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Intramolecular Charge Transfer in 5-Halogen Cytidines Revealed by Femtosecond Time-resolved Spectroscopy
摘要： Ultraviolet radiation induced damage to DNA/RNA can lead to chemical modifications to the nucleosides and understanding the excited states involved is the key to reveal the mechanism of those reactions. 5-Halogen cytidines are metabolized DNA/RNA nucleoside byproducts that exhibit very important biological functions in the process of nucleic acid methylation as well as DNA/RNA damage repairing. However, despite the accumulation of knowledge about their biological functions, effect of halogen substitution on the excited states of canonical nucleoside have not received much attention. In this work, excited-state dynamics of 5-fluorocytidine, 5-chlorocytidine and 5-bromocytidine is investigated. Excitation at 295 nm results a bifurcation event that leads to sub-picosecond decay to ground state and population of intramolecular charge transfer states which have several to tens of picosecond lifetimes. The results elucidate the general excited state relaxation pathways in 5-halogen cytidines and the intrinsic charge transfer state may affect the halogen bonding that stabilizes DNA and protein structures when 5-halogen cytidines are excited.
关键词： Excited-state dynamics,Femtosecond Time-resolved Spectroscopy,DNA/RNA damage,5-Halogen Cytidines,Intramolecular Charge Transfer
更新于2025-09-19 17:13:59
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Dynamic Ligand Surface Chemistry of Excited PbS Quantum Dots
摘要： The ligand shell around colloidal quantum dots mediates the electron and energy transfer processes that underpin their use in optoelectronic and photocatalytic applications. Here, we show that the surface chemistry of carboxylate anchoring groups of oleate ligands passivating PbS quantum dots undergoes significant changes when the quantum dots are excited to their excitonic states. We directly probe the changes of surface chemistry using time-resolved mid-infrared spectroscopy that records the evolution of the vibrational frequencies of carboxylate groups following excitation of the electronic states. The data reveal a reduction of the Pb?O coordination of carboxylate anchoring groups to lead atoms at the quantum dot surfaces. The dynamic surface chemistry of the ligands may increase their surface mobility in the excited state and enhance the ability of molecular species to penetrate the ligand shell to undergo energy and charge transfer processes that depend sensitively on distance.
关键词： surface chemistry,PbS quantum dots,ligand shell,colloidal quantum dots,time-resolved mid-infrared spectroscopy
更新于2025-09-19 17:13:59
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Time-resolved measurements of two-color laser light emitted from GaAs/AlGaAs-coupled multilayer cavity
摘要： We measured the two-color laser oscillation from a GaAs/AlGaAs-coupled multilayer cavity at 18 °C–42 °C using current injection. We confirmed simultaneous lasing by detecting the sum frequency generation signal generated by the two-color laser light, and performed time-resolved measurement using a streak camera with a spectrometer. From the observed time transient of the spectra at various temperatures, it is clarified that the temperature change of the device, induced by current injection, modulates the effective cavity length. Therefore, the temperature control of the device is a key factor in stable two-color lasing and THz wave generation.
关键词： time-resolved measurements,sum frequency generation,THz wave generation,GaAs/AlGaAs-coupled multilayer cavity,two-color laser
更新于2025-09-19 17:13:59
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Thiophenylazobenzene - an alternative photoisomerization controlled by lone pair???π interaction
摘要： Azoheteroarene photoswitches have attracted attention due to their unique properties. We present here the stationary photochromism and the ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB) – an alternative sulfur-based azoheteroarene design. TphAB demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)- and (Z)-isomer absorption bands, allowing for highly selective photoconversion. The (Z)-isomer of TphAB adopts an unusual orthogonal geometry where the plane of the thiophene is perfectly perpendicular to that of the benzene. This geometry is stabilized by a rare lone pair???π interaction between the S-atom and the benzene. The photoisomerization of TphAB occurs on the sub-ps to ps timescale and is governed by the lone pair???π interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.
关键词： time-resolved spectroscopy,photochromism,photoswitch,thiophenylazobenzene,isomerization mechanism
更新于2025-09-19 17:13:59
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Role of Emissive and Non‐Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots
摘要： Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4-dinitrotoluene (DNT) is studied in toluene. We observed two types of QD-DNT complex formations; (i) non-emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non-emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady-state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady-state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD-DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns(cid:2)1 to 0.072 ns(cid:2)1) of intrinsic quenching rate with increasing the QD particle size (from (cid:3) 3.2 nm to (cid:3) 5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from (cid:3) 0.1–0.2 to (cid:3) 1.2–1.7, as the DNT concentration is increased from (cid:3) 1 mm to 17.5 mm. Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.
关键词： CdTe quantum dot,photo-induced electron transfer,donor-acceptor complex,time-resolved spectroscopy,fluorescence quenching
更新于2025-09-19 17:13:59
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Proton release process during the S2-to-S3 transition of photosynthetic water oxidation as revealed by the pH dependence of kinetics monitored by time-resolved infrared spectroscopy
摘要： Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II via light driven cycle of intermediates called S states (S0–S4). Clarifying how electron and proton transfer reactions are coupled with each other in the S2→S3 transition, which occurs just before the O-O bond formation, is crucial to understand the water oxidation mechanism. Here, we investigated the pH dependence of the kinetics of the S2→S3 transition using time-resolved infrared (TRIR) spectroscopy to identify the proton release phase in this transition. TRIR measurements of YD-less PSII core complexes from the D2-Y160F mutant of Thermosynechococcus elongatus showed that the last phase in this transition (τ = ~350 μs at pH 6) was strongly pH dependent, and its time constant at pH 5 was larger than that at pH 8 by a factor of more than three. In contrast, the earlier phase with a ~100 μs time constant was virtually independent of pH. These results strongly support the view that proton release is a rate-limiting step of the proton-coupled electron transfer in the last phase of the S2→S3 transition. This proton release enables electron transfer by removing an excessive positive charge from the catalytic center and hence lowering its redox potential.
