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The Electronic and Vibrational Spectroscopy of Metal Phthalocyanines and Metal Phthalocyanine Chlorides Isolated in Low Temperature Solids
Henchy, Christopher
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. E...
Keyword(s): Electronic; Vibrational; Spectroscopy; Metal Phthalocyanines; Metal Phthalocyanine Chlorides; Low Temperature Solids
Publication Date:
Type: Doctoral thesis
Peer-Reviewed: No
Institution: Maynooth University
Citation(s): 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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First Indexed: 2019-01-11 06:05:49 Last Updated: 2019-01-11 06:05:49