Molecular Design of Highly Efficient Heavy-Atom-Free Triplet BODIPY Derivatives for Photodynamic Therapy and Bioimaging

Novel BODIPY photosensitizers were developed for imaging-guided photodynamic therapy. The introduction of a strong electron donor to the BODIPY core through a phenyl linker combined with the twisted arrangement between the donor and the BODIPY acceptor is essential for reducing the energy gap between the lowest singlet excited state and the lowest triplet state (ΔE_ST ), leading to a significant enhancement in the intersystem crossing (ISC) of the BODIPYs. Remarkably, the BDP-5 with the smallest ΔE_ST (ca. 0.44 eV) exhibited excellent singlet oxygen generation capabilities in both organic and aqueous solutions. BDP-5 also displayed bright emission in the far-red/near-infrared region in the condensed states. More importantly, both in vitro and in vivo studies demonstrated that BDP-5 NPs displayed a high potential for photodynamic cancer therapy and bioimaging.

Near-Infrared Photoactive Aza-BODIPY: Thermally Robust and Photostable Photosensitizer and Efficient Electron Donor

We report herein the synthesis of aza-BODIPY substituted with strongly electron-donating p-(diphenylamino)phenyl substituents (p-Ph₂ N-) at 3,5-positions. The presence of p-Ph₂ N- groups lowers the energy of the singlet excited state (E_s ) to 1.48 eV and induces NIR absorption with λ_abs at 789 nm in THF. The compound studied is weakly emissive with the emission band (λ_f ) at 837 nm and with the singlet lifetime (τ_S ) equal to 100 ps. Nanosecond laser photolysis experiments of the aza-BODIPY in question revealed T₁ →T_n absorption spanning from ca. 350-550 nm with the triplet lifetime (τ_T ) equal to 21 μs. By introducing a heavy atom (Br) into the structure of the aza-BODIPY, we managed to turn it into a NIR operating photosensitizer. The photosensitized oxygenation of the model compound-diphenylisobenzofuran (DPBF)-proceedes via Type I and/or Type III mechanism without formation of singlet oxygen (¹ O₂ ). As estimated by CV/DPV measurements, the p-Ph₂ N- substituted aza-BODIPYs studied exhibits oxidation processes at relatively low oxidation potentials (E_ox ¹ ), pointing to the very good electron-donating properties of these molecules. Extremely high photostability and thermal robustness up to approximately 300 °C are observed for the p-Ph₂ N- substituted aza-BODIPYs.

Fluorescence upconversion by triplet-triplet annihilation in all-organic poly(methacrylate)-terpolymers

Fluorescence upconversion by triplet-triplet annihilation is demonstrated for a fully polymer-integrated material, i.e. in the limit of restricted diffusion. Organic sensitizer and acceptor are covalently attached to a poly(methacrylate) backbone, yielding a metal-free macromolecular all-in-one system for fluorescence upconversion. Due to the spatial confinement of the optically active molecular components, i.e. annihilator and sensitizer, UC by TTA in the constrained polymer system in solution is achieved at exceptionally low averaged annihilator concentrations. However, the UC quantum yield in the investigated systems is found to be low, highlighting that only chromophores in specific local surroundings yield upconversion in the limit of restricted diffusion. A photophysical model is proposed taking the heterogeneous local environment within the polymers into account.

One-pot synthesis of N-confused porphyrin-dipyrrin conjugates and their optical properties

A conjugated N-confused porphyrin-dipyrrin system NCP-IN-X (X = H, Br, Cl, OMe, COOEt) has been reported via N-Confused tetraphenylporphyrin (NCTPP) as a π-enlarged pyrrole and substituted trimethylindole subunits in 75-82% yields by one pot. All the NCP-INs show red-shifted absorption and emission in the NIR range compared with NCTPP in a series nonpolar and polar solvents. Interesting, relative higher emission in DMF can be observed. The structures of dipyrrin-NCP conjugates are characterized by ¹H-NMR and high resolution mass spectrum (HR-MS). Four frontier molecular orbital and simulated stick absorption spectra are calculated by DFT and TD-DFT calculation which is in good agreement with the experiments.

