Bistability of valence tautomeric donor−acceptor dyads formed by covalently linking a ferrocene-based electron-
donor and the perchlorotriphenylmethyl radical, as the electron-acceptor, is tuned via a chemical modification of the ferrocene
group. Specifically, the methylation of the ferrocene unit increases the strength of the donor group stabilizing the zwitterionic
state in polar solvents and leading to an intriguing coexistence of neutral and zwitterionic species in solvents of intermediate
polarity. Bistability in the crystalline phase is demonstrated by temperature dependent Mössbauer spectra. This complex and
interesting behavior is quantitatively rationalized in the framework of a bottom-up modeling strategy. Optical spectra in solution
are first analyzed to extract and parametrize an effective two-state molecular model, which is then adopted to rationalize the
observed bistability in the solid state as due to cooperative electrostatic interchromophore interactions.
