The modulation effect of pi-pi interactions on the electronic and photochromic properties of viologen complexes containing N , N '-bis(carboxyethyl)-4,4'-bipyridinium.

Two viologen complexes containing N , N '-bis(carboxyethyl)-4,4'-bipyridinium (BCEbpy) were prepared, namely [Zn(H 2 O) 6 ]·(BCEbpy)·( p -BDC)·3H 2 O (1) and [Co(H 2 O) 6 ]·(BCEbpy)·( p -BDC)·3H 2 O (2) ( p -H 2 BDC = 1,4-benzenedicarboxylic acid), and their crystal structures, photochromism, frontier molecular orbitals, Hirshfeld surfaces and 2D fingerprint plots were investigated. The modulation effects of pi-pi interactions were explored on the electronic and photochromic properties of compounds 1 and 2. Due to the existence of photo-response viologen radicals, both complexes 1 and 2 display excellent photo-response properties in the sequence 1 < 2. The results indicate that compound 1 exhibits intramolecular electron transfer; compound 2 exhibits both intramolecular and intermolecular electron transfer, which is mainly due to the change of electronic and steric structures caused by pi-pi interactions with a faster photo-response rate than that of compound 1. The donor-acceptor modes, matching principles and inter/intramolecular atom-atom close contacts were illustrated by the density functional theory (DFT)-B3LYP/6-311(d,p) method.