A DFT study of the optical properties of substituted Zn(II)TPP complexes

A series of Zn tetraphenylporphines and Zn porphines with conjugated functional groups at the β-position have been studied using DFT and TDDFT methods (B3LYP). Comparison of calculated optical transitions with experimental data shows that the calculations predict trends in the optical absorption spectra with change of functional group and with increase in conjugation chain length. The effect of the electron-withdrawing substituent is to decrease the configuration interaction effect by breaking the degeneracy of the two lowest unoccupied MOs. Other configuration interaction effects come into play involving the LUMO+2 which in the substituted porphyrins is lowered in energy and the HOMO-2 which is raised in energy. Interrupting the conjugation of the functional group is shown to mitigate the breakdown of the configuration interaction.