A novel non-fluorescent excited state intramolecular proton transfer phenomenon induced by intramolecular hydrogen bonds: an experimental and theoretical investigation

Photoexcitation Dynamics of 4-Aminopthalimide in Solution Investigated Using Femtosecond Time-Resolved Infrared Spectroscopy

Excited-state intramolecular proton transfer (ESIPT) reactions are crucial in photoresponsive materials and fluorescent markers. The fluorescent compound 4-aminophthalimide (4-AP) has been reported to exhibit solvent-assisted ESIPT in protic solvents, such as methanol, wherein the solvent interacts with 4-AP to form a six-membered hydrogen-bonded ring that is strengthened upon excitation. Although the controversial observation of ESIPT in 4-AP has been extensively studied, the molecular mechanism has yet to be fully explored. In this study, femtosecond infrared spectroscopy was used to investigate the dynamics of 4-AP in methanol and acetonitrile after excitation at 350 and 300 nm, which promoted 4-AP to the S1 and S2 states, respectively. The excited 4-AP in the S1 state relaxed to the ground state, while 4-AP in the S2 state relaxed via the S1 state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S1 state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S1 state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S1 state. Upon the excitation of 4-AP to the S2 state, the transition to Enol-4-AP in the S1 state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S2 to the S1 state. The internal S2 → S1 conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S2 state. The detailed excitation dynamics of 4-AP clearly reveal the molecular mechanism underlying its negligible ESIPT, despite the fact that it forms a favorable structure for solvent-assisted ESIPT.

Concerning for the solvent-polarity-dependent conformational equilibrium and ESIPT mechanism in Pz3HC system: A novel insight

In this report, we propose a new insight into the interaction between the solvent-polarity-dependent conformational equilibrium and excited state intramolecular proton transfer (ESIPT) behavior of Pz3HC system in four different polar solvents (polarity order: ACN > THF > TOL > CYC). Using quantum chemistry method, we first announce a coexistence mechanism between Pz3HC-1 and Pz3HC-3 in the ground state in four solvents based on the Boltzmann distribution. In particular, Pz3HC-1 is the principal configuration in non-polar solvent, but Pz3HC-3 is the principal configuration in polar solvent. In addition, the simulated fluorescence spectra interprets the negative solvatochromism effect of Pz3HC-1 and Pz3HC-3 in four solvents. The evidence from intramolecular hydrogen bonding (IHB) parameters and electronic perspective collectively confirms the light-induced IHB enhancement and intramolecular charge transfer (ICT) properties in Pz3HC-1 and Pz3HC-3, which raises the likelihood of the ESIPT process. Combining the calculation of potential energy curve (PEC) and intrinsic reaction coordinate (IRC), we demonstrate that the ESIPT ease of Pz3HC-1 in different polar solvents obeys the order of CYC > TOL > THF > ACN, while the order of ESIPT ease in Pz3HC-3 is opposite. Notably, the ESIPT process of Pz3HC-3 in CYC solvent is accompanied by the twisted intramolecular charge transfer (TICT) process. In addition, we also reveal that the enol* and keto* fluorescence peaks of Pz3HC-3 in CYC solvent are quenched by ISC and TICT process, respectively. Our work not only provides a satisfactory explanation of the novel dynamics mechanism for Pz3HC system, but also brings light to the design and application of new sensing molecules in the future.

Photogenerated carrier dynamics of Mn2+ doped CsPbBr3 assembled with TiO2 systems: Effect of Mn doping content

In recent years, all-inorganic perovskite materials have become an ideal choice for new thin film solar cells due to their excellent photophysical properties and have become a research hotspot. Studying the ultrafast dynamics of photo-generated carriers is of great significance for further improving the performance of such devices. In this work, we focus on the transient dynamic process of CsPbBr3/TiO2 composite systems with different Mn2+ doping contents using femtosecond transient absorption spectroscopy technology. We used singular value decomposition and global fitting to analyze the transient absorption spectra and obtained three components, which are classified as hot carrier cooling, charge transfer, and charge recombination processes, respectively. We found that the doping concentration of Mn2+ has an impact on all three processes. We think that the following two factors are responsible: one is the density of defect states and the other is the bandgap width of perovskite. As the concentration of doped Mn2+ increases, the charge transfer time constant shows a trend of initially increasing, followed by a subsequent decrease, reaching a turning point. This indicates that an appropriate amount of Mn2+ doping can effectively improve the photoelectric performance of solar cell systems. We proposed a possible charge transfer mechanism model and further elucidated the microscopic mechanism of the effect of Mn2+ doping on the interface charge transfer process of the CsPbBr3/TiO2 solar cell system.

