Синтез і спектральні властивості мероціанінів на основі 9H-флуорен-2,7-дикарбонітрилу

Di-, tetra-, and hexamethine merocyanine dyes bearing donor heterocyclic end groups of different electron-donating abilities and the 9H-fluorene-2,7-dicarbonitrile moiety as the acceptor end group have been synthesized. Their UV/Vis absorption spectra have been studied in solvents of varying polarity, and their electronic nature and vertical transitions have been investigated via (TD)-DFT calculations. The results indicate that the electronic structure of these merocyanines approaches the neutral polyene limit, becoming increasingly polyene-like in low-polarity solvents and upon increasing the polymethine chain length, which indicates the weak electron-acceptor ability of the 9H-fluorene-2,7-dicarbonitrile moiety. Nevertheless, longer vinylogs, especially those containing the 4H-pyran donor end group, exhibit the inverse solvatochromic behavior, which is highly unusual for such weakly dipolar merocyanines. A possible explanation for this effect has been proposed although its rigorous verification would require higher-level quantum-chemical calculations with solvent effects taken into account.