Halogen Bonding Facilitates Intersystem Crossing in Iodo-BODIPY Chromophores

BODIPY chromophores can serve as organic-based triplet photosensitizers for a wide range of applications. To perform this function, the formation of the triplet state is critical, and a better understanding of how to modulate the formation of the triplet state could lead to further advances in BODIPY-based sensitizers for solar energy conversion and photodynamic therapy. In this work we investigate the ability of halogen bonding, a noncovalent solvent interaction, to facilitate intersystem crossing in a diiodo-BODIPY. Ultrafast transient absorption spectroscopy is applied to diiodo-BODIPY in the presence of pyridine-based halogen bonding solvent molecules to determine the rate constants for intersystem crossing. We find that halogen bonding facilitates the formation of the triplet state by increasing the intersystem crossing rate constant of diiodo-BODIPY. The results are interpreted in terms of the Marcus expression for intersystem crossing. Quantum chemical calculations show that halogen bonding acts to alter both the spin-orbit coupling terms and the relative energetics of the singlet and triplet states.

AIE nanodots scaffolded by mini-ferritin protein for cellular imaging and photodynamic therapy

Photodynamic therapy (PDT) is one of the most elegant cancer treatment strategies that can be controlled by a beam of light with non-invasion, precise control, and high spatiotemporal accuracy. An ideal photosensitizer (PS) is the key to ensure the efficacy of PDT. Due to their hydrophobic and rigid planar structures, most traditional PSs are prone to aggregate under physiological conditions, which causes fluorescence quenching and significantly reduces reactive oxygen species (ROS) generation. Fortunately, the emergence of aggregation-induced emission (AIE) dyes offers a potential opportunity to overcome these limitations. When AIE PS molecules are in the aggregation state, the fluorescence intensity and ROS production can be increased. We herein use red AIE PS molecules to prepare stable AIE nanodots for cell imaging and PDT via a simple method with a highly negatively charged mini-ferritin protein as the scaffold. The as-prepared protein-AIE nanodots show strong fluorescence emission and efficient singlet oxygen generation, with good stability, relatively long wavelengths of absorption and emission, and negligible dark toxicity. The mini-ferritin-AIE system may be useful in developing novel functional probes for tumour nanotheranostics.

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.

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

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

Enhanced triplet state generation through radical pair intermediates in BODIPY-quantum dot complexes

Generation of triplet excited states through radical pair intermediates has been extensively studied in molecular complexes. Similar schemes remain rare in hybrid structures of quantum dot-organic molecules, despite intense recent interest of quantum dot sensitized triplet excited state generation. Herein, we demonstrate that the efficiency of the intersystem crossing from the singlet to the triplet state in boron dipyrromethene (BODIPY) can be enhanced in CdSe quantum dot-BODIPY complexes through a radical pair intermediate state consisting of an unpaired electron in the quantum dot conduction band and that in oxidized BODIPY. By transient absorption spectroscopy, we show that the excitation of BODIPY with 650 nm light leads to the formation of a charge separated state by electron transfer from BODIPY to CdSe (with a time constant of 6.33 ± 1.13 ns), competing with internal conversion to the ground state within BODIPY, and the radical pair state decays subsequently by back charge recombination to generate a triplet excited state (with a time constant of 158 ± 28 ns) or the ground state of BODIPY. The overall quantum efficiency of BODIPY triplet excited state generation was determined to be (27.2 ± 3.0)%. The findings of efficient triplet state formation and intermediate radical pair states in this hybrid system suggest that quantum dot-molecule complexes may be a promising platform for spintronics applications.

Direct sulfonylation of BODIPY dyes with sodium sulfinates through oxidative radical hydrogen substitution at the α-position

A series of α-sulfonated BODIPYs were efficiently synthesized from sodium sulfinates via a radical process, and were demonstrated as new fluorescent probes for selective biothiol detection.

