A Naphtho-p-quinodimethane Exhibiting Baird's (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence.

Abstract

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 μs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.