Photoinduced Structure Research Articles

Photo-induced intermolecular electron transfer-effect of acceptor molecular structures

Photo-induced electron transfer versus molecular structure of acceptors is investigated using ultrafast time-resolved transient grating spectroscopy. Typical laser dyes Rhodamine 101 (Rh101) and Rhodamine 6G (Rh6G) in electron donor solvent—aniline are adopted as the objects. The forward electron transfer time constant from aniline to the excited singlet state of two Rhodamine dyes and subsequent back electron transfer from two dyes to aniline are measured. The experimental results denote that Rh6G presents faster electron transfer rates with aniline in both forward electron transfer and back electron transfer processes. With chemical calculation and qualitative analysis, it is found that the flexible molecular geometry of Rh6G leads to stronger electron coupling with donor solvent and further gives rise to larger electron transfer rates.

Nano Periodic Structure Formation in 4H–SiC Crystal Using Femtosecond Laser Double-Pulses

The photo-induced periodic nano structure inside 4H–SiC has been induced by a femtosecond double pulse train. The alignment of the periodic structure is in the direction independently from crystal orientation. In particular, FE-SEM analysis revealed that the periodic structure on the fractured surface can be classified into two categories of the polarization-dependent and polarization-independent.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide.

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

Unravelling the Correlation of Electronic Structure and Carrier Dynamics in CuInS2 Nanoparticles

In this work, we report the direct correlation of photoinduced carrier dynamics and electronic structure of CuInS2 (CIS) nanoparticles (NPs) using the combination of multiple spectroscopic techniques including steady-state X-ray absorption spectroscopy (XAS), optical transient absorption (OTA), and X-ray transient (XTA) absorption spectroscopy. XAS results show that CIS NPs contain a large amount of surface Cu atoms with ≪four-coordination, which is more severe in CIS NPs with shorter nucleation times, indicating the presence of more Cu defect states in CIS NPs with smaller size particles. Using the combination of OTA and XTA spectroscopy, we show that electrons are trapped at states with mainly In or S nature while holes are trapped in sites characteristic of Cu. While there is no direct correlation of ultrafast trapping dynamics with NP nucleation time, charge recombination is significantly inhibited in CIS NPs with larger particles. These results suggest the key roles that Cu defect sites play in carri...

Structural Dynamics upon Photoexcitation in a Spin Crossover Crystal Probed with Femtosecond Electron Diffraction

AbstractPhotoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single‐crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time‐dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high‐spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal–ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.

Structural Dynamics upon Photoexcitation in a Spin Crossover Crystal Probed with Femtosecond Electron Diffraction.

Photoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single-crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time-dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high-spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal-ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.

Photo-induced structural changes in Ge-Sb-Se films

Amorphous Ge-Sb-Se thin films have been prepared by the radio-frequency (RF) magnetron co-sputtering deposition technique, and their intrinsic photosensitivity and photo-induced structural changes have been investigated. The results show a crossover from photodarkening (PD) to photobleaching (PB) in the films when the film compositions change from Se-deficient to rich. Further Raman analysis on these as-prepared thin films irradiated with a laser of wavelength 655nm in every five minutes provides direct evidence of photo-induced structure rearrangements.

Direct and real time probe of photoinduced structure transition in colossal magnetoresistive material

We report a direct and real time measurement of photoinduced structure phase transition in single crystal La0.84Sr0.16MnO3 using femtosecond electron diffraction. The melting of orthorhombic lattice ordering under femtosecond optical excitation is found involving two distinct processes with different time scales, an initial fast melting of orthorhombic phase in about 4 ps and a subsequent slower transformation in 90 ps and longer timescales. The fast process is designated as the initial melting of orthorhombic phase induced by the Mn-O bond change that is most likely driven by the quenching of the dynamic Jahn-Teller distortion following the photo-excitation. The slow process is attributed to the growing of newly formed structure domain from the photo-excited sites to the neighboring non-excited orthorhombic sites.

Biological deoxycholic acid–coumarin conjugates: photo-switchable structures and self-assembly morphology

Construction of functional materials and supramolecular chemistry systems using biological molecules has been paid considerable attention. In this Letter, a deoxycholic acid–coumarin (DAC) conjugate with both segments originating from biological molecules was developed. The photo-induced structure transformation was thoroughly investigated. This method can provide a simple and highly-efficient strategy to construct complex bile acid derivatives. With the amphiphilic rigid steroidal skeleton, the DAC conjugates and their corresponding dimers exhibit excellent self-assembly properties, which can also be conveniently adjusted by the photo-induced conversion process. Interestingly, both hydrophilic and hydrophobic binding cavities were demonstrated to exist in the assemblies, leading to the possibility to introduce various functional molecules into the systems simultaneously. Additionally, the fluorescence intensity of DAC can also be adjusted with the structure conversion. Based on these properties, this biologically originated molecule can be used as controlled-release materials and has the potential to observe the real-time condition in living cells.

