The probe HBT-BA1 based on excited-state intramolecular proton transfer (ESIPT) can achieve rapid optical responses and show promising potential for formaldehyde (FA) detection. However, the ESIPT-mediated luminescence regulation mechanism remains completely unexplored, particularly regarding how environmental conditions determine the subsequent reaction pathways of key intermediates. In this work, the systematic theoretical study on the environment-dependent, dual-pathway FA sensing mechanisms was performed by using quantum chemical methods. Under neutral condition, the probe captures FA and undergoes spontaneous photocyclization to emit strong deep-red light at 711 nm, with a large Stokes shift of up to 392 nm. Frontier molecular orbitals (FMOs) analysis confirms that cyclization reduces the energy gap of electronic transition and enhances electronic delocalization, thereby facilitating long-wavelength emission. In acidic environments, the intermediate undergoes acid-catalyzed hydrolysis to release the classic ESIPT fluorophore HBT, which spontaneously protonates to yield HBT+ with bright blue-violet emission at 413 nm. Overall, this work fully elucidates the environment-dependent recognition mechanism at molecular level, which provides important theoretical guidance for developing environment-adaptive probes for precise FA detection.

Diarylethene, as a classical photochromic molecule, often suffers from limitations in its reactive efficiency due to competitive nonradiative decay pathways. The competition between excited-state intramolecular proton transfer (ESIPT) and photocyclization is central to the function of the novel diarylethene derivative studied here. Incorporating an intramolecular hydrogen bond into the π-linker is found to advance ESIPT, which effectively suppresses photocyclization by providing a dominant nonradiative relaxation pathway. This mechanism is unraveled through systematic quantum chemical calculations of the ground- and excited-state potential-energy curves, with reaction barriers quantified by transition-state theory. The analyses of excited-state aromaticity and electronic structure provide a fundamental understanding of the reactivity and clarify the different properties and functions of molecules before and after cyclization. Disrupting the hydrogen bond via esterification conclusively validates the mechanism: it blocks the ESIPT pathway, turns off the nonradiative channel, and enables the efficient photocyclization and photochromic properties. This study elucidates a novel molecular-level strategy for modulating the photocyclization of diarylethene and establishes a foundational strategy for crafting intelligent photoresponsive materials.

Recurrent failures of rail fastener clips have been increasingly observed at a small-radius curve section of a Hong Kong metro line, compromising operational safety of the train. To illustrate the underlying mechanisms, this study adopts a two-pronged methodology that integrates field measurements with numerical simulations. Firstly, wheel-track resonance characteristics are systematically investigated using friction-induced self-excitation vibration theory coupled with synchronised field measurements of rail vibration and corrugation. To characterise the clip's modal parameters, a refined finite element (FE) model validated by on-site modal tests is developed, revealing that clip natural frequencies at 560 Hz and 1050 Hz are directly matching dominant wheel-track excitation frequency bandwidths of 500-600 Hz and 1000-1200 Hz. Secondly, we propose a novel framework to efficiently calculate the clip fatigue damage in frequency domain. It utilises the power spectral density (PSD) function of rail acceleration and clip stress in the track system, significantly reducing the FE model scale and enhancing the simulationmeasurement correlation. The principal findings indicate: (1) The concentrated stress of the gauge-side clips is 13 % higher than the outer-side clips of the clip; (2) The fatigue failure rates of inner rail gauge-side clips is at least 4.7 times higher than that of other positions; and (3) Critical fracture initiation points are situated at the root of the rear arch's inner surface. These results closely correspond with field observations, validating the practical engineering value of the proposed methodology for dynamic fatigue damage assessment.

Metro line has the characteristics of short station distance, low operating speed, frequent traction /braking, high acceleration and deceleration, small curve radius and various track types. The wheel-rail low adhesion behavior, wear and damage problems are prominent under such a complicated operation condition. This article compares and analyzes the wheel-rail adhesion characteristic on four kinds of tracks, including the concrete slab bed track, steel-spring floating slab track, elastic support block track and trapezoidal sleeper track, revealing the adhesion degradation mechanism of metro wheel-rail system under different operating conditions, such as traction and braking. The results indicate that there are differences in the creepage response and the lowest limit of the wheel-rail adhesion coefficient on different track structures, while the maximum adhesion coefficient of the four track structures under traction/braking conditions shows a similar trend with the variation in speed and adhesion conditions. Traction and braking loads, as well as wheel-rail adhesion conditions, exert a relatively minor influence on the normal contact stress distribution of the wheel-rail interface, but significantly affect shear stress distributions. This study is shown to enhance the understanding of the combined effects of adhesion degradation and track types on the wheel-rail contact stress and surface damage.

