Photoexcitation Mechanism Research Articles

Photoactive Chitosan Switching on Bone-Like Apatite Deposition

The work was focused on the synthesis and characterization of the chitosan-g-fluorescein (CHFL) conjugate polymer as a biocompatible amphiphilic water-soluble photosensitizer, able to stimulate hydroxyapatite deposition upon visible light irradiation. Fluorescein (FL) grafting to chitosan (CH) chains was confirmed by UV-vis analysis of water solutions of FL and CHFL and by Fourier transform infrared spectroscopy (FTIR-ATR) analysis of CHFL and CH. Smooth CHFL cast films with 4 microm thickness were obtained by solvent casting. Continuous exposure to visible light for 7 days was found to activate the deposition of calcium phosphate crystals from a conventional simulated body fluid (SBF 1.0x) on the surface of CHFL cast films. EDX and FTIR-ATR analyses confirmed the apatite nature of the deposited calcium phosphate crystals. CHFL films preincubated in SBF (1.0x) solution under visible light irradiation and in the dark for 7 days were found to support the in vitro adhesion and proliferation of MG63 osteoblast-like cells (MTT viability test; 1-3 days culture time). On the other hand, the mineralization ability of MG63 osteoblast-like cells was significantly improved on CHFL films preincubated under visible light exposure (alkaline phosphatase activity (ALP) test for 1, 3, 7, and 14 days). The use of photoactive biocompatible conjugate polymer, such as CHFL, may lead to new therapeutic options in the field of bone/dental repair, exploiting the photoexcitation mechanism as a tool for biomineralization.

Photoexcitation of a Light-Harvesting Supramolecular Triad: A Time-Dependent DFT Study

We present the first time-dependent density functional theory (TDDFT) calculation on a light-harvesting triad carotenoid-diaryl-porphyrin-C(60). Besides the numerical challenge that the ab initio study of the electronic structure of such a large system presents, we show that TDDFT is able to provide an accurate description of the excited-state properties of the system. In particular, we calculate the photoabsorption spectrum of the supramolecular assembly, and we provide an interpretation of the photoexcitation mechanism in terms of the properties of the component moieties. The spectrum is in good agreement with experimental data, and provides useful insight on the photoinduced charge-transfer mechanism which characterizes the system.

Photo-induced charging effect and electron transfer to the redox species on nitrogen-doped TiO2 under visible light irradiation

Energy levels for sub-band structures of the nitrogen-doped TiO2 (N-TiO2) and photo-excitation mechanism for the visible light response were investigated by photo-electrochemical and spectroscopic measurements. It was demonstrated that the photo-excitation from the N-doping level of N-TiO2 causes the accumulation of electrons into the sub-band level at the potential energy of ca. +0.35V vs. NHE (pH 2.5) under visible light irradiation. Subsequently, electron transfer does not take place from the photo-charged N-TiO2 under visible light irradiation into such redox species as methyl viologen (MV2+), H+, Cu2+ ions, but into O2 molecules, Pt4+, Ag+ and Au3+ ions by way of the sub-band level. These findings shed light on a mechanism for the photocatalytic reactions on the N-TiO2 under visible light irradiation.

Microsolvation of the Sodium and Iodide Ions and Their Ion Pair in Acetonitrile Clusters: A Theoretical Study

The structural and thermodynamic properties of Na+(CH3CN)n, I-(CH3CN)n, and NaI(CH3CN)n clusters have been investigated by means of room-temperature Monte Carlo simulations with model potentials developed to reproduce the properties of small clusters predicted by quantum chemistry. Ions are found to adopt an interior solvation shell structure, with a first solvation shell containing approximately 6 and approximately 8 acetonitrile molecules for large Na+(CH3CN)n and I-(CH3CN)n clusters, respectively. Structural features of Na+(CH3CN)n are found to be similar to those of Na+(H2O)n clusters, but those of I-(CH3CN)n contrast with those of I-(H2O)n, for which "surface" solvation structures were observed. The potential of mean force calculations demonstrates that the NaI ion pair is thermodynamically stable with respect to ground-state ionic dissociation in acetonitrile clusters. The properties of NaI(CH3CN)n clusters exhibit some similarities with NaI(H2O)n clusters, with the existence of contact ion pair and solvent-separated ion pair structures, but, in contrast to water clusters, both types of ion pairs adopt a well-defined interior ionic solvation shell structure in acetonitrile clusters. Whereas contact ion pair species are thermodynamically favored in small clusters, solvent-separated ion pairs tend to become thermodynamically more stable above a cluster size of approximately 26. Hence, ground-state charge separation appears to occur at larger cluster sizes for acetonitrile clusters than for water clusters. We propose that the lack of a large Na+(CH3CN)n product signal in NaI(CH3CN)n multiphoton ionization experiments could arise from extensive stabilization of the ground ionic state by the solvent and possible inhibition of the photoexcitation mechanism, which may be less pronounced for NaI(H2O)n clusters because of surface solvation structures. Alternatively, increased solvent evaporation resulting from larger excess energies upon photoexcitation or major solvent reorganization on the ionized state could account for the observed solvent-selectivity in NaI cluster multiphoton ionization.

