BODIPY Research Articles

Novel BODIPY Dyes with a Meso-Benzoxadiazole Substituent: Synthesis, Photophysical Studies, and Cytotoxic Activity Under Normoxic and Hypoxic Conditions

Background/Objectives: Novel boron dipyrromethene derivatives with a heterocyclic, benzoxadiazole substituent were obtained as potential candidates for the photodynamic therapy (PDT) of cancers. Photochemical properties (e.g., singlet oxygen generation quantum yields (ΦΔ), absorption, and emission spectra) and cytotoxic activity studies in normoxic and hypoxic conditions were performed to verify the potential of novel BODIPYs as photosensitizers for PDT. Methods: Obtained dyes were characterized using mass spectrometry and various NMR techniques. The relative method with Rose Bengal as a reference and 1,3-diphenylisobenzofuran as a singlet oxygen quencher was used to determine ΦΔ values. The in vitro studies were conducted on human ovarian carcinoma (A2780) and human breast adenocarcinoma (MDA-MB-231) cells. Results: Photochemical studies showed that the presence of benzoxadiazole moiety only slightly affected the localization of the absorption maxima but resulted in fluorescence quenching compared with meso-phenyl-substituted analogs. In addition, brominated and iodinated analogs revealed a high ability to generate singlet oxygen. Anticancer studies showed high light-induced cytotoxicity of BODIPYs containing heavy atoms with very low IC50 values in the 3.5–10.3 nM range. Further experiments revealed that both compounds also demonstrated phototoxic activity under hypoxic conditions. The most potent cytotoxic effect in these conditions was observed in the iodinated BODIPY analog with IC50 values of about 0.3 and 0.4 μM for A2780 and MDA-MB-231 cells, respectively. Conclusions: The results of this study highlighted the advantages and some potential drawbacks of BODIPY compounds with heavy atoms and benzoxadiazole moiety as a useful scaffold in medicinal chemistry for designing new photosensitizers.

Thiophene engineering of near-infrared D-π-A nano-photosensitizers for enhanced multiple phototheranostics and inhibition of tumor metastasis.

Phototherapy including photothermal therapy (PTT) and photodynamic therapy (PDT) is widely used for cancer treatment because of its non-invasiveness, spatiotemporal controllability, and low side effects. However, the PTT and PDT capabilities of photosensitizers (PSs) compete so it's still a crucial challenge to simultaneously enhance the PDT and PTT capabilities of PSs. In this work, donor-π-acceptor (D-π-A)-based boron dipyrromethene (BODIPY) dyes were developed via molecular engineering and applied for enhanced phototherapy of triple-negative breast cancer. With thiophene engineering and iodine addition, D-π-A BDP dyes possessed a low energy gap between the singlet and triplet states (ΔES1-T1). After the BDP dyes were prepared into nanoparticles (NPs), the BDP4 NPs showed increased generation of type I and II reactive oxygen species (ROS) as well as a high photothermal conversion efficiency (44%). Furthermore, folate (FA)-modified BDP4 NPs achieved high tumor targeting via near-infrared bioimaging. With these advantages, BDP4 NPs with FA achieved total tumor eradication and tumor metastasis suppression via a single injection and 808nm laser irradiation. This work provided a rational design of D-π-A PSs for simultaneously enhancing their photodynamic and photothermal performance, achieving efficient cancer therapy.

Exploring the impact of substituents on the photophysical properties of boron dipyrromethene dyes for enhanced photovoltaic performance

Abstract Boron dipyrromethene (BODIPY) dyes are known for their strong fluorescence and excellent photostability, making them vital for various photoelectric applications. This study explores how different substituents affect the photophysical properties of BODIPY dyes to improve their performance in dye-sensitized solar cells (DSSCs). We examine six distinct BODIPY dye structures with unique electron-withdrawing (NO₂, CHO, Br) and electron-donating (OCH₃, NPh₂) substituents, along with a hydrogen-substituted variant, and report key photovoltaic parameters, including open-circuit voltage (Vₒc), short-circuit current density (Jₛc), fill factor (FF), and energy conversion efficiency (Ω). Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we analyze the dyes' light-harvesting efficiency, electron injection efficiency, and overall energy conversion efficiency. Our results indicate that electron-donating groups, especially NPh₂ and OCH₃, significantly enhance Voc, Jsc, FF, and Ω, leading to improved energy conversion efficiencies. In contrast, while electron-withdrawing substituents increase chemical stability, they typically result in lower photovoltaic performance. This research underscores the importance of strategic substituent selection in optimizing BODIPY dyes for enhanced photoelectric performance, offering valuable insights for designing efficient photovoltaic materials.

