Ultra-fast spectroscopy investigation on organic molecules two-photon absorption

In this thesis, ultra-fast spectroscopy was utilized to investigate the structureproperty of organic materials with high two-photon absorption (TPA) cross-section for potential applications. In summary, four significant areas have been worked on including detailed background reviews of TPA, theoretical expression and modification of two TPA cross-section measuring techniques, two experimental setups for the sensitive determination of TPA cross-section and TPA properties of two newly synthesized compound series with high TPA cross-section.;The structure-property of 1D D-pi-D quadrupolar oligophenylenevinylenes (OPVs) OPV(n) showed they had high TPA cross-sections (>1000GM) and also high fluorescence quantum yield (>80%). For example, OPV(6) in CHCl3 had 1203GM at excitation of 750nm with 84% quantum efficiency. The relationship of TPA cross-sections and conjugation length was not linear and was closer to quadratic. However, enlarging conjugation length caused red-shifts both in the absorption and emission spectra. In addition, polarization analysis of OPV(n) showed that amplified spontaneous emission (ASE) appeared in the vertical emission. The amplification of vertical emission to horizontal emission was about 3.66 for vertical excitation and 1.19 for horizontal excitation at peak power of 0.475GW/cm2. However, the amplified signal was not too strong to form a lasing effect. The vertical ASE excitation was more efficient because OPV(n) was probably sensitive on gravitational force. Therefore, OPV(n) complexes can be applied as horizontal indicators.;The structure-property of multi-dipolar zig-zag oligoaryleneethynylenes (OAEs) DA(n) and D-Ar-A(n) can easily be modified or enhanced by incorporating or extending with various central aryleneethynyl moieties within the dipolar units. The experimental results showed that OAEs exhibited a linear increase in TPA cross-section with increased number of covalently linked dipolar units without red-shifting the absorption and emission spectra. Hence, TPA cross-section can easily be enhanced at a specific wavelength by means of increasing the number of dipolar unit within a zig-zag molecule. Furthermore, the largest TPA cross-sections was obtained for D-TF-A(4) in DCM solvent to be 1522.3GM at 750nm which showed potential for practical applications.