Ultrafast molecular rotor and proton transfer dynamics in solution

Open Access
Author:
Breffke, Jens
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 26, 2014
Committee Members:
  • Mark Maroncelli, Dissertation Advisor
  • Mark Maroncelli, Committee Chair
  • John B Asbury, Committee Member
  • James Bernhard Anderson, Committee Member
  • John H Golbeck, Committee Member
Keywords:
  • fluorescence
  • solvent effect
  • ultrafast spectroscopy
  • malononitriles
  • Thioflavin T
  • 4-diethylamino-3-hydroxyflavone
Abstract:
Dynamic solvent effects on several charge transfer reactions in different molecular rotors and an excited state intramolecular proton-transfer reaction have been studied in a range of representative solvents. Combining broadband Kerr-gated emission spectroscopy and time-correlated single photon counting techniques enables determination of fluorescence lifetimes over femtosecond to nanosecond times. Using conventional solvents with well-known physical properties (viscosity, dielectric constant, solvation time, etc.) allows one to discern what types of solvent effects influence the solute's dynamics. When these experiments are complemented by quantum chemical calculations, a better understanding of reaction mechanisms and solvent dependencies can be gained. Three studies have are described in this dissertation to provide new information and insights. The first study involves three naphthalene malononitrile derivatives which belong to the family of molecular rotors. Previous studies of other malononitriles remained inconclusive with respect to the fluorescence deactivating mechanism. By tuning the rotational barrier and inhibiting rotating degrees of freedom through structural design and by measuring in a series of eleven representative solvents new experimental and computational information supporting a deactivation mechanism consisting of an excited-state double bond isomerization contrary to the more accepted concept of a single bond rotation were provided. The second study looks at Thioflavin T, a popular fluorescence sensor for fibrillization kinetics. In solution Thioflavin T has a viscosity dependent quantum yield. A recent publication indicates a fast auto-degradation which produces a fluorescent impurity which could have led to wrong conclusions about the photophysics. After intensive purification Thioflavin T has been measured here in 18 representative solvents and quantum mechanical calculations have been performed on the rotation dynamics. We find viscosity alone as insufficient to describe the fluorescence quenching but determined a solvation time dependence as well as a difference between protic (alcohols) and aprotic solvents. The third study is the ultrafast excited-state intramolecular proton transfer (ESIPT) reaction of the 3-hydroxyavone derivative DEAHF was studied. Recent literature suggested a proton-tunneling in the Frank-Condon state leading to prompt tautomer formation. However, solvent relaxation induces a reaction barrier which also results in a slow component to the tautomerization process. A binary mixture of acetonitrile and propylene carbonate provides a solvent system having constant polarity but solvation times which vary by a factor of ten between acetonitrile to propylene carbonate. Spectroscopic analysis confirms that the excited-state equilibrium does not change within this mixture series. Time-resolved measurements showed no evidence of a non-zero concentration of the tautomer form at time zero. However, our data clearly shows a slowing in the tautomer formation at early times with decreasing solvation time, suggesting a reaction barrier build-up upon solvation dynamics.