Spectroscopic properties of chlorophylls and their derivatives. Influence of molecular structure on the electronic state

The spectroscopic properties of chlorophylls (Mg-Chl a and H₂-Chl a), metallochlorophyll a and b (M-Chl a and b, M = Ni, Cu and Zn), zinc(II) pyromethylpheophorbide a (Zn-PMP a) and zinc(II) chlorin e₆ trimethyl ester (Zn-Chl e₆ TME) were studied by circular dichroism (CD), magnetic circular dichroism (MCD), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The electronic absorption spectra were assigned in terms of the spectral analysis of CD and MCD. The absorption maximum of the Q_y band varied linearly with the electronegativity of the central metal M and the energy level of the macrocyclic π orbitals in M-Chl a. The semiempirical MO calculation demonstrated that the deviation of H₂-Chl a from the general trend is induced by rising of the energy levels of next HOMO and second next HOMO in H₂-Chl a. A comparison of the UV-VIS and MCD spectral data in zinc(II) chlorophylls revealed that the substituents on the pyrrole β position and the ring V govern the position of the Q and B bands. The spectroscopic behavior was explained on the basis of the energy level of the macrocyclic π orbitals calculated by the conventional MO method.