Environmental effects on the photophysics and photochemistry of some aromatic molecules

Abstract

Aromatic molecules, especially polycyclic hydrocarbons, form a key cornerstone of molecular photophysics and have led to the identification of most of the main concepts, including excimer and exciplex formation. Derivatives of these compounds, most notably those including heteroatoms in the molecular framework, have expanded the field and provided many important applications. As such, the subject remains vibrant and progressive and there remains considerable interest in understanding the fundamental photophysical properties of aromatic molecules in solution and solid phases. In this thesis, we report the photophysical and photochemical properties of selected aromatic molecules with the intention of exposing the underlying environmental effects. The compounds have either been synthesized by specialist research groups or obtained from commercial sources. Chapter 1 presents a summary of the photophysical properties of aryl hydrocarbons and considers some of the better known examples of how selective substitution affects these properties. The chapter draws heavily on literature citations and seeks to introduce some of the subtleties of the field, focussing on fundamental aspects. The key objective is to enquire into how small structural changes influence the photophysics of these molecules and to consider the further effects caused by a change in environment. Basic theoretical considerations, such as the energy-gap law and the Strickler-Berg expression, are covered and the importance of spin-orbital coupling is highlighted. Chapter 2 is the first discussion chapter and deals with the structural dynamics and barrier crossing observed for a fluorescent O-doped polycyclic aromatic hydrocarbon. These materials are finding increasing applications in the emerging field of molecular-based organic electronics. It is shown that the oxygen atoms incorporated into the molecular backbone introduce unexpected flexibility such that the molecule functions as a fluorescent rotor. Temperature- and viscosity-dependence studies are used to calculate torsional barriers for the target molecule. Chapter 3 continues the discussion by way of considering the quest for highly fluorescent chromophores and introduces 1H,3H-isochromeno[6,5,4-mna]xanthene-1,3-dione. This compound, synthesized in-house, shows classical photophysical behaviour and has been studied in a range of organic solvents and at different temperatures. As a small molecule, with a relatively high dipole moment, this highly planar, rigid dye absorbs at around 420 nm, which is ideal for excitation with a blue laser diode, and is extremely stable towards prolonged illumination. Under near-UV excitation, the dye readily sensitises free-radical polymerisation, forming a plastic film with excellent optical v transparency. Weakly structured emission is observed with a small Stokes' shift and remains essentially insensitive to changes in solvent polarity. For example, in tetrahydrofuran the fluorescence quantum yield is 0.96 while the excited-singlet state lifetime is 7.4 ns. Quantum chemical calculations provide further insight into the electronic nature of the dye in solution. Chapter 4 represents a departure from the above approach and considers the fluorescence quantum yield of cresyl violet, a well-known emission standard for the red region. It is shown that this compound is highly susceptible to self-association in all solvents other than the smaller alcohols. The fluorescence quantum yield depends markedly on concentration, solvent polarity, temperature, etc. and we have attempted to quantify these various factors. The photophysical properties of cresyl violet are highly complicated and great caution should be exercised when using this compounds as a reference for fluorescence quantum yields. Chapter 5 describes exciton migration and surface trapping for a photonic crystal displaying charge-recombination fluorescence. The compound of interest is highly polar and the emission properties in solution depend markedly on the polarity of the surrounding solvent. Somewhat unusually, single crystals of this compound are reasonably fluorescent under near-UV illumination due to relatively slow charge recombination. The crystal can be doped with Rhodamine B, but the added dye adsorbs into a surface layer rather than interchelating into the bulk of the crystal. Excitation of the crystal leads to intense fluorescence from Rhodamine B even at astonishingly low dopant levels. Kinetic studies are used to formulate a mechanism for exciton migration and trapping. Chapter 6 describes the results collected for some water-soluble food dyes. The motivation for this work stems from a growing awareness of the need to avoid any kind of toxic compounds when developing practical applications. In fact, there are many dyes that have been used in the food industry for centuries and are believed to be essentially harmless. The cost and inconvenience inherent to testing new reagents for toxicity seems certain to cause renewed interest in these ancient dyes. Here, we explore the kinetics for photochemical bleaching of Phloxine B, Erythrosine, and Riboflavin in water. Some observations are raised regarding the relative stabilities and breakdown mechanisms. One of the systems studied here, namely the photolysis of Erythrosine, will form the basis of an undergraduate laboratory practical experiment. Chapter 7 summarises the experimental methods used throughout this work. Materials were obtained through other sources but all the spectroscopic studies were completed in-house. Fluorescence spectroscopy has been the main workhorse and we have amplified the work by examining the effects of the surrounding medium on the emission properties of the target compound. Temperature has been varied from 77K to over 200 0C using different experimental set-ups while special adapters have vi been designed for handling polymeric films or crystals. In all cases, the spectroscopic data were removed from the operating computer and analysed separately using homemade procedures. These are explained in the chapter.