Excitation Transfer Research Articles

A ‘Turn-on’ fluorogenic probe for selective and specific detection of Hg(II) ions

Mercury (Hg2+) and its associated compounds have drawn serious concern from the scientific communities for their extreme toxicity to human beings. The present article introduced a chromone-benzoxazole embracing an effective fluorogenic probe, (E)-3-(((4-(benzo[d]oxazol-2-yl) phenyl) imino) methyl)-2-methoxy-2H-chromen-4-ol (BPMC) for selective and specific detection of Hg2+ ions having detection and quantification limit in the µM range. A significant photoluminescence enhancement has been observed from BPMC solution (water-DMSO mixture, 50 % v/v) displaying low to highly intense blue-violet photoluminescence under a 365 nm UV lamp due to the inhibition of intramolecular charge transfer (ICT) and excited state intramolecular proton transfer (ESIPT) processes involved in BPMC. Furthermore, to achieve on-site detection of Hg2+ ions and investigate the practical utility of our developed probe, we have fabricated a BPMC-stained paper-strips-based test kit and demonstrated its practical effectiveness for on-spot detection. The details of the detection mechanism have been revealed through 1H NMR, FT-IR, and high-resolution mass spectrometric analysis. The present report evokes a broader perspective for tailoring ICT and ESIPT-based chromo-fluorogenic probes to accelerate their real-world application and environmental monitoring.

Unidirectional photoisomerisation in a dual channel proton transfer molecule rhodamine B based Schiff Base: A ratiometric pH sensor

In this article we have presented the synthesis, characterisation and spectral study of a newly designed fluorophore, (E)-2-((3-(benzo[d]oxazol-2-yl)-5-bromo-2-hydroxybenzylidene)amino)-3′,6′-bis(diethylamino)spiro [isoindoline-1,9′-xanthen]-3-one (RHHBO) which has two possible six member intramolecular hydrogen bonded networks capable for dual channels proton transfer reaction − one towards the N atom of oxazole moiety and the other towards the N atom of rhodamine moity. Details steady state and time resolved spectral analysis of RHHBO and a comparison of spectral features of control compounds 2-(2-hydroxyphenyl)benzoxazole (HBO), (E)-4-bromo-2-(hydrazonomethyl)phenol (HBN) and (E)-2-((5-bromo-2-hydroxybenzylidene)amino)-3′,6′-bis(diethylamino)spiro[isoindoline-1,9′-xanthen]-3-one (RH) having only single proton transfer site reveal that excited state intramolecular proton transfer (ESIPT) in RHHBO occurs towards the rhodamine side but not towards the oxazole side. Interestingly, RHHBO shows different fluorescence colour in acidic and basic medium and hence can be used ratiometric emission based pH sensor.

Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge‐Transfer Exciton Coupling

AbstractA family of chiral perylene diimides (PDIs) was newly developed as excellent circularly polarized luminescence (CPL) materials. They are asymmetrically derivatized with a double‐alkyl‐chained L‐ or D‐glutamate unit and a linear or branched alkyl chain. When water is added to the tetrahydrofuran (THF) solution of glutamate‐PDI‐linear‐alkyl chain compounds, kinetically formed H‐aggregates are formed in globular nanoparticles (NPs). These NPs undergo spontaneous transformation into thermodynamically stable nanotubes via helical nanostructures, which showed structured broad spectra originating from the strong coupling of delocalized Frenkel excitations (FE) and charge transfer excitations (CTE). Significant enhancement of circular dichroism (CD), fluorescence quantum yield, and circularly polarized luminescence (CPL) with luminescence dissymmetry factor (glum) are observed during the transformation of NPs to the FE/CTE‐coupled helical and tubular structures. This transformation process is significantly accelerated by applying physical stimuli, i.e., ultrasonication or adding helical aggregates as seed crystals, a feature unique to living supramolecular polymerization. Meanwhile, the branched chain‐containing PDIs only form H‐aggregates and did not show FE/CTE hybrid exciton states with living supramolecular polymerization properties. This study unveils that suitably designed chiral PDI derivatives show FE/CTE coupling accompanied by high fluorescence quantum yields, enhanced chiroptical properties, and supramolecular living polymerization characteristics.