Ultrafast Imaging of Jahn-Teller Distortion and the Correlated Proton Migration in Photoionized Cyclopropane

The Jahn-Teller (JT) distortion is one of the fundamental processes in molecules and condensed phase matters. For photoionized organic molecules with high symmetry, the JT effect leads to geometric instability in certain electron configurations and thus has a significant effect on the subsequent isomerization and proton migration processes. Utilizing the femtosecond pump-probe Coulomb explosion method, we probe the isomerization dynamics process of a monovalent cyclopropane cation (C3H6+) caused by proton migration and reveal the relationship between proton migration and JT distortion. We found that the C3H6+ cation evolves from the D3h symmetric equilateral triangle geometry either to the acute triangle via two elongated C-C bonds (JT1) or to the obtuse triangle via a single elongated C-C bond (JT2). The JT1 pathway does not involve proton migration, while the JT2 pathway drives proton migration and can be mapped into the indirect dissociation channel of Coulomb explosion. The time-resolved experiment indicates that the delay time between those two JT pathways can be as large as ∼600 fs. After the JT distortion, the cyclopropane cation undergoes a subsequent structural evolution, which brings a greater variety of dissociation channels.

Ultrafast Dynamics Across Pressure-Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady-state photoluminescence, steady-state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES-I to ES-II and ES-II to ES-III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES-II and ES-III is observed within the pressure range of 1.08-1.68 GPa. The pressure-induced fluorescence quenching in ES-II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES-II to ES-III in which radiative recombination becomes non-existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure-induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

Effect of external electric fields on the ESDPT process and photophysical properties of 1,8-dihydroxy-2-naphthaldehyde

In this work, by capitalizing on the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, it has been systematically studied that the excited state double intramolecular proton transfer (ESDPT) process and the photophysical properties of 1,8-dihydroxy-2-naphthaldehyde (DHNA) are affected by the distinct external electric fields (EEFs). The obtained intramolecular hydrogen bond (IHB) parameters containing bond lengths and angles, as well as infrared (IR) vibrational spectra demonstrate that IHB strength changes in the distinct EEFs. Moreover, not only do the potential energy surfaces (PESs) indicate that the ESDPT process of DHNA is stepwise, but also increasing the positive EEF results in a decrease in the energy barrier accordingly, while vice versa. The absorption and fluorescence spectra also undergo a corresponding red or blue shift in the EEF; for instance, when the EEF changes from +10 × 10-4 a.u. to +20 × 10-4 a.u., the fluorescence peak undergoes a blue shift from 602 nm to 513 nm in the keto2 form. In a nutshell, the ESDPT process of DHNA can be influenced by the EEF, which will serve as a reference in regulating and controlling proton transfer that causes luminescence.

Multifunctional Aryl Sulfonium Decavanadates: Tuning the Photochromic and Heterogeneous Oxidative Desulfurization Catalytic Properties Using Salicylaldehyde-type Functional Moieties on Counterions

