Regioselective Counterion Association Governs Organohalogenochromism in Ferrocenyl Ionic Push-Pull Chromophores

Abstract

Two organometallic cationic D-π-A dyes incorporating ferrocene donors and pyridinium or quinolinium acceptors were synthesized and evaluated for their organohalogenochromic (OHC) behavior in halogenated and non-halogenated solvents. Both dyes exhibited pronounced bathochromic shifts in halogenated media that cannot be explained solely by conventional solvent polarity effects. Triflate titration experiments demonstrated that ICT transition energies are directly controlled by counterion association, while ¹H NMR measurements revealed a regioselective interaction centered at the azaheterocyclic acceptor. Benesi–Hildebrand analysis yielded association constants of 4.82 × 10^–2 M^–1 and 1.38 × 10^–2 M^–1 for the pyridinium and quinolinium derivatives, respectively. Complementary T₁ measurements and TD-DFT calculations further supported a solvent-dependent ion-pairing model. Collectively, the results indicate that organohalogenochromism arises from solvent-controlled modulation of ion-pair equilibria, highlighting regioselective counterion association as a key factor governing the optical response of ferrocenyl ionic push-pull chromophores.