Dajun Ding

Ground-state and excited-state multiple proton transfer via a hydrogen-bonded water wire for 3-hydroxypyridine

In this work, the multiple proton transfer reactions in the ground and first excited states of 3-hydroxypyridine (3HP) in aqueous solution have been investigated by using density functional theory and time dependent density functional theory methods. The effect of a water wire (H2O⋯H2O⋯H2O) on the multiple proton transfer has been elucidated for 3HP. We have confirmed that the multiple proton transfer process can take place more easily in the excited state than in the ground state, and the zwitterionic cluster 3-pyridone-(H2O)3 (3Py-(H2O)3) is the optimal proton transfer product. For the case of neutral 3HP-(H2O)3 and zwitterionic 3Py-(H2O)3 clusters, the intermolecular hydrogen bonds are demonstrated to be significantly strengthened and weakened respectively in the excited state in comparison with those in the ground state, and these have been discussed in detail. Moreover, a conclusive description of the proton transfer cycle has been presented on the basis of our calculated transition energies and potential energy profiles. This photocycle can reasonably explain the absence of fluorescence of the 3HP species in the excited state.

New insights into the solvent-assisted excited-state double proton transfer of 2-(1H-pyrazol-5-yl)pyridine with alcoholic partners: A TDDFT investigation

Excited-state double proton transfer (ESDPT) in the hydrogen-bonded 2-(1H-pyrazol-5-yl)pyridine with propyl alcoholic partner (PPP) was theoretically investigated by time-dependent density functional theory (TDDFT) method. Great changes have taken place for the calculated geometric structures, the electron density features and vibrational spectrum of PPP system in S0 and S1 state. Our results have demonstrated that ESDPT reaction happens within the system upon photoexcitation. We also found that the ESDPT process is facilitated by the electronically excited state intermolecular hydrogen bond strengthening. Particularly, after the photoexcitation from HOMO(π) to the LUMO(π(∗)), the rearrangement of electronic density distribution of frontier molecular orbitals (MOs) on pyridine and the pyrazol moieties exhibits a very important positive factor for the ESDPT. Furthermore, by the investigation of the stretching vibrations of NH and OH groups, the infrared (IR) spectroscopic results provide us not only a theoretical evidence of ESDPT, but also a considerable clue to characterize the nature of intermolecular reaction. In addition, efforts have also been devoted towards calculating the absorption peak, which shows good consistency with the experimental result of the studied system.

Different mechanisms of ultrafast excited state deactivation of coumarin 500 in dioxane and methanol solvents: experimental and theoretical study

Solute–solvent intermolecular photoinduced intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states are proposed to account for the unusual properties of coumarin 500 (C500) in 1,4-dioxane (Diox) and methanol (MeOH) solvents. Our femtosecond transient absorption experiment on C500 shows that in Diox, there exists a single exponential component with a time constant of τ1 ∼ 1.4 ps, however in MeOH two exponential components with lifetimes of τ1 ∼ 0.5 ps and τ2 ∼ 8.0 ps are observed. DFT and TDDFT methods were used to optimize the geometries of complexes C500–Diox and C500–(MeOH)3 in the ground and excited states, respectively. They show that the rapid decay time could be due to ICT and ICT → TICT could be responsible for the slow decay time. Strengthening of the hydrogen bond N–H⋯O–H and the weakening of the hydrogen bond N⋯H–O in the excited state of the C500–(MeOH)3 complex could facilitate the process of ICT from the 7-NHEt group to the CF3 group and induce the formation of the TICT state in hydrogen bonding with MeOH. Together, the experimental and theoretical results reveal that C500 exhibits unusual deactivation mechanisms in Diox and MeOH solvents.

Free-Standing Optically Switchable Chiral Plasmonic Photonic Crystal Based on Self-Assembled Cellulose Nanorods and Gold Nanoparticles.

Photonic crystals incorporating with plasmonic nanoparticles have recently attracted considerable attention due to their novel optical properties and potential applications in future subwavelength optics, biosensing and data storage device. Here we demonstrate a free-standing chiral plasmonic film composed of entropy-driven self-co-assembly of gold nanoparticles (GNPs) and rod-like cellulose nanocrystals (CNCs). The CNCs-GNPs composite films not only preserve the photonic ordering of the CNCs matrix but also retain the plasmonic resonance of GNPs, leading to a distinct plasmon-induced chiroptical activity and a strong resonant plasmonic-photonic coupling that is confirmed by the stationary and ultrafast transient optical response. Switchable optical activity can be obtained by either varying the incidence angle of lights, or by taking advantage of the responsive feature of the CNCs matrix. Notably, an angle-dependent plasmon resonance in chiral nematic hybrid film has been observed for the first time, which differs drastically from that of the GNPs embed in three-dimensional photonic crystals, revealing a close relation with the structure of the host matrix. The current approach for fabricating device-scale, macroscopic chiral plasmonic materials from abundant CNCs with robust chiral nematic matrix may enable the mass production of functional optical metamaterials.

