Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.

Theoretical studies of the dynamics of the abstraction reaction, H' + HBr (v=0,j=0) --> H'H + Br, have been performed with quasiclassical trajectory method (QCT) on a new ab initio potential energy surface (Y. Kurosaki and T. Takayanagi, private communication). The calculated QCT cross sections are in good agreement with earlier quantum wave packet results over most of the collision energy range from 0.1 to 2.6 eV, and the state-resolved rotational distributions of the product H'H molecule are quantitatively consistent with the experimental results. Comparisons of the QCT-calculated rotational-state-resolved cross sections on different potential energy surfaces show that the characteristics of the potential energy surface in the region far away from the minimum energy path have a large influence on the title abstraction reaction dynamics, and the indirect reactions that do not follow the minimum energy path have little influence on the differential cross sections (DCS). The DCSs are mainly governed by the direct reactions that do follow the minimum energy path, at both low and high collision energies. The degree of the rotational alignment of the product H'H molecule is strong at high collision energies, which means that the influence of the indirect reactions on the product rotational alignment is negligible, whereas the distribution of P(varphi(r)) is sensitive to the indirect reactions at high collision energies. With increasing collision energy, the polarization of the product rotational angular momentum decreases and the molecular rotation of the product prefers an in-plane reaction mechanism rather than the out-of-plane mechanism.

Photocatalysis of methanol (CH3OH) on anatase (A)-TiO2(101) has been investigated using temperature programmed desorption (TPD) method with 266 nm light at low surface temperatures. Experimental results show that CH3OH adsorbs on the A-TiO2(101) surface predominantly in molecular form, with only a small amount of CH3OH in dissociated form. Photocatalytic products, formaldehyde (CH2O) and methyl formate (HCOOCH3), have been detected under 266 nm light irradiation. In addition to H2O formation, H2 product is also observed by TPD spectroscopy. Experimental results indicate that H2 product is formed via thermal recombination of H-atoms on the BBO sites from photocatalysis of CH3OH on the A-TiO2(101) surface, and H2 production on the A-TiO2(101) surface is significantly more efficient than that on the rutile (R)-TiO2(110) surface.

Studies on the dynamical stereochemistry of the Cl+H2 reaction and its isotopic variants, especially the isotope effect on the product polarization, have been performed at a collision energy of 6.0 kcal/mol on two potential energy surfaces, i.e., G3 surface [T. C. Allison et al., J. Phys. Chem. 100, 13575 (1996)] and BW2 surface [W. Bian and H.-J. Werner, J. Chem. Phys. 112, 220 (2000)]. Quantum mechanical and quasiclassical trajectories calculations of the polarization-dependent differential cross sections for the Cl+H2 reaction have been carried out on the BW2 potential energy surface, and the results indicate that the quasiclassical approximation in general does as good as exact quantum mechanics. Calculations also show that the rotational alignment of the HCl product obtained on the BW2 surface for Cl+H2 reaction is stronger than that calculated on the G3 surface, which implies that the effect of van der Waals force on product polarization is quite weak. The distributions of P(θr) and P(φr) derived from the Cl+H2 and its isotopic reactions indicate that the isotope effect on the product polarization calculated on the G3 potential energy surface is distinct, whereas the isotope effect on the product polarization computed on the BW2 surface is indistinct.

It is well established that adding methanol to water could significantly enhance H 2 production by TiO 2 . Recently, we have found that methanol can be photocatalytically dissociated on TiO 2 (110) at 400 nm via a stepwise mechanism. However, how molecular hydrogen can be formed from the photocatalyzed methanol/ TiO 2 (110) surface is still not clear. In this work, we have investigated deuterium formation from photocatalysis of the fully deuterated methanol (CD 3 OD) on TiO 2 (110) at 400 nm using a temperature programmed desorption (TPD) T iO 2 has attracted enormous interest in heterogeneous catalysis, photocatalysis, solar energy devices, etc. 1−8 Photocatalytic water splitting by TiO 2 is especially attractive because of its potential application in clean hydrogen production. 9 A previous study found that pure TiO 2 is not active for hydrogen production from pure water. 10 Adding methanol to pure water, however, can dramatically enhance hydrogen production. 11 Because of the apparently crucial role in hydrogen production, the photochemistry of methanol has been extensively investigated on single crystal TiO 2 surfaces 12−31 and TiO 2 powders. 32−35 Although investigations on powder TiO 2 with methanol steam 32−35 and a water−methanol mixture 11 show that hydrogen can be produced from methanol by reaction,the detailed mechanism of gaseous hydrogen formation from methanol photocatalysis on TiO 2 remains unknown. In a recent study, 28 we have shown that the elementary photocatalytic dissociation of CH 3 OH on TiO 2 (110) without any other coadsorbed species occurs in a stepwise mechanism in which the O−H dissociation proceeds first and is then followed by C−H dissociation to form formaldehyde (CH 2 O) with only methanol adsorption on TiO 2 (110),where Ti 5C refers to a five-coordinated Ti 4+ (Ti 5C ) site, and H BBO refers to an H atom adsorbed on a bridge-bonded oxygen (BBO) site on the TiO 2 (110) surface. From our experiment, we have found that both dissociation steps are photoinitiated. This means that at low temperature photocatalytic dissociation products from CH 3 OH, i.e., CH 2 O and H atoms on BBO sites, are all left on the TiO 2 surface after laser irradiation, whereas Henderson and co-workers found that molecular CH 3 OH is not photoactive on TiO 2 (110) using a Hg lamp as the surface photocatalysis source. 26 In our experiment, 28,36 we used a femtosecond laser source that has considerably higher photon flux than the Hg lamp used in ref 26, in addition to the highly sensitive mass spectrometric detector with a vacuum background of 1 × 10 −12 Torr. We believe this makes our experiment much more sensitive in detecting TPD products. Further oxidation of CH 3 OH on TiO 2 (110) to form methyl formate has also been observed in three different laboratories. 36−38 However, the important question of how hydrogen molecules are formed from the photocatalysis of methanol on TiO 2 (110) remains unanswered. In order to understand the mechanism of hydrogen formation, the photocatalytic chemistry of CD ...

We review our developed visualization method of charge transfer (CT) for chemical enhancement mechanism on surface‐enhanced Raman scattering (SERS) and tip‐enhanced Raman spectroscopy (TERS). Firstly, we describe our visualization method of charge difference density, which provides direct visual evidence for photoinduced CT. And then, using the visualization method of CT, we interpreted the mechanism of SERS and TERS. Photoinduced charge transfer in the processes of SERS and TERS can be clearly seen. Our visualization method provides a visual and easy understanding way for the mechanism of SERS and TERS. Copyright © 2014 John Wiley & Sons, Ltd.

Using the ultrafast pump-probe transient absorption spectroscopy, the femtosecond-resolved plasmon-exciton interaction of graphene-Ag nanowire hybrids is experimentally investigated, in the VIS-NIR region. The plasmonic lifetime of Ag nanowire is about 150 ± 7 femtosecond (fs). For a single layer of graphene, the fast dynamic process at 275 ± 77 fs is due to the excitation of graphene excitons, and the slow process at 1.4 ± 0.3 picosecond (ps) is due to the plasmonic hot electron interaction with phonons of graphene. For the graphene-Ag nanowire hybrids, the time scale of the plasmon-induced hot electron transferring to graphene is 534 ± 108 fs, and the metal plasmon enhanced graphene plasmon is about 3.2 ± 0.8 ps in the VIS region. The graphene-Ag nanowire hybrids can be used for plasmon-driven chemical reactions. This graphene-mediated surface-enhanced Raman scattering substrate significantly increases the probability and efficiency of surface catalytic reactions co-driven by graphene-Ag nanowire hybridization, in comparison with reactions individually driven by monolayer graphene or single Ag nanowire. This implies that the graphene-Ag nanowire hybrids can not only lead to a significant accumulation of high-density hot electrons, but also significantly increase the plasmon-to-electron conversion efficiency, due to strong plasmon-exciton coupling.

We describe the chemical and electromagnetic enhancements of surface-enhanced resonance Raman scattering (SERRS) for the pyridine molecule absorbed on silver clusters, in which different incident wavelength regions are dominated by different enhancement mechanisms. Through visualization we theoretically investigate the charge transfer (CT) between the molecule and the metal cluster, and the charge redistribution (CR) within the metal on the electronic intracluster collective oscillation excitation (EICOE). The CT between the metal and the molecule in the molecule-metal complex is considered as an evidence for chemical enhancement to SERRS. CR within the metal on EICOE is considered as an evidence for the electromagnetic enhancement by collective plasmons. For the incident wavelength from 300 to 1000 nm, the visualized method of charge difference density can classify the different wavelength regions for chemical and electromagnetic enhancement, which are consistent with the formal fragmented experimental studies.