Core substituted naphthalene diimides as sensors and in single molecule spectroscopy

Abstract

This research thesis explores core substituted naphthalene diimides in their roles as both chemosensors and as single molecule dyes; a combination of these applications ultimately leads to the ability to examine single molecule sensing events. The absorption and emission properties of new core substituted naphthalene diimide based sensors are investigated. Additionally, the underlying mechanisms involved during the sensor-target binding are explored and the corresponding optical signals analysed. Wide-field and confocal microscopy is then used to establish the potential for core substituted naphthalene diimides to be used in single molecule applications. Core substituted naphthalene diimides are first explored as fluorescent sensors for detecting protons (Chapter 3). Two new core substituted naphthalene diimides, containing tertiary amines, are studied using both steady state and time-resolved measurements. Optical absorption and emission wavelength shifts are observed upon protonation of the sensor, as well as changes in the fluorescence lifetimes. The sensing mechanism is attributed to photoinduced electron transfer, where the tertiary amine acts as an electron donor and the naphthalene diimide core acts as an electron acceptor. This photoinduced electron transfer process is hindered upon protonation, causing changes in emission. Furthermore, the emission properties of one of the proton sensors is also found to be highly dependent on solvent polarity, which leads to further examination of that sensor. Time-resolved measurements reveal an equilibrium between the locally excited, and charge separated states, which results in delayed fluorescence. The proton sensing abilities of core substituted naphthalene diimides are then explored over a range of proton concentrations, resulting in changes in the absorption and emission properties. Finally, the polarity dependence of the sensor was investigated using fluorescence lifetime imaging microscopy. The versatility of core substituted naphthalene diimides is investigated through the study of two new cation sensors (Chapter 4). Several crown ether based sensors are synthesised and then their responses to a range of cations are observed by measuring their absorption and emission spectral properties. These sensors, which differ only in the size of the crown ether, respond extremely differently to their target analyte. Differences are seen in the absorption and emission behaviour between the two sensors upon binding to the analyte. Computation calculations reveal a significant difference in the binding mechanisms of the two crown ether based sensors. Core substituted naphthalene diimides are then explored at the single molecule level (Chapter 5). Initial steady state and time-resolved measurements on two core substituted naphthalene diimides, with fluorinated side chains, show unexpectedly high quantum yields and long lifetimes. The single molecule studies demonstrate the potential for core substituted naphthalene diimides to be used in single molecule applications. These molecules show a high photon output before photobleaching and only very few intermittencies. Autocorrelation studies show that there is a low yield of triplet formation which indicates that the substituted fluoride atoms do not create a heavy-atom effect. Finally, the sensing abilities of a core substituted naphthalene diimide based proton sensor were studied at the ultimate level of sensitivity - the single molecule level (Chapter 6). Upon protonation, the sensor responds with a change of intensity, emission wavelength and decay kinetics. Confocal microscopy is used to observe these changes at the single molecule level. The emission properties of the proton sensor, in the neutral and protonated states, are measured. With the monitoring of photon count rate, emission decay kinetics and emission spectral wavelengths, unambiguous proton sensing events were observed.