Benzo[e]indolium derivatives in aqueous solutions: Reaction with bisulfite and successive interaction with Cu²⁺ and...

A new benzo[e]indolium derivative 1 including pyridyl and thienyl groups was synthesized and characterized, which failed to response to Hg²⁺ or Cu²⁺ in aqueous system. However, it is interesting that when it reacted with bisulfite in HEPES buffer, the in situ generated ensemble as 1-SO₃H displayed dramatic absorption and fluorescence changes after adding Cu²⁺ or Hg²⁺, which were contrary to the changes of 1 upon the addition of bisulfite ions. By contrast, the other two derivatives 2 and 3 showed the almost similar trends of spectral changes, which were short of ligating atoms N or S. The further investigation of ¹HNMR spectra changes of 1 showed that C-SO₃H bond may be interrupted because the good binding capacity of Hg²⁺ and Cu²⁺ with O of C-SO₃H weaken the binding force of C-SO₃H, which resulted in the recovery of ethylene with benzo[e]indolium block. The DFT calculations results further confirmed it.