Studies of Temperature-Dependent Excimer-Monomer Conversion in Dendrimeric Antenna Supermolecules by Fluorescence Spectr
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Studies of Temperature-Dependent Excimer-Monomer Conversion in Dendrimeric Antenna Supermolecules by Fluorescence Spectroscopy Youfu Caoa, Jeffrey S. Mooreb and Raoul Kopelmana* a)
Department of Chemistry, University of Michigan, Ann Arbor, MI 48109
b)
Department of Chemistry, University of Illinois, Urbana, IL 61801
* Correspondence Author: [email protected] Abstract: Phenylacetylene (PA) dendrimer labeled with perylene (See Fig 1) is discovered to exhibit temperature-dependent emission spectra in certain organic solvents over the temperature range of 20-65°C. The monomer signal is increasing rapidly when temperature increases, while the excimer signal decreases slowly. Models of excimer formation and weakly associated pairs (M+M) dissociation dynamics are included, and the equilibrium constants at different temperatures are calculated. This behavior suggests potential applications in fluorescence-based thermometry.
W 15-per W31-per W63-per
nanostar
Fig 1: The molecular structures of perylene-substituted phenylacetylene (PA) dendrimers (W15per, W31-per, W63-per and nanostar), the colors (blue, green, orange, red) represent the emission energy of the corresponding subunits.
T7.27.1
Introduction Phenylacetylene (PA) dendrimers 1 and their perylene-substituted derivatives, both compact and extended series, have been investigated both experimentally2, 3 and theoretically4, 5, focusing on their photophysical and photochemical properties. In those perylene-substituted derivatives, perylene molecules are attached to the locus, functioning as energy traps. These energy traps help understand the PA dendrimers. Due to the high efficiency of energy transfer in these molecules and their Cayley tree (or Bethe tree) structures and large molecular sizes, they have potential applications in drug delivery systems6, 7, optical nano-probes8, 9, near field microscopy (single molecule light sources) 10, 11 and photoelectric devices. Synthetic chemists are building more supermolecules with similar structures12, trying to bring them into industrial and scientific applications. In this paper we report quite-unexpected temperature-dependent emission spectra in these big molecules, over the temperature range of 20−65°C, for the “Nanostar”, one typical PA dendrimer composed of 5 generations. Experiments In the experiment, a 110-6M Nanostar solution in spectrophotometric grade n-hexane (Purchased from Fisher, used as received) is bubbled with N2 to avoid excited state quenching by oxygen. The sample is then filled into an airtight quartz cell, preventing solvent evaporation and oxygen contacts, keeping the concentration constant. Steady state fluorescence spectra are taken on a JOBIN YVON-SPEX Instruments S.A. FluoroMax-2 fluorometer with temperature control. The excitation wavelength is 310nm, the absorption peak of the outermost linear PA chains of 2 units 13. The temperature of the sample cell was increased by 5°C, beginning from 20°C. The fluorescence spectra are taken after 30min at each temperature, to allow enough time for
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