Fluorescence Properties of (<i>E</i>,<i>E</i>,<i>E</i>)‑1,6-Di(<i>n</i>‑naphthyl)-1,3,5-hexatriene (<i>n</i> = 1, 2): Effects of Internal Rotation

The fluorescence spectroscopic properties of (E,E,E)-1,6-di­(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λa) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λf) (1, 545 nm; 2, 453 nm) and quantum yields (ϕf) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λex), whereas the spectrum of 2 was weakly λex-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λa = 437–438 nm, λf = 496–505 nm, ϕf = 0.04–0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck–Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)–CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar–CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.