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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 helicene product. This discovery aspired further investigations
into the mechanism of helicene formation and to the first isolation of the previously
proposed intermediate of the two-step SNAr 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.