Synthesis, crystal structure, and optical properties of a three-dimensional quaternary Hg-In-S-Cl chalcohalide: Hg7InS6Cl5

A crystalline three-dimensional (3D) quaternary chalcohalide, Hg(7)InS(6)Cl(5) (1), has been synthesized through a solid-state reaction under medium temperature. It is the first example in the family of the Hg-IIIA-Q-X (Q = S, Se, Te; X = F, Cl, Br, I) systems. Compound 1 features a 3D network and has an optical band gap of 2.54 eV.     

Optical label switching of DRZ/DPSK orthogonal signal generated by photonic-crystal fiber

We demonstrate orthogonal label switching by using a dark-return-to-zero (DRZ) payload and differential phase-shift keying (DPSK) label generated by a dispersion-flattened photonic-crystal fiber. The high extinction ratio of both the payload and label improves the receiver margin. The DRZ payload introduces little cross talk to the DPSK label due to the RZ-like output of the demodulated DPSK. Simulations are performed to study the eye-closure penalty of the payload and label at different DRZ pulse widths. We compare the DRZ/DPSK with the RZ and DPSK signals numerically at the same data rate and show that the DRZ/DPSK has a strong tolerance to the polarization-mode dispersion. The DRZ/DPSK has a more compact spectrum suitable for the strong filtering requirements in WDM systems.     

A new cross-linking route via the unusual collision kinematics of hyperthermal protons in unsaturated hydrocarbons: the case of poly(trans-isoprene)

The bombardment of a approximately 18 nm film of poly(trans-isoprene) molecules (approximately 300,000 carbon atoms per molecule) with a mass-separated proton beam at 10 eV has been shown to induce highly efficient cross-linking of several macromolecules per incident proton. In this new method using physical means to conduct synthetic chemistry, the hyperthermal protons preferentially cleave C-H bonds because of their unusual kinematics in hydrocarbons, and the carbon radicals thus created initiate a polymerization chain reaction of the isoprene C[double bond, length as m-dash]C bonds. The method produces ultrathin polymeric films via cross-linking with a dry process having high chemical selectivity and reactivity but requiring no thermal cost and no chemical additives, conditions that are commonly required in the fabrication of microelectronics and photonics.     

Femtosecond time- and wavelength-resolved fluorescence and absorption spectroscopic study of the excited states of adenosine and an adenine oligomer

By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)(20)) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)(20) oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of approximately 0.13 ps and approximately 0.45 ps associated with the L(a) and L(b) pipi* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the npi* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two pipi* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)(20) KTRF and TA spectra reveals temporal evolution from an ultrafast "A-like" state (with a approximately 0.39 ps decay time) to a relatively long-lived E(1) "excimer" (approximately 4.3 ps decay time) and an E(2) "excimer-like" (approximately 182 ps decay time) state. The "A-like" state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)(20) provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)(20). The rapid transformation of the locally excited (dA)(20) component into the delocalized E(1) "excimer" state which then further evolves into the E(2) "excimer-like" state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.     

Microscopic simulations of molecular cluster decay: Does the carrier gas affect evaporation?

We develop a kinetic theory of cluster decay by considering the stochastic motion of molecules within an effective potential of mean force (PMF) due to the cluster. We perform molecular dynamics simulations on a 50-atom argon cluster to determine the mean radial force on a component atom and hence the confining potential of mean force. Comparisons between isolated clusters and clusters thermostatted through the presence of a 100-atom helium carrier gas show that the heat bath has only a slight effect upon the PMF. This confirms the validity of calculations of cluster properties using isolated cluster simulations. The PMF is used to calculate the atomic evaporation rate from these clusters, and results are compared with the predictions of the capillarity approximation together with detailed balance, both components of the classical theory of aerosol nucleation.     

Unusual Through-Space Interactions between Oxygen Atoms that Mediate Inverse Morphochromism of an AIE Luminogen

We have studied the photophysics of tetrafurylethene, an aggregation-induced emission luminogen with exceptionally short intramolecular O−O distances of 2.80 Å and a significant red-shifted morphochromism (27 nm) when going from the aggregate to the crystal. The short O−O distances, which are substantially smaller than the sum of the van der Waals radii (3.04 Å), are due to the fact that the oxygen atoms act as an electronic bridge connecting the furan rings on opposite ends of the central double bond, giving rise to a circular delocalization of the π-electron density across the rings. In the excited state the O−O distance is further reduced to 2.70 Å; the increased O−O interaction causes a narrowing of the HOMO–LUMO gap, resulting in the red morphochromism of the emission. Our results show the structural origin of the red-shifted emission lies in close O−O contacts, paving the way for understanding the clusteroluminescence of oxygen-rich non-conjugated systems that emit visible light.     

Studies on the Extension of Sequence‐independence and the Enhancement of DNA Triplex Formation

A more elaborate sequence‐independent triple‐helix formation viability study was carried out and extended from a recombination‐like triple‐helical DNA motif of a previous study (J. Mol. Recognition 14, 122–139 (2001)). The intended triple‐helix was formed by mixing one part of a DNA hairpin duplex and one part of a single (or third) strand identical to one of the duplex strands and complementary to the other strand. In contrast to the common purine and pyrimidine motifs in triple‐stranded DNA, the strands of the recombination‐like motif are not monotonously built from pyrimidine only, or purine only, in the sequence. The stability of the recombination‐like motif triplexes with varying sequences was monitored by UV thermal melting curves. The results showed that the order of the stability of the R‐form DNA base triads (J. Mol. Biol., 239, 181–200 (1994)) is G*(G ○ C) > C*(C ○ G) > A*(A ○ T) >T*(T ○ A) (the Watson‐Crick base pair is denoted in the parentheses) in 200 mM NaCl, at pH 7. In an attempt to increase the stability of the triplex in the recombination‐like motif, we replaced cytidine by 5‐methylcytidine (^mC) of the third strand. There is a general trend that ^mC modification stabilizes the complex (<2 °C per ^mC). The complex is furthermore stabilized by Mg²⁺ ion. The Tm increases from 7 to 2 °C from less stable to highly stable triplex by 20 mM Mg²⁺ ion in solution.     

Product formation from the reaction of the NO3 radical with isoprene and rate constants for the reactions of methacrolein and methyl vinyl ketone with the NO3 radical

Using a relative rate method, rate constants for the gas-phase reactions of the NO₃ radical with methacrolein and methyl vinyl ketone were determined to be (4.4 ± 1.7) × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹ and <6 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹, respectively, at 296 ± 2 K. The molar formation yields of methacrolein and methyl vinyl ketone from the gas-phase reaction of the NO₃ radical with isoprene at 296 ± 2 K and atmospheric pressure of air were measured to be 0.035 ± 0.014 each. The tropospheric implications of these kinetic and product data are discussed, and it is concluded that the nighttime NO₃ radical reactions with methacrolein and methyl vinyl ketone are not important. However, during nighttime the formation of methacrolein and methyl vinyl ketone from the reaction of isoprene with the NO₃ radical may dominate over their formation from the O₃ reaction with isoprene. Atmospheric pressure ionization tandem mass spectrometry (API-MS/MS) was used to investigate the products of the reactions of the NO₃ radical with isoprene and isoprene-d₈, and C₅-nitrooxycarbonyl(s) (e.g., O₂NOCH₂C(CH₃) (DOUBLEBOND) CHCHO), C₅-hydroxynitrate(s) (e.g., O₂NOCH₂C(CH₃)(DOUBLEBOND) CHCH₂OH), C₅-nitrooxyhydroperoxide(s) (e.g., O₂NOCH₂C(CH₃)(DOUBLEBOND) CHCH₂OOH), and C₅-hydroxycarbonyl(s) (e.g., HOCH₂CH(DOUBLEBOND) C(CH₃)CHO) and their deuterated analogs were observed from these reactions. © 1996 John Wiley & Sons, Inc.