Surface-enhanced resonance Raman scattering and background light emission coupled with plasmon of single Ag nanoaggregates

We investigated the optical properties of isolated single aggregates of Ag nanoparticles (Ag nanoaggregates) on which rhodamine 6G molecules were adsorbed to reveal experimentally a correlation among plasmon resonance Rayleigh scattering, surface-enhanced resonance Raman scattering (SERRS), and its background light emission. From the lack of excitation-laser energy dependence of background emission maxima we concluded that the background emission is luminescence, not Raman scattering. The polarization dependence of both SERRS and background emission was the same as that of the lowest-energy plasmon resonance maxima, which is associated with a longitudinal plasmon. From the common polarization dependence, we identified that the lowest-energy plasmon is coupled with both SERRS and background emission. In addition, we revealed that the lowest-energy plasmon with a higher quality factor (Q factor) yields larger SERRS and background emission intensity. Also, we identified that the Q factor dependence of the SERRS intensity was similar to that of the background emission intensity. This similarity directly supported us to demonstrate an enhancement of both SERRS and background emission by coupling with a common plasmon radiative mode.     

Elucidation of interaction between metal-free tetraphenylporphine and surface Ag atoms through temporal fluctuation of surface-enhanced resonance Raman scattering and background-light emission

We have observed simultaneously temporal fluctuation of surface-enhanced resonance Raman scattering (SERRS) and its background-light emission from single Ag nanoaggregates that were adsorbed with metal-free tetraphenylporphine (H(2)TPP) molecules. We found that temporally stable SERRS spectra showed clearly a SERRS band that is attributed to a stretching mode of a chemical bond between a carbon atom and a non-hydrogenated nitrogen atom (C(alpha)-N). This stretching mode was not observed in regular resonance Raman spectra which are free from surface enhancement. On the other hand, we also found that temporally unstable SERRS spectra did not clearly show a C(alpha)-N stretching mode in SERRS bands. Furthermore, temporally stable SERRS spectra were accompanied by temporally stable background-light emission. Kobayashi et al. [J. Phys. Chem. 1985, 89, 5174] reported that formation of an Ag-N bond between surface Ag atoms and non-hydrogenated N atoms in a pyrrole ring enhances the intensity of a C(alpha)-N stretching mode. Thus, the observed relationship between clear appearance of a C(alpha)-N stretching mode and temporal stability of SERRS plus background-light emission strongly suggests that formation of a stable Ag-N bond suppresses fluctuation of both SERRS and background-light emission. Furthermore, the observed relationship implies that chemical contribution to SERRS is stabilization of H(2)TPP molecules that are adsorbed on SERRS-active sites by formation of Ag-N bonds. Additionally, we attributed background-light emission to luminescence of complexes between H(2)TPP molecules and surface Ag atoms considering possible formation of Ag-N bonds, synchronized SERRS intensity with background-light emission intensity, blue-shifted background-light emission maxima from normal fluorescence maxima, and previous reports related to electronic structures of H(2)TPP molecules on Ag surfaces.     

Quenching of photoluminescence in conjugates of quantum dots and single-walled carbon nanotube

Development of quantum dot (QD) based device components requires controlled integration of QDs into different photonic and electronic materials. In this regard, introduction of methods for regular arrangement of QDs and investigation of properties of QD-based assemblies are important. In the current work we report (1) controlled conjugation of CdSe-ZnS QDs to sidewall-functionalized single-walled carbon nanotube (SWCNT) templates (2) and the effect of conjugation of QDs to SWCNT on the photoluminescence (PL) properties of QDs. We identified that PL intensity and lifetime of QDs are considerably reduced after conjugation to SWCNT. The origin of the quenching of the PL intensity and lifetime was discussed in terms of F?rster resonance energy transfer (FRET). FRET involves nonradiative transfer of energy from a photoexcited QD (energy donor) to a nearby SWCNT (energy acceptor) in the ground state. This was examined by varying the density of QDs on SWCNT and conjugating smaller and bigger QDs to the same SWCNT. We estimated the FRET efficiency in QD-SWCNT conjugates from the quenching of the PL intensity and lifetime and identified that FRET is independent of the density and type of QDs on SWCNT but inherent to QD-SWCNT conjugates.     

