In this thesis the optical and excited state dynamics of PPY as well as the optical anisotropy of this polymer are studied. Both the PPY thin film and solution of various concentrations are used. Through studying the optics and dynamics, we clarified the controversy about the aggregate or excimer emission versus the triplet emission by giving evidence that the formation of aggregates or excimers is impossible. We showed that the photoluminescence (PL) from PPY thin film is due to triplet excitation, whereas singlet state recombination is responsible for the PL in the solution case. We ruled out the possibility of excimer and aggregates formation by observing the time-integrated PL spectra as a function of PPY concentrations. We cannot see any new radiative pathways showing coexistence o...[ Read more ]

In this thesis the optical and excited state dynamics of PPY as well as the optical anisotropy of this polymer are studied. Both the PPY thin film and solution of various concentrations are used. Through studying the optics and dynamics, we clarified the controversy about the aggregate or excimer emission versus the triplet emission by giving evidence that the formation of aggregates or excimers is impossible. We showed that the photoluminescence (PL) from PPY thin film is due to triplet excitation, whereas singlet state recombination is responsible for the PL in the solution case. We ruled out the possibility of excimer and aggregates formation by observing the time-integrated PL spectra as a function of PPY concentrations. We cannot see any new radiative pathways showing coexistence of excimers or aggregates emission at high polymer concentration. Both singlet and triplet states have distorted pyridyl ring structures in which the nitrogen atom bends up out of the planar ring. This enhanced n and π states mixing and intersystem crossing in PPY. Likewise, oxygen is also an essential part for intersystem crossing and triplet state relaxation. The PPY form complexes with oxygen that enhance the triplet emission due to spin orbit coupling. Triplet emission in PPY solution is not possible because of the much reduced oxygen concentration and protonation of PPY by the solvent molecules preventing the formation of PPY-O₂ complexes. Picosecond time-resolved spectra of PPY thin film show a red shift in PL peak in longer delay time, whereas in the solution case no red shift is observed. This result clearly shows evidence for evolution of singlet emission (450nm emission peak) to triplet emission (520 emission peak) for the thin film case. Furthermore, by means of transmittance difference spectroscopy, we found that the rod shaped PPY molecules on the spin-casted film are aligned radially outward from the center of the film. This observation is a breakthrough since most people assumed polymer molecules are randomly distributed on the film.