| Popis: |
As the limitations of fossil fuels and the effects they have on the environment become more apparent, the need for new alternative energy sources is becoming increasingly obvious. One potential source of renewable energy converts solar energy using organic and inorganic based photovoltaic (PV) cells into electrical energy. In this dissertation, I have used fluorescence spectroscopy to study two photoactive organic compounds that have been shown to undergo self-assembly, and two organic-inorganic hybrids that were also designed for use in PV cells. I also developed a new technique to study sub-picosecond fluorescence anisotropy. The organic compounds studied in this work were amphiphilic N,N-disubstituted naphthalene diimides that were shown to self-assemble into highly ordered aggregates that eventually formed nanostructures such as twisted nanotapes, twisted nanoribbons, and nanotubes. In this work, picosecond time correlated single photon counting (TCSPC) was used to investigate the fluorescence lifetimes and time-dependent fluorescence anisotropies nanostructures. For two of these compounds (Dipeptide B and Bola 1), the fluorescence lifetimes (tfl ~153 ps for Dipeptide B and tfl ~313 ps for Bola 1) were over an order of magnitude longer than that of a simple naphthalene diimide (N,N-dibutyl naphthalene diimide, tfl ~16.4 ps). The lifetime results were consistent with energy migration within highly ordered nanostructures. Further, the time-resolved fluorescence anisotropy results indicated an ultrafast depolarization of the fluorescence signal that could only be attributed to energy migration along a twisted or circular structure.The second portion of this work involved organic-inorganic hybrids consisting of a capped cadmium sulfide nanoparticle forming nanocomposites with a homo-poly(phenylene-ethynylene) polymer and an alternating co-poly(phenylene-ethynylene) polymer. In this work, picosecond TCSPC was used to investigate the average luminescence lifetimes of the nanoparticles, the two polymers and the two composites at two different excitation wavelengths (290 nm for the nanoparticle and 325 for the polymers). Generally, the average luminescence lifetimes were found to be longer with an excitation of 325 nm compared to the excitation at 290 nm. The individual nanoparticles and polymers had the greatest changes in the luminescence lifetimes with the different excitation wavelengths, but a significant difference in the luminescence lifetimes was not observed with the polymer nanocomposites with the change in excitation wavelength. The third and final portion of this dissertation involved the design of a new technique to study sub-picosecond fluorescence anisotropy. This new technique employs the current femtosecond laser spectroscopic setup with the addition of an optical Kerr gating medium and a Photoelastic Modulator (PEM). The optical Kerr gating medium used the non-linear properties of a solvent’s birefringence to obtain sub-picosecond accuracy and the PEM was used to change the polarization of the excitation beam to the parallel (I_¿) and perpendicular (I_¿) polarizations needed to observe time-resolved fluorescence anisotropy. Coumarin 540A was used to test the system and I observed the initial anisotropy of Coumarin 540A to be 0.31 and had an anisotropic decay of r(t) = 0.08 exp(-t/3.2 ps) + 0.23 exp(-t/44.1 ps). |
