Fine-tuning directionality of ESIPT behavior of the asymmetric two proton acceptor system via atomic electronegativity

Influence of atomic electronegativity on ESIPT behaviour for the BTDI and its derivatives: Theoretical exploration

In this work, we theoretically explored the influence of atomic electronegativity on excited-state intramolecular proton transfer (ESIPT) behavior among novel fluorescent probes BTDI and its derivatives (BODI and BSeDI). A thorough examination of the optimized structural parameters and infrared vibrational spectra reveals an enhancement in intramolecular hydrogen bonding within BTDI and its derivatives upon light excitation. This finding is further reinforced by topological analysis and interaction region indicator scatter plots, which underscores the sensitivity of atomic electronegativity to variations in hydrogen bonding strength. With regards to absorption and fluorescence spectra, the decrease in atomic electronegativity leads to a pronounced redshift, primarily attributed to the narrowing of the energy gap. Additionally, an analysis of potential energy curves and the exploration of intrinsic reaction coordinate paths based on transition state structures afford a deeper understanding of the mechanism underlying ESIPT and being modulated through the manipulation of atomic electronegativity. We anticipate that this work on atomic electronegativity regulating ESIPT behavior will serve as a catalyst for novel fluorescent probes in the future, offering fresh perspectives and insights.

Atomic electronegativity-dependent intramolecular hydrogen bond and fluorescence characteristics of novel scaffold-based fluorophore: a TD-DFT study

In this work, fluorescent properties and excited-state intramolecular proton transfer (ESIPT) processes of 2,5-bis(benzo[d]thiazol-2-yl)phenol (BTP) and its derivatives (BOP and BSeP) with different heteroatom atoms (O and Se) have been systematically explored by the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated absorption and fluorescence emission peaks agree well with the experimental values in acetonitrile. From the data of structures, topological parameters, reduced density gradient analyses, and infrared (IR) vibrational frequencies, the intramolecular hydrogen bonds (IHBs) of BTP and its derivatives are enhanced upon light-excitation. The potential energy curves show that the ESIPT process occurs in BTP and its derivatives after surmounting 0.167-0.306 eV energy barrier. The strength of intramolecular hydrogen bond, HOMO-LUMO energy gap, and red-shifted value of absorption and fluorescence emission wavelengths are dependent on the electron-withdrawing ability of heteroatom from O to S and Se. We believe that this work can pave the way for developing a new ESIPT-based fluorophore with better luminescent properties.

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.

Unveiling the effects of atomic electronegativity on the ESIPT mechanism and luminescence property of new coumarin benzothiazole Fluorophore: A TD-DFT exploration

The excited-state intramolecular proton transfer (ESIPT) mechanism, photophysical properties of 8-(benzo[D] thiazole-2-yl)-7-hydroxy-2H-benzopyran-2-one (L-HKS) and the effect of O/Se atomic substitution on L-HKS have been studied in detail based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The S atom in the thiazole ring of L-HKS has been replaced by O/Se atom (denoted to L-HKO/L-HKSe) to analyze the effects of atomic electronegativity on the intramolecular H-bond, absorption/emission spectrum and ESIPT process. Through the analysis of series of calculated results, it can be found that the intramolecular H-bonds at normal form and tautomer form are enhanced and weakened in the S₁ state, respectively, which is favorable to ESIPT process. The potential energy curves revealed that the ESIPT process is much easier to occur gradually from L-HKO to L-HKS and L-HKSe, as the electron-withdrawing ability of atom (from O to S and Se) is weakened. The atomic substitution also has an effect on the photophysical properties. From L-HKO to L-HKS, the emission peak at tautomer form red-shifts 70 nm. The energy gaps of the three compounds follow the decreased order of L-HKO (4.866 eV) > L-HKS (4.753 eV) > L-HKSe (4.371 eV) with the weakened electron-withdrawing ability of atom (from O to S and Se), which leads to the gradual red-shift of the absorption spectra from L-HKO to L-HKS and L-HKSe.

Theoretical Study on the Atom-Substituted Quinazoline Derivatives with Faint Emission as Potential Sunscreens

Two novel compounds (HQS and HQSe) with excited-state intramolecular proton transfer (ESIPT) properties were designed based on the compound 2-(2-hydroxy-3-ethoxyphenyl)-3H-quinazolin-4-one (HQ). The parameters related to the ESIPT properties and electronic spectra of HQ and its derivatives were calculated using density functional theory and time-dependent density functional theory methods. The obtained geometric configurations, infrared vibrational spectra, and reduced density gradient scatter plots have shown that the intramolecular hydrogen bond O₁···H₁-N₁ has been weakened upon photoexcitation. Moreover, from the scanned potential energy curves, it can be found that the ESIPT processes of the three compounds have no energy barriers. It is noteworthy that HQS and HQSe can strongly absorb light in the UVA region (∼340 nm) and exhibit weak fluorescence emission in the visible light region, which comes from the keto configuration. The special optical properties of HQS and HQSe can promote their application as potential sunscreen agents.