Computational studies on structural and excited-state properties of modified chlorophyll f with various axial ligands

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll f (Chlide f), Chlide a, Chlide b, and axial ligated (with imidazole, H(2)O, CH(3)OH, CH(3)COOH, C(6)H(5)OH) Chlide f molecules. The computed differences among the Q(x), Q(y), B(x), and B(y) band absorbance wavelengths of Chlide a, b, and f molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl f molecules. Our computations provide evidence that the red shift in the Q(y) band of chl f is due to the extended delocalization of macrocycle chlorin ring because of the presence of the -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (lowest unoccupied molecular orbital (LUMO) of the Chlide f and LUMO-1 of the Chlide b). All the absorption bands of Chlide f shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide f-imidazole acts as a good electron donor and Chlide f-CH(3)COOH acts as a good electron acceptor.