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Electron transfer from photoexcited naphthalene-1,4:5,8-bis(dicarboximide) radical anion to Mn(bpy)(CO)3X and Re(bpy)(CO)3X CO2 reduction catalysts linked via a saturated methylene bridge
摘要： Supramolecular systems that connect a naphthalene-1,4:5,8-bis(dicarboximide) (NDI) radical anion donor to Mn(bpy)(CO)3Br or Re(bpy)(CO)3Cl CO2 reduction catalysts via a methylene bridge have been synthesized and studied by femtosecond transient visible, near-infrared and mid-infrared spectroscopy. The use of the methylene bridge to link NDI to the complexes does not affect the reduction potentials of the metal complexes. Selective photoexcitation of NDI?? to 2*NDI?? results in ultrafast reduction of the bipyridine (bpy) ligands on both the Mn and Re complexes to form Mn(I)(bpy??)(CO)3X and Re(I)(bpy??)(CO)3X in near unity quantum yield, respectively. The initial formation of Mn(I)(bpy??)(CO)3X is unexpected based on previous electrochemical data that indicates the Mn(I) center is reduced at a more positive potential than the bpy ligand. Moreover, the rate of forward electron transfer in the Mn complex was found to be faster than in the Re complex, while the rate of the back electron transfer in the Re complex was faster than in the Mn complex.
关键词： Electrochemistry,CO2 reduction,Electron transfer,Radical anions,Femtosecond spectroscopy,Solar energy
更新于2025-09-23 15:23:52
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The binding energy of biexcitons in alloy ZnxCd1a??xS quantum dots detected by femtosecond laser spectroscopy
摘要： Biexcitons localized at ZnxCd1?xS quantum dots (x = 0.37 or 0.45) with a diameter of ~ 45?? synthesized by two different methods were studied by femtosecond laser spectroscopy. The spectral features of ultrafast transient absorption spectra at the short-time delay of 70?fs are associated with the three lowest energy transitions of quantum dots. The shapes of the transient absorption bands were modeled by fitting to linear absorption. The spectral positions of the absorption components of the excited state in the transient spectra take into account the energy of the biexciton coupling. By fitting the experimental transient absorption spectra of ZnxCd1?xS QDs, the binding energies of biexcitons were determined. The biexciton binding energies vary from 16.6 to 37?meV depending on the biexciton transition excited at the ZnxCd1?xS quantum dot.
关键词： ZnxCd1?xS,Biexciton,Quantum dots,Femtosecond spectroscopy
更新于2025-09-23 15:19:57
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How the methyl group position influences the ultrafast deactivation in aromatic radicals
摘要： Excited xylyl (methyl–benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry. Depending on the substitution we find different deactivation channels after excitation into the D3(2A00) state (310 nm, 4 eV). While the ortho and para isomer exhibit deactivation rates similar to the benzyl radical, meta-xylyl sticks out and depletes twice as fast into the vibrationally hot ground state. We found that a ring deformation mode rather than the methyl pseudorotation enables access to a conical intersection, which is responsible for the faster deactivation. Transitions in the photoelectron spectrum can be assigned to several Rydberg series with mostly d angular momentum components. Absorption of two 4 eV photons triggers hydrogen loss reactions on a femtosecond timescale.
关键词： photoelectron spectroscopy,mass spectrometry,conical intersection,Rydberg states,femtosecond spectroscopy,ultrafast deactivation,xylyl radicals
更新于2025-09-19 17:15:36
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Formation of a supramolecular charge-transfer complex. Ultrafast excited state dynamics and quantum-chemical calculations
摘要： The formation of a supramolecular complex of bis(18-crown-6)stilbene (1) and 4,4’-bipyridine with two ammoniopropyl N-substituents (3) and the substitution reaction between 1·3 and alkali and alkaline-earth metal perchlorates have been studied using absorption, steady-state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1·(Mn+)2 complexes in acetonitrile was demonstrated. The weak long-wavelength charge-transfer absorption band of 1·3 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability and substitution constants were calculated. The relaxation scheme of the 1·3 singlet state excited by a 25 fs laser pulse was proposed. It includes very fast vibrational relaxation and direct (τCT-d = 0.32 ps) and back (τCT-b = 0.51 ps) electron transfer resulting in complete fluorescence quenching. The quantum-chemistry calculations revealed the species taking part in the ET process and elucidated the mechanism of relaxation of the excited complex.
