Photophysics of substituted alkylamino naphthalene diimides: An experimental and theoretical study

2017-03-21T00:44:09Z (GMT) by Alex Kehridakis
It has been observed from experiments that bulk measurements on SANDI molecules have long fluorescence lifetimes showing a narrow distribution in the range of 10-12 ns, whereas single molecule measurements on the same molecules are often shorter and show a more wide distribution from 2-12 ns. This thesis investigates this variation in fluorescence lifetimes for SANDI molecules when comparing bulk fluorescence lifetime measurements to single molecule measurements. It is hypothesised that this observation is the result of rotation of the substituted group of SANDI molecules which affects the electron donation of the substituted group to the core and therefore the photophysical properties of the molecule such as fluorescence lifetime and fluorescence intensity. The SANDI molecule that was chosen for the experimental analysis section of this project is the yellow emitting monobutyl SANDI which is a readily synthesized efficient emitter that displays suitable photophysical properties for analysis. The structure of the molecule is a naphthalene diimide core with a single butyl amino group attached at the 2 position of the core.<br> <br>    This research focuses on using single molecule fluorescence microscopy methods in conjunction with theoretical calculations to investigate the physical basis for these observations. It is believed that for measurements at room temperature in solvent, the rotation of the substituted group is rapid and the ensemble averaged conformation that is sampled has the average equilibrium angle between the substituted group and the NDI core. The sample preparation method of spin coating in order to embed the single molecules in a polymer film is considered to be an important factor, because spin coating can result in a sample containing a range of ‘locked’, non-equilibrium conformations with varied fixed angles of the substituted group with respect to the NDI core. In order to undertake spectroscopic evaluation of the monobutyl SANDI molecule, single molecule techniques such as wide-field microscopy, defocused wide-field microscopy and confocal microscopy were adopted. The conventional wide-field technique allows for an understanding of the overall brightness distribution of molecules in a film, whereas the defocused wide-field technique allows for the differentiation of single emitter from multiples and the selection of molecules in the most optimal state based on their orientation in the sample film. Confocal microscopy is used to obtain fluorescence lifetime, fluorescence intensity and wavelength data at the single molecule level.<br> <br>    <b>Chapter 1</b> is an introduction into the world of fluorescence including an explanation of fluorescent molecules highlighting their desired properties, specific families of molecules that are fluorescent, their uses and developments. The spectroscopic methods and theoretical calculations applied are introduced with an explanation of their usefulness and examples of their applications. The hypothesis and aims are also outlined in order to provide an understanding of the overall objectives of the thesis.<br> <br>    <b>Chapter 2</b> describes the experimental methods and techniques with thorough explanations of the way in which fluorescence properties are quantified, configurations of the spectroscopy instruments used for data acquisition and the procedure used for data analysis. The method of sample preparation for each experimental measurement is also explained.<br> <br>    <b>Chapter 3</b> is the results section of the thesis that presents and explains the experimental results which were obtained. These results include the initial bulk measurements such as the steady state and quantum yield data, bulk fluorescence lifetime measurements as well as the single molecule fluorescence measurements which were obtained using conventional wide-field, defocused wide-field and confocal microscopy.<br> <br>    <b>Chapter 4</b> presents the theoretical calculations undertaken to support the experimental data. The chapter initially describes the theoretical procedures, followed by an in depth analysis of the rotational profiles, absorption and emission spectra and molecular orbital data obtained. A comparison of the theoretical and experimental results is also made.<br> <br>    The thesis finalizes with a conclusion that summarizes the findings of chapters 1-4 and also suggestions are made in regards to potential future work.