Nanogratings Embedded in Al2O3-Dy2O3 Glass by Femtosecond Laser Irradiation

The self-assembled nanogratings embedded in number of materials, which are aligned according to the laser polarization, is an intriguing phenomenon induced by femtosecond light pulses. Curiously, the orientation of such periodic nanostructure is different between a glass and an indirect bandgap semiconductor. Furthermore a material in which nanogratings can be induced is limited. Here we report that a polarization-dependent periodic nanostructure can be successfully induced in Al2O3-Dy2O3 binary glass with a low glass formation ability. In the case of this binary glass, we confirmed that the self-organization and partial-crystallization were simultaneously induced by the laser irradiation. Such photoinduced structure in Al2O3-Dy2O3 glass exhibit periodic modulation in magnetic properties in nanoscale.

Effect of pulsed light on structure and immunoreactivity of gluten

The effect of pulsed light (from 1.75 to 26.25Jcm−2) on selected properties of wheat gluten powder and aqueous suspension (absorbance, particle size and microstructure, free sulfhydryl content, protein fractions, protein electrophoretic mobility and immunoreactivity) was investigated. Gluten photoreactivity was strongly affected by hydration. While minor photo-induced structure modifications were observed in gluten powder, pulsed light induced the development of browning and promoted partial depolymerisation of hydrated gluten proteins by disulphide exchange. These changes were associated with a significant decrease in immunoreactivity, suggesting that pulsed light could be exploited to efficiently modify structure and thus functionality of gluten.

A split-beam probe-pump-probe scheme for femtosecond time resolved protein X-ray crystallography.

In order to exploit the femtosecond pulse duration of X-ray Free-Electron Lasers (XFEL) operating in the hard X-ray regime for ultrafast time-resolved protein crystallography experiments, critical parameters that determine the crystallographic signal-to-noise (I/σI) must be addressed. For single-crystal studies under low absorbed dose conditions, it has been shown that the intrinsic pulse intensity stability as well as mode structure and jitter of this structure, significantly affect the crystallographic signal-to-noise. Here, geometrical parameters are theoretically explored for a three-beam scheme: X-ray probe, optical pump, X-ray probe (or “probe-pump-probe”) which will allow experimental determination of the photo-induced structure factor amplitude differences, ΔF, in a ratiometric manner, thereby internally referencing the intensity noise of the XFEL source. In addition to a non-collinear split-beam geometry which separates un-pumped and pumped diffraction patterns on an area detector, applying an additional convergence angle to both beams by focusing leads to integration over mosaic blocks in the case of well-ordered stationary protein crystals. Ray-tracing X-ray diffraction simulations are performed for an example using photoactive yellow protein crystals in order to explore the geometrical design parameters which would be needed. The specifications for an X-ray split and delay instrument that implements both an offset angle and focused beams are discussed, for implementation of a probe-pump-probe scheme at the European XFEL. We discuss possible extension of single crystal studies to serial femtosecond crystallography, particularly in view of the expected X-ray damage and ablation due to the first probe pulse.

A phosphorescent molecular "butterfly" that undergoes a photoinduced structural change allowing temperature sensing and white emission.

A butterfly-like phosphorescent platinum(II) binuclear complex can undergo a molecular structure change in which the Pt-Pt distance shortens upon photoexcitation, which leads to the formation of two distinct excited states and dual emission in the steady state, that is, greenish-blue emission from the high-energy excited state at the long Pt-Pt distance and red emission from the low-energy excited state at the short Pt-Pt distance. This photoinduced molecular structure change has a strong dependence on the molecule's surrounding environment, allowing its application as self-referenced luminescent sensor for solid-liquid phase change, viscosity, and temperature, with greenish-blue emission in solid matrix and rising red emission in molten liquid phase. With proper control of the surrounding media to manipulate the structural change and photophysical properties, a broad white emission can be achieved from this molecular butterfly.

A Phosphorescent Molecular “Butterfly” that undergoes a Photoinduced Structural Change allowing Temperature Sensing and White Emission

AbstractA butterfly‐like phosphorescent platinum(II) binuclear complex can undergo a molecular structure change in which the Pt–Pt distance shortens upon photoexcitation, which leads to the formation of two distinct excited states and dual emission in the steady state, that is, greenish‐blue emission from the high‐energy excited state at the long Pt–Pt distance and red emission from the low‐energy excited state at the short Pt–Pt distance. This photoinduced molecular structure change has a strong dependence on the molecule’s surrounding environment, allowing its application as self‐referenced luminescent sensor for solid–liquid phase change, viscosity, and temperature, with greenish‐blue emission in solid matrix and rising red emission in molten liquid phase. With proper control of the surrounding media to manipulate the structural change and photophysical properties, a broad white emission can be achieved from this molecular butterfly.

