Singlet Fission in Concentrated TIPS-Pentacene Solutions: The Role of Excimers and Aggregates.

The excited-state dynamics of 6,13-bis(triisopropylsilylethynyl)pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M-1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.

Melamine detection in liquid milk based on selective porous polymer monolith mediated with gold nanospheres by using surface enhanced Raman scattering.

A rapid and fast detection of trace amounts of melamine in milk is reported by using Gold Nano Spheres embedded monolith conjugates. Monolith was synthesized by the polymerization of Glycidyl Methacrylate (GMA) and Ethylene Dimethacrylate(EDMA) (cross linker and functional monomer), Cyclohexanol (Porogen formation) and 2, 2-Dimethoxy-2-phenyl-acetophenone (photo-initiator) on gold coated silicon wafer. In order to gauge the influence of monolith on SERS signal activity, three shapes of gold nanoparticles namely Gold Nano Spheres (GNSs), Gold Nanorods (GNRs) and Triangular Gold Nanoprisms (GNPrs) were immobilized on monolithic surface and analyzed by the signal molecule Rhodamine (R6G). The Raman Enhancement efficiency of the above three shapes incorporated with monolith was monitored by calculating their maximum enhancement factors. Among three morphologies, Gold Nano Spheres integrated in GMA-EDMA Monolith Sensor (GNS@GEMS) was found more effective for detection of R6G than two others and was therefore projected in analysis of melamine sensing in commercial milk. A linear relationship of regression model (R2 = 0.99) was observed between melamine SERS intensity (at 710 cm-1) and varied concentrations (6.5 < mg/L < 0.125). The lower limit of detection (LOD) and limit of quantification (LOQ) for melamine was determined 0.11 mg/L and 0.38 mg/L respectively. The time window for detection of melamine was 10 min and hence our method is compatible with the FDA's tolerance limit in USA and China (1 mg/L).