Correlated Rotations and Unusual Fluorescence Properties of peri ‐Substituted, Axially Chiral Naphthyl Ketones

The dynamics of the two rotors in 8‐(dimethylamino)naphthyl ketones possessing a tetrahedral amino group and a trigonal planar carbonyl group has been investigated by 1 H NMR spectroscopy. The dynamic system is described by an aryl−nitrogen and an aryl−carbonyl rotation and a possible amine inversion. As the two rotors, which are twisted in the ground state, are tightly interlocked, the question arises of how strongly the processes might be correlated and how strongly the electrostatic interactions would influence the barriers. In the case of the isopropyl, tert ‐butyl , and benzyl ketones, the rotors are coupled sufficiently strongly that correlated rotations become energetically preferred over independent rotations of the individual groups. For the methyl and trimethoxyphenyl ketone, the Ar−N and Ar−C barriers could be distinguished because of the lesser degree of interlocking of the substituents. The low barriers and the similar Ar−C and Ar−N rotations, despite the differences in substituents, are due to the pseudo‐rotation and inversion of the tetrahedral NMe 2 group. Such a process dramatically reduces the steric and electrostatic hindrance in the transition state and also lessens an energetically costly twist of the naphthalene plane. The larger steric hindrance of a complete inversion partly compensates for the weaker electrostatic repulsion, and so a trigonal‐planar transition state is suggested. This additionally allows larger peri distances. The 1,8‐donor/acceptor‐substituted naphthalenes showed unusual fluorescence properties; increases in solvent polarity led to large anomalous Stokes shifts and to a decrease in fluorescence. Methyl 8‐(dimethylamino)naphthalene‐1‐carboxylate produced an anomalous Stokes shift of Δ $\tilde {\nu}$ = 18354 cm −1 .