A BODIPY-Containing Covalent Organic Framework as a Highly Porous Photosensitizer for Environmental Remediation and Pollutants Adsorption.

The direct incorporation of borondipyrromethene (BODIPY) subunits into the structural backbone of covalent organic frameworks (COFs) gives facile access to porous photosensitizers but is still a challenging task. Here, we introduceβ‑ketoenamine-linkedBDP‑TFP‑COF, which crystallizes in AA‑stacking mode withhcbtopology. A comprehensive characterization reveals high crystallinity and enhanced stability in a variety of solvents, excellentmesoporosity (SABET=1042m2g-1), broad light absorption in the visible region, and red emission upon the exfoliation of few-layer COF nanosheets. The versatility of multifunctional BODIPY-COFs is highlighted in various applications. Pollutants Bisphenol A (BPA,qmax=426mgg-1) and Methylene Blue (MB,qmax=96mgg-1) have been efficiently removed from H2O. Fluorescence quenching or enhancement of exfoliatedBDP‑TFP‑COFnanosheets have been utilized for dual-mode sensing of MB or NEt3, respectively. Ultimately, the photosensitizing effect of the BODIPY units is retained in the COF. Thus,BPD‑TFP‑COFwas established as a metal-free triplet photosensitizer, which efficiently oxidized a mustard gas simulant under visible light irradiation.

Multicomponent Polymerization toward Poly(BODIPY-sulfonamide)s as Unique SO2 Generators for Sonodynamic and Gas Combination Therapy.

In this work, a multicomponent polymerization (MCP) approach involving bipyrroles, sulfonyl azides, and diynes was developed to afford a library of poly(bipyrrole-sulfonylimide)s (PPSIs) in high yields and molecular weights, which were further modified to form unique sulfur dioxide (SO2) generators. Bipyrroles served as carbon-based nucleophiles to undergo Cu-catalyzed C-C coupling during the MCP. Upon post-MCP modification by transforming the bipyrrole unit to boron dipyrromethene (BODIPY) and the sulfonylimide moiety to sulfonamide, poly(BODIPY-sulfonamide)s (PBSAs) were obtained as potent anticancer therapeutic agents. This study disclosed the decomposition and SO2-releasing ability of PBSAs under ultrasound (US) irradiation. Furthermore, one of these polymers having a reactive oxygen species-cleavable thioketal linker was modified with triethylene glycol chains to give PBSA-EG, which was further studied for in vitro and in vivo US-induced anti-cancer therapy. Upon tail veil administration of PBSA-EG nanoparticles into tumor-bearing mice followed by US irradiation, remarkable tumor suppression was observed. This study demonstrates that the innovative and efficient MCP method can construct polymers bearing the BODIPY-sulfonamide moieties, which show a great potential as safe and potent materials for combined gas and sonodynamic therapy against cancer.

Recent advances in the development of enantiopure BODIPYs and some related enantiomeric compounds.

During the process of developing smart chiroptical luminophores, small chiral organic dyes have emerged as candidates of utmost importance. In this regard, the chiral variants of boron dipyrromethene (BODIPY) serve as suitable molecules owing to their excellent photophysical properties such as high fluorescence quantum yields, narrow emission bandwidths with high peak intensities, high photo and chemical stability, and higher molar extinction coefficients. Thus, the last decade observed an influx of research from various research groups for the induction of chirality in originally achiral BODIPY. Among these, the generation of chiral centers at various positions in BODIPY favored the synthetic accessibility towards this particular chiral pool, which in turn is found to be applicable in various areas like photodynamic therapy, bio-imaging, dye-sensitized solar cells, optoelectronics, fluorescent indicators, dye lasers, and chiral sensing. This review summarizes these various aspects of creating stereogenic centers at various positions, like α, β, meso, or at boron, in BODIPYs.

Conditional Relay Activation of Theranostic Prodrug by Pretargeting Bioorthogonal Trigger and Fluorescence-guided Visible Light Irradiation.

