Ultrafast nonlinear optical response and high density excitation effects of the stacking fault exciton in BiI3

On the exciton states localized at a two-dimensional stacking fault interface in a layered crystal BiI₃, some nonlinear optical phenomena clearly appear reflecting large transition probability. The optical Stark shift and other high density exciton effect on the energy-shift and the spectral broadening are observed with clear separation under intense laser pumping by time-resolved measurements. The optical Stark shift is analyzed based on the dressed exciton model. The ultrafast optical response faster than 3 ps for the pump-laser field is confirmed on the Stark shift. Degenerate four-wave-mixing signals show fairly long dephasing time of 40 ps in this system. The dephasing probability depends linearly on the pump-laser intensity in the same manner as that of the spectral line-broadening reflecting the relaxation process. The dephasing mechanisms are understood by the exciton scattering at high density in parallel with the spectral changes. The blue-shift due to the high density excitons are discussed on the basis of exciton-exciton interaction in connection with a phase-space filling theory in two-dimensional systems.     

$H(2)$-Unknotting Number of a Knot

An $H(2)$-move is a local move of a knot which is performed by adding a half-twisted band. It is known an $H(2)$-move is an unknotting operation. We define the $H(2)$-unknotting number of a knot $K$ to be the minimum number of $H(2)$-moves needed to transform K into a trivial knot. We give several methods to estimate the $H(2)$-unknotting number of a knot. Then we give tables of $H(2)$-unknotting numbers of knots with up to 9 crossings.     

Reduction of the 1,2,5-thiadiazole ring of 3,6:12,15-di-1,4-benzo[6.6](3,4)-1,2,5-thiadiazolo- and 3,5:11,13-di-1,3-benzo-[6.6](3,4)-1,2,5-thiadiazolocyclophanes. Selective preparation of cis- and trans-[23]cyclophane-1,2-diacetoamide

The effect of the [2.2.2]cyclophane ring structure on the reduction of 1,2,5-thiadiazole ring incorporated in cyclophanes 1a-c and 2a-c was investigated. When reduced by sodium metal in ethanol followed by acetylation, para[2³]cyclophane 1 gave a mixture of the expected cis- and trans-diamides, 3 and 4 , in which 4 was the major product. On the other hand, reduction of 1 with lithium aluminum hydride proceeded in a cis-selective manner and gave 3 as a major product after a treatment of the reduced products with acetic anhydride. The reduction of metacyclophane 2 , which is less strained than 1 , proceeded exclusively in cis-fashion and a subsequent treatment of the reduction product with acetic anhydride gave only cis-diamide 6 .     

Preparation of [23]cyclophane-1,2-diones by the reaction of 3,6;12,15-Di-1,4-benzo[6.6](3,4)-1,2,5-thiadiazolocyclophanes with grignard reagents

3,6;12,15-Di-1,4-benzo[6.6](3,4)-1,2,5-thiadiazolocyclophanes 1a-c were prepared starting from 3,4-di-p-tolyl-1,2,5-thiadiazole 3 and converted into [2³]cyclophane-1,2-diones 2a-c by the reaction with Grignard reagents.     

Electric-field enhancement inducing near-infrared two-photon absorption in an indium-tin oxide nanoparticle film

An enhancing factor: the enhancement of the electric properties of a dye molecule (IR26) by indium-tin oxide nanoparticles (ITO NPs, see picture) has been shown by measuring the near-infrared two-photon-excited transient absorption spectra. The dye molecule was excited much more efficiently in the presence of an ITO NP layer.     

