Optical spectroscopy of porphyrin-like systems (1999)
AuthorsPrince, Barry J.show all
This thesis presents the results of absorption, magnetic circular dichroism (MCD), selective excitation, fluorescence-detected MCD (FDMCD), total luminescence (TL), magnetic circularly polarised luminescence (MCPL) and spectral hole-burning (HB) studies of zinc porphyrin derivatives isolated in argon matrices. These spectroscopic studies have focused on the lowest-energy * transition of these molecules; the Q transition. These studies have been driven by two main areas of interest in matrix-isolated metalloporphyrins; the degree to which intermolecular coupling occurs in such matrices, and the role of the Jahn-Teller (JT) effect in these systems. Spectra collected mark the first examples of the application of the selective MCPL and FDMCD, the latter of which has the potential, not only to provide superior resolution over conventional MCD, but also, in some cases, to give an enhancement in S/N. The construction of a spectrometer was necessary for the collection of these spectra and comprised a significant portion of the work. The development of a new matrix deposition technique, in conjunction with selective excitation, TL and HB studies of the Q-band of zinc phthalocyanine, has provided evidence of the existence of interacting pairs of molecules in this matrix, The population of these 'coupled-pair' sites, which absorb to the red side of the Q-band origin, increases if the matrix in annealed, and at the same time the efficiency of HB is also increased, These results also provide information regarding the mechanism of HB in these systems, which is thought to involve intermolecular triplet-state charge separation followed by singlet recombination. Three approaches were adopted for the study of the JT effect in the Q (1Eu) states of these systems. A program (JTCFZ) was written to generate and solve the E b vibronic Hamiltonian matrix and to simulate the absorption and MCD spectra for an E A transition of a system with a single effective JT-active vibrational mode in the D4h pointgroup. Although this model provides insight into many of the observed characteristics of these systems, its inability to account for the finer details of the spectra of a number of systems points to an inadequacy of the single-mode approach. Normal coordinate analysis was used in conjunction with a semi-empirical molecular orbital (MO) calculation to approximate the JT displacements (JT) of the Q state along all potentially JT-active modes of ZnPc. The results indicate that there are many modes with significant JT values, reinforcing the view that the effective singlemode model cannot adequately describe these systems. This conclusion is further supported by selective FDMCD spectra of ZnPc/Ar and ZnTBP/Ar for which the sign of totally symmetric vibrational overtones is opposite to that of JT-active overtones. As predicted by the calculations, many JT-active overtones of varying intensities appear throughout the entire Q-band envelopes of these systems. FDMCD spectra of ZnPc/Ar has also provided evidence for the assignment of a transition (Q') underlying the higher vibrational overtones of the Q band. The spectra collected clearly demonstrate that a previous assignment of this transition to 1A1u 1A1g is not correct and, with additional support from semi-emperical MO calculations, it has been reassigned as 1Eu 1A1g.