Multifunctional materials based on polyoxovanadates (POVs) have rarely been reported. Herein, we used aryl sulfonium counterions (ASCIs) bearing a salicylaldehyde-type functionality to tune the properties of decavanadate ([V10O28]6-)-based hybrids for their application in photochromism and heterogeneous oxidative desulfurization (ODS) catalysis. The counterions FHPDS ((3-formyl-4-hydroxyphenyl)dimethylsulfonium), DFHPDS ((3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium), and EFPDS ((4-ethoxy-3-formylphenyl)dimethylsulfonium) were clubbed with the decavanadate cluster to generate the hybrids (FHPDS)4[H2V10O28](H2O)4 (HY1), (DFHPDS)4[H2V10O28](H2O)3 (HY2), and (EFPDS)4[H2V10O28](H2O)6 (HY3). The photochromic properties of these hybrids were tested under 365 nm irradiation, which showed a color change from yellow to green. Different hybrids exhibited different photocoloration half-life (t1/2) values in the range of 0.77-28.38 min, suggesting the dependence of the photocoloration properties upon functional groups on the counterions. The hybrid HY2, having a 2,6-diformyl phenol moiety on the ASCI, exhibited an impressive t1/2 of 0.77 min. UP to 70% reversibility of photocoloration was achieved for the best photochromic hybrid HY2 in 48 h at 70 °C under an oxygen atmosphere. Theoretical and experimental data suggested that some of these aryl sulfonium POVs follow a different e--h+ stabilization mechanism than traditional sulfonium POM hybrids. Further, the salicylaldehyde-type ASCIs control the solubility of the decavanadate hybrids, which enables their application as heterogeneous catalysts for the selective oxidation of various sulfides. The nature of the substituents on the ASCIs also affected their catalytic activities; the counterion that facilitates the reversible V4+/V5+ switching enhances the catalytic ODS efficiency of the hybrids. Using HY2 as the catalyst, up to 99% conversion and 96% selectivity toward sulfones were achieved in dibenzothiophene (DBT) oxidation. The present study suggests a new promising approach for controlling POVs' photoresponsive and catalytic properties by using ASCIs bearing salicylaldehyde-type functional moieties.

Dangling Water Molecules Bridge for ESIPT in Aggregated TMP: A Theoretical Study

We present a theoretical study on the occurrence of excited-state proton transfer in an aggregated structure of 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP) exclusively in water among polar solvents, as reported in a recent experiment (Bhattacharyya, A. New J. Chem. 2019, 43, 15087). Our extensive investigation of the TMP monomer and dimer implementing density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, in three different solvents, namely, water, methanol, and dimethyl sulfoxide (DMSO), with explicit inclusion of solvent molecules demonstrated the existence of both enol and keto forms of the TMP dimer in the excited state, but only in water; this confirmed the experimental emission spectra completely and simultaneously validated the aggregation-induced emission phenomenon. Further analysis of various parameters such as potential energy scan (PES) of the hydroxyl (O-H) bond involved in hydrogen bonding, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), and infrared (IR) stretching frequencies of both the monomer and dimer forms of TMP in different solvents clearly indicated the geometry of the dimer, with the arrangement of the solvent molecules to be the sole reason for the excited-state charge transfer. The bridging alignment of water molecules in between the stacked units of the TMP dimer results in intermolecular interactions, ultimately leading to intermolecular proton transfer in the excited state.

ESIPT-induced spin-orbit coupling enhancement leads to tautomer fluorescence quenching of the 10-HHBF molecule

We present novel insights into the interplay between excited state intramolecular proton transfer (ESIPT) and spin-orbit coupling (SOC) in the 10-hydroxy-11H-benzo[b]fluoren-11-one (10-HHBF) molecule, utilizing the time-dependent density functional theory approach and femtosecond transient absorption spectroscopy. Our discoveries entail a reassessment of the luminescence mechanism for 10-HHBF, characterizing it as an ESIPT fluorophore. Additionally, we demonstrate that the molecule undergoes intersystem crossing (ISC) following proton transfer, which quenches the fluorescence of the proton-transferred state, thus resulting in the absence of dual emission and a limited spectral range of fluorescence. Furthermore, our investigation reveals that 10-HHBF displays an SOC enhancement feature induced by ESIPT, which facilitates the ISC process. This trait serves as a barrier to the application of 10-HHBF in single-molecule white light emitters (SMWLEs). Our findings underscore the notable influence of the ESIPT-induced spin-orbit interaction enhancement on luminescent properties, which necessitates consideration in the design of SMWLEs.

