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

Abstract

Abstract Several sulfur‐bridged heterocyclic carbocations of the acridinium and triangulenium family were prepared using 9‐(2,6‐dimethoxyphenyl)‐1,8‐dimethoxy‐thioxanthenium tetrafluoroborate [ 7 ][BF 4 ] as key synthetic intermediate. Thanks to simple yet not so facile aromatic nucleophilic substitution reactions, thiachromenoacridinium, azaoxa‐ and dioxathiotriangulenium salts (e.g., [ 8 ][BF 4 ], [ 9 ][BF 4 ], and [ 10 ][BF 4 ], respectively) were obtained in moderate to good yields. Importantly, from the crystal structural analysis of 9‐(2,6‐dimethoxyphenyl)‐1,8‐dimethoxy‐9 H ‐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 S N Ar reactions. A comparison between electronic absorption spectra of the sulfur‐bridged dyes and their aza analogues was also carried out. A ca. 50 nm shift towards the low energy region was found to occur upon the insertion of the sulfur atom. Copyright © 2010 John Wiley & Sons, Ltd.