Ultrafast studies of a photochromic oxazine in solution

This dissertation is composed of two studies. Ultrafast studies of a photochromic oxazine are the main project of the dissertation. An independent study that describes the anisotropic behavior of the phenylene ethynylene dendrimers is also presented.;Photochromic molecules alter their structure and electronic properties upon excitation by optical light sources. The photogenerated form of the photochromic molecule can return to its original form either thermally or photodynamically. If the reverse reaction is thermal, the photochromic cycles of the molecules can be achieved simply turning on and off the light source. Photochromic molecules with the ability to restore the output level in each cycle are good candidates for molecular switches. The rate of switching cycles is an important property of the photochromic molecules that needs to be known for the implementation of the molecular switches.;In this study, we worked with a novel photochromic oxazine which exhibits photochromism by a ring opening reaction. The C-O bond cleavage in the ring opening reaction occurs very fast and has to be investigated by ultrafast spectroscopy. The ultrafast measurements are complemented with preliminary steady state experiments.;To explore the electronic properties of the states involved in the ring opening mechanism, the experiments are performed in the presence and the absence of benzophenone, a good triplet sensitizer, and also they are measured in solutions of different solvents (acetonitrile and hexane).;Two distinctive transient absorption bands are observed in the experiments. One of the bands, which is formed at around 505 nm, raises within 250 femtosecond (fs) and decays with 2.2 ps and 3.7 ps time constants. This band is assigned to a charge separated state. The second band, which is absorbing at around 440 nm, is attributed to the open form of the photochromic molecule. The open form of the molecule is produced indirectly from the charge separated state with 12.5 ps time constant and directly from first excited with 6.3 ps time constant and remains unchanged at least 650 ps. Although, the band belonging to a charge separated state appears and disappears within the rise time of the ring opened form, it was proved that this state does not make a significant contribution to the production of the open form of the photochromic oxazine.;The minor study that completes the dissertation is on the anisotropic behaviors of unsymmetrical phenylene ethyneylene dendrimers. The unsymmetrical Phenylne Ethynylene denderimers we studied exhibited low excitation anisotropy values along the excitation spectra at room temperature and at 77K confirming energy transfer from the longer conjugated segments to shorter ones with different transition moments. The excitation anisotropy of these molecules at 77 K exhibited a complex behavior suggesting the presence of more than one electronic state contributing to the anisotropic behavior.