关键词： Mn4CaO5 cluster,proton-coupled electron transfer,S2→S3 transition,photosynthetic water oxidation,time-resolved infrared spectroscopy
更新于2025-09-16 10:30:52
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Annealing effect on laser-induced magnetization dynamics in Co/Ni-based synthetic antiferromagnets with perpendicular magnetic anisotropy
摘要： We report a comprehensive study of annealing treatment modulation on the laser-induced ultrafast magnetic behaviors in antiferromagnetically exchange-coupled [Ni/Co]4/Ru/[Co/Ni]3 multilayers with perpendicular magnetic anisotropy (PMA). Magnetic hysteresis loops indicate that the uniaxial PMA ?eld Hkeff decreases monotonously with the increase in annealing temperature Ta, but the variation of interlayer coupling ?eld Hex is rather complicated. Time-resolved magnetic-optical Kerr effect (MOKE) measurements demonstrate that the laser-excited demagnetization and precession process relies signi?cantly on Ta. Upon laser impulsion, the MOKE signal immediately shows a nonchanging transient increase and decrease with H increasing for low Ta, but only the ultrafast decreasing behavior for high Ta. From the subsequent dynamic precession spectra, the optical and acoustic precession modes are identi?ed. By ?tting the ?eld-dependent frequency curves via the deduced dispersion relations, both Hkeff and Hex are determined and their variation trends agree well with the results from the static magnetic measurement. Moreover, it is found that the critical ?eld where the ultrafast signal decrease occurs is dependent on the co-effect of Hkeff and Hex, whereas the maximum ?eld at which the optical mode precession disappears shares the same trend as Hex. The magnetic damping of acoustic mode is seen to increase with Ta due to the increased inhomogeneities. Our ?ndings provide a deep understanding of the magnetic properties in synthetic antiferromagnetic multilayers with high annealing temperatures, which will be helpful for designing advanced spintronic devices.
关键词： time-resolved magnetic-optical Kerr effect,laser-induced ultrafast magnetic behaviors,annealing treatment,synthetic antiferromagnets,perpendicular magnetic anisotropy
更新于2025-09-16 10:30:52
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Time-resolved interference microscopy for studying nonideal plasma formed by high-power femtosecond laser pulses
摘要： Measurement results of characteristic size of plasma density inhomogeneity on the surface of bulk iron target formed under action of femtosecond laser pulses with intensity of 1016 W/cm2 are presented. Investigations are performed using time-resolved interference microscopy technique. Femtosecond laser pulses with high time contrast of 107 generated by chromium:forsterite laser system are applied. Efficacy of the chosen technique is demonstrated; the size of plasma inhomogeneity is shown to be less than 30 nm.
关键词： femtosecond laser pulses,time-resolved interference microscopy,plasma density inhomogeneity,nonideal plasma
更新于2025-09-16 10:30:52
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Influence of target temperature on femtosecond laser-ablated brass plasma spectroscopy
摘要： femtosecond LIBS, a high-temperature and low-density plasma with high emission can be generated by increasing the target temperature. The results showed that, compared with the experimental condition of 25 oC, the spectral intensity of the femtosecond LIP was enhanced with more temperature target. In addition, the electron temperature and density were calculated by Boltzmann equation and Stark broadening, indicating that the changes in the electron temperature and density of femtosecond LIP with the increase of the target temperature were different from each other. By increasing the target temperature, the electron temperature increased while the electron density decreased. Therefore, in femtosecond LIBS, a high-temperature and low-density plasma with high emission can be generated by increasing the target temperature. The increase in the target temperature can improve the resolution and sensitivity of femtosecond LIBS.
关键词： LIBS,femtosecond laser,emission enhancement,laser-induced breakdown spectroscopy,target temperature,time-resolved spectroscopy
更新于2025-09-16 10:30:52
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Implementation of data-cube pumpa??probe KPFM on organic solar cells
摘要： An implementation of pump–probe Kelvin probe force microscopy (pp-KPFM) is reported that enables recording the time-resolved surface potential in single-point mode or over a 2D grid. The spectroscopic data are acquired in open z-loop configuration, which simplifies the pp-KPFM operation. The validity of the implementation is probed by measurements using electrical pumping. The dynamical photoresponse of a bulk heterojunction solar cell based on PTB7 and PC71BM is subsequently investigated by recording point-spectroscopy curves as a function of the optical power at the cathode and by mapping 2D time-resolved images of the surface photovoltage of the bare organic active layer.
关键词： Kelvin probe force microscopy (KPFM),time-resolved measurements,bulk heterojunctions,organic photovoltaics,pump–probe configuration,photocarrier dynamics
更新于2025-09-16 10:30:52