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

The spectroscopic properties of chlorophylls (MgChl a and H2Chl a), metallochlorophyll a and b (MgChl a and b, M  Ni, Cu and Zn), zinc(II) pyromethylpheophoribide 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 Qy 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.