Tuning the Photophysical Properties of Nickel and Zinc Complexes of N-Confused Tetraphenylporphyrin via Trans-Cis Isomerization.

Porphyrins are detected in many biological systems and have significant roles in some important artificial systems, while the N-confused porphyrins present very interesting photophysical and chemical properties, which differ from those observed in porphyrins. In the present study, metal (M) complexes of tetraphenylporphyrin (TPP), N-confused TPP (NCTPP), and the ethenyl-pyrazine derivative of NCTPP (NCTPP-p), i.e., M-TPP, M-NCTPP, and M-NCTPP-p, where M = Zn and Ni, were studied via density functional theory (DFT) and TD-DFT calculations. The photophysical properties of molecules and their absorption spectra are studied. It has been found that the M affects the relative stability of the M-NCTPP and M-NCTPP-p tautomers, resulting in different tautomers having the lowest energy structure, while for the M-NCTPP-p molecule, there are cis isomers, which are lower in energy than the corresponding trans isomers due to the van der Waals interactions. The global minima of the nickel complexes have the H atom of a reversed pyrrole attached to C (), while the zinc complexes have the H atom outside of the porphyrin core attached to N (). For M-NCTPP-p, the global minimums are (Ni) and (Zn). The absorption main peaks of M-NCTPP-p are red-shifted compared to M-NCTPP up to 80(135) nm for the Soret(Q) bands. The different isomers present shifts of their main absorption peaks up to 50(180) nm. Additionally, the vertical de-excitation energies from selected excited states are also investigated. Overall, the selection of the metal and the peripheral group lead to different lowest values in the energy structure, affect its UV-vis absorption spectrum, and thus they tune the photophysical properties of the M-NCTPP complexes.