Nano Trek Beyond: Driving Nanocars/Molecular Machines at Interfaces

In 2016, the Nobel Prize in Chemistry was awarded for pioneering work on molecular machines. Half a year later, in Toulouse, the first molecular car race, a “nanocar race”, was held by using the tip of a scanning tunneling microscope as an electrical remote control. In this Focus Review, we discuss the current state‐of‐the‐art in research on molecular machines at interfaces. In the first section, we briefly explain the science behind the nanocar race, followed by a selection of recent examples of controlling molecules on surfaces. Finally, motion synchronization and the functions of molecular machines at liquid interfaces are discussed. This new concept of molecular tuning at interfaces is also introduced as a method for the continuous modification and optimization of molecular structure for target functions.     

Biomimetic reductive amination under the continuous-flow reaction conditions

This study present a full account of continuous-flow reaction conditions for biomimetic reductive amination of fluorinated carbonyl compounds to corresponding amines and amino acids of biomedical importance. We demonstrate that simple silica-adsorbed DBU can be used as efficient catalysts for on-column 1,3-proton shift reaction, a key transformation in the biomimetic reductive amination process. This new on-column process features operationally convenient conditions, higher chemical yields, enantioselectivity and purity of the corresponding products as compared with traditional in-flask reactions. Moreover the removal of base-catalyst, the most delicate problem of the in-flask reactions, is not an issue in the on-column process, as the silica-adsorbed DBU or polymer-bound guanidine remains on the column and can be reused. This feature renders the overall process substantially more economical and synthetically efficient, in particular, for large-scale synthesis of the corresponding fluorinated amines and amino acids target.     

Adjacent- versus remote-site electron injection in TiO2 surfaces modified with binuclear ruthenium complexes

Nanocrystalline thin films of TiO2 cast on an optically transparent indium tin oxide glass were sensitized with ruthenium homo- and heterobinuclear complexes, [LL'Ru(BL)RuLL']n+ (n = 2, 3), where L and L' are 4,4'-dicarboxy-2,2'-bipyridine (dcb) and/or 2,2'-bipyridine (bpy) and BL is a rigid and linear heteroaromatic entity (tetrapyrido[3,2-a:2',3'-c:3",2"-h:2'",3'"-j]phenazine (tpphz) or 1,4-bis([1,10]phenanthroline[5,6-d]imidazol-2-yl)benzene (bfimbz)). The photophysical behavior of the RuII-RuII diads in solution indicated the occurrence of intercomponent energy transfer from the upper-lying Ru --> bpy charge-transfer (CT) excited state of the Ru(bpy)(2) moiety to the lower-lying Ru --> dcb CT excited state of the Ru(bpy)(dcb) (or Ru(dcb)(2)) subunit in the heterobinuclear complexes. These sensitizer diads adsorbed on nanostructured TiO2 surfaces in a perpendicular or parallel attachment mode. Adsorption was through the dcb ligands on one or both chromophoric subunits. The behavior of the adsorbed species was studied by nanosecond time-resolved transient absorption and emission spectroscopy, as well as by photocurrent measurements. In the TiO2-adsorbed samples where BL was bfimbz, the electron injection kinetics was very fast and could not be resolved because an electron is promoted from the metal center to the dcb ligand directly linked to the semiconductor. In the TiO2-adsorbed samples where BL was tpphz, for which, in the excited state, a BL localization of the lowest-lying metal-to-ligand charge transfer (MLCT) is observed, slower injection rates (9.5 x 10(7) s(-1) in [(bpy)(2)Ru(tpphz)Ru(bpy)(dcb(-))](3+)/TiO2 and 5.5 x 10(7) s(-1) in [(bpy)(dcb)Ru(tpphz)Ru(bpy)(dcb(-))](3+)/TiO2) were obtained. Among the systems, the heterotriad assembly [(bpy)(2)Ru(bfimbz)Ru(bpy)(dcb(2-))](2+)/TiO2 gave the best photovoltaic performance. In the first case, this was attributed to a fast electron injection initiated from a dcb-localized MLCT; in the second case, this is attributed to improved molecular orientation on the surface, which was due to rigidity and, at the same time, linearity of the heterotriad system, resulting in a slower charge recombination between the injected electron and the hole.