Stability of <Superscript>244–260</Superscript>Fm isotopes against alpha and cluster radioactivity

Taking Coulomb and proximity potentials as the interacting barrier we have studied the cold valley in the radioactive decay of ^244–260Fm isotopes. It is found that in addition to alpha particle minima, other minima occur at S, Ar and Ca clusters. We have computed the half-lives and other characteristics of different clusters emitted from these parents treating parent, daughter and emitted cluster as spheres. Our study reveals that most of these parents are unstable against alpha and heavy cluster (⁴⁶Ar, ^48,50Ca) emissions and stable against light cluster emission, except ⁸Be from ^244–248Fm isotopes. The most probable clusters from these parents are predicted to be ⁴⁶Ar, ^48,50Ca which indicate the role of doubly or near doubly magic clusters (Z = 20, N = 28) and also stress the role of doubly magic ²⁰⁸Pb daughter. The computed half-lives for alpha decay are in good agreement with the experimental data. It is found that the presence of neutron excess in the parent nuclei slows down the cluster decay process. The effect of quadrupole (β ₂) and hexadecapole (β ₄) deformations of parent and fragments on half-lives are also studied. It is found that inclusion of β ₂ and β ₄ reduces the height and shape of the barrier (increases barrier penetrability) and hence the half-life decreases.     

Synthesis of heterocycle appended spiro(oxindole-3,2’-pyrrolidine) derivatives from heterocyclic ylidenes and azomethine ylide through 1,3-dipolar cycloaddition reactions

Spiro(oxindole-3,2’-pyrrolidine) is a privileged scaffold displaying a wide spectrum of biological activities. A series of these molecules containing heterocyclic rings attached to the pyrrolidine unit has been synthesized utilizing the classical 1,3-dipolar cycloaddition reaction of azomethine ylide with heterocyclic ylidenes. The method portrays a two-step functionalization of thiophene/furan with spiropyrrolidine oxindoles in the second/third positions. Biological activity evaluation for antibacterial, cell toxicity, and anticancer potential were done. Compounds exhibited no antibacterial activity. However, they were found to be non-cytotoxic upto 100?µg/ml and to exhibit potent anticancer activity by activating pro-apoptotic genes p53 and caspase 7.     

[8+3]‐Cycloaddition of Tropones with Azaoxyallyl Cations

Although azaoxyallyl cations are widely used as 1,3‐dipoles for various cycloaddition reactions leading to nitrogen‐containing heterocycles, their application in higher‐order cycloaddition reaction remains scarce. Herein, we present the [8+3]‐cycloaddition reaction of tropones with in situ generated azaoxyallyl cations allowing the one‐step construction of cycloheptatriene‐fused 1,4‐oxazinones in moderate to good yields. This base‐promoted new carbon‐oxygen and carbon‐nitrogen bond‐forming reaction takes place under mild conditions in the absence of transition metal catalysts.     

Structural and spectroscopic investigations on the quenching free luminescence of europium oxalate nanocrystals

The structural features leading to the intense quenching free luminescence exhibited by europium oxalate nanocrystals, poly[[hexaaquatri‐μ₂‐oxalato‐dieuropium] 4.34‐hydrate], {[Eu₂(C₂O₄)₃(H₂O)₆]·4.34H₂O}_n, is the focal point of this report. Europium oxalate nanocrystals were synthesized by a simple microwave‐assisted co‐precipitation method. Powder X‐ray diffraction analysis revealed the monoclinic structure of the nanocrystals and the phase purity. The morphology and particle size were examined by transmission electron microscopy (TEM) analysis. Luminescence measurements on a series of samples of La_2–xEu_x(C₂O₄)₃·10H₂O, with x varying in the range 0.1 to 2, established the quenching free nature exhibited by the europium oxalate nanocrystals. A single‐crystal structure analysis was carried out and the quenching free luminescence is explained on the basis of the crystal structure. A detailed photoluminescence characterization was carried out using excitation and emission studies, decay analysis, and CIE coordinate and colour purity evaluation. The various spectroscopic parameters were evaluated by Judd–Ofelt theoretical analysis and the results are discussed on the basis of the crystal structure analysis.