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

The spectroscopic properties of chlorophylls (Mg-Chl a and H-2-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 e(6) trimethyl ester (Zn-Chl e(6) 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 pi orbitals in M-Chl a. The semiempirical MO calculation demonstrated that the deviation of H-2-Chl a from the general trend is induced by rising of the energy levels of next HOMO and second next HOMO in H-2-Chl a. A comparison of the W-VIS and MCD spectral data in zinc(II) chlorophylls revealed that the substituents on the pyrrole beta 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 pi orbitals calculated by the conventional MO method. (C) 1997 Elsevier Science B.V.