Xian Zhao

Solvent effects on the electronic structure of a newly synthesized two-photon polymerization initiator

Time-dependent hybrid density functional theory in combination with polarized continuum model has been applied to study the solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption processes, of a newly synthesized asymmetrical charge-transfer (CT) two-photon absorption (TPA) organic molecule. The TPA cross section calculated from a generalized two-state model and solvatochromic shift of the CT state are found to be solvent dependent, for which a nonmonotonic behavior with respect to the polarity of the solvents has been observed. The calculated properties are in good agreement with the experimental data available. The character of the CT state is visualized by plotting its charge density difference from ground state, in which an excess of electron density on the donor side of the molecule is found. This implies that the excited molecule is ready to donate its electron to the surroundings. The energetic aspect of the electron donation is discussed by examining the solvent dependence of the molecular ground state oxidation potential. The importance of the electron correlation for describing the two-photon absorption is also demonstrated.

Vibronically Resolved Electronic Circular Dichroism Spectra of (R)-(+)-3-Methylcyclopentanone: A Theoretical Study

The vibrationally resolved electronic circular dichroism (ECD) spectra of the two dominant conformers of (R)-(+)-3-methylcyclopentanone in gas phase are computed by density functional response theory, with a full account of Franck-Condon and Herzberg-Teller vibrational contributions at the harmonic level. Proper inclusion of the latter contributions was made possible by the recent implementation of effective-scaling computations of vibrational overlaps and of analytical gradients of time dependent DFT. The Coulomb-attenuated Becke three parameters Lee-Yang-Parr (CAM-B3LYP) functional reproduces both the position and the intensity of the experimental peaks, providing a remarkable improvement over the spectra obtained with the popular hybrid B3LYP functional, and allowing a confident assignment of the CD fine vibrational structure. Franck-Condon and Herzberg-Teller contributions are discussed in detail. The computed decrease of the CD intensity in the gas phase upon increase of the temperature of the sample follows the trend observed experimentally in different solvents.

Synthesis, structure and properties of a new two-photon photopolymerization initiator

A new two-photon free-radical photopolymerization initiator, (E,E)-4-{2-[p′-(N,N-di-n-butylamino)stilben-p-yl]vinyl}pyridine (abbreviated to DBASVP), has been synthesized. Quantum chemistry calculations showed that the new initiator possesses a large delocalized π electron system, a large change in dipole moment on transition to the excited state and a large transition moment. The calculated two-photon absorption cross-section is as high as 881.34 × 10−50 cm4 s photon−1. The single-photon and two-photon absorption and fluorescence properties in various solvents have been investigated carefully. The new initiator exhibits outstanding solvent-sensitivity, which experimentally interprets the excellent electron delocalized properties of the molecule. A microstructure has been fabricated under irradiation at 800 nm using a 200 fs, 76 MHz Ti:sapphire femtosecond laser.

Theory for Vibrationally Resolved Two-Photon Circular Dichroism Spectra. Application to (R)-(+)-3-Methylcyclopentanone†

A harmonic adiabatic approach in combination with density functional response theory for computing two-photon vibronically resolved circular dichroism spectra of chiral molecules is presented. It includes both Franck-Condon and Herzberg-Teller contributions and it takes fully into account frequency changes and Duschinsky effects. Model calculations have been performed for two dominant conformers of (R)-(+)-3-methylcyclopentanone in the gas phase. It is found that the Herzberg-Teller contribution can introduce a sign change in two-photon circular dichroism of a single excited electronic state of a conformer. The change survives after Boltzmann averaging, and it might be amenable to experimental verification. Interesting interference effects between Franck-Condon and Herzberg-Teller contributions are revealed and analyzed in detail. Results obtained within the more approximate and less computationally intensive linear coupling vibronic model are also given for comparison.

Crystal growth and physical properties of UV nonlinear optical crystal zinc cadmium thiocyanate, ZnCd(SCN)4

Abstract Zinc cadmium thiocyanate, ZnCd(SCN) 4 (ZCTC) single crystals of optical quality have been grown from aqueous solution by controlled evaporation at 40 °C. The morphology of the crystal was indexed. Raman spectroscopy, refractive indices, optical transmission and optical damage threshold of the ZCTC crystal were all performed at room temperature. Violet and ultraviolet (UV) light output by frequency doubling of a 808 nm GaAlAs laser diode and a continuous wave Ti:sapphire laser at 760 nm were achieved. The dielectric constants, piezoelectric strain constants, electroptic coefficients, and direct current resistivities have also been measured.