Bioorthogonalized light-responsive click-and-uncage platform has enabled precise cell surface engineering and timed payload release, but most of such photoactivatable prodrugs have "always-on" photoactivity leading to the dark toxicity. On the other hand, the conditionally activatable photocage is limited to the application of fluorogenic probe/photosensitizer liberation. Herein, we devise a conditionally activatable theranostic platform based on the tetrazine (Tz)-boron-dipyrromethene (BODIPY) construct, in which tetrazine serves as a quencher motif to disable both the fluorescence and photoresponsivity of BODIPY. The uncaged BODIPY upon the bioorthogonal transformation of the tetrazine reinstates fluorescence on-target to guide the subsequent visible light irradiation thereby releasing the therapeutic payload in situ. The proof of concept for such "light where is lit up"-activated theranostics was demonstrated by the tetrazine-BODIPY conjugated SN-38 (1) with masked fluorescence, up to 70 times-reduced cytotoxicity, and good photostability. Upon an inverse electron-demand Diels-Alder (IEDDA) reaction between tetrazine unit on 1 and a biotinylated trans-cyclooctene (TCO) as a pretargeted trigger on tumor cell surface, the green fluorescence from BODIPY was fully restored for cell-selective imaging, which guided the green light irradiation (500~520 nm) to activate the caged drug SN-38 exerting potent cytotoxicity against tumor cells, offering high spatiotemporal precision of disease diagnosis and therapy.

Design, Synthesis, and Chemical Characterization of Pyrene‐Substituted BODIPY Dyes

In this study, novel di‐ and tetra‐pyrenylated BODIPY (boron‐dipyrromethene) dyes (Bdpy2 and Bdpy4) were synthesized via the Suzuki‐Miyaura cross‐coupling reaction, incorporating pyrene units at specific positions on the BODIPY core. Comprehensive structural characterization was achieved through FT‐IR, UV‐Vis, MALDI‐mass spectrometry, and NMR techniques. The optical properties were extensively analyzed in different solvents, revealing Q‐band absorption maxima at 531 nm for Bdpy2 and 533 nm for Bdpy4 in THF, along with red‐shifted emission peaks due to increased π‐π stacking interactions. Bdpy4 displayed a fluorescence quantum yield of 0.68 and a fluorescence lifetime of 3.88 ns, surpassing Bdpy2's values of 0.61 and 3.44 ns, respectively. These improved values in Bdpy4 highlight the enhanced energy transfer efficiency provided by the additional pyrene groups. Theoretical TD‐DFT calculations supported the experimental findings, indicating effective energy transfer from pyrene to the BODIPY core. These findings suggest that the high fluorescence efficiency and extended lifetimes of pyrenylated BODIPY derivatives make them promising candidates for optoelectronic applications, including bioimaging and energy harvesting.

Chemical feature-based machine learning model for predicting photophysical properties of BODIPY compounds: density functional theory and quantitative structure-property relationship modeling.

Boron-dipyrromethene (BODIPY) compounds have unique photophysical properties and have been applied in fluorescence imaging, sensing, optoelectronics, and beyond. In order to design effective BODIPY compounds, it is crucial to acquire a comprehensive understanding of the relationships between the structures of BODIPY and the corresponding photoproperties. Fifteen molecular descriptors were identified to be strongly correlated with the maximum absorption wavelength. The developed ML/QSPR model exhibited good predictive performance, with coefficients of determination (R2) of 0.945 for the training set and 0.734 for the test set, demonstrating robustness and reliability. A posterior analysis of some of the selected descriptors in the model provided insights into the structural features that influence BODIPY compound properties; meanwhile, it also emphasizes the importance of molecular branching, size, and specific functional groups. This work shows that applied combined cheminformatics and machine learning approach is robust to screen the BODIPY compounds and design novel structures with enhanced performance. In the present study, all the BODIPY models studied were fully optimized, and the corresponding absorption spectrum was obtained at DFT/TDDFT//B3LYP/6-311G(d,p) level. All the above calculations were executed by the Gaussian 16 program. Based upon the theoretical computational results, the machine learning-based quantitative structure-property relationship (ML/QSPR) model was employed for predicting the maximum absorption wavelength (λ) of BODIPY compounds by combining hand-crafted molecular descriptors (MD) and explainable machine learning (EML) techniques using Scikit-learn python library. A dataset of 131 BODIPY compounds with their experimental photophysical properties was used to generate a diverse set of molecular descriptors capturing information about the size, shape, connectivity, and other structural features of these compounds using Chemaxon and Alvadesc software. A genetic algorithm (GA) variable selection together with the multi-linear regression (MLR) method were applied to develop the best predictive model using the Genetic Selection python library.

