Rigidified Push–Pull Dyes: Using Chromophore Size, Donor, and Acceptor Units to Tune the Ground State between Neutral and the Cyanine Limit

A series of rigidified dyes with either dodecyloxy or piperidyl donors in combination with either malononitrile or barbiturate acceptors were synthesized. To analyze the influence of varying the donor (piperidyl vs. dodecyloxy), chromophore (mono‐, bis‐, or tricyclic system), and acceptor moiety (methyl barbiturate vs. malononitrile) on the ground‐state polarity, a combination of several methods was employed: UV/Vis absorption and emission, quantum chemical computations, 1H as well as 13C NMR spectroscopy, and cyclic voltammetry measurements. Depending on the acceptor and donor moieties, the ground state was shifted from a neutral form to the cyanine limit and further to a zwitterionic structure. When the dye had strong piperidyl donor and barbiturate acceptor substituents, the ground state was close to the cyanine limit, which resulted in strong cyanine‐like absorption properties with a dominant 0–0 transition. When the dye combined a weak dodecyloxy donor with a malononitrile or barbiturate acceptor, the neutral resonance form significantly contributed to the ground state, leading to weaker but broader absorption spectra featuring transitions to higher vibronic states. The ground‐state polarity of rigidified dyes was probed by using UV/Vis absorption and emission, DFT calculations, 1H and 13C NMR spectroscopy, and CV measurements. Variation of donor (dodecyloxy and piperidyl), acceptor (malononitrile and barbiturate), and chromophore sizes influenced the absorption maxima, relative strength of vibronic transitions, solvatochromism as well as the redox behavior owing to a shift of S0 from a neutral form over the cyanine limit to a zwitterionic state (see figure).