Theoretical investigation on the electronic properties and UV–Vis absorption spectra of expanded porphyrin mono-Cu(II) complexes

Abstract

The metalation chemistry of expanded porphyrins exhibits wide-ranging applications in numerous fields. In the work, we have investigated the electron spin density, charge transfer, frontier molecular orbitals and UV–Vis absorption spectra of four expanded porphyrin mono-Cu(II) complexes (Cu(II)@P, Cu(II)@HP, Cu(II)@OP and Cu(II)@d-OP) using density functional theory (DFT) method and time-dependent DFT. The results show that Cu(II)@HP has different electronic properties and absorption spectra compared with other complexes. What is more interesting is that the single electrons of Cu(II)@HP are not only distributed on the Cu atom and the four coordination N atoms, but also on the uncooridnation conjugated pyrroles. However, the single electrons of other complexes are mainly distributed on the Cu atom and the four coordination N atoms. This phenomenon can be successfully explained by the specific mechanism of Cu(II)@HP unlike the Zn(II)@HP. Meanwhile, the calculated values of extended charge decomposition analysis indicate that beta electrons of Cu(II)@HP have more transfer, and its frontier molecular orbitals demonstrate greater involvement of Cu(II) ion compared with other complexes. The particular properties give rise to the different absorption spectra of Cu(II)@HP. The maximum wavelength of Cu(II)@HP displays an unusual red-shift (over 100nm) compared with Cu(II)@d-OP. The absorption spectra of Cu(II)@HP can be assigned to π→π∗ transition and metal-to-ligand charge transfer characteristic. Emission wavelengths corresponding maximum absorption wavelengths of Cu(II)@HP is 610nm that show Stokes shift with 27nm. The aim of this work is to provide valuable information to contribute to synthesis of new metal complexes of expanded porphyrins and optical materials.