Utilizing Selective Chlorination to Synthesize New Triangulenium Dyes with Redshifted Absorption and Emission

Access to functionalization of a new site in the triangulenium core structure has been achieved at an early stageby chlorination with N-chlorosuccinimide (NCS), giving rise to two new triangulenium dyes (1 and 2). By introducing the chlorine functionalities in the acridinium precursor, positions complementary to those previously accessed by electrophilic aromatic substitution of the dyes are functionalized. The chlorination is highly selective giving only one regioisomer for both mono- and dichlorination. For the monochlorinated acridinium compound a highly selective ring-closing reaction was discovered generating only a single regioisomer of the cationic [4]helicene product. This discovery aspired further investigations into the mechanism of [4]helicene formation and to the first isolation of the previously proposed intermediate of the two-step S_NAr reaction key to all aza bridged triangulenium and helicenium synthesis. The fully ring closed mono- and di-chlorinated triangulenium dyes 1 and 2 show a redshift in absorption and emission relative to the non-chlorinated analogues, while still maintaining relative high quantum yields of 36% and 41 %, and long fluorescence lifetimes of 15 ns and 16 ns, respectively.