From homonuclear to heteronuclear: a viable strategy to promote and modulate phosphorescence

The use of weakly emissive homonuclear Pt(ii) precursors to fabricate Pt–M heteronuclear complexes with intermetallic contacts paves a unique avenue to achieve highly efficient phosphorescence as well as modulate emission energy.

Photoinduced direct hydration of dipyridylacetylenes in acidic aqueous solution

The straightforward and catalyst-free photoinduced hydration reaction of dipyridylacetylenes in acidic aqueous solution was achieved upon UV irradiation at room temperature.

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.

Making triplets from photo-generated charges: observations, mechanisms and theory

Photo-excitation of electron donor–acceptor systems can lead to the generation of a charge separated state (CT). Sometimes the charge recombination occurs mainly to the local triplet excited state (T₁). How does the spin flip?

Multi-state amine sensing by electron transfers in a BODIPY probe

Photoinduced electron transfer sets up the BODIPY probe for multi-state amine sensing by single-electron transfer then collisional quenching.

Finding the optimal exchange–correlation functional to describe the excited state properties of push–pull organic dyes designed for thermally activated delayed fluorescence

In view of designing organic dyes for TADF, TD-DFT calculations are performed on 10 prototypical donor-acceptor compounds and are benchmarked against riCC2 calculations, demonstrating that modified range-separated hybrids perform best.

A facile strategy to realize a single/double photon excitation-dependent photosensitizer for imaging-guided phototherapy against HeLa cancer cells at separate irradiation channels

A facile strategy has been proposed to realize two-photon phototherapy against HeLa cancer cells with simultaneous one-photon fluorescence imaging modality at separate irradiation channels.

Diketopyrrolopyrrole–fullerene C60 architectures as highly efficient heavy atom-free photosensitizers: synthesis, photophysical properties and photodynamic activity

New diketopyrrolopyrrole–C₆₀ architectures were synthesized as potential heavy atom-free photosensitizers in photodynamic inactivation of microorganisms.

Fluorescence resonance energy transfer-based drug delivery systems for enhanced photodynamic therapy

In this Review, recent advances in fluorescence resonance energy transfer-based drug delivery systems for enhanced photodynamic therapy are described, and the current challenges and perspectives in this emerging field are also discussed.

pH stimulus-disaggregated BODIPY: an activated photodynamic/photothermal sensitizer applicable to tumor ablation

A pH activated photodynamic/photothermal sensitizer applicable to tumor ablation.

Bioapplications of small molecule Aza-BODIPY: from rational structural design to in vivo investigations

This review highlights the empirical design guidelines and photophysical property manipulation of Aza-BODIPY dyes and the latest advances in their bioapplications.

An exceptionally long-lived triplet state of red light-absorbing compact phenothiazine-styrylBodipy electron donor/acceptor dyads: a better alternative to the heavy atom-effect?

Heavy atom-free dyads showing a red light-absorbing and exceptionally long-lived triplet state based on a spin–orbital charge transfer intersystem crossing mechanism.

Polyoxometalate-based room-temperature phosphorescent materials induced by anion–π interactions

A series of polyoxometalate-based host–guest materials emit strong red room-temperature phosphorescence attributed to intermolecular charge-transfer states which was caused by unorthodox anion–π interactions.

Near-IR absorbing J-aggregates of a phenanthrene-fused BODIPY as a highly efficient photothermal nanoagent

A phenanthrene-[b]-fused BODIPY exhibited well-defined J-aggregates in both pure hydrocarbon solution and aqueous solution, and was developed as a highly efficient photothermal nanoagent.

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.

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

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

BODIPYs revealing lipid droplets as valuable targets for photodynamic theragnosis

Endowing BODIPY PDT agents with the ability to probe lipid droplets is demonstrated to boost their phototoxicity, allowing the efficient use of highly fluorescent dyes (poor ROS sensitizers) as phototoxic agents. Conversely, this fact opens the way to the development of highly bright ROS photosensitizers for performing photodynamic theragnosis (fluorescence bioimaging and photodynamic therapy) from a single simple agent. On the other hand, the noticeable capability of some of the reported dyes to probe lipid droplets in different cell lines under different conditions reveals their use as privileged probes for advancing the study of interesting lipid droplets by fluorescence microscopy.