Oxidative stress-induced cytotoxicity of HCC2998 colon carcinoma cells by ZnO nanoparticles synthesized from Calophyllum teysmannii

The field of green synthesis, namely using plant extracts for the production of metal nanoparticles, is rapidly gaining traction. Therefore, this study investigated the process of producing zinc oxide nanoparticles (ZnO NPs) using a water-based extract derived from the stem bark of Calophyllum teysmannii. Notably, this is the first documented utilization of this particular plant source. The presence of a distinct Ultraviolet-Visible (UV-Vis) absorption peak at 372 nm provided evidence for the creation of ZnO nanoparticles. The X-ray Diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FESEM) investigations indicated that the nanoparticles exhibited sizes ranging from 31.5 to 59.9 nm and had spherical morphologies. Energy Dispersive X-ray Diffractometer (EDX) analysis verified the elemental composition of the ZnO nanoparticles, whereas the Fourier Transform Infrared (FTIR) spectra showed clear peaks, demonstrating their production. The FTIR examination of the C. teysmannii extract revealed peaks at around 3370 cm− 1, indicating the presence of phenolic compounds. These chemicals are likely responsible for the reduction and stabilization of the ZnO NPs. The high-resolution X-ray Photoelectron Spectroscopy (XPS) spectra clearly revealed separate peaks corresponding to Zn 2p and O 1s, providing confirmation of the chemical states and bonding contexts. The Raman Spectroscopy analysis revealed a distinct peak at around 425 cm⁻¹, confirming the presence of the wurtzite structure. The harmful effects of ZnO nanoparticles on HCC2998 (a kind of human colon cancer) and Vero (a type of monkey kidney epithelial) cells were evaluated using 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT), dichlorodihydrofluorescein diacetate (DCFH-DA), and boron-Dipyrromethene (BODIPY) assays. The cancer cells underwent cell death due to oxidative stress in a dose-dependent manner, as confirmed by microscopic and flow cytometry investigations.

A New Strategy for Photo‐Electrochemical Reduction of Carbon Dioxide Using a Carbazole‐BODIPY Based Metal‐Free Catalyst

AbstractIn this study, a cross‐linked boron dipyrromethene (BODIPY) photocatalyst containing a carbazole donor group designed for photoelectrocatalytic carbon dioxide (CO2) reduction is synthesized and characterized. The BODIPY‐based system, coated onto a platinum surface, is evaluated for its electrochemical and photocatalytic performance under light illumination. Cyclic voltammetry (CV) and chronoamperometry measurements reveals enhanced photocurrent responses, confirming the catalyst's ability to effectively drive CO2 reduction. Gas chromatography/mass spectrometry (GC‐MS) analysis identifies the formation of ethanol (C2H5OH) as a major reaction product, showing that its yield increased with extended reaction times. Additionally, the photocatalyst demonstrates remarkable performance with significantly increasing turnover numbers (TON) and turnover frequencies (TOF) over time, indicating stable and sustained catalytic activity. With a Faradaic efficiency of 34.79% at a potential of ‐1.15 V, this BODIPY system exhibits both high activity and long‐term stability. The combination of efficient electron transfer and visible light absorption by the carbazole‐BODIPY donor‐acceptor structure positions this system as a highly promising candidate for sustainable CO2 conversion applications.

Synthesis of a β-Nitrile-BODIPY from its β-Aldoxime Through Dehydration by Triphosgene: Comparative Spectroscopic Characterization

A high yielding (84%) transformation of a β-aldoxime-Boron dipyrromethenes (BODIPY) to its β-nitrile was achieved through a dehydration reaction employing in situ generated phosgene (from triphosgene and triethylamine). The two new organic dyes were comparatively and comprehensively characterized by 1H and 13C nuclear magnetic resonance, high-resolution mass spectrometry, infrared spectroscopy, Ultraviolet-vis absorption, and fluorescence emission spectroscopy (both steady-state and time-correlated). The significantly reduced emission and shorter excited-state lifetimes of β-oxime- BODIPY, compared to the β-nitrile derivative, were attributed to non-radiative decay processes facilitated by C=N bond isomerization in the excited state. This study highlights the effectiveness of triphosgene in facilitating the seamless, rapid conversion of the β-aldoxime to β-nitrile functionality in the model 1,3,5,7-tetramethyl-BODIPY dye under mild reaction conditions.. KEYWORDS :Aldoxime, Boron dipyrromethenes dyes, Dehydration reaction, Fluorescence, Nitrile, Phosgene.

Computational Investigation of Meso-Substituted, Heavy Atom-Free BODIPY Derivatives as Photosensitizers: Insights From TDDFT and Dynamics Studies.

