Small is Beautiful: Electronic Origin and Synthetic Evolution of Single-Benzene Fluorophores

Stepwise Modulation of Bridged Single-Benzene-Based Fluorophores for Materials Science

In recent years, researchers studying fluorogenic samples have steadily shifted from using large, expensive, poorly soluble fluorophores with complex synthetic sequences to smaller, simpler π scaffolds with low molecular weight. This research article presents an in-depth study of the photophysical properties of five bridged single-benzene-based fluorophores (SBBFs) investigated for their solution and solid-state emission (SSSE) properties. The compounds O4, N1O3, N2O2, N3O1, and N4 are derived from a central terephthalonitrile core and vary in the amount of oxygen and nitrogen bridging atoms. These minimalized emitters show full-color tunable emission properties and exhibit moderate-to-high photoluminescence quantum yield values reaching up to 0.78 in dimethyl sulfoxide (DMSO). In addition to demonstrating excellent compatibility in poly(methyl methacrylate) (PMMA) films and additive manufacturing using stereolithography (SLA), white light emission was achieved in both solution and 3D-printed materials by controlling the mixing ratio of the compounds. Employing density-functional theory (DFT), well-correlating theoretical absorption and emission wavelengths were calculated as average values of the different possible conformers. Furthermore, cellular internalization of the substances was accomplished using Pluronic® F-127 nanoparticles. Overall, this study emphasizes the remarkable properties of single-benzene-based emitters, showcasing their accessibility and potential applications in biomedical fields and materials science.

Precision Molecular Engineering of Compact Near-Infrared Fluorophores

Organic fluorophores with near-infrared (NIR) emission and reduced molecular size are crucial for advancing bioimaging and biosensing technologies. Traditional methods, such as conjugation expansion and heteroatom engineering, often fail to reduce fluorophore size without sacrificing NIR emission properties. Addressing this challenge, our study utilized quantum chemical calculations and structure-property relationship analysis to establish an iterative design approach and enable precision engineering for compact, single-benzene-based NIR fluorophores. These newly developed fluorophores exhibit emissions up to 759 nm and maintain molecular weights as low as 192 g/mol, approximately 50% of that of Cy7. Additionally, they display unique environmental sensitivity─nonemissive in aqueous solutions but highly emissive in lipid environments. This property significantly enhances their utility in wash-free imaging of live cells. Our findings mark a substantial breakthrough in fluorophore engineering, paving the way for more efficient and adaptable imaging methodologies.

Molecular and Electronic Structure and Properties of the Single Benzene-Based Fluorophores Containing Guanidine Subunit

The Gibbs energies of protonation (ΔGp) and the basic photophysical properties for single-benzene fluorophores (SBFs) containing guanidine and/or amino subunits and the changes that occur upon protonation were modeled by the TDDFT approach. The calculated ΔGp energies for amino SBFs in the S1 state range from 985 to 1100 kJ mol-1 which are below the values for guanidines. The protonation of the guanidine-SBFs induces a hypsochromic shift of the absorption and the emission maxima with the Stokes shift of > 100 nm in both cases. Isomerization through the ESIPT process is less probable than in amino-SBFs due to the unfavorable thermodynamics. Still, if it occurs, it leads to a strong red shift of the emission by > 150 nm. Aromaticity indices point to strong antiaromatic character of the examined guanidino-SBFs in the FC region which decreases upon geometrical relaxation and ESIPT. The excited state proton transfer occurs in guanidine-SBF/phenol complexes in the S1 state, stabilizing CT states and fluorescence quenching.

Positional effects of electron-donating and withdrawing groups on the photophysical properties of single benzene fluorophores

We investigated how the positional arrangement of electron-donating (amino) and electron-withdrawing (ester) groups in single benzene-based fluorophores influences their emission properties. By synthesizing 26 regioisomeric fluorophores, we achieved wavelength modulation from 322 to 539 nm, revealing key correlations between functional group positioning and photophysical behavior.

Amino-Terephthalonitrile and Amino-Terephthalate-Based Single Benzene Fluorophores - Compact Color Tunable Molecular Dyes for Bioimaging and Bioanalysis

This review article discusses the emerging amino-terephthalonitrile (Am-TN) and amino terephthalate-based single benzene fluorophores (SBFs) for their highly emissive nature and potential for numerous technical applications. Am-TN-SBFs are a new class of SBFs having amine as the electron donating (EDG) and dinitrile as the electron withdrawing group (EWG). The beauty of these Am-TN-SBFs lies in excellent intramolecular charge transfer between the EDG and EWG. The placement of two nitrile groups in para-position on the benzene ring allows better charge transfer from the donating amines to the linear nitrile group leading to the strongly emissive nature. We also outline here the latest developments in the well-known family of amino terephthalate SBFs reported in the last 2 to 3 years. Amino terephthalate SBFs have esters as the EWG and amine as the EDG. These have intramolecular H-bonding between the EDG and EWG which is responsible for their emissive behavior.

Urea-fused and π-extended single-benzene fluorophores with ultralarge Stokes shifts

The excited-state tautomer equilibrium of the urea-fused single-benzene fluorophore was synthetically modulated to produce exceptionally large Stokes shifts (>12 400 cm-1). The key N-H⋯N hydrogen bonding motif utilizes an endogenous proton for long-wavelength emission or an exogenous proton for acid-base chemistry, the balance of which is exploited for fluorescence switching in the solid state.

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.

Monophenyl luminescent material with dual-state emission and pH sensitivity for cell imaging

Dual-state emission (DSE) luminescent materials are a newly discovered category of luminescent materials that exhibit efficient light emission in multiple states, including dilute solutions, highly concentrated solutions, aggregated states and solid states. These materials effectively address the aggregation-caused quenching (ACQ) observed in traditional organic luminescent materials with large conjugated planes, as well as the limitations of aggregation-induced emission (AIE) materials, which typically do not emit light in dilute solutions. The design and development of DSE luminescent materials for organelle imaging applications has attracted considerable interest. In this context, this study presents the design and synthesis of a novel luminescent compound, DMSS-AM, characterised by intramolecular hydrogen bonding and a D-π-A structure. As a monophenyl luminescent material, DMSS-AM exhibits DSE properties with fluorescence quantum yields of 22.1% in solution and 14.0% in the solid state. In particular, it exhibits unique pH-responsive properties, facilitating the targeted detection of lysosomal pH changes. Confocal laser scanning microscopy imaging of cells demonstrated that DSE emitters at both low and high concentrations do not affect image quality for bio-imaging applications. This advance is expected to significantly broaden the applicability of DSE luminescent materials in future applications.