Density-functional studies of plasmons in small metal clusters

We study the formation of plasmon modes of small gold clusters by modeling the excitation spectra. The shape change of the longitudinal mode as a function of cluster size is studied using time-dependent Kohn-Sham theory and Gaussian basis sets. The presence of d electrons in gold atoms affect the plasmon formation process, resulting in a high excitation energy for transverse mode and a complicated spectra profile in general. The transverse mode can still be identified with the help of a frozen-orbital approximation.

Optimally enhanced optical emission in laser-induced air plasma by femtosecond double-pulse

In laser-induced breakdown spectroscopy, a femtosecond double-pulse laser was used to induce air plasma. The plasma spectroscopy was observed to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to femtosecond single-pulse laser. In particular, the optical emission intensity can be optimized by adjusting the delay time of femtosecond double-pulse. An appropriate pulse-to-pulse delay was selected, that was typically about 50 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy, plasma channel, and so on.

Angular distributions of fragment ions in dissociative ionization of CH2I2 molecules in intense laser fields

The interaction of CH2I2 with a 90 fs 800 nm or 400 nm linearly polarized laser field in the intensity range from 1 × 1014 to 1.4 × 1015 W cm−2 was studied by means of a time-of-flight mass spectrometer. Extensive fragmentation was observed at both wavelengths. The kinetic energy released in the Coulomb explosion was obtained by the peak splitting method. The angular distributions were determined for various fragment ions. It was found that the multiple charged iodine ions exhibit a markedly anisotropic distribution, while single charged fragments result a nearly isotropic angular distribution. It has been inferred that the strongly anisotropic angular recoil distributions of multiple charged fragment ions are due to geometric alignment.

Experimental and theoretical investigations of ionization/dissociation of cyclopentanone molecule in a femtosecond laser field

The ionization/dissociation mechanism of cyclopentanone has been experimentally investigated in molecular beam by irradiating with intense 394 and 788 nm laser fields with pulse duration of 90 fs. The range of laser intensities varied from 3 x 10(13) to 4 x 10(14) W/cm(2). For both wavelengths, the singly charged parent ion is observable while the doubly charged one cannot be found easily, although the fragmentation pattern supports its presence. Meanwhile, the extent of fragmentation at 788 nm is less than that in the 394 nm case. We quantitatively analyze the ionization processes of cyclopentanone in intense femtosecond laser by comparing the calculation results of ionization rate constants obtained from Ammosov-Delone-Krainov, Keldysh, and Keldysh-Faisal-Reiss (KFR) theories based on hydrogenlike atom model. We also compare the experimental and theoretical results; the generalized KFR theory is found to be useful in predicting the ionization yields of singly and doubly charged cyclopentanone ion. To interpret the dissociation patterns of the cyclopentanone ions, we have used the Rice-Ramsperger-Kassel-Marcus theory with the potential surfaces obtained from the ab initio quantum chemical calculations.

An experimental and theoretical study of solvent hydrogen-bond-donating capacity effects on ultrafast intramolecular charge transfer of LD 490

The excited-state intramolecular charge transfer (ICT) of LD 490 were investigated in different hydrogen-bond-donating solvents (α scale) on the basis of the Kamlet-Taft solvatochromic parameters (π*, α, β). The femtosecond transient absorption spectra and the kinetics decay rate reveal that with an increase of solvents α capacity, the long-lived picosecond process, which is attributed to the ICT, becomes much faster. Combining with time-dependent density functional theory (TDDFT) calculations, we demonstrate that the enhancement of α acidity substantially increases the electronegativity of the carbonyl oxygen in LD 490, which strengthen excited-state intermolecular hydrogen bonding interactions and consequently facilitate the ICT process.

Thermal analysis of double-layer metal films during femtosecond laser heating

In this paper, the primary interest is the heat effect of the bottom-layer metal on the temperature distribution of the top-layer metal in a double-layer metal structure during femtosecond laser irradiation. The evolution of the surface electron and lattice temperature depends a lot on the thermal parameters of the substrate. The damage threshold can be increased by using a substrate material with high electron–lattice coupling factor. Next, we choose chrome as the bottom-layer material. The results of modeling show that the surface lattice temperature of top-layer gold can be reduced remarkably. For a fixed entire thickness of the double-layer film, there is an optimal proportion of top and bottom layers for which the damage threshold is the highest possible. Also, for increasing the damage threshold, a substrate with higher melting temperature should be chosen.