Light-induced excited spin state trapping effect on [Fe(mepy)3tren](PF6)2solvated crystals

A spin-crossover solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N has been prepared and its switching properties have been compared to those reported for the non-solvated solid. The thermal spin transition occurs at 88 K with the opening of a 3.5 K wide hysteresis loop, while a fairly steep transition at 215 K without hysteresis has been previously reported for the non-solvated analogue. This feature has been rationalized by the analysis of the high-spin (HS) and low-spin (LS) structures, evidencing a relative stabilization of the high-spin state, as well as strong intermolecular interactions in the solvated compound. The photoswitching of the solvated solid, based on the light-induced excited spin state trapping effect, leads to a quantitative transformation from the low-spin to the high-spin state at 10 K. The long lifetime of the metastable HS state at 10 K allows the measurement of the photo-induced HS structure, where the cooperative interactions are enhanced, compared to those of the thermally populated HS structure. Then,the HS-to-LS relaxations have been studied between 45 and 60 K. They are sigmoidal in shape and can be well fitted in the frame of the mean-field approximation. The relative stability of the photo-induced HS state in this family of spin crossover compounds is not directly related to their thermal spin transition temperature. This unexpected observation is rationalized by a careful analysis of their structural characteristics.

Signal to noise considerations for single crystal femtosecond time resolved crystallography of the Photoactive Yellow Protein.

Femtosecond time resolved pump-probe protein X-ray crystallography requires highly accurate measurements of the photoinduced structure factor amplitude differences. In the case of femtosecond photolysis of single P63 crystals of the Photoactive Yellow Protein, it is shown that photochemical dynamics place a considerable restraint on the achievable time resolution due to the requirement to stretch and add second order dispersion in order to generate threshold concentration levels in the interaction region. Here, we report on using a 'quasi-cw' approach to use the rotation method with monochromatic radiation and 2 eV bandwidth at 9.465 keV at the Linac Coherent Light Source operated in SASE mode. A source of significant Bragg reflection intensity noise is identified from the combination of mode structure and jitter with very small mosaic spread of the crystals and very low convergence of the XFEL source. The accuracy with which the three dimensional reflection is approximated by the 'quasi-cw' rotation method with the pulsed source is modelled from the experimentally collected X-ray pulse intensities together with the measured rocking curves. This model is extended to predict merging statistics for recently demonstrated self seeded mode generated pulse train with improved stability, in addition to extrapolating to single crystal experiments with increased mosaic spread. The results show that the noise level can be adequately modelled in this manner, indicating that the large intensity fluctuations dominate the merged signal-to-noise (I/σI) value. Furthermore, these results predict that using the self seeded mode together with more mosaic crystals, sufficient accuracy may be obtained in order to resolve typical photoinduced structure factor amplitude differences, as taken from representative synchrotron results.

Generalized model for photoinduced surface structure in amorphous thin films.

We present a generalized model to explain the spatial and temporal evolution of photoinduced surface structure in photosensitive amorphous thin films. The model describes these films as an incompressible viscous fluid driven by a photoinduced pressure originating from dipole rearrangement. This derivation requires only the polarizability, viscosity and surface tension of the system. Using values of these physical parameters, we check the validity of the model by fitting to experimental data of As2S3 and demonstrating good agreement.

Photoinduced conductivity change and structure transformation

Photoinduced conductivity change and structure transformation

A photo-induced excited state structure of a hetero-bimetallic ionic pair complex, Nd(DMA)4(H2O)4Fe(CN)6·3H2O, analyzed by single crystal X-ray diffraction

The excited state crystal structure of the ionic complex (Nd(DMA)(4)(H(2)O)(4)-Fe(CN)(6)·3H(2)O (DMA = dimethylacetamide) has been determined at 15 K upon UV illumination by single crystal X-ray diffraction. Significant structural changes are observed around the Fe site in the excited state. These changes are similar to those observed for a related molecular compound exhibiting photomagnetic properties.

Photoinduced Morphology Switching of Polymer Nanoaggregates in Aqueous Solution

A novel photosensitive C-PNIPAAm comprising hydrophilic PNIPAAm conjugated with a relatively short but very hydrophobic coumarin part was designed and prepared using a coumarin-containing disulfide derivative (C-S-S-C) as transfer agent in the presence of Bu(3)P and water. It was found that C-PNIPAAm can form polymer micelles in aqueous solution. And the micellar morphology in aqueous solution can be photoswitched into hollow spheres according to the photodimerization of coumarin end groups upon 365 nm irradiation and reform the micellar morphology after the subsequent photoscission of dimers upon 254 nm. This instant morphology changing phenomenon was successfully monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. TEM observations showed the small spherical shape of micelles with diameter at 30-50 nm before photo-cross-linking, the big vesicles with diameter at 200-350 nm after photo-cross-linking, and the small micelles with diameter at 30-50 nm after the subsequent photo-de-cross-linking in the first irradiation cycle. The reason for this significant morphology switching can be attributed to the reversible photoinduced amphiphilic structure transformation between the telechelic "hydrophobic end-hydrophilic chain" structure and the ABA type of "hydrophilic chain-hydrophobic center-hydrophilic chain" one upon alternating irradiation.