Photoinduced charge transfer assisted through external electric field and ternary hydrogen bonding strategies

Understanding the mechanisms governing interfacial charge transfer in photoactive layer is crucial for optimizing photogenerated charge separation efficiency. In this study, the interfacial charge transfer process is regulated by applying external electric field (Fext) and ternary hydrogen bonding strategies. We observe significant changes in the excited state properties and charge transfer parameters at the interface under Fext conditions. This facilitates an in-depth exploration of the effects of Fext on the microscopic details of the interface at the molecular level. Implementing the ternary hydrogen bonding strategy significantly enhances the electron-attracting ability of fullerene. Comparative analysis of the PTB7-Th:PC71BM and PTB7-Th:C7:PC71BM systems under Fext = 0, reveals that the ternary hydrogen bonding strategy increases the charge transfer pathway and significantly improves the intermolecular charge separation rate (KCS). Additionally, we evaluated the relationship between hydrogen bond strength and Fext, demonstrating that Fext significantly enhances the hydrogen bond strength between C7:PC71BM. These studies provide deeper insights into the role of Fext and ternary hydrogen bonding strategies in charge transfer process, providing a valuable theoretical framework for designing more efficient organic solar cells (OSCs).

Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge-Transfer Exciton Coupling.

A family of chiral perylene diimides (PDIs) was newly developed as excellent circularly polarized luminescence (CPL) materials. They are asymmetrically derivatized with a double-alkyl-chained L- or D-glutamate unit and a linear or branched alkyl chain. When water is added to the tetrahydrofuran (THF) solution of glutamate-PDI-linear-alkyl chain compounds, kinetically formed H-aggregates are formed in globular nanoparticles (NPs). These NPs undergo spontaneous transformation into thermodynamically stable nanotubes via helical nanostructures, which showed structured broad spectra originating from the strong coupling of delocalized Frenkel excitations (FE) and charge transfer excitations (CTE). Significant enhancement of circular dichroism (CD), fluorescence quantum yield, and circularly polarized luminescence (CPL) with luminescence dissymmetry factor (glum) are observed during the transformation of NPs to the FE/CTE-coupled helical and tubular structures. This transformation process is significantly accelerated by applying physical stimuli, i.e., ultrasonication or adding helical aggregates as seed crystals, a feature unique to living supramolecular polymerization. Meanwhile, the branched chain-containing PDIs only form H-aggregates and did not show FE/CTE hybrid exciton states with living supramolecular polymerization properties. This study unveils that suitably designed chiral PDI derivatives show FE/CTE coupling accompanied by high fluorescence quantum yields, enhanced chiroptical properties, and supramolecular living polymerization characteristics.

An insight into the role of ESIPT/TICT-based antioxidant flavone analogues in fluoro-probing diabetes-induced viscosity changes: a unified experimental and theoretical endeavour.

Potent antioxidants, like 3-hydroxy flavones, attracted considerable attention due to their excited state intramolecular proton transfer (ESIPT)-based fluorescence behaviour. This article is an interesting demonstration of a series of synthetic 3-hydroxy flavone analogues having high antioxidant activity as molecular rotor-like viscosity probes. Among these flavone analogues, 4'-N,N-dimethylamino-3-hydroxy flavone (3) is the most potent one, showing the twisted intramolecular charge transfer (TICT)-dependent fluoroprobing activity toward the blood viscosity changes associated with diabetes and free fatty acids (FFA)-induced nuclear viscosity changes of MIN6 cells. The TICT dynamics of (3), which instigates its viscosity probing activity, was comprehended with the help of DFT-based computational studies. Abnormal cellular viscosity changes are the pathological traits for various diseases, and non-toxic flavone-based viscosity probes can be useful for diagnosing such pathological conditions.