Visible-Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents

This study reports on the photocatalytic transformation of fluoroquinolone antibacterial agents (ciprofloxacin, enrofloxacin, norfloxacin, and flumequine) in aqueous titanium dioxide (TiO2) suspensions irradiated with ultraviolet (UV; lambda > 324 nm) or visible light (lambda > 400, > 420, or > 450 nm). Visible-light-mediated fluoroquinolone degradation is unexpected from direct photolysis or established TiO2 band gap photoexcitation mechanisms, which both require UV light. Visible-light-mediated photocatalysis requires an appropriate conduction band electron acceptor (e.g., O2, BrO3-), but is not dependent upon hydroxyl radical, superoxide, or other reactive oxygen species generated upon TiO2 band gap excitation. The process slows considerably when fluoroquinolone adsorption is inhibited. Whereas fluoroquinolone decomposition in UV-irradiated TiO2 suspensions is accompanied by mineralization, no changes in dissolved organic carbon occur during visible-light-photocatalyzed degradation. Results are consistent with a proposed charge-transfer mechanism initiated by photoexcitation of surface-complexed fluoroquinolone molecules. Complexation to the TiO2 surface causes a red shift in the fluoroquinolone absorption spectrum (via ligand-to-metal charge transfer), enabling photoexcitation by visible light. Fluoroquinolone oxidation then occurs by electron transfer into the TiO2 conduction band, which delivers the electron to an adsorbed electron acceptor. The lack of organic carbon mineralization indicates formation of stable organic byproducts that are resistant to further degradation by visible light. In UV-irradiated TiO2 suspensions, the charge-transfer mechanism acts in parallel with the semiconductor band gap photoexcitation mechanism.

Mechanism of Carrier Photogeneration and Carrier Transport in Molecular Crystal Tetracene

Models for the carrier photoexcitation mechanism in molecular crystals have been established initially on the bases of measurements on oligoacenes and later applied to conjugated polymers as well. These models emphasize the localized nature of photoexcitations and describe carrier generation as a secondary process involving exciton dissociation. The results of our photoconductivity studies of single crystal tetracene are at variance with these widely accepted models, and in fact indicate that the photocarrier quantum efficiency appears independent of temperature, photon energy, and light intensity, thus featuring the hallmarks of direct interband carrier photogeneration and coherent carrier transport at band states.

Secondary photoelectron emission experiments on p-, intrinsic, and n-type diamond

Total photoyield spectroscopy (TPYS) is applied to characterize the surface electronic properties of hydrogen-terminated and reconstructed diamond. All samples (n-, p-type doped, undoped) show an onset of electron emission at around 4.4 eV. Obviously, direct excitation of valence-band electrons into the vacuum, which is on hydrogen-terminated diamond 1.1 eV below the conduction band minimum, takes place in the close vicinity of the surface. This photoexcitation on hydrogen-terminated diamond with negative electron affinity is discussed applying different annealing treatments in UHV on diamond. Hydrogen evaporation results in a shift of the TPY spectrum. Taking into account the energy dependent properties of TPY spectra, an electron affinity of + 1.17 ± 0.05 eV is calculated for the clean (001) diamond surface. After a re-hydrogenation of the surface the properties recover completely, which results in a comparable spectrum as before hydrogen evaporation. Applying annealing treatments, where only a small fraction of hydrogen evaporates, results in a shift of the sub-band TPY onsets towards high energies. An indication that with decreasing hydrogen-termination of the diamond surface a decrease of negative electron affinity is detected. These data are discussed in detail taking into account a new photoexcitation mechanism.

