Sarah J. Schmidtke

Structural and Spectroscopic Studies of the Photophysical Properties of Benzophenone Derivatives

The effect a solvent has on the photophysical properties of a series of benzophenone derivatives, all FDA approved for use in sunscreens, is examined. Experimentally significant differences in the solvatochromic behavior are found to be dependent upon the substituents on the parent benzophenone molecule. The spectral trends do not appear to originate from only changes in the solvent polarity but indicate that specific solvent-solute interactions influence the absorbance energies of some benzophenones. Computational investigations examine the structure and electronic excitation energies of the molecules. Specific interactions of the solvent and solute are modeled to evaluate structural changes that result from solvent-solute complexation and the impact of the changes upon absorbance properties. The viability of an intramolecular excited state proton transfer is theoretically evaluated. The combination of experimental and computational analysis provides a more complete understanding of the molecular level origin of the unique photophysical properties of this class of UV absorbers.

pH-dependent spectral properties of para-aminobenzoic acid and its derivatives.

The local environment dictates the structural and functional properties of many important chemical and biological systems. The impact of pH on the photophysical properties of a series of para-aminobenzoic acids is examined using a combination of experimental spectroscopy and quantum chemical calculations. Following photoexcitation, PABA derivatives may undergo an intramolecular charge transfer (ICT) resulting in the formation of a zwitterionic species. The thermodynamics of the excited state reaction and temperature-dependence of the radiative emission processes are evaluated through variable temperature fluorescence spectroscopy carried out in a range of aqueous buffers. Quantum chemical calculations are used to analyze structural changes with modifications at the amine position and different protonation states. The ICT is only observed in the tertiary amine, which calculations show has more sp(2) character than the primary or secondary amines. Thermodynamic analysis indicates the ICT reaction is driven by entropy.

Pyridylpyrrolides as alternatives to cyclometalated phenylpyridine ligands: synthesis and characterization of luminescent zinc and boron pyridylpyrrolide complexesElectronic supplementary information (ESI) available: Beer?s Law plots and emission peak area vs. concentration plots for complexes; comparison of excitation and absorbance spectra; effect of triplet quencher isoprene on luminescence intensity. See http://www.rsc.org/suppdata/dt/b3/b315523d/

The synthesis, structure, and properties of six luminescent pyridylpyrrolide complexes and the first structural characterization of pyridylpyrrolide metal complexes are reported. A series of new zinc complexes, bis(pyridylpyrrolyl)zinc, (R2PyrPy)2Zn (R = Me, Et, iPr, tBu, and Ph), that vary in their substituents on the pyrrole ring (Me, Et, iPr, tBu, and Ph), were prepared. Pyrrole substitution produced small structural changes in the complexes and affected the fluorescence properties very little. The zinc complexes were found to be luminescent, emitting at 495 nm (Phi = 0.32, 0.32 0.31, 0.19 and 0.57, respectively). A boron analog, (Me2PyrPy)BF2, was prepared and was found to share the luminescent properties with the zinc complexes, emitting at 505 nm (Phi = 0.22), but not their water-sensitivity. A total of four crystal structures are reported, tBu2PyrPyH, (Me2PyrPy)2Zn, (tBu2PyrPy)2Zn, and (Me2PyrPy)BF2. tBu2PyrPyH crystallizes as a doubly hydrogen bonded dimer with non-coplanar pyridine and pyrrole rings. The solid-state structures of (Me2PyrPy)2Zn and (tBu2PyrPy)2Zn revealed that despite the large change in steric bulk, the two compounds have very similar structures. The structure of (Me2PyrPy)BF2 showed changes that are expected with the interaction between a smaller atom (B as compared to Zn). Molecular orbital calculations were performed on Me2PyrPyH, (Me2PyrPy)BF2, and (Me2PyrPy)2Zn using Gaussian 98 methods. It was found that the main transition (HOMO-LUMO) for all three molecules is a pi-->pi* transition and that in the inorganic complexes, the metal atom (zinc or boron) present has very little effect on transition, evidence that the optical properties are largely ligand based and that the B or Zn atoms main effect is lowering of the LUMO relative energy.

2-(2?-Pyridyl)pyrroles: Part I. Structure and energetics of pyridylpyrroles, their dimers, complexes and excited statesElectronic supplementary information (ESI) available: NOESY spectra for Me2PyrPy and 1?1 Me2PyrPy?MeOH (Figs. 1S and 2S). See http://www.rsc.org/suppdata/cp/b4/b401824a/

Structural and energetic evaluations of substituted 2-(2′-pyridyl)pyrroles were performed through a combination of computational and experimental methods. In conjunction with experimental absorbance and fluorescence studies, the data were analyzed with respect to hydrogen bonding ability, complex formation with alcohols, dimerization, excited state behavior, and potential for proton transfer. Experimental and theoretical evidence show the importance of the pyrrole substituent groups in both structural and spectral properties of the molecules and their complexes. Low temperature NMR experiments and full solution phase DFT optimizations support the formation of a cyclically bridged alcohol complex for the 3,5-dimethyl-2-(2′-pyridyl)pyrrole species with a nearly coplanar ring system, whereas the full DFT optimization of the 3,5-di-tert-butyl-2-(2′-pyridyl)pyrrole:methanol complex shows a break in the planarity of the ring system.

2-(2?-Pyridyl)pyrroles: Part II. Spectroscopic investigation of pyridylpyrrole alcohol complexesElectronic supplementary information (ESI) available: Titration data fit with single or double complexation model (Figs. S1?S8). See http://www.rsc.org/suppdata/cp/b4/b401859c/

A homologous series of 2-(2′-pyridyl)pyrroles were studied as a possible model system for intermolecular hydrogen bonding interactions in molecules that can act as both hydrogen bond donors and acceptors. Steady state spectroscopic methods were used to assess the importance of intermolecular hydrogen bonding interactions in dilute solutions of 2-(2′-pyridyl)pyrroles in the absence and presence of alcohols. The absorption and fluorescence properties of such solutions were investigated to determine the relevance of such hydrogen bonding interactions in the ground and excited state behavior of the 2-(2′-pyridyl)pyrroles. Over the concentration range studied, no evidence was found for the formation of hydrogen-bonded 2-(2′-pyridyl)pyrrole dimers. However, it was determined that 3,5-dimethyl-2-(2′-pyridyl)pyrrole forms weak (K ∼ 5–12 M−1) 1∶1 hydrogen-bonded complexes with methanol and t-butyl alcohol in the ground state, and 3,5-di-tert-butyl-2-(2′-pyridyl)pyrrole forms both 1∶1 and 1∶2 complexes with the same alcohols. Despite the formation of hydrogen-bonded 2-(2′-pyridyl)pyrrole alcohol complexes, no experimental evidence was found for excited state proton transfer in such complexes.