Photophysical properties of 2’-hydroxychalcones of 2-cinnamoyl-4-nitro-1-naphthol series

2’-Hydroxychalcones of 2-cinnamoyl-4-nitro-1-naphthol series were obtained by the condensation between 2-acetyl- 4-nitro-1-naphthol and benzaldehydes. The presence of the 4-positioned nitro group in the 1-hydroxy-2-naphthyl fragment contributes to the excited state intramolecular proton transfer (ESIPT) fluorescence of these 2’-hydroxy- chalcones in the solid state. Compound with dimethyl- amino substituent, 2-(4-dimethylaminocinnamoyl)-4-nitro- 1-naphthol, also demonstrates ESIPT in solution.

Deciphering TADF Mechanisms in Metal-Free Organic Emitters BrA-HBI: Utilizing Optimally Tuned Range-Separated Functionals for Insight

Abstract Metal-free organic emitters, characterized by their thermally activated delayed fluorescence (TADF) properties, offer considerable promise for the creation of highly efficient organic light-emitting diodes (OLEDs). Recently, Shao et al. presented a novel excited state intramolecular proton transfer (ESIPT) system BrA-HBI, demonstrating an emission quantum yield of up to 50% [Adv. Funct. Mater. 32, 2201256 (2022)]. However, many open issues cannot be answered solely by experimental means only and require detailed theoretical investigations. For instance, what causes the activation of TADF from the Keto* tautomer and leads to fluorescence quenching in the Enol* form? Herein, we provide a theoretical investigation on the TADF mechanism of the BrA-HBI molecule by optimally tuned range-separated functionals. Our findings reveal that ESIPT occurs in the BrA-HBI molecule. Moreover, we have disclosed the reason for the fluorescence quenching of the Enol* form and determined that the T 2 state plays a dominant role in the TADF phenomenon. In addition, double hybrid density functionals method was utilized to verify the reliability of optimally tuned range separation functionals on the calculation of the TADF mechanism in BrA-HBI. These findings not only provide a theoretical reference for development of highly efficient organic light-emitting diodes, but also demonstrate the effectiveness of the optimally tuned range-separated functionals in predicting the luminescence properties of TADF molecules.

Porous hierarchical sphere g-C3N4 with coordinated Co-ions as an efficient and stable self-Fenton photocatalyst for the removal of antibiotic

Photocatalysis-self-Fenton reaction is a promising antibiotics-contaminated wastewater treatment method, but it is still limited by the leakage and sluggish cycling of metal-ions activating sites. Herein, a novel porous hierarchical sphere g-C3N4 with coordinated Co-ions (HCN-Co) was successfully synthesized by a supramolecular modified calcination strategy. The hierarchical sphere structure and coordinated Co-ions of HCN-Co exhibited a synergistic effect on enhancing visible-light absorption, stabilization of Co-ions, charge transfer and surface reaction, which leaded to increased H2O2 generation and activation under visible-light irradiation. Consequently, HCN-Co displayed remarkable and stable photocatalysis-self-Fenton degradation of ciprofloxacin (CIP) with an optimal removal percentage of 99.9 % under visible-light irradiation for 50 min. In this photocatalysis-self-Fenton reaction, H2O2 was generated from 2-step single-e− oxygen reduction reaction and activated to hydroxyl radical (OH) by Co2+ refreshing from interface charge transfer (IFCT) excitation. This work presents a viable approach and provides valuable insights for developing efficient photocatalysis-self-Fenton system for wastewater remediation.

