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

Abstract

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll ƒ (Chlide ƒ), Chlide a, Chlide b, and axial ligated (with imidazole, H₂O, CH₃OH, CH₃COOH, C₆H₅OH) Chlide ƒ molecules. The computed differences among the Q_x, Q_y, B_x, and B_y band absorbance wavelengths of Chlide a, b, and ƒ molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl ƒ molecules. Our computations provide evidence that the red shift in the Q_y band of chl ƒ 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 ƒ and LUMO-1 of the Chlide b). All the absorption bands of Chlide ƒ shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide ƒ -imidazole acts as a good electron donor and Chlide ƒ -CH₃COOH acts as a good electron acceptor.