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, H2O, CH3OH, CH3COOH, C6H5OH) Chlide f molecules. The computed differences among the Qx, Qy, Bx and By 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 an evidence that the red shift in Qy band of chl f is due to the extended delocaliztion of macrocycle chlorin ring due to the presence of -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (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 -CH3COOH acts as a good electron acceptor.