The Photophysics of Three Naphthylmethylene Malononitriles

The solvent dependence of the photophysical properties of three naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedinitrile (1-MN), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2-(3,4-dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-MN), was studied in order to determine their potential utility as fluidity probes and to make comparisons to the better studied benzylidene malononitriles. Density functional calculations were used to understand the possible conformational states related to rotation about the vinyl-aromatic bond ("τ"). Absorption and emission frequencies, extinction coefficients, fluorescence quantum yields, and fluorescence lifetimes were measured in 11 representative solvents. Both the computational and experimental results indicate that the S0 → S1 transitions of these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result from relaxed S1 states which do not differ qualitatively from the Franck-Condon states reached by absorption. In 2-MN, time-resolved emission reveals the presence of two ground-state conformers ("a" and "b" differing by ∼180° rotation about τ) coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer. Fluorescence lifetimes vary from ∼1 ps in 1-MN to ∼200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a), the lifetimes vary systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence decay in naphthylmethylene malononitriles is likely to be the same as that of the benzylidene malononitriles-twisting about the double bond in S1, which leads to rapid internal conversion via a conical intersection with S0.