Targeted Photoconvertible BODIPYs Based on Directed Photooxidation-Induced Conversion for Applications in Photoconversion and Live Super-Resolution Imaging

A BODIPY-tagged trivalent glycocluster for receptor-targeting fluorescence imaging of live cells

Multivalent glycoclusters have been extensively used as a targeting agent for drug delivery. However, tools capable of investigating their dynamic interactions with a target receptor remain elusive. Here, we synthesized fluorescently-tagged galactoclusters for the fluorescence imaging of cells that overly express the asialoglycoprotein receptor (ASGPr). A trivalent galactoside was synthesized, to which a boron dipyrromethene (BODIPY) dye was conjugated. The resulting fluorescent glycocluster was used for the targeted fluorescence imaging of liver cancer cells with a high ASGPr expression level. The trivalent probe was also demonstrated to be applicable for super-resolution imaging of ASGPr-mediated ligand endocytosis and the dynamic intracellular translocation to the lysosomes. As such, this study provides a suitable chemical tool for the study of receptor dynamics using fluorescently tagged glycoclusters.

Spontaneously blinking spiroamide rhodamines for live SMLM imaging of the plasma membrane

We have developed spontaneously blinking fluorescent probes based on the reversible spirolactamization of rhodamine, to efficiently image the plasma membrane (PM) of live cells with enhanced resolution using SMLM. This study demonstrates that the blinking efficiency of spiroamide PM probes is not solely governed by their pKa; the presence of a charged polar group on the amide should also be taken into account.

A Photoactivatable Plasma Membrane Probe Based on a Self-Triggered Photooxidation Cascade for Live Cell Super-Resolution Microscopy

Super-resolution imaging based on the localization of single emitters requires a spatio-temporal control of the ON and OFF states. To this end, photoactivatable fluorophores are adapted as they can be turned on upon light irradiation. Here, we present a concept called self-triggered photooxidation cascade (STPC) based on the photooxidation of a plasma membrane-targeted leuco-rhodamine (LRhod-PM), a non-fluorescent reduced form of a rhodamine probe. Upon visible light irradiation the small number of oxidized rhodamines, Rhod-PM, acts as a photosensitizer to generate singlet oxygen capable of oxidizing the OFF state LRhod-PM thereby switching it to its ON state. We showed that this phenomenon is kinetically favored by a high local concentration and propagates quickly when the probe is embedded in membrane bilayers. In addition, we showed that the close proximity of the dyes favors the photobleaching. At the single-molecule level, the concomitant activation/bleaching phenomena allow reaching a single-molecule blinking regime enabling single-molecule localization microscopy for super-resolution of live cellular membranes and their thin processes including filopodia and tuneling nanotubes.

Crystallization-Induced Emission Enhancement or Quenching? Elucidating the Mechanism behind Using Single-Molecule-Based Versatile Crystals

It is challenging to predict optical properties of fluorescent dyes, especially in the crystalline state, owing to the uncertainty in conformation, packing, and coupling. Herein, we elucidate the decisive role of molecular conformation and molecular packing in the fluorescence emissions of some crystalline materials based on experimental results and theoretical calculations. Two homologous fluorophores (Ph-MP and Ph-HP) were synthesized, and they both exhibited interesting crystallization-induced emission enhancement and quenching. Although the homologues show almost the same fluorescence behavior in the solid state, on-off emission of their crystals depends upon different factors. Emission of the Ph-MP crystals is governed by the twisted intramolecular charge transfer effect, while emission of the Ph-HP crystals relied on π-π stacking. Based on this understanding, application of single-molecule-based versatile crystals in information encryption was demonstrated. It is believed that the evidence and unveiled mechanism for the effect of crystallization on emission will contribute to development in high-performance luminescent materials.

Synthesis, structure, and spectral and electrochemical properties of new visible to NIR absorbing 3-pyrrolyl BODIPY derivatives

