A bichromophoric molecule consisting of two coumarins linked by a pentakis(ethylene oxide) spacer can efficiently bind Pb2+ ions in acetonitrile and in propylene carbonate. The resulting changes in photophysical properties are reported with special attention to photoinduced electronic energy transfer. Steady-state data allow one to determine the transfer efficiency and stoichiometry of the complex, whereas subpicosecond time-resolved experiments by the excite-and-probe technique provide information on the transfer kinetics. The stoichiometry of the complex is 1:1 in acetonitrile and 1:3 (ligand:metal) in propylene carbonate. A significant increase in efficiency and rate of energy transfer for the complex is observed in acetonitrile, whereas there is almost no change in propylene carbonate. The results are discussed in terms of a possible structure of the complex which is different in the two solvents.
The hydroboration of acetylene with HBCl2 yields the products (Cl2B)2CHMe (1a), (Cl2BCHMe)2BCl (2a), (Cl2BCHMeBCl)2CHMe (3a), and (ClBCHMe)3 (4a). The crystal structure of 4a as well as of the pentaiodo derivative 2b were determined. A mechanism for the formation of the different hydroboration products is proposed. The 1,3,5-triborapentane 2a serves as precursor for the synthesis of derivatives of 1,3,5-triboracyclohexanes, 1,3,5-tribora-2-aza-cyclohexanes, 1,3,5-tribora-2-phospha-cyclo-hexanes, and 1,3,5-tribora-2-thia-cyclohexanes. Their constitutions have been derived from spectroscopic studies. The crystal structure of the 1,3,5-tribora-2-oxa-cyclohexane derivative was also determined.
Quantum-size semiconductor microcrystallites of ZnCdS and CdS in aqueous and isopropanol solution were optically excited using intense sub-picosecond UV laser pump pulses. Broad-band optical probing monitored the yield of electron emission and the build-up of primary photochemical products. With typical pump fluences of femtosecond light pulses multiple photons are absorbed by one quantum-dot leading to a quantum yield of up to 0.3 of photogenerated electrons. The yield curves, as a function of pump fluence, are described by a simple kinetic model which involves the trapping-rate constant and a maximum number of electrons which can be emitted per particle. This model is shown to be consistent with thermodynamic considerations.
By excite-and-probe spectroscopy with subpicosecond time resolution, we have measured the intramolecular electronic energy transfer in bichromophoric coumarins linked by a flexible polymethylene chain. The transfer proceeds on a time scale between 1 and 20 ps depending on the polymethylene chain length. The results can be well described by a dipole-dipole interaction model that takes into account the statistical distribution of intramolecular distances between the two chromophores.
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