For photochromic molecules, the ability to switch reversibly between two states is controlled largely by structural effects. The photodimerization of 5-methyluracil, commonly known as thymine, a member of the pyrimidine family and one of the DNA bases, is a particularly important reaction in photobiology, one which occurs when the molecule is subjected to UV irradiation. Dimerization takes place through the C5=C6 bond of thymine and results in the formation of a cyclobutane ring. Various analogues of thymine behave in a similar manner. In solution, for the majority of molecules in the pyrimidine family, excitation is accompanied by an accumulation of lone electrons at C5 and C6 and a marked reduction in bond order. Dimerization occurs through the generation of a singlet state, followed by intersystem crossing and decay to a triplet state, from which the dimer is formed. The lifetime of the excited triplet state is much longer than the singlet state and energy is normally dissipated by chemical routes, since there is ample opportunity for molecules to diffuse and collide in solution. When a molecule is tethered at an interface, reaction can readily occur through the singlet state. The impact of this is that uracils which do not photoreact in solution, i.e., 5-nitrouracil, will photoreact when immobilized in the solid state. To produce photoactive materials that display both high sensitivity and reversibility, it is necessary to control the orientation of the thymine molecule. Nature dictates this in the case of DNA, and only the cis-syn isomer is formed upon dimerization. The cis-syn dimer exhibits the greatest ease of photosplitting, apparently due to steric repulsion of the methyl groups attached to C5 in thymine. In the case of photoactive materials, the necessary orientation can be achieved through the formation of a thin solid film. Irradiation with monochromatic light at 280 nm leads to dimerization, a process which can be reversed again by irradiation at 240 nm. Depending upon the nature of the molecule this process can be performed reversibly over many cycles. Photosplitting in the case of thin solid films is apparently induced by crystal lattice strain. Inefficiencies in the photosplitting process are caused by misalignment in the thin solid films and/or the formation of a photoinactive conformer. One very attractive way of obtaining the appropriate orientation at an interface is through the formation of self-assembled monolayers (SAMs) or through the formation of thin solid films by alternative techniques such as dip-coating. The first objective of the thesis, was to form such films, exploring how chemical structure influenced the photodimerization process and associated surface physical properties. Photoresponsive surfaces were prepared by attaching synthesized pyrimidine-terminated molecules to flat gold substrates (as thiol self-assembled monolayers) or quartz surfaces (by dip-coating). Both types of films underwent photodimerization (two pyrimidine rings react with one another and form a cyclobutane type dimer through the C5=C6 double bond) when irradiated with light of 280 nm wavelength. The reverse reaction was carried out by irradiating the dimerized surface with light of 240 nm wavelength. The photoinduced chemical changes are accompanied by a change in the physical properties of the surface (e.g., wettability and acidity), and are highly dependent on the structure of the photoactive molecules. The surface dimerization reaction follows a pseudo-first order reaction. The rate constant is determined by the structure of the pyrimidine headgroup. In self-assembled monolayers, uracil derivatives dimerize faster than thymine derivatives due to a reduced steric repulsion near the reaction center. In dip-coated films, however, uracil derivatives appear to be less ordered and, correspondingly, the efficiency of the reaction is lower. The reaction rate is also very sensitive to the ordering within the layer, which can be manipulated through the structure of the tail. In SAMs, faster dimerization occurs with molecules containing flexible chains. In dip-coated films, the presence of a polar group at the chain terminus favors dimerization. The second objective was to explore how photosensitizers could be used to alter the wavelength at which dimerization occurred, giving greater flexibility to the entire process. Thin solid films of thymine and uracil derivatives, incorporating a photosensitizer in the film, were prepared on quartz surfaces using a dip-coating technique. Photosensitized dimerization in this thin solid film was investigated for the first time and compared with the conventional, non-photosensitized reaction. p-Aminobenzoic acid (PABA) and N-dodecyl PABA (PABA-C12) were selected as photosensitizers. It was found that PABA and PABA-C12 were effective in causing the photosensitized dimerization of thymine, upon exposure to light of wavelength 313 nm. Uracil surfaces, however, do not photodimerize under these conditions. The concentration of photosensitizer and the solvent selected in the dip-coating process strongly influenced the dimerization process. The reaction rate of sensitized dimerization follows a pseudo-first order reaction.