Journal of Applied Spectroscopy | 2021
Density Functional Theory Calculations of the Lowest Excited Triplet State of the Closest Analogs of Chlorophyll and Bacteriochlorophyll
Abstract
Density functional theory PBE/TZVP calculations of the geometric structures in the ground singlet state S0 and the first excited triplet state T1 were carried out for Mg pheophorbide a (MgPhe) (i.e., chlorophyll a devoid of the phytol tail ), Mg bacteriopheophorbide a (MgBPhe), Mg chlorin (MgC), and Mg bacteriochlorin (MgBC). A comparison of the bond lengths for the pairs MgC–MgPhe and MgBC–MgBPhe found that the central macrocycle of the first pair is more unstable to splittings of equivalent bond lengths as a result of the S0 → T1 transition and as the molecular structure becomes more complex (mainly due to cyclopentanone ring V formation). The symmetry of MgC in the T1 state is lowered as compared to the C2v symmetry in the S0 state while MgBC has D2h symmetry in both states. These peculiarities are related to the central π-system of the MgC–MgPhe pair being antiaromatic (containing 24 electrons corresponding to the Huckel 4n rule) and the central π-system of the MgBC–MgBPhe pair being aromatic (containing 22 electrons corresponding to the Huckel 4n + 2 rule). The energies of the T1 states of the studied molecules are calculated. The computed $$ {E}_{T_1} $$\n values for MgPhe are 11,400, 10,850, and 10,200 cm–1 for the vertical S0 → T1 transition taking into account optimization of the geometry in the T1 state and changes of the zero-point vibrations at the S0 → T1 transition, respectively, and agree well with the experimental value of 10,310 cm–1 (for chlorophyll a). The computed $$ {E}_{T_1} $$\n values for MgBPhe are 8350, 8100, and 7700 cm–1 while the experimental value is 8190 cm–1 (for bacteriochlorophyll a).