High temperature frictional heat generated at the wheel/rail interface can lead to high thermal stresses and rapid wear of wheel and rail. This study develops a heavy-haul train-track coupled dynamics model that incorporates temperature rise and thermal stress in the contact patch, which enables a more accurate and realistic simulation of the thermo-mechanical contact behavior. The influence of wheel wear evolution on frictional heat transfer characteristics and its effect on wheel/rail dynamic interaction are investigated. Results show that large creepage causes increased frictional heating and thermal stress. Wheel slip during braking induces higher frictional heat and thermal stress in low adhesion conditions, while lower anti-slip control thresholds can reduce temperature rise and thermal stress. Moreover, wheel tread wear will give rise to the maximum temperature and peak thermal stress in the contact areas. The findings can provide valuable insights for theoretical research on wheel/rail frictional thermal stress solutions and wheel wear prediction simulations, while laying a foundation for optimizing anti-slip control systems on locomotives.

Excited-state intramolecular proton transfer (ESIPT) can serve as an effective pathway for inducing thermally activated delayed fluorescence (TADF), but the underlying mechanism by which it triggers TADF remains to be fully elucidated. Herein, we explore the photophysical mechanisms of ESIPT-mediated TADF behavior in salicylaldehyde Schiff base derivatives. Theoretical calculations reveal that the key to truly triggering TADF lies not in the favorable ESIPT kinetic barrier but in whether ESIPT further enhances the intramolecular charge transfer (ICT). The synergistic enhancement effect of ESIPT and ICT reduces the singlet-triplet energy gap (ΔE ST ), thereby facilitating rapid intersystem crossing (ISC) and reverse intersystem crossing (RISC). Furthermore, the reorganization energy (λ) is another crucial factor governing TADF. Even in systems with an extremely small ΔE ST , a large λ significantly suppresses the ISC/RISC. These results provide an important molecular design strategy for developing high-performance ESIPT-TADF materials.

Abstract Wear and rolling contact fatigue (RCF) are regarded as two prominent factors that deteriorate the long-term service performances and aggravate the maintenance costs of metro rails on the small-radius curves; therefore, an effective countermeasure against them is needed. In this work, a long-term rail wear and RCF evolution prediction model based on vehicle–track coupled dynamics and surface material wear and fatigue damage theory is performed, in which an enhanced wheel/rail non-Hertzian contact method is involved. On the other hand, a series of candidate rail profiles are constructed and generated by applying the Gaussian function correction (GFC) method. An improved nondominated sorting genetic algorithm-II (NSGA-II) method is applied for the optimization design of rail profiles, and then we propose two types of rail profiles which take the rail wear or RCF as the optimization objectives. Further, the long-term rail wear and RCF evolution performances subjected to the raw rail profiles and optimally designed ones are compared. The results demonstrate that the optimized rail profiles contribute profitably to alleviate the wheel flange–rail gauge corner frictional contact and, subsequently, can slow down the wear and RCF development of rails on sharp-radius curves. This study can offer a theoretical inspection for wear- and RCF-resistant design of rail grinding profiles.

Recent intriguing concepts have proposed that utilizing metal-organic frameworks (MOFs) as host matrixes can improve the crystallization of 9,10-diphenylanthracene (9,10-DPA) molecules in the solid film while also maintaining its high fluorescence quantum yield. Understanding the fundamental excited state dynamics is essential to the potential applications of these emerging materials, especially considering that photocarrier dynamics are expected to behave differently in the MOF structure due to the restricted degrees of freedom and long-range electronic couplings. Here, we have systematically investigated the carrier dynamics in highly crystalline and ordered DPA-MOF thin films employing transient absorption spectroscopy. Under photoexcitation, the constrained DPA molecules within the framework exhibit an ultrafast structural torsion in several hundred femtoseconds, which alters the energy positions of S1 states and blocks the S0 -> S1 transition, enabling a strong localized excitation and the excimer formation. Sequentially, the localized photocarriers start to redistribute via the intramolecular singlet-to-triplet energy transfer and the interlayer singlet charge transfer. Furthermore, we have demonstrated that the photoinduced torsion and corresponding carrier dynamics can be effectively controlled by the excitation fluence.

