Exploring the Photochromic Performance of Two Viologen-Based Complexes

This research examines the structural characteristics and light-responsive color change behavior of two viologen-derived compounds, (HEQ)(H 2 BTEC)·(H 4 BTEC) 0.5 ·3H 2 O ( 1 ) and [Zn(EQ)(BTEC) 0.5 (H 2 BTEC) 0.5 ·3H 2 O] n ( 2 ) (EQBr is 1-ethyl-4,4′-bipyridinium bromide, and H 4 BTEC is 1,2,4,5-benzenetetracarboxylic acid). Ultraviolet–visible (UV–vis) absorption and electron paramagnetic resonance spectroscopy demonstrate reversible photochromism for both systems, with compound 2 exhibiting a faster response rate and stronger radical signals. Powder X-ray diffraction analysis demonstrated that the structures of both complexes remained intact throughout the color change and recovery cycles, suggesting that the photochromic behavior is a result of photoinduced electron transfer. First-principles calculations provide a clear electronic-level explanation: the band structure of 2 displays a narrower band gap (1.051 vs 1.806 eV in 1 ) and greater band dispersion, while density of states analysis shows enhanced overlap of O-p and N-p orbitals near the Fermi level. These features shortened donor–acceptor distances and promoted efficient charge transfer from carboxyl oxygen to viologen nitrogen centers, thereby accelerating radical generation and stabilizing the photoinduced state. The combined experimental and theoretical evidence highlights that dimensional extension from zero-dimensional (0D) to two-dimensional (2D) enhances orbital coupling and charge mobility, offering a rational strategy for designing high-performance photochromic materials.