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Computational Studies on Structural and Excited-State Properties of Modified Chlorophyll f with Various Axial Ligands

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journal contribution
posted on 10.11.2011, 00:00 by SRKC Sharma Yamijala, Ganga Periyasamy, Swapan K Pati
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 evidence that the red shift in the Qy 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–CH3COOH acts as a good electron acceptor.