Decomposition of water into H 2 and O 2 by a two-step photoexcitation reaction over a Pt–TiO 2 photocatalyst in NaNO 2 and Na 2CO 3 aqueous solution

Both H 2 and O 2 were evolved when a suspension of Pt–TiO 2 in aqueous NaNO 2 was irradiated with UV light with wavelengths greater than 200 nm. The activity and stability were promoted by the addition of Na 2CO 3 to the NaNO 2 solution, and stoichiometric decomposition of water into H 2 and O 2 (H 2/O 2 = 2) took place for a long time (>5 days). A new two-step photoexcitation mechanism (a Z-scheme system) is proposed, whereby the Pt–TiO 2 serves as a photocatalyst for H 2 evolution, the NO 3 - / NO 2 - couple serves as a redox mediator, and O 2 evolution occurs via direct photoexcitation of NO 3 - ion.

Ab Initio Molecular Dynamics Simulations of an Excited State of X-(H2O)3(X = Cl, I) Complex

Upon excitation of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters, the electron transfers from the anionic precursor to the solvent, and then the excess electron is stabilized by polar solvent molecules. This process has been investigated using ab initio molecular dynamics (AIMD) simulations of excited states of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) clusters. The AIMD simulation results of Cl(-)(H(2)O)(3) and I(-)(H(2)O)(3) are compared, and they are found to be similar. Because the role of the halogen atom in the photoexcitation mechanism is controversial, we also carried out AIMD simulations for the ground-state bare excess electron -- water trimer [e(-)(H(2)O)(3)] at 300 K, the results of which are similar to those for the excited state of X(-)(H(2)O)(3) with zero kinetic energy at the initial excitation. This indicates that the rearrangement of the complex is closely related to that of e(-)(H(2)O)(3), whereas the role of the halide anion is not as important.

Photoresponse of surface oxygen defects on TiO 2(1 1 0)

Using scanning tunneling microscopy (STM), the local photoresponse of a rutile TiO 2(1 1 0) surface was investigated on the atomic scale. While the illumination of 325 nm ultraviolet (UV) light excited almost all the areas on the (1 1 0) surface, steps and (1 × 2) ridges that are associated with oxygen deficiencies and a “rosette” structure exhibited higher response intensities. Previously proposed photoexcitation mechanism involving oxygen defect states in the energy band gap is examined by exciting newly created defect sites. These sites did respond to the illumination, but interestingly, when the sites are being paired, only one of them were active for the visible light illumination.

Photoinduced absorption and refraction in azo dye doped PMMA films: the aging effect

We report the nonlinear interferometric and spectral studies of photoinduced changes of the complex refractive index in azo dye doped polymers. A simple theoretical analysis is used that describes rather well the experimental results when a dominant nonlinearity due to the photoexcitation mechanism, is present. The approach is used to study the behavior of chromophores in different aging stages of the host.

A new mechanism of optoacoustic response in semiconductors

Recently grown semiconductors exhibiting ferromagnetic properties can be very promising materials for optoacoustic applications. In these materials, the ferromagnetic phase transition controlled by the density of photoexcited carriers can bring about the generation of sound. The efficiency of this mechanism and the electron-deformation mechanism of photoexcitation of sound are compared. The duration of acoustic pulses generated by the change in the magnetic susceptibility of the sample is estimated.

Mechanism of photoexcitation of oxide-related emission bands in Si–SiO 2 systems

The excitation process of oxide-related centers in magnetron sputtered Si–SiO 2 systems with Si nanocrystals of different sizes is investigated by photoluminescence, EPR and Raman scattering methods. It is found that the photoluminescence spectra of the systems studied contain four bands. The infrared band (1.38–1.54 eV) can be ascribed to exciton recombination in Si crystallites, while the other bands (at 1.7, 2.06 and 2.32 eV) can be attributed to oxide-related defects. Besides intra-defect transitions, the excitation of oxide defects due to light absorption in Si crystallites was found. It is shown that the effectiveness of this excitation channel decreases with the increase of Si crystallite sizes. The mechanism of excitation of oxide-related centers via an Auger process with bound carrier participation is discussed.

Water Splitting Reaction under Visible Light Using Artificial Photosynthesis-photocatalyst System.

