Mechanism of the chemiluminescence of biisoquinolinium salts

Abstract

1,1′-Biisoquinolinium salts (BIQ²⁺2X^–) are known to exhibit chemiluminescence on addition of hydrogen peroxide in an alkaline solution. Reduction of BIQ²⁺ with Na₂S₂O₄–Na₂CO₃ or Na–Hg gave the radical cation BIQ^+˙ and then the neutral two-electron reduced species, 1,1′-biisoquinolylidene (BIQ), which was characterised by mass spectrometry, UV–VIS and ¹H NMR spectroscopy. This electron-rich olefin instantaneously reacted with molecular oxygen showing a blue luminescence in organic solvents. Treatment of BIQ²⁺ or BIQ^+˙ with KO₂ in dimethyl sulfoxide (DMSO) or acetonitrile also caused light emission. The chemiluminescence spectra of both BIQ–³O₂ and BIQ²⁺–KO₂ reactions were the same as that of BIQ²⁺–H₂O₂–OH^– and the main product was an isoquinolinone derivative in either case. In the case of 2,2′-ethylene-bridged BIQ²⁺, the chemiluminescence spectrum coincided with the fluorescence spectrum of the corresponding biisoquinolinone. A plausible reaction pathway for the chemiluminescence of the BIQ²⁺–H₂O₂–OH^– system is as follows: BIQ²⁺ is reduced by electron donors such as OH^– and OOH^– to BIQ, which reacted with molecular oxygen to give a 1,2-dioxetane. Thermal decomposition of this intermediate generates the excited state of isoquinolinone. The ‘photoproducts’ from isoquinolinones were obtained in the chemiluminescence reaction as minor products, confirming the chemical formation of the excited state of the isoquinolinones.