Ultrafast charge transfer in conventional solvents and ionic liquids

This thesis work is aimed at understanding ultrafast intermolecular electron transfer and solvation processes in conventional solvents and ionic liquids. Following a brief introduction in Chapter 1, Chapter 2 explains the main experimental techniques and data analysis methods used in this thesis. Chapter 3 and Chapter 4 are focused on charge transfer reactions in representative ionic liquids in comparison to conventional solvents. Chapter 5 carefully examines a selected system for studying dynamic heterogeneity studies in ionic liquids.;Charge transfer is an important step in many types of chemical and biochemical reactions. The energetics and rates of charge transfer are greatly affected by the polarity and dynamics of the surrounding medium and extensive research has explored the effect of polar solvents on charge transfer. In contrast, despite the large amount of attention on ionic liquids for many applications, few studies have focused on fundamental aspects of charge transfer ionic liquids. I have surveyed the behavior of several solvent-controlled intramolecular charge transfer reactions in representative ionic liquids and compared it to what is found for these same reactions in conventional dipolar solvents using time-resolved fluorescence spectroscopy.;In chapter 3, crystal violet lactone (CVL), which exhibits distinct dual emission in fluid solution as a result of a rapid excited-state charge transfer reaction, was measured in series of conventional solvents as background for ionic liquids studies. Solvatochromic analysis using a dielectric continuum model suggests dipole moments of 9-12 D for the initially excited (LE) state and ∼24 D for the charge-transfer (CT) state. Intensities of steady-state emission as well as kinetic data provide free energies for the LE → CT reaction that range from +12 kJ/mol in nonpolar solvents to -10 kJ/mol in highly polar solvents at 25°C. Reaction rates constants, which lie in the range of 10-100 ns-1 in most solvents, depend on both solvent polarity and solvent friction. In highly polar solvents, this charge transfer reaction was confirmed to follow two-state kinetics as used in typical dual fluorescence probes like aminobenzonitriles. Reaction rates are correlated to solvation times in a manner that indicates the reaction is a solvent-controlled electron transfer on an adiabatic potential surface having a modest barrier.;In Chapter 4, the charge transfer reaction of CVL was studied in ionic liquids. Compared with conventional solvents, the reaction is much more complicated. Equilibrium is not reached in any ionic liquid due to the much slower reaction. Furthermore, in two other well-studied charge transfer probes in several classes of ionic liquids, we also found the reaction kinetics are typically more complicated. Multiexponential decays are observed when reaction times are comparable to solvation times, which are on the nanosecond scale in most ionic liquids at room temperature. Time-resolved spectra were dissected into LE and CT states using non-polar solvents as reference. Reaction times in ionic liquids calculated from LE/CT decays are slower than in conventional solvents approximately in proportion to the larger viscosities and longer solvation times of ionic liquids.;In Chapter 5, C102 in the ionic liquid [N4441][Tf2N] was chosen as a promising system to confirm the red edge excitation dependence observed in previous studies using C153 in [Nip311][Tf2N]. We were not able to approach the far red edge of the ionic liquid in the previous study. This study is also aimed to support the other dynamic heterogeneity observations in ionic liquids. An experimental temperature was first determined to get the complete solvation response function. Decay data was collected at perpendicular angle for better defined time-resolved spectra. Normalized time-resolved spectra show no difference with the ones collected at magic angle, which is evidence for using perpendicular angle for future experimental condition. The asymmetry factor was also fixed when fitting the spectra with log-normal function. Solvation response function was fitted with stretched exponential function and the frequency at time infinity was found to be important in solvation time calculation. No significant trend of solvation time dependence on excitation wavelength was observed considering the uncertainty of TCSPC instrumentations. Other systems or detection system with higher resolution is needed for future studies.;Chapter 6 describes the initial result of an on-going project in our group about the "up-relaxation" behavior of solvation dynamics in slow solvents using nanosecond dye laser. Only one set of up-relaxtion behavior is observed for now using 4-AP in propanol at -90°C. Spectra exciting at far-edge are still too noisy and need to be improved with other instrumentation skill. Probes with longer lifetime and ionic liquids will be studied in the future in a similar way.