Manipulating Charge-Transfer States in BODIPYs: A Model Strategy to Rapidly Develop Photodynamic Theragnostic Agents

On the basis of a family of BINOL (1,1'-bi-2-naphthol)-based O-BODIPY (dioxyboron dipyrromethene) dyes, it is demonstrated that chemical manipulation of the chromophoric push-pull character, by playing with the electron-donating capability of the BINOL moiety (BINOL versus 3,3'-dibromoBINOL) and with the electron-acceptor ability of the BODIPY core (alkyl substitution degree), is a workable strategy to finely balance fluorescence (singlet-state emitting action) versus the capability to photogenerate cytotoxic reactive oxygen species (triplet-state photosensitizing action). It is also shown that the promotion of a suitable charge-transfer character in the involved chromophore upon excitation enhances the probability of an intersystem crossing phenomenon, which is required to populate the triple state enabling singlet oxygen production. The reported strategy opens up new perspectives for rapid development of smarter agents for photodynamic theragnosis, including heavy-atom-free agents, from a selected organic fluorophore precursor.

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

Recently varieties of Bodipy derivatives showing intersystem crossing (ISC) have been reported as triplet photosensitizers, and the application of these compounds in photocatalysis, photodynamic therapy (PDT), and photon upconversion are promising. In this review we summarized the recent development in the area of Bodipy-derived triplet photosensitizers and discussed the molecular structural factors that enhance the ISC ability. The compounds are introduced based on their ISC mechanisms, which include the heavy atom effect, exciton coupling, charge recombination (CR)-induced ISC, using a spin converter and radical enhanced ISC. Some transition metal complexes containing Bodipy chromophores are also discussed. The applications of these new triplet photosensitizers in photodynamic therapy, photocatalysis, and photon upconversion are briefly commented on. We believe the study of new triplet photosensitizers and the application of these novel materials in the abovementioned areas will be blooming.

Unveiling the Photophysical Properties of Boron-dipyrromethene Dyes Using a New Accurate Excited State Coupled Cluster Method

Boron-dipyrromethene (BODIPY) molecules form a class of fluorescent dyes known for their exceptional photoluminescence properties. Today, they are used extensively in various applications from fluorescent imaging to optoelectronics. The ease of altering the BODIPY core has allowed scientists to synthesize dozens of analogues by exploring chemical substitutions of various kinds or by increasing the length of conjugated groups. However, predicting the impact of any chemical change accurately is still a challenge, especially as most computational methods fail to describe correctly the photophysical properties of BODIPY derivatives. In this study, the recently developed coupled cluster method called "domain-based local pair natural orbital similarity transformed equation of motion-coupled cluster singles and doubles" (DLPNO-STEOM-CCSD) is employed to compute the lowest vertical excitation energies of more than 50 BODIPY molecules. The method performs remarkably well yielding an accuracy of about 0.06 eV compared to the experimental absorption maxima. We also provide an estimate to the error made by neglecting vibronic effects in the computed spectra. The dyes selected for investigation here span a large range of molecular sizes and chemical functionalities and are embedded in solvents with different polarities. We have also investigated if the method is able to correctly reproduce the impact of a single chemical modification on the absorption energy. To characterize the method in more specific terms, we have studied four large BODIPY analogues used in real-life applications due to their interesting chemical properties. These examples should illustrate the capacity of the DLPNO-STEOM-CCSD procedure to become a method of choice for the study of photophysical properties of medium to large organic compounds.

