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Subject: search.filters.subject.Chromophores

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    Optical properties of organic NLO chromophores for terahertz applications : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Palmerston North, New Zealand
    (Massey University, 2012) Kutuvantavida, Yasar
    This thesis was motivated by the potential of organic nonlinear optical (NLO) zwitterionic chromophores to be used as active components in THz emitters and detectors. This study presents the results from measurements of the electro-optic (EO) coefficient, photostability and the THz properties of organic NLO chromophores. An EO coefficient of over 700 pm/V was measured for some EO polymers using the Teng-Man modulation ellipsometry technique at low frequencies. A roll off in the value of the EO coefficient was observed for frequencies over 10 kHz and converging to the theoretically estimated value of ∼ 2 pm/V. The large over estimate of the EO coefficient at low frequencies using the Teng-Man method is attributed to contributions from the cavity resonance and interference effects. The EO coefficient was also measured using a new method based on the electric field induced modulation of diffraction gratings. The measured EO coefficient using this technique is an order of magnitude lower than that measured using the Teng- Man method but it is an order of magnitude higher that the theoretically expected value. By investigating DC and AC measurements it was possible to deduce that the large apparent EO coefficient is due to the inverse piezoelectric effect and thin film interference. The large EO coefficient at low frequencies may also have a contribution from dipole clusters. The small EO coefficient at high frequency shows that the chromophores may not be particularly useful for THz applications. However, they have potential in low frequency applications such as modulators, and dense wavelength division multiplexing. The current photodegradation models are shown to be inadequate and the photodegradation can be modeled by considering a distribution in the photodegradation quantum efficiencies. The addition of a singlet oxygen quencher has been found to enhance the photostability of some chromophores by up to 6 times. Structural modifications also affect the photostability. A correlation between the photostability and bond length alternation (BLA) was found where there is a trade-off between the chromophores molecular NLO effect and photostability. Calculations based on the THz dielectric properties showed that the EO polymers have a reasonable coherence length for THz generation using optical rectification. Low wavenumber Raman spectroscopy measurements made on recrystallised compounds exposed some theoretically predicted modes that were not been reported found experimentally. Low temperature and low energy Raman spectroscopy measurements made on some selected organic compounds showed a strong temperature dependence of the low energy vibrational modes. The temperature dependent redshift and line broadening have been primarily attributed to thermal expansion and a distribution in the distance between monomers.
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    Supramolecular helical arrangement of porphyrins along DNA : a thesis submitted in the partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry, Massey University, Palmerston North, New Zealand
    (Massey University, 2010) Stephenson, Adam Wayne Ian
    Porphyrins are useful chromophores and have been used in numerous biological applications including light harvesting, oxygen transport and energy transfer. DNA is a perfect template for the controlled assembly of organic chromophores. By combining DNA and porphyrins in a controlled manner we have developed a novel range of porphyrin-DNA supramolecular constructs for future applications in nanobiotechnology. A number of β-pyrrolic functionalised porphyrin precursors were synthesised to be used as building blocks in the construction of both covalently and non-covalently modified DNAs. Using these porphyrins we have created several lipophilic porphyrin-DNA complexes through non-covalent attachment methods. Using a CuI catalysed azide alkyne cycloaddition (CuAAC) reaction of azido functionalised porphyrins we have developed a versatile approach for the covalent, site specific internal porphyrin insertion into oligonucleotides in a post-synthetic manner. We have investigated a number of duplex structures where porphyrins were located in the major or minor grooves of the duplex. Additionally, porphyrins were studied as intercalating moieties when they were inserted as a bulge in the middle of the duplexes or parallel triplexes. Additionally, when porphyrins were placed in both strands of the duplex they formed a zipper type structure in the minor groove. This resulted in a significant increase in the duplex thermal stability due to the formation of porphyrin H-aggregates. UV-Vis and CD spectroscopy as well as molecular modelling were used to help understand the interactions between porphyrins in the duplex. These findings lay the foundation for the future design of artificial DNA-chromophore supramolecular architectures and for their applications in material science and nanotechnology.