molecules has attracted particular attention because it allows for the development of photosensitizers with programmed triplet state and fluorescence quantum yields. Due to their synthetic accessibility and tunability of optical properties, boron dipyrromethenes (BODIPYs) are so far the most perspective class of photosensitizers operating via this mechanism. This article reviews recently reported heavy-atom-free BODIPY donor-acceptor dyads and dimers which produce long-living triplet excited states and generate singlet oxygen. Structural factors which affect PET and concomitant triplet state formation in these molecules are discussed and the reported data on triplet state yields and singlet oxygen generation quantum yields in various solvents are summarized. Finally, examples of recent applications of these systems are highlighted.

Blue fluorescence from N,O-coordinated BF2 complexes having aromatic chromophores in solution and the solid state

We prepared amide-heterocycle (HC) compounds having various aromatic π-electron systems (Ar), such as phenyl, naphthyl, furyl, thienyl and phenanthryl moieties, and converted them as ligands to difluoroboronated complexes, Ar@HCs. Blue fluorescence from Ar@HCs was observed in solution and the solid state, and the fluorescence quantum yields (Φ_f) and lifetimes (τ_f) were determined. The Φ_f values in CHCl₃ were as small as 0.1 except for the phenanthrene derivatives (0.4-0.6). Observation of the triplet-triplet absorption upon laser flash photolysis of Ar@HCs in solution indicated that the fluorescence process competes with intersystem crossing to the triplet state. Blue fluorescence in the solid state was observed with the Φ_f values of 0.3-0.7. Based on the crystallographic data, the relationship between the crystal structures and emission features of Ar@HCs in the solid state is discussed.

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

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

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

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

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.

Novel ruthenium(ii) and iridium(iii) BODIPY dyes: insights into their application in photodynamic therapy in vitro

Organic-metal complexes are promising molecules for use in photodynamic therapy (PDT). The aim of this study was to investigate in vitro effects of novel Ru(ii) and Ir(iii) BODIPY complexes for PDT. These hybrid organic-metal molecules (Ru-BD and Ir-BD) have been synthesized via reactions of a BODIPY precursor (BD) with a phenanthroline unit bearing Ru(ii) (3) and novel Ir(iii) (4) compounds. The crystal structures of the new distyryl BODIPY (BD) and Ru(ii) complex (3) are also reported. The photophysical and singlet oxygen generation properties of Ru-BD and Ir-BD were investigated in comparison with unsubstituted BODIPY (BD). Moreover, Ru-BD and Ir-BD have been biologically evaluated in vitro in chronic myeloid leukemia and cervical cancer cell lines in terms of photodynamic therapy efficacy in the presence of BD control. These complexes were not toxic in the dark but red light was needed to induce cell death. These data support the fact that Ru-BD could be accepted as a valuable photosensitizer-drug for further PDT treatment.

Selective Cu-Catalyzed Intramolecular Annulation of 3-Aryl/Heteryl-2-(diazoacetyl)-1H-pyrroles: Synthesis of Benzo/Furo/Thieno[e]-Fused 1H-Indol-7-oles and Their Transformations

The Co(III)-catalyzed reaction of 1,3-dicarbonyl compounds with 2-(diazoacetyl)-2H-azirines, prepared by a simplified procedure from 2H-azirin-2-carbonyl chlorides, led in high yields to the formation of 2-(diazoacetyl)pyrroles, while leaving the diazoacetyl function intact. The intramolecular aromatic substitution reaction of 2-(diazoacetyl)pyrroles, catalyzed by Cu(OTf)₂, provided selectively previously unknown benzo[e]- and hetero[e]-fused indol-7-oles in good yields. Formylation of benzo[e]indol-4-ol led selectively to the 5-formyl derivative, which is a good precursor for an unusual salen ligand and its Ni-complex. Triflates prepared from benzo[e]indol-4-oles gave various 4-substituted benzo[e]indoles carrying aryl, 2-thienyl, 2-pyridyl, and alkynyl groups, in excellent yields using cross-coupling reactions. 4-(2-Pyridyl)benzo[e]indoles, upon treatment with BF₃·Et₂O/Et₃N, afforded a new type of fluorescent boron complexes with large Stokes shifts.