Pressure-Induced Tunable Charge Carrier Dynamics in Mn-Doped CsPbBr3 Perovskite

All-inorganic perovskite materials (CsPbX₃) have attracted increasing attention due to their excellent photoelectric properties and stable physical and chemical properties. The dynamics of charge carriers affect the photoelectric conversion efficiencies of perovskite materials. Regulating carrier dynamics by changing pressure is interesting with respect to revealing the key microphysical processes involved. Here, ultrafast spectroscopy combined with high-pressure diamond anvil cell technology was used to study the generation and transfer of photoinduced carriers of a Mn-doped inorganic perovskite CsPbBr₃ material under pressure. Three components were obtained and assigned to thermal carrier relaxation, optical phonon-acoustic phonon scattering and Auger recombination. The time constants of the three components changed under the applied pressures. Our experimental results show that pressure can affect the crystal structure of Mn-doped CsPbBr₃ to regulate carrier dynamics. The use of metal doping not only reduces the content of toxic substances but also improves the photoelectric properties of perovskite materials. We hope that our study can provide dynamic experimental support for the exploration of new photoelectric materials.

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

2-(2'-Hydroxyphenyl)benzoxazole (HBO) substituted at the 5'-position with bipyridylvinylene phenylenevinylene (compound 2) produces both normal and, via an excited-state intramolecular proton transfer (ESIPT) reaction, tautomer emissions in solvents that preserve intramolecular hydrogen bonds. The abundance of the tautomer emission from compound 2 in a poor hydrogen-bonding solvent increases in response to the application of a higher excitation energy. Based on quantum chemical calculations, the excitation-dependent dual emission is consistent with a model in which the ESIPT reaction is more favored in the S₂ than in the S₁ electronically excited state.

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.

Tailored Solvatochromic NIR Phosphorus-Chromophores via Selective P-N and P-C Chemistry in P-Heteropines

Herein we report selective P-C and P-N chemistry as a new synthetic tool for constructing phosphorus (P)-chromophores with rich chemical structures. Our studies reveal that isomeric structures significantly influence the chemical structure and electronic communication of P-heteropines, which results in efficient tunability of the photophysical properties. In particular, isomeric P-chromophores with a protic N-H (indole) are also capable of participating in intramolecular H bonding, offering a new strategy to access a near-infrared chromophore.

The influence of intermolecular hydrogen bonds on single fluorescence mechanism of 1-hydroxy-11H-benzo [b]fluoren-11-one and 10-hydroxy-11H-benzo [b]fluoren-11-one

Solvent effects usually have an essential effect on excited-state intramolecular proton transfer (ESIPT) processes and fluorescence mechanism. This contribution presents new insights into a newly synthesized compound, namely, 10-hydroxy-11H-benzo [b]fluoren-11-one (10-HHBF), and its analogue 1-hydroxy-11H-benzo [b]fluoren-11-one (1-HHBF), which exhibit single-fluorescence properties in protic solvents (methanol, MeOH), using time-dependent density functional theory (TDDFT). The results established four schemes, namely, MeOH-1, MeOH-2, MeOH-3, and MeOH-4, for 1-HHBF and 10-HHBF in MeOH. Absorption and emission spectra showed that the 1-HHBF and 10-HHBF at the conformation MeOH-2, MeOH-3 and MeOH-4 were closer to the experimental values than those at the MeOH-1. Energy barriers indicate the possibility of the ESIPT and ESPT process in 1-HHBF and 10-HHBF under the four schemes. Moreover, reverse PT processes were easy to occur at the conformations of MeOH-2, MeOH-3, and MeOH-4 in the S₁ state. Given the single-fluorescence properties of 1-HHBF and 10-HHBF in the experiment, the conformation MeOH-1 was excluded. Therefore, our contribution proved that MeOH-2, MeOH-3, and MeOH-4 might exist in single fluorescence, and the hydrogen bond at the MeOH-2 position plays a decisive role, indicating the intermolecular hydrogen bonding interaction on the acceptor atom will have a more significant impact on the fluorescence properties of the substance.

