Synthesis and photophysical properties of dimethoxybis(3,3,3-trifluoropropen-1-yl)benzenes: compact chromophores exhibiting violet fluorescence in the solid state.

Dimethoxybis(3,3,3-trifluoropropen-1-yl)benzenes were prepared through palladium-catalyzed double cross-coupling reactions of diiododimethoxybenzenes with CF(3)C≡CZnCl, followed by reduction of CF(3)C≡C groups with LiAlH(4) or H(2) in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2-(MeO)(2)-4,5-(CF(3)CHC=CH)(2) benzenes 1 and 1,3-(MeO)(2)-4,6-(CF(3)CH=CH)(2) benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4-(MeO)(2)-2,5-[(E)-CF(3)CH=CH](2) benzene ((E)-3) ended at 406 nm. These findings indicate that the effective conjugation length of (E)-3 was significantly larger than those of 1 and 2. Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)-3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1-3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1-3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)-3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO-LUMO gaps when compared with the corresponding fluorine-free analogues. In the case of (E)-3, it is suggested that the HOMO-LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp(2))-CF(3) moieties.