This study investigates the structural and electronic properties of BODIPY (BDP) derivatives featuring meso-substituted donors arranged orthogonally, leveraging Time-Dependent Density Functional Theory (TD-DFT). These deriva-tives, selected based on experimental evidence of their quantum yield towards singlet oxygen generation, exhibit intricate excited-state dynamics, transitioning from fluorescence to intersystem crossing (ISC), thereby presenting a promising avenue for applications in photodynamic therapy. Emphasizing heavy-atom-free organic triplet photosensitizers, with BDP dyes highlighted for their exceptional adaptability in photophysical characteristics, our analysis contributes to a deeper understanding of the fundamental design principles governing such photosensitizers. Through a combined approach of static and dynamic calculations, we elucidate the mechanisms underlying the population transfer from singlet to triplet states, thereby providing valuable insights for the development of efficient photodynamic therapy agents.

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.

Catalytic Enantioselective Synthesis of Boron-Stereogenic and Axially Chiral BODIPYs via Rhodium(II)-Catalyzed C-H (Hetero) Arylation with Diazonaphthoquinones and Diazoindenines.

The molecular engineering of boron dipyrromethenes (BODIPYs) has garnered widespread attention due to their structural diversity enabling tailored physicochemical properties for optimal applications. However, catalytic enantioselective synthesis of structurally diverse boron-stereogenic BODIPYs through intermolecular desymmetrization and BODIPYs with atroposelectivity remains elusive. Here, we showcase rhodium(II)-catalyzed site-specific C-H (hetero)arylations of prochiral BODIPYs and polysubstituted BODIPYs with diazonaphthoquinonesand diazoindenines, providing efficient pathways for the rapid assembly of versatile (hetero)arylated boron-stereogenic and axially chiral BODIPYs through long-range desymmetrization and axial rotational restriction modes. The synthetic application of the procedures has been emphasized by the efficient synthesis of BODIPY derivatives with various functions. Photophysical properties, bioimaging, and lipid droplet-specific targeting capability of tailored BODIPYs are also demonstrated, indicating their promising applications in biomedical research, medicinal chemistry, and material science.

Catalytic Enantioselective Synthesis of Boron‐Stereogenic and Axially Chiral BODIPYs via Rhodium(II)‐Catalyzed C−H (Hetero) Arylation with Diazonaphthoquinones and Diazoindenines

AbstractThe molecular engineering of boron dipyrromethenes (BODIPYs) has garnered widespread attention due to their structural diversity enabling tailored physicochemical properties for optimal applications. However, catalytic enantioselective synthesis of structurally diverse boron‐stereogenic BODIPYs through intermolecular desymmetrization and BODIPYs with atroposelectivity remains elusive. Here, we showcase rhodium(II)‐catalyzed site‐specific C−H (hetero)arylations of prochiral BODIPYs and polysubstituted BODIPYs with diazonaphthoquinonesand diazoindenines, providing efficient pathways for the rapid assembly of versatile (hetero)arylated boron‐stereogenic and axially chiral BODIPYs through long‐range desymmetrization and axial rotational restriction modes. The synthetic application of the procedures has been emphasized by the efficient synthesis of BODIPY derivatives with various functions. Photophysical properties, bioimaging, and lipid droplet‐specific targeting capability of tailored BODIPYs are also demonstrated, indicating their promising applications in biomedical research, medicinal chemistry, and material science.

Chemically anchored black phosphorus/chromophore composite luminescent materials towards solid-state photoluminescence and electroluminescence

As a promising two-dimensional (2D) material, black phosphorus (BP) has gained great attention because of its intrinsic physicochemical characteristics. However, BP is easy to be oxidized and form various surface defects, which decreases the charge transport and makes luminescence difficult. Herein, we report a facile strategy to synthesize chemically anchored BP/chromophore composite luminescent materials by integrating 2D BP nanosheets with boron dipyrromethene (BODIPY) dyes through simple sonication reactions. Solids of BP composite materials are brightly fluorescent with 5 orders of magnitude increase of photoluminescence intensity compared with the pure solids of BP and BODIPY. 2D BP nanosheets with large specific surface area and lone pair electrons facilitate chemisorption of BODIPY by forming covalent interactions between P and B/N atoms, which not only passivates the surface of BP and eliminates the oxidation, but also prevents the aggregation caused quenching (ACQ) issue of BODIPY. Furtherly, the organic light-emitting diodes (OLEDs) based on the BP/chromophore composite luminescent materials were first successfully achieved, the electroluminescence spectra of which cover the full spectrum of red, green, and blue visible light. This work provides a brand-new approach to further design and development of high mobility and high luminous efficiency composite luminescent materials and devices based on BP.

