Morphology and photophysical properties of phenyleneethynylene oligomer

The influence of annealing on the morphologies and optical properties of phenyleneethynylene pentamer films has been investigated by using optical microscopy, UV-vis spectroscopy, steady-state and time-resolved photoluminescence (PL). The optical microscopy study shows that the grainy morphology of crystallites with irregular shapes and sizes of the spincoated oligomer films reorganize into well-ordered single crystal-like laths or bamboo-like structures upon melt-crystallization or annealing. The crystalline features of the annealed films were simulated and modeled through molecular mechanics (MM) and molecular dynamics (MD) and further measured by X-ray diffraction. These studies revealed that the molecular backbones form lamellar and interdigitated conformations. Investigation of optical properties of the thermally treated films indicated that aggregates and excimers dominate excitation/emission processes due to molecular closely packing upon annealing, which are evidenced by the broadening, structureless absorption and luminescence spectra, as well as longer lifetime of PL dynamics.