Resonance Raman scattering study and vibrational analysis of metalloporphyrin analogues

Abstract

In this thesis, the electronic, vibrational, vibronic properties of free base and metal complexes of octaethyltetraazaporphyrin (OETAP) were studied by resonance Raman (RR), off-resonance FT-Raman, infrared and UV-visible absorption spectroscopies. The vibrational properties of the nickel complex of OETAP (NiOETAP), a typical four coordinated, fluorescence free D_4h metallo-OETAP, were studied in detail with various spectroscopic methods. The ¹⁵N( 8 ) , ¹³C_α(8), and ²H(40) isotope shifts, together with a normal coordinate analysis, lead to a relative complete assignment for the observed vibrational bands. It has been shown that, unlike in normal metalloporphyrins, two lowest single-electronic π-π* excited states (B and Q states) in NiOETAP are not strongly vibronically coupled. On the other hand, the strong intrastate (Q-Q) vibronic coupling (Jahn-Teller coupling) does exist, inducing very strong depolarized bands in the Q- band excited RR spectra. The free base octaethyltetraazaporphyrin (H₂OETAP) and its ¹⁵N(8) , ¹³C_α(8), and ²H(40) substituted derivatives were studied and compared with NiOETAP. As a result of the symmetry lowering from D_4h to D_2h, splitting of the absorption bands were observed in both UV-visible and infrared spectra from NiOETAP to H₂OETAP. All of the strongly enhanced RR bands of H₂OETAP were found to be polarized (0< p <1/3), implying the predomination of Frank-Condon mechanism. The assignments of the observed vibrational bands of H₂OETAP were also proposed. On the basis of isotope shift and the comparison with NiOETAP, the influence of the interaction between metal ions and OETAP macrocycle on the spectroscopies of the complexes was investigated by extensive metal substitutions. The correlation relationship between the vibrational frequencies and the core-sizes of the complexes was established. Combined analysis of the core-size sensitivities and normal mode compositions reveals that, similar to the normal metalloporphyrins, the dependencies of the frequencies on the core-sizes of the complexes are parallel to the contributions of the C_αN_m stretching coordinate to the individual modes. Spectra were recorded for iron-OETAP complexes in various spin, oxidation, and ligation states. The vibrational frequency shifts upon the changes of spin, oxidation and ligation states were observed and rationalized. The Raman and IR spectroscopies demonstrate that the vibrational states of [Fe(II)OETAP]-(1-MeIm)₂ and [Fe(II)OETAP]-(py)₂ are dramatically different from other metallo-OETAPs, indicating an abnormal ground state geometry for a six coordinated low-spin ferrous complex comparing with other metallo-OETAPs. On the other hand, the parallel study on Co(II)OETAP shows that the Raman spectra of its -(1-MeIm)₂ and -(py)₂ adducts are similar to the normal metallo-OETAPs. In addition, the vibrational and vibronic properties of tetraoxaporphyrin (TOP ²⁺) and octaethyltetraoxaporphyrin (OETOP ²⁺) dications were studied by vibrational spectroscopies. The FT-IR and off-resonance FT-Raman spectra indicate that the "exclusion rule" is hold for TOP ²⁺ and partially hold for OETOP ²⁺, in accordance with the X-ray results of a D_4h symmetry for TOP ²⁺ and a "pseudo D_4h" symmetry for the "porphyrin core" of OETOP ²⁺. The Raman spectra obtained under near-resonance condition with B-band electronic are dominated by strong polarized bands for both TOP ²⁺ and OETOP ²⁺. However, several dp bands were also clearly observed, indicating the possible vibronic coupling. Partial assignments for the observed vibrational bands were proposed by comparing with the well-established vibrational assignments of their normal metalloporphyrin counterparts.

Date of Award	1997
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology