On the synthesis and optical properties of sulfur-bridged analogues of triangulenium cations and their precursors

Several sulfur-bridged heterocyclic carbocations of the acridinium and triangulenium family were prepared using 9-(2,6-dimethoxyphenyl)-1,8-dimethoxy-thioxanthenium tetrafluoroborate [7][BF4] as key synthetic intermediate. Thanks to simple yet not so facile aromatic nucleophilic substitution reactions, thiachromenoacridinium, azaoxa- and dioxathiotriangulenium salts (e. g., [8][BF4], [9][BF4], and [10][BF4], respectively) were obtained in moderate to good yields. Importantly, from the crystal structural analysis of 9-(2,6-dimethoxyphenyl)-1,8-dimethoxy-9H-thioxanthen-9-ol 16, bond lengths between sulfur and carbon atoms were confirmed to be much longer than that of classical C-O or C-N in analogous frameworks. This clearly triggers a distortion of the sulfur-containing six-membered ring which can be invoked to explain the relative lack of stabilization of the resulting carbenium ions. The increased electrophilicity of the central carbon set off a strongly reduced reactivity towards SNAr reactions. A comparison between electronic absorption spectra of the sulfur-bridged dyes and their aza analogues was also carried out. A ca. 50nm shift towards the low energy region was found to occur upon the insertion of the sulfur atom. Copyright (C) 2010 John Wiley & Sons, Ltd.