BODIPY-based fluorescent sensor material for airborne acetone and benzene with aggregation-driven selectivity

Paper highlights advances in design of fluorescent sensors for solvent vapors with boron-dipyrromethene (BODIPY) as a model active component. The present study demonstrates that BODIPY-based fluorescent materials could be successfully utilized for detection of benzene and acetone in gas phase. Concentration of luminophore was found to affect sensory response to benzene (up to 3-fold increase in adjusted conditions). At the same time, sensory response to acetone is concentration-independent, suggesting different underlying response mechanisms and allowing to tune selectivity with choice of concentration. Detection limits achieved for sensory materials are 1.2·104 mg/m3 for acetone and 0.94·104 mg/m3 for benzene. Ultimately, obtained sensor materials could be completely regenerated without need for inert gas purge.

Tetraarylpyrrolo[3,2-b]pyrrole-BODIPY dyad: a molecular rotor for FRET-based viscosity sensing.

With the aim to develop a FRET-based viscosity sensor, two dyad molecules, 4 and 5, comprising tetraarylpyrrolo[3,2-b]pyrrole (TAPP) (donor) and naked boron-dipyrromethene (BODIPY) dyes (acceptor), were designed. Dyads were synthesized via acid-catalyzed multicomponent reactions followed by Sonogashira coupling. In both dyads, the BODIPY and TAPP moieties are linked through phenylethynyl groups, which allow free rotation of the BODIPY dyes; that is, they can act as molecular rotors. This was supported by X-ray crystallographic and DFT-optimized structures. Spectroscopic studies also confirmed the presence of both TAPP and BODIPY dyes in dyads with no electronic interactions that are suitable for fluorescence resonance energy transfer (FRET). Very high energy transfer efficiency (ETE >99%) from the donor TAPP moiety to the acceptor BODIPY moiety on excitation at the TAPP part was observed. However, due to the non-fluorescent nature of naked BODIPY dyes, no fluorescence emission was observed from the BODIPY moiety in both dyads. With increasing solvent viscosities, emission from the BODIPY moieties increases due to the restricted rotation of the BODIPY moieties. Plotting the logarithms of the fluorescent intensity of dyad 5 and the viscosity of the solution showed a good linear correlation obeying a Förster-Hoffmann equation. Non-fluorescent dyad 5 in methanol became greenish-yellow fluorescent in a methanol/glycerol (1:1) solvent. Furthermore, with an increase in the temperature of the methanol/glycerol (1:1) system, as the viscosity decreases, the fluorescence also starts decreasing. Thus, dyad 5 is capable of sensing the viscosity of the medium via a FRET-based "Off-On" mechanism. This type of viscosity sensor with a very large pseudo-Stokes shift and increased sensitivity will be useful for advancing chemo-bio sensing and imaging applications.

Excitation energy transfer based naphthalimide-boron-dipyrromethene dyads as two-photon fluorescent probes for palladium(0) detection and live cell imaging

Increasing industrial demand has accelerated the consumption and contamination of palladium (Pd) worldwide. The implementation of fluorescent probes, especially with two-photon excitation, holds great promise for Pd detection in environmental and biological analysis. In this work, two naphthalimide (NPI)-boron-dipyrromethene (BDP) dyads (named NBH and NBN) were developed as two-photon fluorescent probes for Pd(0) detection based on an excitation energy transfer (EET) strategy, employing NPI as the energy donor and BDP as the acceptor. Spectroscopic studies revealed that both probes exhibited a fast response to Pd(0) in a pronounced fluorescence "ON-OFF" manner. The Pd-catalyzed Tsuji-Trost reaction activated an intramolecular photoinduced electron transfer (PET) process between the aniline and BDP moieties, resulting in the fluorescence quenching of the BDP fluorophore. The reaction-based probes exhibited excellent sensitivity and selectivity for Pd(0) species with a detection limit as low as 2.4 nM. These probes were successfully applied to the determination of Pd residues in drug, soil, and water samples. A simple smartphone-based platform was also constructed to determine Pd(0) via an RGB reader. With negligible cytotoxicity, confocal imaging study validated their feasibility of monitoring Pd(0) in living cells through multiple excitation channels.