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

The density functional theory PBE/TZVP calculations of the geometric structures in the ground singlet state S₀ and the first excited triplet state T₁ have been carried out for molecules of Mg pheophorbide a (MgPhe) (i.e., for chlorophyll α, devoid the phytol “tail”), Mg bacteriopheophorbide α (MgBPhe), as well as Mg chlorin (MgC) and Mg bacteriochlorin (MgBC). It follows from the comparison of the bond lengths for MgC-MgPhe and MgBC-MgBPhe pairs that the central macrocycle of the first pair is more unstable to “splittings” of equivalent bond lengths both as a result of the S₀→T₁ transition and as the molecular structure becomes more complex (mainly due to cyclopentanone ring V formation). It is found that the symmetry of MgC in the T₁ state decreases as compared to the C_2v symmetry in the S₀ state while MgBC has the D_2h symmetry in both states. These peculiarities can relate to that in whole the central n system of the MgC-MgPhe pair is antiaromatic (contains 24 electrons which corresponds to the Htickel 4n rule), and the central n system of the MgBC-MgBPhe pair is aromatic (consists of 22 electrons and satisfies the Htickel 4n+2 rule). The energies of the T₁ state of the molecules studied are calculated. For MgPhe the computed values of E _T1 are 11400, 10850, and 10200 cm^-1 for the vertical S₀→T₁ transition, taking into account the optimization in the geometry in the T1 state and additionally taking into account the changes of the zeropoint vibrations at S₀→T₁ transition, respectively, that is in very good agreement with the experimental value of 10310 cm^-1 (for chlorophyll a). For MgBPhe, the corresponding calculated values ET1 are 8350, 8100, 7700 cm^-1, while the experimental one is 8190 cm^-1 (for bacteriochlorophyll α).

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