Ruthenium(II) Conjugates of Boron-Dipyrromethene and Biotin for Targeted Photodynamic Therapy in Red Light

The ruthenium(II) complexes [RuCl(L¹)(L³)]Cl (1), [RuCl(L¹)(L⁴)]Cl (2), [RuCl(L²)(L⁴)]Cl (3), [RuCl(L¹)(L⁵)]Cl (4), and [RuCl(L²)(L⁵)]Cl (5) of NNN-donor dipicolylamine (dpa) bases (L⁴, L⁵) having BODIPY (boron-dipyrromethene) moieties, NN-donor phenanthroline derivatives (L¹, L²), and benzyldipicolylamine (bzdpa, L³) were prepared and characterized by spectroscopic techniques and their cellular localization/uptake and photocytotoxicity studied. Complex 1, as its PF₆ salt (1a), has been structurally characterized with help of a single-crystal X-ray diffraction technique. It has a RuN₅Cl core with the Cl bonded trans to the amine nitrogen atom of bzdpa. The complexes showed intense absorption spectral bands near 500 nm (ε ≈ 58000 M^-1 cm^-1) in 2 and 3 and 654 nm (ε ≈ 80000 M^-1 cm^-1) in 4 and 5 in 1/1 DMSO/DPBS (v/v). Complex 5 having biotin and PEGylated-disteryl BODIPY gave a singlet oxygen quantum yield (Φ_Δ) of ∼0.65 in DMSO. Complex 5 exhibited remarkable PDT (photodynamic therapy) activity (IC₅₀ ≈ 0.02 μM) with a photocytotoxicity index (PI) value of >5000 in red light of 600-720 nm in A549 cancer cells. The biotin-conjugated complexes showed better photocytotoxicity in comparison to nonbiotinylated analogues in A549 cells. The complexes displayed less toxicity in HPL1D normal cells in comparison to A549 cancer cells. The emissive BODIPY complexes 3 and 5 (Φ_F ≈ 0.07 in DMSO) showed significant mitochondrial localization.

Elucidating J-Aggregation Effect in Boosting Singlet-Oxygen Evolution Using Zirconium-Porphyrin Frameworks: A Comprehensive Structural, Catalytic, and Spectroscopic Study

Metal-organic frameworks (MOFs) are powerful toolkits to directly correlate structure-function relationships due to their well-defined structures. In this work, 5,15-di(3,4,5-trihydroxyphenyl)porphyrin (DTPP) and 5,10,15,20-tetra(3,4,5-trihydroxyphenyl)porphyrin (TTPP) are reacted with zirconium ions to afford two MOFs (Zr-DTPP and Zr-TTPP) with acid and base tolerance in the pH range of 1.0-14.0. Powder X-ray diffraction investigation combined with Rietveld refinement reveals the J-aggregated porphyrin building blocks confined by benzene-1,2,3-trisolate-zirconium chains in the newly prepared Zr-DTPP. Electron spin-resonance, singlet-oxygen determination, and sulfides oxidation experiments demonstrate a much better singlet-oxygen evolution of J-aggregated Zr-DTPP than that of unaggregated Zr-TTPP reported previously, in good contrast to the weaker photocatalytic capability disclosed for DTPP than that for TTPP in solution, consummating the theory of photosensitizer J-aggregation in boosting heterogeneous photoinduced singlet-oxygen generation.

Towards Efficient and Photostable Red-Emitting Photonic Materials Based on Symmetric All-BODIPY-Triads, -Pentads, and -Hexads

The development of efficient and stable red and near-IR emitting materials under hard radiation doses and/or prolonged times is a sought-after task due to their widespread applications in optoelectronics and biophotonics. To this aim, novel symmetric all-BODIPY-triads, -pentads, and -hexads have been designed and synthesized as light-harvesting arrays. These photonic materials are spectrally active in the 655-730 nm region and display high molar absorption across UV-visible region. Furthermore, they provide, to the best of our knowledge, the highest lasing efficiency (up to 68 %) and the highest photostability (tolerance >1300 GJ mol^-1 ) in the near-IR spectral region ever recorded under drastic pumping conditions. Additionally, the modular synthetic strategy to access the cassettes allows the systematic study of their photonic behavior related to structural factors. Collectively, the outstanding behavior of these multichromophoric photonic materials provides the keystone for engineering multifunctional systems to expedite the next generation of effective red optical materials.

