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First Line of Title TIME RESOLVED AND STEADY STATE EXPERIMENTS WITH PRODAN AND LAURDAN SOLUTIONS by Matthew J. Phillips A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Science in Chemistry and Biochemistry Spring 2019 © 2019 Phillips All Rights Reserved TIME RESOLVED AND STEADY STATE EXPERIMENTS WITH PRODAN AND LAURDAN SOLUTIONS by Matthew J. Phillips Approved: __________________________________________________________ Lars Gundlach, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: __________________________________________________________ Brian Bahnson, Ph.D. Chair of the Department of Chemistry and Biochemistry Approved: __________________________________________________________ John Pelesko, Ph.D. Interim Dean of the College of Arts and Sciences Approved: __________________________________________________________ Douglas J. Doren, Ph.D. Interim Vice Provost for Graduate and Professional Education ACKNOWLEDGMENTS A huge thank you to Dr. Lars Gundlach, whose mentorship and guidance made this work and my professional development possible. My colleagues Samantha Doble, Joseph Avenoso, Han Yan, Mercury Li, Meng Jia, who are always willing to share their knowledge and expertise, and make lab awesome. The support and love of my friends and family iii TABLE OF CONTENTS LIST OF TABLES ......................................................................................................... v LIST OF FIGURES ....................................................................................................... vi ABSTRACT ................................................................................................................. vii Chapter 1 INTRODUCTION .............................................................................................. 1 2 EXPERIMENTAL ............................................................................................. 6 2.1 Lamellar Films ........................................................................................... 6 2.2 Steady State Spectra .................................................................................. 6 2.3 Transient Absorption Spectroscopy .......................................................... 7 2.4 Time Correlated Single Photon Counting ................................................. 9 3 RESULTS AND DISCUSSION ....................................................................... 11 4 CONCLUSION ................................................................................................ 22 REFERENCES ............................................................................................................. 23 iv LIST OF TABLES Table 1: Fluorescence lifetimes of PRODAN in various solvents ............................... 14 v LIST OF FIGURES Figure 1: Moiety of PRODAN derivatives. For PRODAN, R = CH3. For laurdan, R=C9H19..................................................................................................... 2 Figure 2: Mechanism of intramolecular charge transfer in PRODAN upon excitation. .................................................................................................. 3 Figure 3: Diagram of TA measurement set up. The pump pulse, black, crosses with the probe pulse, gray, at the sample, followed by a pinhole to allow only the probe pulse to reach detection. [10] ............................................ 8 Figure 4: Measurement apparatus used for TA experiments with lamellar films. In the center of the cuvette lies the film on a thin glass medium. The pipette tip water reservoir humidifies the cuvette. .................................... 9 Figure 5: emission spectra of various PRODAN solutions excited with 350 nm light 12 Figure 6: Emission spectra of PRODAN solutions with 410 nm excitation. ............... 13 Figure 7: Normalized absorbance spectra of PRODAN solutions ............................... 14 Figure 8: TA maps of PRODAN in various solvents. A:methanol, B: ethanol, C: dimethyl sulfoxide, D: acetonitrile, E: acetone. The color scale, in units of OD, is common for all graphs in its row. For A and B Sapphire white light generation was used, and for C, D and E Calcium fluoride was used to generate white light. ............................................... 16 Figure 9: PRODAN emission maximum over time with sapphire white light generation for ethanol and methanol, and calcium fluoride white light generation for all others. .......................................................................... 18 Figure 10: TA maps of PRODAN loaded lamellar film (A) and a blank film (B) ....... 20 Figure 11: IR region of PRODAN emission spectra, presumably phosphorescence. .. 