2-{[3-(1H-benzoimidazol-2-yl)-2-hydroxy-5-methylbenzylidene] amino}-benzoic acid (H2BIo) based on proton transfer can serve as the fluorescent probe for detecting heavy metal ions. The excited-state intramolecular proton transfer (ESIPT) reaction mechanism of the H2BIo chromophore with an intramolecular asymmetric double hydrogen bond in different solvents are investigated. The reaction barrier of the ESIPT along hydrogen bond O1-H2···N3 is higher than that of ESIPT along O4-H5···N6, which indicates that the double ESIPT is a stepwise process. The time-evolving non-adiabatic excited-state dynamic simulations shows that the sequence ESIPT reactions on a time scale: the ESIPT along O1-H2···N3 is faster than the ESIPT along O4-H5···N6. The analyses of electron structure and spectra indicate that the double ESIPT couples with electron transfer, significantly enhances the fluorescence signal, thereby improving the performance of the fluorescent probe in detecting heavy metal ions.

The stability of protein secondary structure is the basis for the realization of biological functions, and temperature induces conformational changes in proteins by disrupting the equilibrium of the internal hydrogen bond network. This study systematically elucidates the temperature-induced unfolding mechanism of aprotinin based on one-dimensional infrared spectroscopy (1D IR) and two-dimensional infrared spectroscopy (2D IR) simulated by molecular dynamics (MD) simulations. The simulations showed that the characteristic absorption peaks of the amide I band in the IR spectra of the aprotinin were blueshifted and weakened with increasing temperature, indicating that the hydrogen bond breaking leads to the secondary structure transformation of the aprotinin. The 2D IR simulations of aprotinin reveal that when the temperature rose to 333 K, the vibrational coupling peak of the β-sheet at (1625.4 cm−1, 1653.4 cm−1) disappears. At 353 K, new coupling peaks corresponding to random coil and β-turn appear at (1639.7 cm−1, 1672.2 cm−1), and these signals disappear at 373 K, indicating complete unfolding of aprotinin. During the heating process, the 2D IR spectral signal of the aprotinin shifted from excited-state absorption (ESA) to ground-state bleaching (GSB), reflecting the energy transfer process within the protein, and its dynamics process and spectral broadening rate were greatly affected by temperature. The unfolding pathway of aprotinin was again obtained by MD simulation and analyzed in comparison with infrared spectroscopy, and the unfolding behavior of the aprotinin was resolved at the atomic level. In this work, the MD simulation of aprotinin was carried out by GROMACS, using the OPLS/AA force field and SPC/E water model, with a simulation duration of 500 ns and a time step of 2 fs. After the simulation, the C = O stretching vibrations of peptide bonds were extracted from the trajectory files to calculate the autocorrelation function and third-order nonlinear response function, which were Fourier transformed to obtain the 1D and 2D IR spectra of aprotinin. The DSSP program in GROMACS was used to quantitatively analyze the secondary structure content of aprotinin at various temperatures, and the conformational changes under different temperature conditions were visually analyzed by using VMD software.

Recently, researchers studied how the aromaticity of it-linker affects cyclization reactions and found that varying aromaticity can regulate these reactions. Inspired by this, the different aromatic it-linker with proton donor-acceptor blocks will be investigated to explore the influence of aromaticity on cyclization and proton transfer reactions. In this work, the quantum chemical calculation combined with density functional theory (DFT) and the multi-configurational self-consistent field (MCSCF) methods are employed to unveil the photophysical and photochemical processes. The established potential energy curves (PECs) and the calculated minimum energy pathways (MEPs) indicate that the excited state proton transfer (ESPT) and photocyclization processes vary with different it-linker. The complete-active-space self-consistent-field (CASSCF) calculations reveal that a non-radiative channel will be opened via the conical intersection (CI) between the ground and the first excited states during photocyclization. The time evolved excited state nonadiabatic dynamics are simulated to demonstrate the ultrafast photocyclization reactions impede the ESPT. Finally, the aromaticity of various it-linker is evaluated using nucleus-independent chemical shifts (NICS) aromaticity indices. The study concludes that excited-state antiaromaticity relief facilitates the cyclization reaction and ESPT process.

Surface enhanced fluorescence (SEF) plays a key role in sensing and cell imaging. However, in order to prevent cytotoxicity, the concentration of noble metal nanoparticles (Ag) needs to be lowered, which leads to poor SEF performance. Herein, different Ag@C (core-Ag, shell-carbon) nanoparticles were synthesized, and Ag@C nanoparticles prepared under 4 h heating (AC4.0) had the largest SEF enhancement factor (EFsef). Dilution of AC4.0 markedly reduces EFsef. Subsequently, Ag@C-GSH nanoparticles (ACG) were prepared by modifying glutathione (GSH) on the surface of diluted AC4.0. EFsef of ACG was improved at pH 4, and similar result for diluted AC4.0 at pH 8. This recovery of EFsef at low nanoparticle concentrations was attributed to a mechanism of electric field strength enhancement: acid-induced protonation of glutathione mediates aggregation of ACG via hydrogen bonding, which amplifies electric field strength at "hot spots".

