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    The Electronic and Vibrational Spectroscopy of Matrix-Isolated Phthalocyanines – Experiment and Theory

    Murray, Ciaran Anthony (2010) The Electronic and Vibrational Spectroscopy of Matrix-Isolated Phthalocyanines – Experiment and Theory. PhD thesis, National University of Ireland Maynooth.

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    Phthalocyanines (Pcs) are robust macrocyclic tetrapyrrolic molecules having distinctive dark blue to green colours. Despite their importance in industry, their optical spectroscopy has yet to be fully characterised. Low-temperature solids consisting of the rare gases or inert materials like nitrogen offer an ideal environment in which to study these molecules. The resulting spectral bands in inert cryogenic matrices are narrow and well resolved with relatively small shifts from the gas phase values. The aims of the work presented in this thesis are to record the vibrational and luminescence spectroscopy for free-base phthalocyanine (H2Pc) and zinc phthalocyanine (ZnPc) isolated in various inert low-temperature matrices and with the assistance of extensive theoretical calculations, assign the spectral features observed. The infrared absorption spectra of matrix-isolated ZnPc and H2Pc have been recorded in the region from 400 to 4000 cm-1 in solid N2, Ar, Kr and Xe. The isotopomers HDPc and D2Pc were synthesised in an attempt to resolve the conflicting assignments that currently exist in the literature for the N-H bending modes in H2Pc spectra. Comparison of the spectroscopic results obtained with isotopic substitution and with predictions from large basis set ab initio density functional theory (DFT) calculations, allow identification of the in-plane (IP) bending mode of H2Pc at 1045 cm-1 and the out-of-plane (OP) bending mode at 765 cm-1. The antisymmetric N-H stretching mode observed at ~3310 cm-1 in low temperature solids is slightly blue shifted from, but is entirely consistent with the literature KBr data. The vibrational modes of the experimental spectra of H2Pc and its deuterium isotopomers were correlated using DFT and the ratios of the lighter H atom frequencies to those of their heavier D atom equivalents (νH/νD) were determined. With the exception the N-H stretches, the recorded H/D isotope shifts in all the N-H vibrations are complex with the IP bending modes exhibiting small νH/νD ratios (the largest value is 1.089) while one of the observed OP modes has a νH/νD ratio < 1. DFT results reveal the small ratios arise in particular from strong coupling of the N-H IP bending modes with IP stretching modes of C-N bonds. The unexpected behaviour of νH/νD ratios is analysed through the examination of the continuous theoretical evolution of the frequencies with the mass of H. A consequence of this frequency increase in the heavier isotopomer is that the direction of the N-D OP bend is reversed from the N-H OP bend. The Raman scattering data recorded for ZnPc and H2Pc in KBr discs are also analysed and found to be quite similar and moreover, identical to the visible fluorescence of matrix-isolated Pcs. The visible absorption, emission and excitation spectra of ZnPc and H2Pc trapped in Ne, N2, Ar, Kr and Xe matrices have been recorded. The visible absorption peaks for the Q band of ZnPc and the Qx and Qy transitions of H2Pc have revealed matrix shifts from the gas phase values and distinct sites of isolation. The spectral positions of the vibronic bands in emission with pulse dye laser excitation for both Pcs have been identified. A comparison of the fluorescence spectra with Raman spectra in KBr pellets has revealed very strong similarities. This is entirely consistent with the selection rules and points to the occurrence of only fundamental vibrational transitions in the emission spectra corresponding to those in the ground state. This favourable comparison between Raman and fluorescence spectra has allowed the vibronic modes of ZnPc and H2Pc coupling to the electronic emission to be assigned using DFT calculated vibrational spectra. A mirror symmetry between the vibronic structures of ZnPc has been observed in emission and excitation, indicating similar geometries in the ground and excited states. The vibrational modes of ZnPc in the excited state have therefore been assigned using the same DFT Raman vibrations determined for the ground state. For the excitation spectra of H2Pc, the mirror symmetry with emission has been seen to breakdown after ~950 cm-1 due the onset of the higher energy Qy state absorption. A matrix dependence has been found for the Qx-Qy energy splitting varying from 916 cm-1 in Xe and 985.3 cm-1 in Ar. In a comparison of the H2Pc excitation and emission spectra recorded in this work and those previously reported for D2Pc, a tentative assignment of the vibrational mode in the Qx state coupling to the Qy has been made using DFT to a weakly Raman active mode of B1g symmetry consisting of an in-plane bending motion of the central N-H bonds. The vertical excitation energies and oscillator strengths of H2Pc and ZnPc as well as those of free-base and zinc tetraazaporphyrin (TAP), tetrabenzoporphyrin (TBP) and porphine (P) have been calculated with linear-response time-dependent DFT utilizing the B3LYP hybrid functional and 6-311++G(2d,2p) basis set. The theoretical results for the lowest energy transition have been compared to experimental data and have been found to correctly predict many of the trends apparent for these molecules. One of the major discrepancies that has been observed between the TD-DFT and experimental transition energies was the underestimation of the theoretically determined Qx-Qy splitting of H2Pc and the relative intensities of these bands. Laser induced fluorescence spectroscopy of H2Pc and ZnPc phthalocyanines trapped in rare gas and nitrogen matrices reveals a quite unexpected phenomenon with a moderate increase in the laser intensity. For these molecules in all matrices, except for ZnPc in Xe, a huge increase in the intensity of a one particular emission band has been observed when pumping the S1 ← S0 transition. The band involves a vibrational mode of the ground state, located at 1550 and 1525 cm−1 for H2Pc and ZnPc, respectively. This vibration has been assigned in both phthalocyanines using DFT to the most intense Raman active mode involving an out-of-phase stretching of the C-N-C bonds in the tetrapyrrole ring. Many of the characteristics of amplified emission (AE) are exhibited by this vibronic transition and the threshold conditions have been investigated. In light of the success of the DFT Raman spectra to correctly predict both the positions and relative intensities of the vibronic emission bands of H2Pc and ZnPc, the optimized geometries, vibrational frequencies and Raman scattering intensities have been calculated for a selection of other structurally related tetrapyrrolic molecules and their potential for exhibiting AE assessed. Excitation scans recorded for the AE band show greatly enhanced site selectivity compared to what is obtained in normal fluorescence excitation scans.

    Item Type: Thesis (PhD)
    Keywords: Electronic and Vibrational Spectroscopy; Matrix-Isolated Phthalocyanines;
    Academic Unit: Faculty of Science and Engineering > Chemistry
    Item ID: 2585
    Depositing User: IR eTheses
    Date Deposited: 17 Jun 2011 16:53

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