Vinylogous series of merocyanines have been synthesized with indole, benzimidazole, and benzo[cd]indole as the donor termini and 1,2-diphenyl-3,5-pyrazolidinedione as the acceptor terminal group. Their absorption and fluorescence spectra have been studied in different polarity solvents. From the analysis of their solvatochromism, vinylene shifts, Brooker's deviations, and spectral band shapes, the effects of structural modifications, viz. the donor group strength and the polymethine chain length, on their electronic structures have been revealed. Based on the obtained data, the acceptor strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue has been re-evaluated, showing – contrary to the literature data – that it is not a record-strong acceptor. The (TD)-DFT/B3LYP calculations of the studied dyes have been performed in three different solvents using the PCM solvation model. From the analysis of TD-B3LYP electronic transitions, the low fluorescence of the studied dyes has been explained by the fast competitive unimolecular non-radiative decay associated with the low-lying non-fluorescent ππ*-excited state involving the molecular orbital localized on the acceptor terminal group.