Flavin-type molecules have good biocompatibility and spectral properties, and they are widely studied in photophysics and electrochemistry, most of them are derivatives of Alloxazine (All). Some scholars have studied the spectral properties of All in water-organic mixtures at the molecular level. Nevertheless, the photophysical properties and reaction mechanism of simple flavin-water in an aprotic solvent remain poorly explored. Our research delves into the proton transfer mechanism of All, as well as investigates the effects of substituents and DMSO concentration on its photophysical properties. It is found that the intermolecular hydrogen bond between the solvent and solute is crucial for excited-state proton transfer (ESPT). ESPT occurs spontaneously when at least two water molecules and All to form three intermolecular hydrogen bonds (All-2H2O). The time-resolved excited-state non-adiabatic dynamics illustrate that the long-range ESPT of All-2H2O is an ultrafast kinetic reaction, the bond cleavage of proton donors N-H first occurs on a time scale of less than 100 fs. It indicates the hydrogen-supplying capacity of the proton donor is dominant in the ESPT process. Moreover, F substitution (All-F) and 50 % DMSO concentration transform the first excited state of All into a bright state with allowed transitions. This work will offer valuable theoretical insights for developing and applying flavin-type based on ESPT.

Multifunctional photochromic diarylethylethene derivatives have attracted much attention from researchers due to their unique physicochemical properties originated from photocyclization. Yu et al. synthesized a terthiazolebased diarylethenes (ap-P) featuring intramolecular hydrogen bonds in experiment. They supposed that there was a competitive relationship between photocyclization and excited-state intramolecular proton transfer (ESIPT). To gain deeply insight into the connection between two reactions, the geometry optimization, the calculation of electron spectra, and the construction of minimum energy paths of ESIPT and cyclization were carried out under solvent and gas phases. These studied were based on closed shell and unrestricted open shell density functional theory, as well as other quantum chemistry methods. Moreover, qualitative and quantitative analyses of the electron structure revealed that the competition between two reactions is fundamentally driven by changes in aromaticity and electron delocalization. Furthermore, it was theoretically proved that the regulation of ESIPT is realized by changing environmental parameters, leading to a novel method for precise control of photocyclization reaction. It is expected to provide theoretical guidance and prediction for the flexible application of multifunctional photochromic molecular switches in photoelectric materials, biomedicine and other fields.

Herein, the intramolecular long-range proton transfer reaction mechanism of the HQBT chromophore in different solvents is investigated employing density functional theory and complete active space self-consistent field methods. The results show that HQBT successfully undergoes the first excited state intramolecular proton transfer (ESIPT) under photoexcitation. Subsequently, through the first cis-trans isomerization process of dihedral angle torsion, a minimum energy conical intersection (MECI) is formed between the ground state and the first excited state. The MECI continues the second cis-trans isomerization to generate the trans-keto structure. At this point, a new intramolecular hydrogen bond is formed and experiences the second ESIPT, thus achieving long-range transport of hydrogen protons. This careful theoretical research has significant guiding significance for the design of intelligent and efficient tautomeric molecular switches in the future.

Recently, the solvation preferences of N-confused tetraphenylporphyrin (NCTPP) within dichloromethane mixed solvent systems have been investigated. Findings indicate that the significance of interactions between solvent molecules is comparable to that between the solute and solvent molecules. However, there are limited reports on the photoinduced tautomerism mechanism of NCTPP. In this paper, the effects of N-confusion and substituents on the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of NCTPP derivatives are studied by density functional theory and time-dependent density functional theory. Initially, the molecular geometric structure is fully optimized. Electronic structure analysis indicates that the molecule has the local excitation characteristic of ππ* transition. Subsequently, the observed large Stokes shift in emission spectra is attributed to electron perturbations in the π-conjugated ring, induced by N-confusion. Aromaticity analysis confirms that this shift is due to the enhanced localization of electron rearrangement. Finally, potential energy curves are constructed, and the ESIPT reaction rate is computed, along with excited-state non-adiabatic dynamics simulations, demonstrating the dynamic equilibrium of ESIPT. Theoretical studies reveal that N-confusion outside the ring, coupled with benzene substitution, induces electron rearrangement, resulting in enhanced electron localization and a pronounced Stokes shift. This alteration in photophysical properties opens new avenues for applications in photochemistry and materials science.

