# Stimuli-Responsive Thiele’s Hydrocarbon Derivatives: Potential Inversion, Strong Electronic Coupling, and Influence of Brønsted/Lewis Acids and Bases

## TL;DR

This paper explores new nitrogen-containing hydrocarbon derivatives that change properties in response to stimuli, offering potential for functional materials.

## Contribution

The study introduces new diradicaloid systems with tunable properties and demonstrates their stimuli-responsive behavior.

## Key findings

- Increasing nitrogen atoms in the molecular backbone leads to consistent changes in absorption and redox properties.
- Compound 2PhRC is a borderline class II–III mixed-valence species with strong electronic coupling and low thermodynamic stability.
- Coordination with acids reversibly alters the optical responses of 2Py and 2Pz.

## Abstract

Stimuli-responsive
diradicaloid systems often display fascinating
and tunable electrochemical, optical, and magnetic properties. Herein,
we present the design and synthesis of a series of nitrogen-containing
Thiele’s hydrocarbon derivatives (2

Ph
, 2

Py
, and 2

Pz
), based on phenyl, pyridinyl,
and pyrazinyl spacers, with tunable electrochemical, electronic, and
optical properties. By systematically increasing the number of nitrogen
atoms in the molecular backbone, we observed consistent trends in
both their absorption and their redox properties. A detailed investigation
on the different oxidation states of 2

Ph
, which displays potential inversion, was also performed, including
the intermediate radical cationic state (2

Ph

RC). This intermediate represents an
intriguing example of a mixed-valent species whose thermodynamic stability,
as calculated by the comproportionation constant (K
c), is diminishingly small, whereas the electronic coupling
is strong, making it a borderline class II–III mixed-valence
compound. External modulation through coordination with Brønsted
and Lewis acids reversibly altered the optical responses of 2

Py
and 2

Pz
. This work highlights both structural and stimuli-responsive
strategies for tuning organic diradicaloid systems, thereby opening
new avenues for their applications in functional materials and coordination
chemistry.