Emissively enhanced novel azo compounds featuring with ESIPT core

This study presents the synthesis and detailed characterization of novel azo compounds featuring Excited State Intramolecular Proton Transfer (ESIPT) cores derived from orcinol, resorcinol, and 2-naphthol. Using a comprehensive suite of spectroscopic techniques, including Nuclear Magnetic Resonance (NMR), Liquid Chromatography-Mass Spectrometry (LC-MS), Ultraviolet-Visible (UV–Vis) spectroscopy, and fluorescence spectroscopy, we explored the unique photophysical properties of these compounds. Notably, the synthesized compounds exhibited exceptionally large Stokes shifts, with values recorded at 204 nm, 251 nm, and 168 nm for compounds 1A, 2A, and 3A, respectively. These shifts indicate significant potential for applications in areas such as optical sensing and fluorescent imaging. Furthermore, Density Functional Theory (DFT) calculations were employed to investigate the nonlinear optical (NLO) properties of the enol and keto tautomeric forms of these compounds. The findings revealed that compound 2A, in particular, demonstrated impressive first-order hyperpolarizability, especially in chloroform, underscoring its suitability for advanced nonlinear optical applications. The study highlights the pivotal role of the ESIPT mechanism, where efficient intramolecular hydrogen bonding facilitates proton transfer upon excitation, leading to enhanced fluorescence properties. This phenomenon is particularly pronounced in compound 2A, where dual O—H interactions contribute to its remarkable fluorescence behavior, making it a standout candidate for use in high-performance optical materials. These findings not only advance our understanding of azo compounds and their versatile photophysical characteristics but also open new avenues for their application in cutting-edge technologies such as optical sensors, imaging agents, laser materials, and other fluorescence-based devices. By strategically leveraging the ESIPT process and the unique structural features of these compounds, this research paves the way for future innovations in the field of advanced optical materials and photonic devices.

Uncovering excited state behaviours associated with electron-withdrawing CN and electron-donating TPA moieties on SAA derivatives: a theoretical study

Inspired by the excellent photochemical characteristics exhibited by salicylaldehyde azine (SAA) derivatives, our current endeavour primarily revolves around delving into the intricacies of photo-induced excited state reactions for its electron-donating and electron-withdrawing moieties derivatives (i.e. CN-SAA-CN, CN-SAA-TPA and TPA-SAA-TPA). Our simulations reveal push–pull-substituted-dependent photoexcitation that could significantly affect excited state intramolecular proton transfer (ESIPT) reaction for SAA derivatives. Constructing potential energy surfaces (PESs), we unveil the ultrafast ESIPT mechanism due to the low potential barriers. Additionally, by comparing the differences of barriers among CN-SAA-CN, CN-SAA-TPA and TPA-SAA-TPA, we present regulative manner for SAA derivatives by substituting electron-donating and electron-withdrawing moieties. We sincerely anticipate that the modulation of push–pull substitutions on excited state behaviours will pave the way for ground-breaking advancements in luminescent materials.

Acoustic properties and attenuation of coupled shaft-submarine hull system under various excitation transfer paths

Pump-jet propulsor excitation transfers to submarine hull along rotor-shaft and duct-stator paths simultaneously. The investigations on the effects of excitation transfer paths on structural vibration and acoustic radiation of submarine are limited. The present work aims to investigate vibro-acoustic characteristics of coupled shaft-submarine hull system utilizing a theoretical wavenumber analysis method and conduct acoustic design. The energy functional of the coupled structure-fluid system of the research object is first developed, and the displacement components of the jointed shell and the acoustic pressure are expanded by the Fourier series along circumferential direction. This allows for obtaining vibro-acoustic responses in the circumferential wavenumber-frequency domain, by which the predominant wavenumbers contributing to acoustic radiation are identified. The discussions reveal that the modes n = 0 and n = 1 respectively dominate the acoustic radiation under axial and vertical rotor loads. The acoustic radiations under duct-stator load are mainly contributed by mode n = 0, and the higher order modes n = 1 and n = 2 determine several acoustic peaks. Furthermore, two acoustic design schemes are proposed to suppress the wavenumbers with high radiation efficiency. It is proven that the design of the symmetric inner foundation and the application of new material are two efficient ways to improve acoustic performance of the submarine.

