Exploring of indole derivatives for ESIPT emission: A new ESIPT-based fluorescence skeleton and TD-DFT calculations

Selective RNA binding and imaging with imidazopyrazine-based fluorescent molecule

We report the synthesis and characterization of novel imidazopyrazine-based fluorescent molecules 5a and 5b targeting RNA and DNA binding. Molecule 5b showed superior photophysical properties with stable fluorescence and high quantum yield in various solvents. UV-Vis and fluorescence spectroscopy revealed strong RNA binding with time-dependent fluorescence quenching and increasing absorbance, suggesting groove binding or π-π stacking interactions. Furthermore, agarose gel electrophoresis further confirmed selective RNA binding of 5b. Imaging studies demonstrated that 5b penetrated into viable MCF-7 cells and selectively stained RNA and retained fluorescence for up to 8 h under ambient conditions. These findings advance the study of RNA dynamics in living cells, highlighting the potential of 5b for RNA-specific bioimaging and sensing applications.

Comparative Study of Fluorescence Emission of Fisetin, Luteolin and Quercetin Powders and Solutions: Further Evidence of the ESIPT Process

Fisetin and Luteolin are important flavonoids produced in plants and known for their antioxidant, anti-inflammatory, neuroprotective, and analgesic properties. They are also good candidates for different types of biosensors. The model used to describe the fluorescence (FL) emission of these flavonoids involves an excited-state intermolecular proton transfer (ESIPT) process that causes a change in the molecule configuration and a corresponding decrease in the emission energy. Due to the different molecular structures of Fisetin and Luteolin, only one possible proton transfer within the molecule is allowed for each of them: transfer of the H3 proton for Fisetin and of the H5 for Luteolin. Here, we compare their calculated emission wavelengths, obtained using TDDFT/M06-2X/6-31++G(d,p), with their FL emission spectra measured on the corresponding powders and solutions and show that the experimental data are consistent with the presence of the ESIPT process. We also compare the emission wavelengths found for Fisetin and Luteolin with those calculated and measured for Quercetin, where, under photoexcitation, the transfers of both H3 and H5 protons are possible. We analyze the difference in the processes associated with the H3 and H5 proton transfers and discuss the reason for the predominance of the H5 proton transfer in Quercetin. Additionally, a new system of notation for flavonoid molecules is developed.

Theoretical investigation of the excited-state intramolecular double proton transfer process of 2,2'-(benzo[1,2-d:4,5-d']bis(thiazole)-2,6-diyl)diphenol

In this work, the excited state intramolecular double proton transfer (ESIDPT) mechanism of 2,2'-(benzo[1,2-d:4,5-d']bis(thiazole)-2,6-diyl)diphenol (BTAP) is proposed using density functional theory (DFT) and time-dependent DFT (TDDFT). The changes in bond lengths, bond angles and IR vibrational spectra associated with two intramolecular hydrogen bonds of BTAP upon photoexcitation indicate that the hydrogen bonds are strengthened in the excited state, facilitating the ESIDPT process. Investigation of the constructed S1-state potential energy surface proposes that BTAP prefers a stepwise ESIDPT mechanism. Electronic spectra and frontier molecular orbitals (FMOs) are also presented to illustrate the luminescent properties of BTAP.

Design and Synthesis of ESIPT-Based Imidazole Derivatives for Cell Imaging

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent molecules offer several exciting applications and are utilized most frequently as a cell imaging agent. Because of this, four distinct imidazole derivatives with ESIPT emission have been synthesized, and their fluorescence characteristics have been assessed in a variety of settings. Measurements using fluorescence spectroscopy have shown a promising candidate for cell staining, and potential candidate was specifically investigated for cell imaging uses in HT-29, MDA-MB-231, and HaCaT. Cytotoxicity of candidate molecule (1d) was analyzed using HT-29 and HaCaT cell lines, and at a dosage of 160 μM, HT-29 and HaCaT cell lines showed no signs of important cell toxicity. When spectroscopically measured, compound 1d showed no fluorescence ability in phosphate-buffered saline (PBS) solution. However, after 8 h of incubation in several cell lines, excellent fluorescence characteristics were seen in the green and red filters.

The photodynamic approach to the molecular-level origin of metal-guided photochromism and ultrafast absorption spectroscopy

Metal-guided photochromic material (photochromic complex) is one of the latest versions of photo-responsive materials due to their smart behaviour and promising real-world applications. The present work explores the molecular-level origin of metal-guided photochromism using a photodynamic approach and ultrafast absorption spectroscopy, to address all existing lacunas. Here, rhodamine B (RhB) dye containing the Schiff base zinc complex is considered a representative photochromic complex for both theoretical treatment and experimental observations. Detailed theoretical studies, including geometry optimization, frontier molecular orbital (FMO) analysis, transition state (TS) identification, and natural bond orbital (NBO) analysis, along with spectral studies, are employed to investigate the photodynamic equilibrium (enol-form keto-form). This equilibrium is regulated by the interplay of intrinsic factors (push-pull effect) and extrinsic factors (such as UV-light, the phenolic-OH group, metal ions, and solvents). The potential energy surface (PES) of the photo-conversion (enol →enol*→keto*→ meta-stable keto) is evaluated. While, the PES of the reversion (meta-stable keto →enol) is constructed based on the studies of thermo-reversion and photo-reversion. Finally, the theoretical findings related to the photodynamic equilibrium are validated by direct experimental evidence obtained through femtosecond transient absorption (fs-TA) spectroscopy.

Heterocyclic molecules with ESIPT emission: synthetic approaches, molecular diversities, and application strategies

Excited-state intramolecular proton transfer (ESIPT) is one of the most essential emission processes in most circumstances because of its dual emission band in most cases and its high Stokes shifts. These distinguishing properties make ESIPT-based probes more suitable for a variety of applications, including analyte sensors, solid-state sensing mechanisms, optical technologies, and biomarkers for endogenous or exogenous compounds in various settings. As a result, researchers around the world are working on ESIPT emissions and developing different scaffolds for various applications or industry demands. This field of study is rapidly expanding and there is a need for an up-to-date review of synthesis methodologies and applications. This paper provides the highlights of ESIPT-based heterocyclic scaffolds, synthesis strategies, and application scenarios in the literature from 2017 to 2023.

ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^OH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of L^OH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L^H,OH), a monohydroxy congener of L^OH,OH. Both L^OH,OH and L^H,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for L^H,OH and two for L^OH,OH) strongly affects the luminescence mechanism and color of the emission: L^H,OH emits in the light green region, whereas L^OH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T₁ → S₀ phosphorescence and not related to ESIPT. At the same time, L^OH,OH also exhibits S₁ → S₀ fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for L^H,OH due to the absence of the hydroxy group in the imidazole moiety.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

Spectroscopic and mechanistic insights into solvent mediated excited-state proton transfer and aggregation-induced emission: introduction of methyl group onto 2-(o-hydroxyphenyl)benzoxazole

2-(2-Hydroxy-5-methylphenyl)benzoxazole(HBO-pCH3), a solvatochromic benzoxazole-based probe, exhibited a typical dual fluorescence phenomenon, high fluorescence quantum yield, red-shifted emission and large Stokes’ shift via the ESIPT in solvents.

Regulation of conjugate rigid plane structures for achieving transformation of fluorescence recognition properties

Regulation of a conjugate rigid plane structure based on bisbenzimidazole derivatives to research the structure-effective relationship between conjugate systems size and fluorescence sensing properties.