Tuning Wavefunction Mixing in Push–Pull Molecules: From Neutral to Zwitterionic Compounds

A new push–pull molecule, containing an (alkylamino)thiophene as electron donor and 1,3-bis(dicyanomethylidene)-2- indenylindene as electron acceptor, has been synthesized and characterized in the solid state by X-ray crystallography, in solution by UV/Vis and NMR spectroscopy and by theoretical calculations. The quinoidal zwitterionic form of this compound was underscored both in the solid state and in moderately polar solvents. Experimental and theoretical analyses were also performed on two similar push–pull molecules, selectively chosen to vary basic chemical features such as the esonance energy of the aromatic group and the steric hindrance of the attractor. Comparative analysis of the three compounds has shown that steric hindrance, resonance en- ergy and the polarity of the environment play a significant role in stabilizing one limiting resonance form over the other. By varying these chemical variables, fine tuning of the electronic ground state of a push–pull molecule, from the neutral aromatic form to the charge-separated zwitterion, passing through a form in which the two limiting forms contribute almost equally (the cyanine limit), can be achieved.