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    The Electronic and Vibrational Spectroscopy of Metal Phthalocyanines and Metal Phthalocyanine Chlorides Isolated in Low Temperature Solids


    Henchy, Christopher (2016) The Electronic and Vibrational Spectroscopy of Metal Phthalocyanines and Metal Phthalocyanine Chlorides Isolated in Low Temperature Solids. PhD thesis, National University of Ireland Maynooth.

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    Abstract

    Phthalocyanines (Pcs) are an important class of dye molecules, capable of incorporating a wide range of metal atoms into their macrocyclic cavity and can be heavily substituted on their outer carbons. These highly stable molecules play an important role not only in the commercial dye industry, but have also fuelled the development of many other technologies due to their attractive physical and chemical properties. Given their potential as light harvesting molecules, dye lasers and in photodynamic therapies, it is important to have a detailed understanding of the optical properties of the phthalocyanines. The matrixisolation technique provides an excellent opportunity to perform a detailed spectroscopic study of these molecules. Isolation of a molecule in an inert gas solid such as the rare gases or N2 at cryogenic temperatures allows for the acquisition of highly resolved spectra with very narrow bandwidths and the absence of hot bands or rotational bands. Furthermore, the positions of the bands are only slightly shifted from gas phase values due to the weak interaction between the host and the guest species. The aim of the work presented in this thesis is to provide novel insights into the spectroscopy of matrix-isolated phthalocyanines, with particular attention given to the gallium phthalocyanine chloride (GaPcCl) molecule. The most significant findings in the current work pertain to the luminescence spectroscopy and amplified emission of matrix-isolated GaPcCl. A vibrational analysis is performed on the ground electronic state of the molecule (as well as the related molecules MgPc and AlPcCl), which is used to assign the vibronic bands observed in emission and excitation. The infrared absorption spectra of matrix-isolated MgPc, AlPcCl and GaPcCl are recorded in the region from 400 to 4000 cm-1 in solid Ar and N2. Comparison of the spectroscopic results with predictions from large basis set ab initio density functional theory (DFT) calculations allow for mode assignments to be made. The most intense bands in the spectra correspond to A2u (A1) and Eu (E) modes associated with out-of-plane and in-plane motions of the macrocycle and hydrogens of MgPc (AlPcCl/GaPcCl). The high frequency modes arising from the C-H stretching modes were not observed in either matrix, but have been tentatively assigned in results from KBr discs. Two metal dependent vibrational modes were observed for AlPcCl; one at 491 (488) cm-1 in Ar (N2), and another at 519 cm- 1 in both Ar and N2. A single metal dependent vibrational mode was observed in MgPc, located at 505 (504) cm-1 in Ar (N2). The remaining metal dependent modes for these two molecules, and all of those from GaPcCl, lie in the low frequency region below 400 cm-1 and out of the range of the infrared detectors. The Raman scattering data recorded for MgPc, AlPcCl and GaPcCl in KBr discs are also analysed and found to be quite similar. The Raman active modes tend to correspond to in-plane distortions of the macrocycle, with the most intense bands being of B1g (B2) symmetry for MgPc (AlPcCl/GaPcCl). The Raman scattering spectra of GaPcCl are noted for being very similar to the fluorescence data. The visible luminescence spectra of GaPcCl trapped in N2, Ar, Kr and Xe matrices are recorded and analysed. The visible absorption in the region of the Q band clearly show the effect of matrix shifts in different hosts, although no gas phase data exists for comparison. Resolved vibronic bands are observed to the blue of the 0-0 transition of the Q band, where there is evidence for site structure. The less resolved B bands are recorded in the UV region, and are weaker than the fully allowed Q transition. A set of very weak bands are observed to the red of the B bands in each host. The vertical excitation energies and oscillator strengths of GaPcCl (as well as a number of other M-Pc and M-TAP molecules) are calculated with TD-DFT utilizing the B3LYP hybrid functional and 6-311++G(2d,2p) basis set. The theoretical results correctly predict the trend of the strong Q band in the visible and the weaker B band in the UV region of the spectrum. The predicted bands are typically blue-shifted with respect to the experimental results. A number of very weak bands were consistently predicted in the 350 – 500 nm region for all of the M-Pcs and MTAPs. These features are also observed in the experimental absorption spectra of matrixisolated GaPcCl and warrant further investigation. Emission spectra of matrix-isolated GaPcCl are recorded with pulsed dye laser excitation. Vibronic bands in emission extend up to ~1600 cm-1 from the 0-0 transition. A comparison of the absorption (excitation) and emission spectra shows obvious mirror symmetry, indicating the molecule has a similar structure in its ground and excited states. A vibrational analysis of the excited electronic state is performed given the similar vibronic