Photophysical properties of fluorescent nucleobase P-analogues expected to monitor DNA replication

In this work, we computationally design a series of fluorescent purine analogues based on the 2-amino-8-(1'-β-D-2'-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one (P) to monitor the DNA replication process with merely a minimal perturbation to the natural structure of nucleic acid. The P-modified fluorescent probes present red-shifted absorption spectra and enhanced photoluminescence due to the additional π-conjugation resulting from the fluorophore modification and the ring-expansion. Efficient fluorescence quenching of P-analogues occurs upon pairing with the complementary 6-amino-5-nitro-3-(1'-β-D-2'-deoxyribofuranosyl)-2(1H)-pyridone (Z) due to the nonradiative relaxation from the low-lying dark excited state to the ground state of Z moiety. Especially, the P3 and the P7, which have high fluorescence intensity in both gas and liquid phases, are proposed as the sensors for studying conformational switching in the presence and absence of a complementary sequence. Also examined are the influences of hydration and the linking to deoxyribose on absorption and emission processes. Besides, the potential phosphorescence emission of these modified base pairs is taken into account by constructing the relaxed potential energy curves of S₀, T₁ and S₁ states.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696-1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

Theoretical Insights into Excited-State Intermolecular Proton Transfers of 2,7-Diazaindole in Water Using a Microsolvation Approach

The detailed excited-state intermolecular proton transfer (ESInterPT) mechanism of 2,7-diazaindole with water wires consisting of either one or two shells [2,7-DAI(H₂O)_n; n = 1-5] has been theoretically explored by time-dependent density functional theory using microsolvation with an implicit solvent model. On the basis of the excited-state potential energy surfaces along the proton transfer (PT) coordinates, among all 2,7-DAI(H₂O)_n, the multiple ESInterPT of 2,7-DAI(H₂O)₂₊₃ through the first hydration shell (inner circuit) is the most easy process to occur with the lowest PT barrier and a highly exothermic reaction. The lowest PT barrier resulted from the outer three waters pushing the inner circuit waters to be much closer to 2,7-DAI, leading to the enhanced intermolecular hydrogen-bonding strength of the inner two waters. Moreover, on-the-fly dynamic simulations show that the multiple ESInterPT mechanism of 2,7-DAI(H₂O)₂₊₃ is the triple PT in a stepwise mechanism with the highest PT probability. This solvation effect using microsolvation and dynamic simulation is a cost-effect approach to reveal the solvent-assisted multiple proton relay of chromophores based on excited-state proton transfer.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d']bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E_(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S₁ state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S₁ state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

ESIPT on/off switching and crystallization-enhanced emission properties of new design phenol-pyrazole modified cyclotriphosphazenes

Cyclophosphazene-based high-efficiency excited state intramolecular proton transfer (ESIPT) active and inactive molecules were prepared depending on the different bonding patterns of the difunctional phenol-pyrazol reagent.

In Situ Ligand Formation-Driven Synthesis of a Uranyl Organic Framework as a Turn-on Fluorescent pH Sensor

A uranium-based metal-organic framework, [(UO₂)(H₂DTATC)] (HNU-39, H₄DTATC = 5,5'-(9,10-dihydroxy-4a,9,9a,10-tetrahydroanthracene-9,10-diyl)diisophthalic acid) was successfully prepared by a hydrothermal method. The structure of HNU-39 comprises UO₈ hexagonal bipyramids linked by doubly protonated DTATC ligands, forming a ribbon arrangement. It is worth noting that the DTATC ligand was transformed in situ from 5,5'-(anthracene-9,10-diyl)diisophthalic acid (H₄DPATC) during the synthesis of HNU-39. Research on fluorescence properties has shown that HNU-39 exhibits fluorescence turn-on response under alkaline conditions and could be used as a potential pH sensor. Moreover, HNU-39 can also be successfully applied for pH sensing in real samples from a sewage treatment plant. The sensing mechanism can be interpreted as OH^- ions reacting with the protons in the organic ligand of HNU-39.

A Selective Fluorescence Turn-On Probe for the Detection of DCNP (Nerve Agent Tabun Simulant)

Diethylcyanophosphonate (DCNP) is a simulant of Tabun (GA) which is an extremely toxic chemical substance and is used as a chemical warfare (CW) nerve agent. Due to its toxic properties, monitoring methods have been constantly come under the spotlight. What we are proposing within this report is a next-generation fluorescent probe, DMHN1, which allows DCNP to become fully traceable in a sensitive, selective, and responsive manner. This is the first fluorescent turn-on probe within the dipolar naphthalene platform induced by ESIPT (excited state intramolecular proton transfer) suppression that allows us to sense DCNP without any disturbance by other similar G-series chemical weapons. The successful demonstrations of practical applications, such as in vitro analysis, soil analysis, and the development of an on-site real-time prototype sensing kit, encourage further applications in a variety of fields.

