Effect of the Aza-N-Bridge and Push-Pull Moieties: A Comparative Study between BODIPYs and Aza-BODIPYs

Near-Infrared Absorbing Aza-BODIPY Dyes for Optoelectronic Applications

Organic dyes that absorb light in the visible to near-infrared region have garnered significant interest, owing to their extensive utility in organic photovoltaics and various biomedical applications. Aza-boron-dipyrromethene (Aza-BODIPY) dyes are a class of chromophores with impressive photophysical properties such as tunable absorption from the visible region towards near infrared (NIR) region, high molar absorptivity, and fluorescence quantum yield. In this review, we discuss the developments in the aza-BODIPYs, related to their synthetic routes, photophysical properties and their applications. Their design strategies, modifications in chemical structures, mode/position of attachment, and their impact on photo-physical properties are reviewed. The potential applications of aza-BODIPY derivatives such as organic solar cells, photodynamic therapy, boron-neutron capture therapy, fluorescence sensors, photo-redox catalysis, photoacoustic probes and optoelectronic devices are explained.

Ratiometric fluorescence nanoscopy and lifetime imaging of novel Nile Red analogs for analysis of membrane packing in living cells

Subcellular membranes have complex lipid and protein compositions, which give rise to organelle-specific membrane packing, fluidity, and permeability. Due to its exquisite solvent sensitivity, the lipophilic fluorescence dye Nile Red has been used extensively to study membrane packing and polarity. Further improvement of Nile Red can be achieved by introducing electron-donating or withdrawing functional groups. Here, we compare the potential of derivatives of Nile Red with such functional substitutions for super-resolution fluorescence microscopy of lipid packing in model membranes and living cells. All studied Nile Red derivatives exhibit cholesterol-dependent fluorescence changes in model membranes, as shown by spectrally resolved stimulated emission depletion (STED) microscopy. STED imaging of Nile Red probes in cells reveals lower membrane packing in fibroblasts from healthy subjects compared to those from patients suffering from Niemann Pick type C1 (NPC1) disease, a lysosomal storage disorder with accumulation of cholesterol and sphingolipids in late endosomes and lysosomes. We also find small but consistent changes in the fluorescence lifetime of the Nile Red derivatives in NPC1 cells, suggesting altered hydrogen-bonding capacity in their membranes. All Nile Red derivatives are essentially non-fluorescent in water but increase their brightness in membranes, allowing for their use in MINFLUX single molecule tracking experiments. Our study uncovers the potential of Nile Red probes with functional substitutions for nanoscopic membrane imaging.

Rational design and investigation of nonlinear optical response properties of pyrrolopyrrole aza-BODIPY-based novel push-pull chromophores

Intramolecular charge transfer (ICT)-based chromophores are highly sought after for designing near-infrared (NIR) absorbing and emitting dyes as well as for designing materials for nonlinear optical (NLO) applications. The properties of these 'push-pull' molecules can easily be modified by varying the electronic donor (D) and acceptor (A) groups as well as the π-conjugation linker. This study presents a methodical approach and employs quantum chemical analysis to explore the relationship between the structural features, electro-optical properties, and the NLO characteristics of molecules with D-π-A framework. The one- and two-photon absorption (2PA), linear polarizability (α), and first hyperpolarizability (β) of some novel chromophores, consisting of a dimeric aza-Boron Dipyrromethene (aza-BODIPY) analogue, called, pyrrolopyrrole aza-BODIPY (PPAB), serving as the acceptor, have been investigated. The electronic donors used in this study are triphenylamine (TPA) and diphenylamine (DPA), and they are conjugated to the acceptor via thienyl or phenylene π-linkers. Additionally, the Hyper-Rayleigh Scattering (βHRS), which enables direct estimation of the second-order NLO properties, is calculated for the studied chromophores with 1064 nm excitation in acetonitrile. The β value shows a significant increase with increasing solvent polarity, indicating that the ICT plays a crucial role in shaping the NLO response of the studied molecules. The enhancement of the 2PA cross-section of the investigated molecules can also be achieved by modulating the combinations of donors and linkers. The results of our study indicate that the novel D-π-A molecules designed in this work demonstrate considerably higher hyperpolarizability values than the standard p-nitroaniline, making them promising candidates for future NLO applications.

