Variation of Donor and Acceptor Strength in Analogues of Brooker’s Merocyanine and Generalization to Various Classes of Charge Transfer Compounds

Derivatives of Brooker’s merocyanine (BM) have been investigated, which possess different donors and acceptors and therefore vary their donor–acceptor strength SDA. The 00 energies have been extracted from the spectra and compared. Under basic conditions, where the neutral (merocyanine) form is present, the absorption energies for all compounds are similar, whereas there is a large difference for acidic conditions where the cationic (cyanine) form is present. This behavior could be explained by a simple theoretical model involving the dependence of the excitation energy ΔE01 on SDA. This model can be generalized to describe in a consistent way two different well-known classes of neutral chromophores with a certain degree of charge separation, namely merocyanine I (TICT) and merocyanine II (often betainic) compounds. Merocyanines I are characterized by a medium polar aromatic ground state and a zwitterionic quinoid excited state and hence positive solvatochromism, whereas merocyanines II are formally characterized by a zwitterionic aromatic ground state and a less polar quinoid excited state and, accordingly, by negative solvatochromism. On increasing the donor–acceptor strength SDA sufficiently, merocyanines II can, however, move to the so-called overcritical region with the excited state dominated by the zwitterionic valence bond structure. For many of the merocyanine II molecules investigated here, a weakly positive solvatochromic behavior is observed indicating that the ground state contains less of the zwitterionic valence bond wave function than the excited state and that these compounds belong to the overcritical region. The fluorescence spectra have been analyzed in terms of the Franck–Condon model and confirm these conclusions.