Ultrafast carrier dynamics in all-inorganic CsPbBr3 perovskite across the pressure-induced phase transition

The excited-state carrier dynamics of lead halide perovskites play a critical role in their photoelectric properties, and are greatly affected by lattice structural changes. In this work, the carrier dynamics of all-inorganic CsPbBr₃ peroveskite, as a function of pressure, are investigated using in situ high-pressure femtosecond transient absorption spectroscopic experiments. Compression is found to drive crystal structural evolution, thereby markedly changing the behavior of charge carriers in CsPbBr₃. Before the phase transition, simultaneous prolonging of the carrier relaxation and Auger recombination is achieved alongside a narrowing in the bandgap. The results favor improved efficiency and photovoltaic performance.

Pressure-Induced Tunable Electron Transfer and Auger Recombination Rates in CdSe/ZnS Quantum Dot-Anthraquinone Complexes

Electron transfer (ET) and Auger recombination (AR) processes in quantum dots (QDs) are key mechanisms for the advance of QD-based devices. However, it still remains a challenge to promote ET and suppress AR simultaneously. Here, we use in situ high-pressure ultrafast transient absorption spectroscopy to explore the impact of pressure on the ET between CdSe/ZnS and anthraquinone (AQ) and AR dissolved in cyclohexane. Remarkably, under compression, ET lifetimes are shorten, while suppression of AR lifetimes is present. The promotion of ET is attributed to the shortened distance between CdSe/ZnS and AQ induced by pressure. We rationalize that for the AR suppression, pressure may enhance the formation of an alloy layer at the core/shell interface. These findings indicate that compression is an effective approach to promote ET and suppress AR simultaneously. This study highlights a brand-new approach for modulating ET and AR and provides new routes toward QD-based applications.

A Schiff Base Fluorescence Enhancement Probe for Fe(III) and Its Sensing Applications in Cancer Cells

We report a new Schiff base fluorescent probe which senses ferric ion, Fe(III), with a significant fluorescence enhancement response. The probe showed high sensitivity (0.8 ppb), and fast response time (<10 s) of Fe(III) in aqueous media. In addition, the probe showed the ability to sense Fe(III) in a HeLa cancer cell line, with very low cytotoxicity. As a new bio-imaging probe for Fe(III), it gave bright fluorescent images in confocal laser scanning microscopy (CLSM).

Tunable electron transfer rate in a CdSe/ZnS-based complex with different anthraquinone chloride substitutes

We use femtosecond transient absorption spectroscopy to study ultrafast electron transfer (ET) dynamics in a model donor and acceptor system using CdSe/ZnS core/shell structure quantum dots (QDs) as donors and anthraquinone (AQ) molecules as acceptors. The ET rate can be enhanced by decreasing the number of chlorine substituents in the AQ molecules because that increases the driving force, which is the energy level offset between the conduction band energy of CdSe/ZnS and the lowest upper molecular orbital potential of AQ derivatives, as confirmed by cyclic voltammetry measurements. However, the electronic coupling between the QDs and AQ derivatives, and the sum of reorganization energy of AQ molecules and solvent calculated by density functional theory are not the main reasons for the change in ET rate in three systems. Our findings provide new insights into selecting an acceptor molecule and will be useful in tuning ET processes for advanced QD-based applications.

Low-Lying Excited States of hqxcH and Zn-hqxc Complex: Toward Understanding Intramolecular Proton Transfer Emission