Intersystem Crossing in Naphthalenediimide-Oxoverdazyl Dyads: Synthesis and Study of the Photophysical Properties

Oxoverdazyl (Vz) radical units were covalently linked to the naphthalenediimide (NDI) chromophore to study the effect of the radical on the photophysical properties, especially the radical enhanced intersystem crossing (REISC), which is a promising approach to develop heavy-atom-free triplet photosensitizers. Rigid phenyl or ethynylphenyl linkers between the two moieties were used, thus REISC and formation of doublet (D₁ , total spin quantum number S=1/2) and quartet states (Q₁ , S=3/2) are anticipated. The photophysical properties of the dyads were studied with steady-state and femtosecond/nanosecond transient absorption (TA) spectroscopies and DFT computations. Femtosecond transient absorption spectra show a fast electron transfer (<150 fs), and ISC (ca. 1.4-1.85 ps) is induced by charge recombination (CR, in toluene). Nanosecond transient absorption spectra demonstrated a biexponential decay of the triplet state of the NDI moiety. The fast component (lifetime: 50 ns; population ratio: 80 %) is assigned to the D₁ →D₀ decay, and the slow decay component (2.0 μs; 20 %) to the Q₁ →D₀ ISC. DFT computations indicated ferromagnetic interactions between the radical and chromophore (J=0.07-0.13 eV). Reversible formation of the radical anion of the NDI moiety by photoreduction of the radical-NDI dyads in the presence of sacrificial electron donor triethanolamine (TEOA) is achieved. This work is useful for design of new triplet photosensitizers based on the REISC effect.

Upconversion-like Photolysis of BODIPY-Based Prodrugs via a One-Photon Process

Photochemical reactions at lower energy than the absorption window are currently achieved by multi-photon processes, including two-photon absorption and photon upconversion, which have limited energy utilization efficiency. Here, we report a one-photon strategy based on triplet-triplet energy transfer (TTET) between a photosensitizer and a photocleavable molecule to achieve photolysis at low energy. To verify this concept, we chose platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) as the photosensitizer and synthesized a boron-dipyrromethene (BODIPY)-based prodrug as the photocleavable molecule. Photolysis of the prodrug is achieved by TTET upon excitation of PtTPBP at 625 nm with a photolysis quantum yield of 2.8%. Another demonstration shows an unexpected higher photolysis quantum yield than the direct excitation at 530 nm. This strategy opens a new path for achieving photolysis at long wavelengths, benefiting the applications in biological studies, photopharmacology, and photoresponsive drug delivery.

Neutral iridium(iii) complexes bearing BODIPY-substituted N-heterocyclic carbene (NHC) ligands: synthesis, photophysics, in vitro theranostic photodynamic therapy, and antimicrobial activity

The synthesis, photophysics, and photobiological activities of a series of novel neutral heteroleptic cyclometalated iridium(iii) complexes incorporating boron dipyrromethene (BODIPY) substituted N-heterocyclic carbene (NHC) ligands (Ir1-Ir5) are reported. The effect of the substitution position of BODIPY on the NHC ligands, either on C4 of the phenyl ring (Ir1-Ir3) or C5 of the benzimidazole unit (Ir4 and Ir5), and its linker type (single or triple bond) on the photophysical properties was studied. Ir1-Ir5 exhibited BODIPY-localized intense 1IL (intraligand transition)/1MLCT (metal-to-ligand charge transfer) absorption at 530-543 nm and 1,3IL/1,3CT (charge transfer) emission at 582-610 nm. The nanosecond transient absorption results revealed that the lowest triplet excited states of these complexes were the BODIPY-localized 3π,π* states. Complexes Ir4 and Ir5 exhibited blue-shifted 1IL absorption and 1,3IL/1,3CT emission bands compared to the corresponding absorption and emission bands in complexes Ir1 and Ir3. However, replacing the methyl substituents on N3 of benzimidazole in complexes Ir1 and Ir4 with oligoether substituents in Ir3 and Ir5, respectively, did not impact the energies of the low-energy absorption and emission bands in the corresponding complexes. Water-soluble complexes Ir3 and Ir5 have been explored as photosensitizers for in vitro photodynamic therapy (PDT) effects toward human SKMEL28 melanoma cells. Ir3 showed no dark cytotoxicity (EC50 > 300 μM) but good photocytotoxic activity (9.66 ± 0.28 μM), whereas Ir5 exhibited a higher dark cytotoxicity (20.2 ± 1.26 μM) and excellent photocytotoxicity (0.15 ± 0.01 μM). The phototherapeutic indices with visible light (400-700 nm) activation were >31 for Ir3 and 135 for Ir5. Ir3 and Ir5 displayed 1O2 quantum yields of 38% and 22% in CH3CN, respectively, upon 450 nm excitation. Ir5 was more effective at generating reactive oxygen species (ROS) in vitro. Ir5 was also active against Staphylococcus aureus upon visible light activation, with a phototherapeutic index of >15 and EC50 value of 6.67 μM. These photobiological activities demonstrated that these neutral Ir(iii) complexes are promising in vitro PDT reagents, and substitution at C5 on the benzimidazole group of the NHC ligand was superior to C4 substitution on the phenyl ring.

