Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy

Delocalization of
charges is one of the factors controlling charge
transport in conjugated molecules. It is considered to play an important
role in the performance of a wide range of molecular technologies,
including organic solar cells and organic electronics. Dimerization
reactions are well-suited as a model to investigate intermolecular
spatial delocalization of charges. While dimerization reactions of
radical cations are well investigated, studies on radical anions are
still scarce. Upon dimerization of radical anions with neutral counterparts,
an electron is considered to delocalize over the two molecules. Here,
by using time-resolved infrared (TRIR) detection coupled with pulse
radiolysis, we show that radical anions of 4-n-hexyl-4′-cyanobiphenyl
(6CB) undergo such dimerization reactions, with an electron
equally delocalized over the two molecules. We have recently demonstrated
that nitrile ν­(CN) vibrations respond to the degree
of electron localization of nitrile-substituted anions: we can quantify
the changes in the electronic charges from the neutral to the anion
states in the nitriles by monitoring the ν­(CN) IR shifts.
In the first part of this article, we show that the sensitivity of
the ν­(CN) IR shifts does not depend on solvent polarity.
In the second part, we describe how probing the shifts of the nitrile
IR vibrational band unambiguously confirms the formation of dimer
radical anions, with Kdim = 3 × 104 M–1. IR findings are corroborated by electronic
absorption spectroscopy and electronic structure calculations. We
find that the presence of a hexyl chain and the formation of π–π
interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. The present study provides
clear evidence of spatial delocalization of electrons over two molecular
fragments.