We report the synthesis, characterization, and studies of novel 3-pyrrolyl BODIPY-based Schiff base products 3-6 and 3-pyrrolyl BODIPY-based benzo[d]thiazol-2-yl derivatives 7-8. The Schiff base compounds 3-6 were synthesized via condensation of α-formyl 3-pyrrolyl BODIPY with various amine derivatives, while the Knoevenagel condensation products 7-8 were obtained by reacting α-formyl 3-pyrrolyl BODIPY with 2-(benzo[d]thiazol-2-yl) acetonitrile and bis(benzo[d]thiazol-2-yl) methane, respectively. The compounds were thoroughly characterized by using HR-MS, 1D and 2D NMR spectroscopy, and X-ray crystallography for two compounds. The crystal structures revealed distinct conformational differences between the Schiff base product 3 and the Knoevenagel condensation product 8. In compound 3, the appended pyrrole was oriented towards the BF2-dipyrrin core and placed almost in the same plane, while in 8, the pyrrole was inverted, deviated from the BF2-dipyrrin plane, and aligned with the bis(benzothiazolyl) moiety, which adopted a transoid configuration. The Schiff base compounds 3-6 exhibited absorption bands in the region of 610-665 nm, whereas the benzo[d]thiazol-2-yl derivatives (7 and 8) showed enhanced optical properties with a red shifted absorption band that extended into the NIR region. These structural modifications of the 3-pyrrolyl BODIPY chromophore enable precise tuning of their electronic, absorption, and emission properties from the red to the NIR region. Furthermore, we also synthesized a 3-pyrrolyl BODIPY-Re(I) complex 8-Re(I) using bis(benzo[d]thiazol-2-yl)-3-pyrrolyl BODIPY 8. In the 8-Re(I) complex, the bis(benzothiazolyl) moiety adopted a cisoid configuration with a bite angle of 41.60°, and the Re(I) center exhibited a distorted octahedral geometry with a boat-like conformation. In the 8-Re(I) complex, the Re(I) was coordinated to three axial CO ligands, two nitrogen atoms from the bis(benzothiazolyl) units, and one chloride atom. DFT and TD-DFT studies corroborated our experimental findings, providing deeper insights into these compounds' structural, photophysical, and electronic properties. Overall, this study demonstrates the versatility of 3-pyrrolyl BODIPY derivatives in tuning absorption properties from the visible to the NIR region and their potential in forming stable Re(I) chelates, highlighting their potential in the development of NIR fluorophores and coordination chemistry.

Introducing of an Unexplored Aza-BODIPY Diradicaloids with 4-(2,6-Ditert-butyl)phenoxyl Radicals Located in 1,7-Positions of the Aza-BODIPY Core

We have prepared and characterized two diradicaloid systems 5a and 5b that originated from the oxidation of a 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPY core. The aza-BODIPY diradicaloids were characterized by a large array of experimental and computational methods. The diamagnetic closed-shell state was postulated as the ground state in solution and a solid-state with the substantial thermal population originating from both open-shell diradical and open-shell triplet states observed at room temperature. Transient absorption spectroscopy indicates fast (<10 ps) excited state deactivation pathways associated with the target compounds' diradical character in solution at room temperature. Variable-temperature 1H NMR spectra indicate the solvent dependency of the diradical character in 5a and 5b. The diradicaloids could be stepwise reduced to the mixed-valence radical-anion and dianion states upon consequent single-electron reductions. Similarly, deprotonated 1,7-(4-(2,6-di-tert-butyl)phenol)-substituted aza-BODIPYs can be oxidized to the diradicaloid form. Both mixed-valence and dianionic forms exhibit an intense absorption in the NIR region. Density functional theory (DFT) and time-dependent DFT calculations were used to explain the transformations in the UV-Vis-NIR spectra of all target compounds.

Electron-Deficient Organic Molecules Based on B←N Unit: A N-Type Room-Temperature Chemiresistive Sensors with Moisture Resistance

Organic molecules with tailorable chemical structures, high stability, and solution processability have great potential in the sensing field. Compared with p-type organic small molecules (OSMs), the electron-dominated n-type analogs show superior conductivity when exposed to reducing gases, which can achieve outstanding sensor signal-to-noise ratios. However, inadequate humidity resistance at room temperature hinders the development of such molecules. Herein, an A-D-π-D-A molecular design strategy is proposed based on electron-deficient B←N units, which results in effective intramolecular charge transport and sensitive responses by extending the π-conjugation bridge. As a result, the ST-2BP with A-D-π-D-A configuration shows a prominent sensitivity of 787 (Ra/Rg) in 20 ppm NH3 at room temperature and an almost initial and stable response under different relative humidity conditions, which is the highest among currently reported OSM sensors. Supported by theoretical calculations and in situ FTIR spectra, it is revealed that B←N units, which function as the active centers mediate the specific ammonia adsorption. This study provides a new understanding of the design of high-performance room temperature gas sensing materials by decorating B←N units.