Red-Emitting SBBF (Single-Benzene-Based Fluorophore)-Silica Hybrid Material: One-Pot Synthesis, Characterization, and Biomedical Applications

Aggregation-Induced Modulation of Ground and Excited State Photophysics of 5-(tert-Butyl)-2-Hydroxy-1,3-Isophthalaldehyde (5-tBHI)

5-(tert-Butyl)-2-hydroxy-1,3-isophthalaldehyde (5-tBHI) is a photochromic material susceptible to either excited state proton transfer or excited state intramolecular proton transfer, depending upon the solvent. However, it has also been found to aggregate in the presence of sodium dodecyl sulfate. In this current study, based on the steady-state and time-resolved spectroscopy, supported by crystallography, quantum chemical density functional theory calculation, and molecular dynamics (MD) simulation, we report on the aggregation of this potential single benzene-based emitter (SBBE) in neat solvents as well as solid phase to modulate its photophysics. It has been found that 5-tBHI forms mixed aggregates of different orders, owing to the presence of both enolic and tautomeric forms, to yield tunable emission, although the emission intensity is quenched. These findings suggest that the intramolecular hydrogen bonding of 5-tBHI not only limits intermolecular interactions but also promotes nonradiative deactivation pathways. Hence, designing and structural engineering, with a focus to suppressing intramolecular hydrogen bonding as well as increasing through space conjugation by replacing the aldehydic moieties with bulky aliphatic or aromatic ketonic groups, can be a plausible approach to yielding improved probes with tunable emission and higher fluorescence quantum yields.

Recent advances in minimal fluorescent probes for optical imaging

Fluorescent probes have revolutionized biological imaging by enabling the real-time visualization of cellular processes under physiological conditions. However, their size and potential perturbative nature can pose challenges in retaining the integrity of biological functions. This manuscript highlights recent advancements in the development of small fluorescent probes for optical imaging studies. Single benzene-based fluorophores offer versatility with minimal disruption, exhibiting diverse properties like aggregation-induced emission and pH responsiveness. Fluorescent nucleobases enable precise labeling of nucleic acids without compromising function, offering high sensitivity and compatibility with biochemistry studies. Bright yet small fluorescent amino acids provide an interesting alternative to bulky fusion proteins, facilitating non-invasive imaging of cellular events with high precision. These miniaturized fluorophores promise enhanced capabilities for studying biological systems in a non-invasive manner, fostering further innovations in molecular imaging.

Design of 2-Pyridone Fluorophores for Brighter Emissions at Longer Wavelengths

The recent discovery of blue fluorophores with high quantum yields based on pyridone structures inspired the development of new low-molecular-weight fluorophores with bright emissions at tunable wavelengths, which are highly attractive for various applications. In this study, we propose a rational design strategy for 2-pyridone-based fluorophores with bright emissions at long wavelengths. With a detailed understanding of the positional substitution effects on each carbon atom of the 2-pyridone core, we developed a bright blue fluorophore (λabs =377 nm; λem =433 nm; ϵ=13,200 M-1  cm-1 ; ϕF =88 %) through C3 -aryl and C4 -ester substitutions followed by cyclization. Furthermore, by applying the intramolecular charge transfer (ICT) principle, we invented a bright green fluorophore through C3 - and C4 -diester and C6 -aryl substitutions. The ICT fluorophore based on the pyridone structure shows large molar absorptivity (ϵ=20,100 M-1  cm-1 ), longer emission wavelength (λem =539 nm), high emission quantum yield (ϕF =74 %), and large Stokes shift (Δv=5720 cm-1 ), which are comparable to those of practical fluorescent probes.

meta-Fluorophores: an uncharted ocean of opportunities

meta-Fluorophores (MFs) are unique ultra-light (in terms of molecular weight (MW)) fluorophores exhibiting luminescence with a wide colour gamut ranging from blue to the NIR. Single benzenic MFs are easy to synthesize, are quite bright (with photoluminescence quantum yield (PLQY) as high as 63%) and exhibit very large Stokes shift (as high as 260 nm (8965 cm-1)), with large solvatochromic shift (as high as 175 nm), and very long excited-state-lifetime (as high as 26 ns) for such ultra-light fluorophores. An emission maximum of ≥600 nm has been achieved with an MF in a polar medium having a MW of only 177 g mol-1 and in a nonpolar medium having MW of only 255 g mol-1; therefore, a large-sized π-conjugated para-fluorophore is no longer a prerequisite for red/NIR emission. Structurally varied MFs pave the way for creating an ocean of opportunities and are thus promising for replacing para-fluorophores for different applications, ranging from bioimaging to LEDs.

Miniaturized Chemical Tags for Optical Imaging

Recent advances in optical bioimaging have prompted the need for minimal chemical reporters that can retain the molecular recognition properties and activity profiles of biomolecules. As a result, several methodologies to reduce the size of fluorescent and Raman labels to a few atoms (e.g., single aryl fluorophores, Raman‐active triple bonds and isotopes) and embed them into building blocks (e.g., amino acids, nucleobases, sugars) to construct native‐like supramolecular structures have been described. The integration of small optical reporters into biomolecules has also led to smart molecular entities that were previously inaccessible in an expedite manner. In this article, we review recent chemical approaches to synthesize miniaturized optical tags as well as some of their multiple applications in biological imaging.