Tracking the Intramolecular Charge Transfer Process of 2,6-Substituted D-A BODIPY Derivatives

Three 2,6-electron donor-substituted boron dipyrromethene (BODIPY) exhibiting an intramolecular charge transfer (ICT) character with large Stokes shift and moderate fluorescence quantum yields were designed and synthesized. Broadband femtosecond transient absorption (fs-TA) spectroscopy measurements were performed to directly detect the CT state in nonpolar or less polar solvents and the charge separation (CS) state in more polar solvents. A solid foundation for the fs-TA assignment can be found in electrolysis experiments. In addition, the ICT character of the newly designed compounds was investigated by density functional theory (DFT) calculations. Meanwhile, the reference compounds without the donor groups were synthesized, and their photophysical behaviors and ultrafast time-resolved spectra confirmed that no ICT process occurred regardless of the nature of the solvent. This work emphasizes the importance of decorating the BODIPY core with electron-donating substituents at 2,6-positions to efficiently adjust its photofunctional behaviors demonstrating the ICT character. Importantly, the photophysical processes could be easily regulated by changing the solvent with different polarities.

Red-to-Near-Infrared Emitting PyrrolylBODIPY Dyes: Synthesis, Photophysical Properties and Bioimaging Application

Near-infrared (NIR) fluorophores with characteristics such as deep tissue penetration, minimal damage to the biological samples, and low background interference, are highly sought-after materials for in vivo and deep-tissue fluorescence imaging. Herein, series of 3-pyrrolylBODIPY derivatives and 3,5-dipyrrolylBODIPY derivatives have been prepared by a facile regioselective nucleophilic aromatic substitution reaction (S_N Ar) on 3,5-halogenated BODIPY derivatives (3,5-dibromo or 2,3,5,6-tetrachloroBODIPYs) with pyrroles. The installation of a pyrrolic unit onto the 3-position of the BODIPY chromophore leads to a dramatic red shift of both the absorption (up to 160 nm) and the emission (up to 260 nm) in these resultant 3-pyrrolylBODIPYs with respect to that of the BODIPY chromophore. Their further 5-positional functionalization provides a facile way to fine tune their photophysical properties, and these resulting dipyrrolylBODIPYs and functionalized pyrrolylBODIPYs show strong absorption in the deep red-to-NIR regions (595-684 nm) and intense NIR fluorescence emission (650-715 nm) in dichloromethane. To demonstrate the applicability of these functionalized pyrrolylBODIPYs as NIR fluorescent probes for cell imaging, pyrrolylBODIPY 6 a containing mitochondrion-targeting butyltriphenylphosphonium cationic species was also prepared. It selectively localized in mitochondria of HeLa cells, with low cytotoxicity and intense deep red fluorescence emission.

Attachment of −tBu groups to aza-BODIPY core at 3,5-sites with ultra-large Stokes shift to enhance photothermal therapy through apoptosis mechanism

By the introduction of the −tBu groups into aza-BODIPY core, di-tert-butyl-substituted aza-BODIPYs at 3,5-sites (tBuazaBDPs) were prepared for the first time. Based on the X-ray analysis of CN-tBuazaBDP, this molecular structure is twisted. Near-infrared dye SMe-tBuazaBDP has the ultra-large Stokes shift (152 ​nm) in aza-BODIPY system, combining with the twisted intramolecular charge transfer and the free rotation of the −tBu groups at 3,5-sites. Although the barrier-free rotors of the distal −tBu groups in SMe-tBuazaBDP result in low fluorescence quantum yield, the photothermal conversion efficiency is markedly enhanced. SMe-tBuazaBDP nanoparticles with low power laser irradiation were proven to block cancer cell cycle, inhibit cancer cell proliferation, and induce cancer cell apoptosis in photothermal therapy (PTT). The strategy of “direct attachment of −tBu groups to aza-BODIPY core” gives a new design platform for a photothermal therapy agent.

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Di-tert-butyl-substituted aza-BODIPYs at 3,5-sites (tBuazaBDPs) were prepared for the first time.

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Near-infrared dye SMe-tBuazaBDP has the ultra-large Stokes shift (152 ​nm) in aza-BODIPY system.

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SMe-tBuazaBDP ​nanoparticles can photothermally induce apoptosis as a potential photothermal therapy agent.