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.