Research on the water splitting into H2 and O2 using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis, is introduced. It was found that the H2 evolution took place on a Pt-SrTiO3(Cr-Ta-doped) photocatalyst using an I− electron donor under the visible light irradiation. And the Pt-WO3 photocatalyst showed an excellent activity of the O2 evolution using an IO3− electron acceptor under visible light. Both H2 and O2 evolution was observed with a stoichiometric ratio (H2/O2 = 2) for more than 250 h under visible light using a mixture of the Pt-WO3 and the Pt-SrTiO3(Cr-Ta-doped) powders suspended in NaI aqueous solution. It is the first finding that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I−/IO3− redox mediator. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.

Electron-deformation mechanism of photoexcitation of hypersound in semiconductors in a dc electric field

The effect of a dc electric field on photoexcitation of a hypersonic pulse in a semiconductor via an electron-deformation mechanism is studied. The profiles of acoustic pulses are simulated for different directions of the electric field.

A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis

Abstract We studied the water splitting into H2 and O2 using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis. It was found that the H2 evolution took place on a Pt–SrTiO3 (Cr–Ta-doped) photocatalyst using an I− electron donor under the visible light irradiation. The Pt–WO3 photocatalyst showed an excellent activity of the O2 evolution using an IO3− electron acceptor under visible light. Both H2 and O2 gases evolved in the stoichiometric ratio (H2/O2=2) for more than 250 h under visible light using a mixture of the Pt–WO3 and the Pt–SrTiO3 (Cr–Ta-doped) powders suspended in NaI aqueous solution. It is for the first time that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I−/IO3− redox mediators. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.

Mechanism of Carrier Photoexcitation in Semiconducting Polymers: The Role of Electron Photoemission in “Photoconductivity” Measurements

ABSTRACTUltrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo-excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hν = 6.2 eV indicate an apparent increase in the “photoconductivity” at hν > 3-4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of π-π* absorption, behavior that is inconsistent with a large exciton binding energy.

Space charge-induced reorientation in polymethyne dye-doped nematics under excitation with nanosecond laser pulses

Photoelectric phenomena and orientational nonlinearity have been studied which are induced in planar liquid crystal layers oriented by SiO and doped with polymethyne dyes (0.1 wt.%) under irradiation with nanosecond laser pulses. It was demonstrated that illumination of LC cells with Q-switched ruby laser pulses results in a drastic molecular reorientation which develops during 20 ms. This phenomenon is caused by photo-generation of surface and space charges in the LC cell and spatial distribution of space charges along the grating vector and beam propagation direction both with and without a DC field applied. A model is discussed which explains the orientational mechanism of photo-excitation by emergence of electrohydrodynamic instability at presence of both unipolar injection and anisotropy of conductivity and dielectric permittivity.

Photoluminescence of oligothiophene inclusion compounds

The optical properties and the photoexcitation dynamics of quasi-planar thiophene oligomers included in perhydrotriphenylene are reported. The spectroscopical properties of different oligothiophene conformers are studied with both cw site selective and time resolved photoluminescence measurements. A fast torsional relaxation in the excited state before radiative recombination is proposed as the main photoexcitation mechanism.

Core-hole decay mechanism in Ru studied by Auger–photoelectron coincidence spectroscopy

Abstract In order to explore the photoexcitation and decay mechanism of holes created in the 4p core level in Ru, the N 2 , 3 VV Auger spectrum of the Ru(0001) surface has been measured in coincidence with several features of the photoemission spectra and vice versa. The main features of the core level photoemission spectrum and the Auger spectra can be successfully modelled by atomic-like multiplets of the p 5 d 8 and d 6 configurations respectively, meaning that the core hole is suddenly screened. Furthermore, the photoemission spectrum cannot be simply described by a 4p 1/2 ,4p 3 / 2 spin-orbit doublet, but the angular momentum coupling between the 4p core hole and the 4d open shell must be considered. The Auger electrons emitted in coincidence with the satellite feature, shifted 10 eV below the 4p core level, have a high energy threshold that exceeds the threshold of conventional Auger measurements by an amount corresponding to the downward energy shift of the satellite feature. This indicates that the excitation associated with the satellite feature is long-lived and participates in the Auger decay. Remaining differences between the experimental and the model spectra for the Auger emission could be explained by correlation effects.