Density Functional Theory Calculations of the Lowest Excited Triplet State of the Closest Analogs of Chlorophyll and Bacteriochlorophyll

Density functional theory PBE/TZVP calculations of the geometric structures in the ground singlet state S 0 and the first excited triplet state T 1 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 S 0 → T 1 transition and as the molecular structure becomes more complex (mainly due to cyclopentanone ring V formation). The symmetry of MgC in the T 1 state is lowered as compared to the C 2v symmetry in the S 0 state while MgBC has D 2h 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 T 1 states of the studied molecules are calculated. The computed E T 1 values for MgPhe are 11,400, 10,850, and 10,200 cm –1 for the vertical S 0 → T 1 transition taking into account optimization of the geometry in the T 1 state and changes of the zero-point vibrations at the S 0 → T 1 transition, respectively, and agree well with the experimental value of 10,310 cm –1 (for chlorophyll a). The computed E T 1 values for MgBPhe are 8350, 8100, and 7700 cm –1 while the experimental value is 8190 cm –1 (for bacteriochlorophyll a).