The Stern-Volmer quenching constants (Ksv) for poly(amidoamine)-(PAMAM-) based dendrimers (generations G0-G4) modified with (4, 8, 16, 32 and 64) pendant [Ru(bpy)3] +2 (bpy is 2,2′-bipyridine) chromophores have been measured in the presence of three nitroaromatic quenchers (2,4,6-trinitrotoluene ) TNT, 2,4-dinitrotoluene ) DNT, nitrotoluene ) NT), two negatively charged quenchers (K 3Fe(CN)6, Na4Fe(CN)6), and one energy transfer type quencher (Fe(C5H5)2). The quenching efficiencies were calculated for the dendrimers (G0-G4) in the presence of TNT and were found to peak for dend-16-Ru(bpy)3. The generation dependence of the quenching efficiency mirrors the calculated crowding factor and the variations were attributed to changes in the 3-dimensional structure with generation (size) and the associated changes in the accessibility of the [Ru(bpy) 3] 2+ -pendant groups. Electrostatic attractions between the positively charged dendrimers and the negatively charged quenchers resulted in larger Ksv values when compared to the reference complex, [Ru(bpy)3] 2+ . The addition of electrolyte to an ionic strength of 0.1 M (KCl or NaCl) caused a diminution of the Ksv values by 60%, consistent with electrostatic interactions and charge screening effects.
The interactions of five bis(bipyridyl) Ru(II) complexes of pteridinyl-phenanthroline ligands with calf thymus DNA have been studied. The pteridinyl extensions were selected to provide hydrogen-bonding patterns complementary to the purine and pyrimidine bases of DNA and RNA. The study includes three new complexes [Ru(bpy)(2)(L-pterin)](2+), [Ru(bpy)(2)(L-amino)](2+), and [Ru(bpy)(2)(L-diamino)](2+) (bpy is 2,2'-bipyridine and L-pterin, L-amino, and L-diamino are phenanthroline fused to pterin, 4-aminopteridine, and 2,4-diaminopteridine), two previously reported complexes [Ru(bpy)(2)(L-allox)](2+) and [Ru(bpy)(2)(L-Me(2)allox)](2+) (L-allox and L-Me(2)allox are phenanthroline fused to alloxazine and 1,3-dimethyalloxazine), the well-known DNA intercalator [Ru(bpy)(2)(dppz)](2+) (dppz is dipyridophenazine), and the negative control [Ru(bpy)(3)](2+). Reported are the syntheses of the three new Ru-pteridinyl complexes and the results of calf thymus DNA binding experiments as probed by absorption and fluorescence spectroscopy, viscometry, and thermal denaturation titrations. All Ru-pteridine complexes bind to DNA via an intercalative mode of comparable strength. Two of these four complexes--[Ru(bpy)(2)(L-pterin)](2+) and [Ru(bpy)(2)(L-allox)](2+)--exhibit biphasic DNA melting curves interpreted as reflecting exceptionally stable surface binding. Three new complexes--[Ru(bpy)(2)(L-diamino)](2+), [Ru(bpy)(2)(L-amino)](2) and [Ru(bpy)(2)(L-pterin)](2+)--behave as DNA molecular "light switches."
The photophysics of PAMAM (poly-amidoamine) based dendrimers (generations G0-G4) modified with (4, 8, 16, 32, and 64) pendant-[Ru(tpy) 2 ] +2 (tpy is 2,2′:6′,2′′-terpyridine) and [Ru(bpy) 3 ] +2 (bpy is 2,2′-bipyridine) chromophores have been studied in both fluid solution at 298 K and frozen glasses, at 77 K. The absorption and emission spectra, the excited-state lifetimes and quantum yields have been obtained for both families of metallodendrimers. In general and in analogy to the behavior exhibited by the discrete molecules (i.e., [Ru-(tpy) 2 ] +2 and [Ru(bpy) 3 ] +2 ), the bipyridine derivatives exhibit longer lifetimes and higher quantum yields when compared to the corresponding terpyridine complexes. Some generation dependent changes have also been observed. We have also explored the effects of solvent by comparing results in butyronitrile and dimethylacetamide with the latter being used as a mimic of the dendritic backbone. Our results suggest that for the higher generations the dendritic backbone acts as the solvent in affecting the photophysical behavior.
Boiling-point data are commonly used in introductory chemistry classes to illustrate the effects of intermolecular forces of attraction (IMFs) on familiar physical properties. In this article, we describe how we use boiling-point trends of group IV-VII hydrides to introduce intermolecular forces in our first-year general chemistry classes. Our approach grew out of our experience teaching with the ACS general chemistry textbook (1), which emphasizes conceptual understanding and uses an inquiry-based approach to help students understand chemical principles.A number of parameters can be used as a measure of intermolecular forces, and it could be argued that the enthalpy of vaporization is a more direct, and therefore better, indicator of the strength of these attractive forces. Boiling point is the temperature at which Gibbs energy, ΔG vap , equals zero, so that the boiling temperature, T BP , is the ratio of the enthalpy of vaporization, ΔH vap , to the entropy of vaporization, ΔS vap :Because ΔH vap depends directly on the strength of intermolecular forces, boiling points also incorporate a similar dependence. However, the boiling point also depends on ΔS vap , which theoretically could change the overall dependence of boiling point on IMFs. We explore this question and explain why we believe that it is legitimate to look at boiling-point trends, at least for the group IV-VII hydrides. We also examine the choice of variable to use on the x axis when graphing boiling point or ΔH vap and discuss why the number of electrons, although an imperfect indicator of the total molecular interaction energy, is conceptually more sound than other reasonable alternatives, especially in an introductory chemistry class. Finally, we address the common misconception that dispersion forces are the weakest of the IMFs and then use London dispersion forces to explore the nonlinearity of the boiling-point data for the group IV compounds. Discussion: Trends in Boiling Points of HydridesThe Effects of Intermolecular Forces of Attraction on Boiling PointAll general chemistry courses introduce the different types of intermolecular forces and how they help to explain a variety of physical properties. A graph of the boiling points of the group IV-VII hydrides is often used to illustrate periodic trends that relate to intermolecular forces because it is generally assumed that a higher boiling point is the result of stronger intermolecular attractions. Because the enthalpy of vaporization is a more direct measure of the strength of IMFs in a liquid and differences in IMF strength will change the magnitude of ΔH vap , there is an argument for using ΔH vap rather than boiling point to examine the role of IMFs. In our courses, IMFs are introduced early in the chemistry course, when students have not yet learned about enthalpy, and thus, it makes sense to talk instead about a property (boiling point) with which the students are familiar. To test the validity of using boiling point as an indicator of IMF strength, we compare graphs of boiling points...
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