关键词： supramolecular complex,quantum-chemical calculations,femtosecond spectroscopy,charge-transfer,electron transfer
更新于2025-09-19 17:15:36
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Solvation Controlled Excited-State Planarization in a Pusha??Pull Pyrene Dye
摘要： The excited state deactivation pathway of the push–pull pyrene derivative (EPP), a molecule emitting bright red fluorescence, has been studied in several solvents using quantum chemical calculations, femtosecond time-resolved fluorescence up-conversion spectroscopy and femtosecond transient absorption spectra (fs-TA). From the steady-state spectra, EPP has shown a strong negatively solvatochromic behavior of the absorption spectra and a non-solvatochromic emission spectra, revealing the decreased dipole moment upon excitation. Femtosecond up-conversion measurements have shown interesting rise-decay kinetics at red emission side and fs-TA spectra also feature the SE (Stimulated emission) formed and new ESA (Excited State Absorption) developed in all investigated solvents. The careful analysis of the data gathered from fluorescence up-conversion and fs-TA spectra, supported by DFT quantum chemical calculations and temperature-dependent fluorescence measurements, unambiguously pointed out the excited state relaxation pathway of EPP is controlled by solvent stabilized planar intramolecular charge transfer (PICT) dynamics. This finding confirms the surrounding solvents as a significant factor that affects the rate of excited state conformation changes.
关键词： solvation controlled,push–pull pyrene derivative,excited state deactivation,planar intramolecular charge transfer,femtosecond spectroscopy
更新于2025-09-19 17:13:59
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Theoretical Modelling and Experimental Demonstration of a Mid-Infrared Femtosecond Upconversion System
摘要： Mid-infrared (mid-IR) imaging and spectroscopic techniques have been rapidly evolving in recent years, primarily due to a multitude of application within diverse fields like biomedical imaging, chemical sensing and cancer diagnostics [1]. Most complex chemicals have unique spectral signatures in the mid-IR region, facilitating unambiguous identification based on their absorption features. Owing to the limitations of conventional detectors in the mid-IR spectral range, sum frequency generation (SFG) in a second order nonlinear crystal can be used to convert the mid-IR signal to the visible/near-IR, enabling easy detection using silicon-based detectors. Ultrashort-pulsed upconversion is of particular interest for performing pump-probe experiments and studies of fast relaxation dynamics of chemicals and molecules [2]. In this work, we report the first demonstration of femtosecond mid-IR upconversion imaging using SFG in the Fourier plane of a 4f imaging setup. The theory developed in [3] is not valid in the short-pulsed regime (below approximately 1 picosecond) because it does not account for the reduced interaction length in the crystal (temporal walk-off between the interacting pulses) due to group velocity mismatch (GVM). We develop a theoretical model to describe femtosecond upconversion imaging, considering the broad spectrum associated with femtosecond pulses and reduced interaction length associated with GVM. This model enables the calculation of key parameters of a short-pulsed upconversion imaging system. We find a significant increase in angular and spectral acceptance bandwidth for mid-IR signal in the short-pulsed regimes, due to drastic reduction in interaction length, resulting in exceptionally large upconversion imaging field of view (FoV) as depicted in Fig 1 (a). The resolution of the system is studied experimentally by illuminating a USAF resolution target with a mid-IR signal followed by detection of the upconverted signal. An example of an upconverted image is shown in Fig 1 (b), where a resolution of 14.2 line pair/mm is obtained.
关键词： upconversion,femtosecond,spectroscopy,imaging,mid-infrared
更新于2025-09-11 14:15:04