Deeply coloured and highly fluorescent dipolar merocyanines based on tricyanofuran

Three vinylogous series of merocyanines with the tricyanofuran (TCF) acceptor and different electron-donor groups have been synthesized, and their absorption and fluorescence spectra have been studied in different polarity solvents to reveal the polyene-polymethine transitions of their electronic structures. The TCF dyes with the indole or benzimidazole donor groups have relatively bright fluorescence in the red and near-IR spectral ranges. From the obtained data, the electron-acceptor ability of the TCF group has been estimated to be close to that of the thiobarbituric residue, while the TCF group is less prone to formation of solute-solvent H-bonds in protic media. From the DFT and TD-DFT calculations of the TCF-based dyes, performed using B3LYP, CAM-B3LYP, and M06-2X hybrid functionals, it has been revealed that their two major conformers have very close spectral-fluorescence properties, which explains why they are undistinguishable in the steady-state electronic spectra. However, the calculated polarizabilities and the first hyperpolarizabilities are more dependent on the molecular geometry, with both parameters being greater for the transoid conformer.