Controlling Intramolecular Electronic Communication through the Conformation Changes via Stepwise Oxidations in Dicopper(II) and Dinickel(II) Porphyrin Dimers

Abstract

Herein, we report a shape-dependent intramolecular electronic communication in neutral and oxidized complexes of dinickel(II) and dicopper(II) porphyrin dimers. Upon careful manipulation of the reaction conditions, nano-size molecules having different shapes, namely ‘linear’, ‘cofacially sloping’, and ‘clamshell bucket’, were synthesized by varying the concentration of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Detailed structural, spectroscopic viz. UV-vis-NIR, NMR, EPR, ESI-MS, VT magnetic study, and extensive computational investigation have been exploited to investigate long-range electronic communication. The varying shapes and extent of conjugation of the porphyrin dimers are responsible for several prominent attributes, including colours, polarity, π-conjugation, intensity of NIR absorption bands and narrow HOMO−LUMO gaps, etc. Notably, the addition of carbonyl moieties at the methine bridge in ‘clamshell bucket’ facilitates stronger electronic communication through it followed by the ‘linear’ and ‘cofacially sloping’ complexes. Upon stepwise oxidations, highly stable mono-cation radical and di-cation di-radicals were isolated that exhibited long-range charge/radical delocalization via the bridge to produce strong NIR bands. The spin-density plots for the neutral, 1e–, and 2e– oxidised complexes demonstrate the crucial role of metal ions, the bridge, and their shapes in long-range electronic communication. -Conjugation between two macrocycles in the 2e–-oxidized complexes is better in the linear ‘butterfly-like’ molecule, followed by curved ‘cofacially sloping’ and carbonyl-inserted 'clamshell bucket’, while nickel(II) dominates over copper(II) motifs. Notably, the distinct signatures of the triplet state in the EPR spectra were observed due to the magnetic interaction between the two Cu(II) centres in the 2e–-oxidized complexes. The carbonyl group acts as a good π-mediator in the oxidized complex for through-space intramolecular communication between the two rings.