structure in emission (and absorption/excitation) and the ground state Raman modes. Emission lifetimes are measured and found to be in the 2.3 – 2.6 ns range which is expected for a strongly absorbing dye molecule. The lifetimes are thermally stable and not strongly host dependent but do exhibit a small wavelength dependence. The existence of multiple sites of isolation is revealed in emission, with Ar matrices showing the best examples of this due to the narrow bandwidths of its emission peaks. Sites are also observed in Kr and Xe, but are less resolved than in Ar. The broad emission bands observed in N2 also indicate the presence of multiple but unresolved sites. Sites are also evident in excitation, where the position of the 0-0 band (and thus the vibronic bands) shift depending on which emission wavelength is being monitored. With a moderate increase in the laser intensity, the laser induced fluorescence spectroscopy of GaPcCl trapped in rare gas and nitrogen matrices exhibits some unusual behaviour. In all matrices, a huge increase in the intensity of one particular emission band is observed when pumping the S1 (Q) ← S0 transition. This band involves a vibrational mode of the ground state, located at 1540 cm-1 (for GaPcCl/Ar) and from DFT calculations is assigned to the most intense Raman active mode involving an out-of-phase stretching of the bridging Cα-Nm-Cα bonds. Many of the characteristics of amplified emission (AE) are exhibited by this vibronic transition, and thus the threshold conditions and lifetime of this emission band are investigated. The narrow bandwidths of the AE bands allow for the identification and classification of phonon structures in emission and excitation, as well as the resolution of individual sites. A Wp function is used to analyse resolved phonon structure (the zero phonon line and phonon side bands) of GaPcCl/Ar AE spectra. 2D Excitation-emission spectra are employed to reveal the excitation and emission features in each host material. Ar matrices give the richest spectroscopy, showing a continuum of sites between two dominant species. The isolated monomer is abundant in this host, although a significant amount of aggregates are also present. Kr matrices tend to contain less of the isolated monomer and higher amounts of the aggregate, whereas Xe appears to contain only the aggregate species. Both Kr and Xe contain two dominant features, similar to what is observed in Ar. The N2 matrix differs from the inert gas matrices in that only a single dominant feature is present, although it is evident that this feature contains several unresolved sites located very close in energy. DFT calculations are performed on a series of M-TAPs and M-Pcs to investigate the effect the metal atom has on the structure of the porphyrin scaffold. Two situations are observed – one in which the metal fits comfortably into the macrocycle cavity and another where the metal is forced to sit above the plane of the porphyrin ring. As the metals bind to all four of the pyrrole nitrogen atoms the symmetries of these structures are found to be D4h and C4v for the planar and non-planar molecules respectively. Analysis of several metals with different atomic radii shows that the size of the atom is the sole property governing whether or not the macrocycle can incorporate a given metal. The non-planar structures also show evidence of a doming of the porphyrin macrocycle, which gets progressively more pronounced as the size of the metal atom increases. Where experimental values are available, DFT calculations agree to within 3% of measured bond lengths and angles. Calculations on the M-PcCl and M-Pc+ molecules (M=Al/Ga) show that a counter-ion can pull the metal from the central cavity and cause a small doming effect. DFT calculations are used to analyse the occurrence of reverse isotope shift ratios (ISR) in H/D substitution of the free-base tetrapyrroles, whereby the frequency ratio νH/νD is less than 1. The reverse ISR effect is found to be most evident in the out-of-plane bending modes (B2g and B3u symmetry) involving some N-H motion for the four molecules studied; porphyrin (H2P), tetraazaporphyrin (H2TAP), tetrabenzoporphyrin (H2TBP), and phthalocyanine (H2Pc). This effect was analysed by following the evolution of the normal mode frequencies with incremental variation of the H atom masses from 1 to 2 amu. This method allows direct, unambiguous mode correlations to be established between the light and the heavy isotopologues. When the NH(D) motion is predominant, the H to D frequency evolution decreases in a continuous manner for a particular normal mode. In the case of two modes of the same symmetry and whose frequencies are similar, their frequency evolutions could cross, depending on the extent of NH(D) motion involved in them. The evolution diagrams may show avoided crossings of various extents, which thereby reflects the degree of the NH(D) motion in the modes. The reverse ISR effect is directly correlated to these avoided crossings.

    Item Type: Thesis (PhD)
    Keywords: Electronic; Vibrational; Spectroscopy; Metal Phthalocyanines; Metal Phthalocyanine Chlorides; Low Temperature Solids;
    Academic Unit: Faculty of Science and Engineering > Chemistry
    Item ID: 10419
    Depositing User: IR eTheses
    Date Deposited: 10 Jan 2019 15:24
    URI:

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