21 vi ABSTRACT 2-propionyl-6-dimethylamino naphthalene (PRODAN) is used in biomedical research due to the sensitivity of its emission spectrum to changes in the environment. Typically, the fluorescence intensity at two wavelengths is analyzed to draw conclusions about water content, for example, of the PRODAN’s microenvironment. However, both polarity and hydrogen bonding ability of the environment play roles in PRODAN’s emission behavior. PRODAN was studied in a variety of solvents and mixtures, using steady state spectroscopy and time resolved techniques to bring a new perspective to the discussion of the nature of the excited states of PRODAN and the factors which give rise to them. vii Chapter 1 INTRODUCTION Both PRODAN and laurdan (figure 1) were first synthesized by Gregorio Weber in 1979 for the study of dipolar relaxation1, and have since remained a hot topic in the scientific community. Since, the molecule and many functionalized derivatives have been studied by physical chemists and spectroscopists, and used as a probe by biologists for decades2, 3. PRODAN is a model system for the study of the phenomenon of solvatochromism, a property where a dye changes its color depending upon its surroundings (i.e. the solvent). This makes PRODAN and its derivatives very useful as probes in many biophysical studies. However, we must first understand the mechanism of solvatochromism and why PRODAN displays such a dramatic shift in fluorescence maxima- about 125 nm, or 10 eV difference from cyclohexane to water as the solvent4. Upon excitation, the dipole moment of the molecule increases with charge localizing to the amino and carbonyl residues. This causes reorientation of the solvent dipoles, which lowers the energy of the excited state1. This is the cause of the observed red shift of emission of PRODAN as solvent polarity increases. 1 Figure 1: Moiety of PRODAN derivatives. For PRODAN, R = CH3. For laurdan, R=C9H19 A dye in two situations, first surrounded by hydrocarbons, and second surrounded by water, may or may not enter an identical state upon excitation, and are acted upon by different forces as the solvation sphere re-orients. Different forces are being exerted on these identical dye molecules by different solvents, so distinct emission bands should be observed and follow a trend based on the polarity and H- bonding ability of the solvent. Experimentally this is the case1, 4, and the trend is understood. For PRODAN, there is at least a locally excited (LE) state and an intramolecular charge transfer (ICT) state2. In polar solvent, PRODAN emits from the ICT state shown in figure 2. The primary goal of this study is to determine the dynamics of PRODAN in polar solvents leading up the ICT state. The current model assumes an initial transition to the locally excited state for all solvents2, as absorption changes very little with the solvent. Upon excitation to the locally excited state, 2 relatively little rearrangement of non-polar solvent occurs, but a polar solvent will rearrange its dipoles considerably to stabilize the excited state and allow a transition to the ICT state, where further rearrangement can occur. The greater the stabilization, the lower energy of the resulting transition and thus the degree to which emission is redshifted. However, there has been, and still is, considerable debate as to the nature of the excited state(s) of PRODAN4, 5. Let us first discuss the nature of the ICT state. The charge transfer occurs between the amino and carbonyl groups of PRODAN on either side of the naphthalene ring structure, which acts as a bridge as shown in figure 2. Charge is transferred from the amine to the carbonyl, resulting in a zwitterionic iminium enolate. Figure 2: Mechanism of intramolecular charge transfer in PRODAN upon excitation. Previous literature suggested that a twisting of donor and acceptor CT groups of the molecule was involved in excitation, and that the degree of twisting allowed played a role in the environmental sensitivity of PRODAN6. Experiments with PRODAN derivatized with ring structures to immobilize the donor and acceptor proved to not significantly change the molecules spectroscopic properties5. Thus, it is now understood that the excited state of PRODAN under non-extreme temperatures is 3 planar in polar and non-polar solvents. However it remains unclear whether PRODAN enters the same locally excited state in polar and nonpolar solvents, and if so in polar solvents, how the transition to the ICT state proceeds. The utility of PRODAN, and more commonly laurdan, as a probe is based on the molecules` solvatochromism. The emission of the dye is clearly dependant upon its surroundings, but not enough is understood for truly quantitative methods to be developed based on this property. Still, qualitative or
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