Optical Properties of the High-Pressure Phases of SnO2: First-Principles Calculation

We present a detailed investigation on the optical properties, including dielectric function, reflectivity, absorption, refractive index, and electron energy-loss spectrum, of the high-pressure phase SnO(2) in the rutile, pyrite, fluorite, and cotunnite structures by using the density functional theory (DFT) plane-wave pseudopotential method. The results indicate that with the increasing of pressure the band gaps become larger, the density of states are broader, so the curves of optical properties have a little blue shift. Except that the fluorite phase has some metallic properties, the other three phases exhibit excellent dielectric behavior. Interestingly, the fluorite and cotunnite SnO(2) phases always have some special characteristics, such as higher plasma frequency, which need further fundamental and application research.

Hydrogen saturation stabilizes vacancy-induced ferromagnetic ordering in graphene

Density functional theory calculations are performed to explore vacancy-induced magnetism in graphene. The hydrogen saturation not only stabilizes the vacancy structure but also induces distinct magnetic coupling depending on the defect distribution: weak magnetic coupling between defects on different sublattices and strong coupling between defects on the same sublattice. Ferromagnetic ordering has to be accompanied with a semiconducting property. The interaction integral J between defective spins decreases linearly with the increase of the distance between them. Based on the 2D Ising model and Monte Carlo simulations, the possible highest Curie temperature T(c) of defective graphene is predicted to be lower than 500 K.

Differences in Two-Photon and One-Photon Absorption Profiles Induced by Vibronic Coupling : The Case of Dioxaborine Heterocyclic Dye

A theoretical study of a dioxaborine heterocyclic compound in solution provides a case study for an analysis of the effects induced by the so-called Herzberg-Teller (HT) vibronic coupling on the one-(OPA) and two-photon absorption (TPA) spectra. For TPA, the HT vibronic coupling induces differences in the shapes of the absorption band. The study highlights the importance of vibronic coupling as a potentially important mechanism in absorption spectroscopy, able to explain differences in the OPA/TPA spectra.

Upconversion assisted BiOI/ZnWO4:Er3+, Tm3+, Yb3+ heterostructures with enhanced visible and near-infrared photocatalytic activities

Photocatalytic activities of near-infrared (NIR) photocatalysts can be improved by the formation of heterostructures, and an efficient semiconductor upconversion agent of ZnWO4:Er3+, Tm3+, Yb3+ (ZWOETY) was applied to synthesize the BiOI/ZWOETY (BOI/ZWOETY) composite with a p–n heterostructure. BOI/ZWOETY displays a flower-like structure, and ZWOETY nanoparticles are found to be dispersed homogeneously throughout the BOI surfaces, which is beneficial for the NIR light harvesting. Due to the upconversion luminescence properties of ZWOETY, NIR light can be upconverted to a wide range of light emissions, including red (658 nm), green (526 and 550 nm), blue (483 nm), violet (410 nm), and UV (367 and 380 nm) light, and all these upconversion emissions can be absorbed by BOI for photocatalysis. Under visible and NIR light irradiation, BOI/ZWOETY shows better photocatalytic activities in the degradation of methyl orange (MO) and salicylic acid (SA), compared to pure BOI and BOI/ZWO. The generation of holes contributes to the MO and SA decomposition greatly, and the upconversion properties and the highly efficient separation of electron–hole pairs are responsible for the enhanced photocatalytic activities in BOI/ZWOETY.

Solvent Effects on the Three-Photon Absorption of a Symmetric Charge-Transfer Molecule

We present a theoretical study of the solvent-induced three-photon absorption cross section of a highly conjugated fluorene derivative, performed using density functional (DFT) cubic response theory in combination with the polarizable continuum model. The applicability of the often used two-state model is examined by comparison against the full DFT response theory results. It is found that the simplified model performs poorly for the three-photon absorption properties of our symmetric charge-transfer molecule. The dielectric medium enhances the three-photon absorption cross section remarkably. The effects of solvent polarity and geometrical distortions have been carefully examined. A detailed comparison with experiment is presented.