Uncovering solvent-polarity-associated excited state behaviors for the novel PQ-BSD fluorophore: A theoretical study

In view of the prominent regulated characteristics of electron-donating substituted julolidine-bis(salicylidene)-1,5-diaminonaphthalene (PQ-BSD) derivatives, we are dedicated to exploring photo-induced excited state behaviors of PQ-BSD in solvents. Given the variational solvent polarities, the geometrical changes, infrared (IR) spectral variations, core-valence bifurcation (CVB) indexes as well as charge reorganizations are focused on based on TDDFT method. The strengthened excited-state hydrogen bonding interactions of both hydrogen bonds fully present the excited state intramolecular proton transfer (ESIPT) tendency. Via constructing potential energy surfaces (PESs), the stepwise excited state intramolecular double proton transfer (ESIDPT) process could be revealed for PQ-BSD fluorophore.

Low-Energy Electron Scattering from Ethyl Amine and Propyl Amine: A Comparative Study.

In this study, we investigate the low-energy electron (<20 eV) scattering cross sections for the linear ethyl amine and propyl amine using the R-matrix approach. A theoretical study is conducted, covering various cross sections such as elastic, symmetry-wise elastic, differential, momentum transfer, and electronic excitation. We obtained the ionization cross section using the CSP-ic and BEB methodologies. We unveil the intricate electron-amine interactions within these molecular systems. Additionally, our work extends to a comparative analysis among ethyl amine, propyl amine, and previously studied methyl amine systems. By comparing the obtained results, we discern subtle differences and similarities in the electron scattering behaviors across these structurally related amines. The findings presented herein shed light on fundamental electron-amine interactions, offering valuable insights for diverse fields ranging from chemical kinetics to astrophysics.

Molecular Descriptors of Excited-State Property for High-Throughput Screening of Organic Photofunctional Materials.

The excited-state property determines the occurrence of photofunctions in organic materials. We have developed a fragment frontier molecular orbital model for the donor-acceptor-type (D-A-type) systems and constructed molecular descriptors of the excited-state property with charge transfer (CT) or local excitation (LE) based on the orbital information on constituent D and A fragments. Applying these descriptors, we rapidly screened 1CT or 1LE and 3CT or 3LE molecules from 2500 molecules generated by the binding of 50 donors and 50 acceptors, and the results of 26 molecules were confirmed by available experiments and first-principles calculations. Moreover, the modulation of these descriptors by chemical groups allows the rational design of target excited states. This work is significant for high-throughput screening of excellent organic photofunctional materials from a giant chemical database.

Effect of concentration and wavelength of excitation on the photophysics of salicylate anion in acetonitrile and water

Sodium salicylate (NaSA) molecule exists as salicylate anion in acetonitrile (ACN) and water solvents, and exhibits large Stokes shifted fluorescence due to excited state intramolecular proton transfer (ESIPT), with decay times of ∼5 ns in ACN and ∼3.9 ns in water by 300 nm (absorption maxima) excitation. Previous studies report both ground and excited state enol-keto tautomerization in ACN, but only excited state tautomerization in water at 10−4 M. However, the current work explores the effect of concentration and excitation wavelengths on the photoinduced dynamics of ESIPT in the salicylate anion. On increasing the concentration of NaSA, no change in absorption spectra appears in both the solvents, and emission spectra of NaSA in water remain unaffected by changes in concentration or excitation wavelength, whereas, a slight red shift and decrease in FWHM appears in ACN. Time-domain fluorescence measurements show predominantly single-exponential decay throughout the emission profile by 300 nm excitation above the 10−5 M concentration in both the solvents, while by 375 nm excitation, multi-exponential fluorescence decay is observed at low concentrations, and as the concentration of NaSA increases, this decay behaviour tends to converge towards a single exponential decay. These results suggest that solute–solvent interactions stabilize the ground-state intermolecular hydrogen-bonded species at low concentrations, while higher concentrations weaken these interactions, leading to emission solely from the salicylate anion. Peak fit analysis of excitation spectra confirms enol-keto tautomerization in both the solvents, with the keto form being more stabilized in ACN. These findings underscore that in ACN, behaviour of NaSA is influenced by both concentration and excitation wavelength and contrary to previous reports, the keto form of the molecule is also present in water, though at a very low concentration and an increase in non-radiative transitions in water cause fluorescence quenching.