Excited state intramolecular proton transfer (ESIPT) has been a topic of interest due to its potential to lead to multiple emissions. Although many organic molecules showing ESIPT emission are already known, studies on metal complexes showing ESIPT and their related theoretical understandings are very limited. In this study, we focus on [Zn(hqxc)₂(DMSO)₂] (Zn-hqxc: hqxc = 3-hydroxy-2-quinoxalinecarboxylate, DMSO = dimethyl sulfoxide), which shows ESIPT emission in the solid state, even though the hqxcH ligand does not show ESIPT emission. To gain insights into the role of the zinc atom and the emission mechanisms, we examined excited states of free hqxcH and the Zn-hqxc complex using time-dependent density functional theory calculations. From the results, it was shown that the zinc atom triggers a structural change of the hqxcH ligand from the lactam form (3,4-dihydro-3-oxo-2-quinoxalinecarboxylic acid) to the enol form (3-hydroxy-2-quinoxalinecarboxylic acid), where the latter form has several stable excited states. Several stable geometries were found for singlet and triplet excited states, suggesting that emissions for the Zn-hqxc complex can be both phosphorescence and fluorescence caused by the enol-enol, keto-keto, and keto-enol forms of the two hqcx ligands in the complex. We found that the photophysical properties of the Zn-hqxc complex are dominated by the ligand due to the filled d¹⁰ of Zn(II). The presented results suggest that, to design new ESIPT metal complexes, one possible approach is to combine a metal atom showing ligand centered emission and a ligand that has separate ESIPT and coordination sites.

Hydrazine Exposé: The Next-Generation Fluorescent Probe

Hydrazine (N₂H₄) is one of the most important pnictogen hydride chemicals, and is utilized within a wide spectrum of industries. As a result of its extensive use, hydrazine's monitoring methods have constantly come under fire due to its potential health risk and the subsequent environmental pollution. Fluorometric molecular sensing systems generally report with a major emphasis on the merit of fluorescence analysis. What we are proposing within this report is a next-generation fluorescent probe that allows hydrazine to become fully traceable, within multifarious environments that show fast and intuitional fluorescence transformation. A new sensing moiety, ortho-methoxy-methyl-ether ( o-OMOM) incorporated electron donor (D)-acceptor (A) type naphthaldehyde provides high selectivity and sensitivity amidst its superiority within practical applications for sensing hydrazine. The new probe overcomes most of the drawbacks of currently used fluorescent probes, and due to its successful demonstrations, such as real-time spray-based sensing, soil analysis, and two-photon tissue imaging, its potential for practical application is beyond reproach.

Lighting Up the Invisible Twisted Intramolecular Charge Transfer State by High Pressure

The twisted intramolecular charge transfer (TICT) state plays an important role in determining the performance of optoelectronic devices. However, for some nonfluorescent TICT molecules, the "invisible" TICT state could only be visualized by modifying the molecular structure. Here, we introduce a new facile pressure-induced approach to light up the TICT state through the use of a pressure-related liquid-solid phase transition of the surrounding solvent. Combining ultrafast spectroscopy and quantum chemical calculations, it reveals that the "invisible" TICT state can emit fluorescence when the rotation of a donor group is restricted by the frozen acetonitrile solution. Furthermore, the TICT process can even be effectively regulated by the external pressure. Our study offers a unique strategy to achieve dual fluorescence behavior in charge transfer molecules and is of significance for optoelectronic and biomedical applications.

Asymmetric substitution changes the UV-induced nonradiative decay pathway and the spectra behaviors of β-diketones

Asymmetric substitution has not been termed as an essential factor in studying photo-induced ultrafast dynamics of molecular system. Asymmetric 4-hydroxybut-3-en-2-one (HEO), together with symmetric malonaldehyde (MA) and acetylacetone (AA), have been provided as target sample to study the nonradiative decay (ND) processes of β-diketones. An effective ND pathway of the three molecules is presented that their excited second (S₂) states transfer to first (S₁) state by nonadiabatic surface hopping, and then transfer to triplet (T₁) state by crossing minimum energy crossing point (MECP), after which decay to ground (S₀) state through MECP. More importantly, the asymmetric substitution of HEO induces the proton transfer in the S₁ state and generates a proton-transferred conformer with lowest energy, which does not occur for MA and AA. This change exploits a new ND pathway that the S₁ state decays to the proton transferred T₁ state and then undergoes reverse proton transfer to S₀ state through the MECPs between the three states. The two pathways of HEO give detailed energy and geometric information on surface hopping of S₂/S₁ and MECPs of S₁/T₁/S₀, and interpret the reason of the ND pathway while not spectra emission. This result is significantly different from the previous reported ND pathway of photoisomerization or conical intersection between different states. This work shows that asymmetric substitution changes the molecular structure and then changes their spectra behaviors.