Photoactive organic material discovery with combinatorial supramolecular assembly

Organic semiconductors have received substantial attention as active components in optoelectronic devices because of their processability and customizable properties. Tailoring the organic active layer in these devices to exhibit the desired optoelectronic properties requires understanding the complex and often subtle structure-property relationships governing their photophysical response to light. Both structural organization and molecular orbitals play pivotal roles, and their interactions with each other are difficult to anticipate based upon the structure of the components alone, especially in systems comprised of multiple components. In pursuit of design rules, there is a need to explore multicomponent systems combinatorially to access larger data sets, and supramolecularly to use error correcting, noncovalent assembly to achieve long-range order. This review will focus on the use of supramolecular chemistry to study combinatorial, hierarchical organic systems with emergent optoelectronic properties. Specifically, we will describe systems that undergo excited state deactivation by charge transfer (CT), singlet fission (SF), and Förster resonance energy transfer (FRET). Adopting combinatorial, supramolecular assembly to study emergent photophysics promises to rapidly accelerate progress in this research field.

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.

5-Hydroxymethylfurfural-Derived Boron-Dipyrromethene Immobilized on Resin Support as a Sustainable Catalyst for C-H Arylation of Heterocycles

5-Hydroxymethylfurfural (HMF) was used as a sustainable raw material in the development of a resin-supported boron-dipyrromethene (BODIPY)-based photocatalyst. In the development of the catalyst, the brominated product (HMF-BODIPY-Br) and photocatalyst (HMF-BODIPY-Br-Suc) were isolated under a chromatography-free condition. The photocatalyst was loaded on polymeric resin by bridging alcohol functionality in HMF and amine functionality in polymeric resin using succinic anhydride. The resin-supported photocatalyst was used in light-mediated C-H arylation of various heterocycles using aryldiazonium salt. For representative examples, diazotization and photoarylation were carried out in one pot, and arylated furans were obtained in very good yields. C-H arylation was found to proceed via a photogenerated radical intermediate, and the radical intermediate was trapped by forming an adduct with TEMPO.

Computational Design of Near-Infrared Fluorescent Organic Dyes Using an Accurate New Wave Function Approach

The extensive research focusing on fluorescent organic dyes for bioimaging has made this in vivo method available for a diverse range of applications. One way to enhance this method is to tune the absorption and emission wavelengths of dyes to the near-infrared region where better light penetration and imaging resolution can be achieved. For this purpose, the well-known BODIPY dyes and their derivatives called aza-BODIPY have been the subject of extensive synthetic efforts. The interest in these systems stems from their excellent photophysical properties. Despite numerous studies, the rational design of near-infrared active dyes with desirable properties remains difficult. Here, we present a new wave function-based method for modeling excited states of large molecules, which has numerous theoretical advantages over the most commonly used electronic structure methods. This method is employed to suggest candidates for new dyes with the desired properties and to predict the absorption and fluorescence maxima and luminescence spectra of aza-BODIPY dyes with possible applications in fluorescence imaging.