# Tuning the Photophysical Properties of Nickel and Zinc Complexes of N‑Confused Tetraphenylporphyrin via Trans–Cis Isomerization

## TL;DR

This paper studies how changing metals and molecular structures affects the light-related properties of certain porphyrin complexes.

## Contribution

The study reveals how metal choice and isomerization tune the photophysical properties of N-confused porphyrin complexes.

## Key findings

- Nickel and zinc complexes show different lowest energy tautomers due to metal influence.
- Cis isomers of M-NCTPP-p are more stable than trans isomers due to van der Waals interactions.
- Absorption peaks of M-NCTPP-p are red-shifted compared to M-NCTPP by up to 135 nm for Q bands.

## Abstract

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 = ZnII and NiII, 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 MII 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 (a), while the
zinc complexes have the H atom outside of the porphyrin core attached
to N (b). For M-NCTPP-p, the global minimums are a.cis (NiII) and b.trans (ZnII). 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.