Theoretical uncovering of the chalcogen element regulated ESDPT behaviors for 2,5-bis(2-benzoxazolyl)-hydroquinone derivatives.

Inspired by the captivating allure of exquisitely regulated characteristics exhibited by 2-(2-hydroxyphenyl)-benzoxazole and its derivatives in the realms of photochemistry and photophysics, our current endeavor primarily revolves around delving into the intricacies of photo-induced excited state reactions for derivatives of 2,5-bis(2-benzoxazolyl)-hydroquinone (BBHQ). Given the significant impact of chalcogen element doping, herein we predominantly focus on exploring the excited state behaviors of BBHQ-OO, BBHQ-SS, and BBHQ-SeSe fluorophores. Our simulations, resulting from variations in geometry and vertical excitation charge reorganization, reveal atomic-electronegativity-dependent hydrogen bonding interactions and charge recombination induced by photoexcitation that can significantly enhance the excited state intramolecular double proton transfer (ESDPT) reaction for BBHQ-OO, BBHQ-SS, and BBHQ-SeSe fluorophores. By constructing potential energy surfaces (PESs) and identifying transition states (TS), we unveil the ultrafast stepwise ESDPT mechanism due to the low potential barriers. Additionally, by employing heterosubstituted BBHQ-OS and BBHQ-OSe compounds, we rigorously validate the stepwise ESDPT mechanism regulated by chalcogen atomic electronegativity. We sincerely anticipate that the modulation of solvent polarity on excited state behaviors will pave the way for groundbreaking advancements in luminescent materials.

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.

Theoretical model of donor–donor and donor–acceptor energy transfer on a nanosphere

In this study, we introduce a novel advancement in the field of theoretical exploration. Specifically, we investigate the transfer and trapping of electronic excitations within a two-component disordered system confined to a finite volume. The implications of our research extend to energy transfer phenomena on spherical nanoparticles, characterized by randomly distributed donors and acceptors on their surface. Utilizing the three-body Padé approximant technique, previously employed in single-component systems, we apply it to address the challenge of trapping within our system. To validate the robustness of our model, we conduct Monte Carlo simulations on a donor–acceptor system positioned on a spherical nanoparticle. In particular, very good agreement between the model and Monte Carlo simulations has been found for donor fluorescence intensity decay.

Substituent effects on the ESIPT process of H2O2 sensing product 1,3-bis(bispyridin-2-ylimino) isoindolin-4-ol: A theoretical study

H2O2 plays an important role in a variety of physiological processes, but its overproduction or accumulation can lead to oxidative damage and related diseases, so it is crucial to establish accurate H2O2 detection methods. Fluorescence sensing as a potential method, the design and modulation of its luminescent properties are crucial to the detection results. In this work, we investigated the sensing mechanism of a novel H2O2 fluorescent probe (P1) and examined the role of substituent effects in modulating the fluorescence properties of its reaction products. The results show that the non-fluorescence property of the probe is due to twisted intramolecular charge transfer (TICT) mechanism resulting from a torsion of the excited state molecular conformation, and the fluorescence turn-on upon recognition of H2O2 by the probe is due to the reaction product (BBYI1) following the excited state intramolecular proton transfer (ESIPT) process. When the substituent is an electron-donating group, the energy barrier of the ESIPT reaction is lowered, promoting luminescence, while the electron-withdrawing group has the opposite effect. In summary, the present work may offer a theoretical foundation for the development of more sensitive fluorescent probes for ESIPT-based H2O2 detection.