  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.
