Influence of a second donor and of rotational isomers on the solvatochromic properties of ketocyanine fluorophores

The solvatochromic behaviour of the donor-acceptor-donor system 1,5-bis[4-(diethylamino)phenyl]penta-1,4-dien-3-one (I), and two closely related donor-acceptor systems (1-[4-(diethylamino)phenyl]-5-phenylpenta-1,4-dien-3-one (II) and 1-[4-(diethylamino)phenyl]but-1-en-3-one (III)) was examined in 15 organic solvents. Two rigid analogues of the cyclopentadienone series were examined too. The UV/Vis spectroscopic data were correlated using different polarity scales. It was shown that the presence of a strong second donor induced the Vis absorption maximum to shift to higher wavelengths, and bestowed high sensitivity with respect to the solvent hydrogen-bond donor character. In return, the fluorescence solvatochromism was much reduced. Compounds I, II, and III exist as a mixture of conformational isomers. Therefore, the consequences of this equilibrium upon the solvatochromic properties were investigated through comparison with the blocked analogues. Rigidity resulted in a decrease of sensitivity to solvent variations.