High-pressure engineering presents a promising avenue to enhance the thermoelectric property of SnSe, but the behaviors of electronic/lattice systems under pressure, as well as their connections to the thermoelectric performance, remain poorly understood. This study employs in situ high-pressure ultrafast optical pump-probe spectroscopy to investigate the nonequilibrium dynamics of both Fermi and bosonic particles in SnSe. These results reveal an electronic structure phase transition (EPT) occurring in the low-pressure region, alongside a structural phase transition (SPT) at higher pressures. Notably, at 4 GPa, anomalous behaviors of inter-valley and intra-valley scattering processes are observed, which band structure calculations attribute to a pressure-induced abrupt change in electronic structure, identified as a Lifshitz transition. At 7 GPa, the evolution of electron-phonon scattering lifetime and phonon frequency indicates a structural phase transition from Pnma to Bbmm. These insights into nonequilibrium behaviors of SnSe under high pressure provide a foundational understanding for further optimizing its thermoelectric efficiency.

The unique stereocarborane fluorophore design has attracted widespread attention because of its ability to effectively enhance the performance of thin-film fluorescence sensors. This study provides a detailed theoretical investigation of the excited-state intramolecular proton transfer (ESIPT) reactions of three o-carborane derivatives, NaCBO, PaCBO and PyCBO, using density functional theory and time-dependent density functional theory. We found that as the number of benzene rings in the electron donor groups increases, the interaction between the first excited-state electron donor-acceptor fragments is enhanced, weakening the intramolecular hydrogen bonding strength, which in turn inhibits the ESIPT. Notice that the intramolecular charge transfer (ICT) is proportional to the interaction between electron donor and acceptor fragments with the analysis of electron structure. The higher the ICT level of the target system, the more unstable the keto form generated by ESIPT. Therefore, the ESIPT can be precisely manipulated by rationally adjusting the ICT effect. Furthermore, with increasing solvent polarity, the photophysical properties of the chromophores exhibit non-solvatochromism. This is attributed to the rigid o-carborane structure, which serves as a spatial scaffold to increase the adaptability of the molecule to the environment. This study is expected to provide theoretical guidance and new ideas for preparing high-performance thin-film fluorescence sensing.

Low-friction surface conditions significantly contribute to the reduction of the wheel/rail adhesion capability and the occurrence of wheel/rail slipping behaviors, which may lead to the degradation of mechanical properties and frictional wear damage at the wheel/rail interface. To mitigate these undesirable consequences, modern railway locomotives are equipped with on-board anti-slip control systems. In this study, three different anti-slip controller models, comprising the traditional re-adhesion anti-slip controller and PID-based anti-slip controller with fixed threshold and with optimal threshold, are established. The wheel/rail rolling-slipping performances subjected to different anti-slip control algorithms under changing wheel/rail friction conditions are compared based on train-track interaction simulations. The results demonstrate that the PID-based anti-slip controller with an optimal threshold achieves the maximum utilization of wheel/rail adhesion in the presence of low-friction conditions, outperforming the other two types of anti-slip controllers. Additionally, the adoption of an anti-slip controller with a lower control threshold can effectively reduce the tread wear of locomotive wheels. This research can provide a deep going understanding of optimization design of anti-slip controller on railway vehicles.

The electromagnetic enhancement (EM) mechanism is one of the important theories that need to be mastered study surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS). However, the charge transfer in the chemical enhancement (CM) mechanism has different effects on the two. The study developed simple and environmentally friendly composite structural substrate by depositing silver (Ag) on the rough surface with regular columnar protuberances of bio-material cicada wings (CW) using magnetron sputtering technology. The formation of Ag nano-islands enhanced the fluorescence and Raman signals of rhodamine 6G (R6G), while the addition of silver nanoparticles (AgNPs) quenched the fluorescence. To explore the magical phenomenon, the electromagnetic field distribution on the surface of the substrate was simulated through threedimensional finite difference time domain (3D-FDTD), which verified the experimental results of SEF and SERS. The results calculated by Gaussian16 with Density Functional Theory (DFT) and time-dependent density functional theory (TDDFT) explained the effect of adding AgNPs on the surface of the substrate on the fluorescence intensity. The work not only contributes experimental data and theoretical results to the EM mechanism and CM mechanism in enhanced spectra, but also provides ideas for preparing new SEF and SERS substrates.