Stability and Ultrafast Dynamics of Luminescent Biquinoxen-Bis-σ^H-Adducts

We report the synthesis of two new biquinoxen-σ^H-adducts (3,3'-diisopropoxy-4,4'-dimethyl-3,3',4,4'-tetrahydro-2,2'-biquinoxaline (Mbqn-(OⁱPr)₂) (1) and 3,3'-bis(isopropylthio)-4,4'-dimethyl-3,3',4,4'-tetrahydro-2,2'-biquinoxaline (Mbqn-(SⁱPr)₂) (2)) with the same molecular structure other than the exchange of two oxygen atoms with sulphur atoms. This enables us to directly compare the optical properties and stability of the compounds as a result of this substitution. For freshly prepared solutions of 1, a fluorescence quantum yield of 97% is observed, whereas for 2, the value is much lower at 7%. We furthermore note a decrease in quantum yields for solutions investigated after certain storage times, indicating a reactive channel. We note that this decomposition is much faster for solutions of 2 compared with compound 1. For 1, the decomposition likely proceeds to the biquinoxen dipseudobase via an equilibrium, whereas for 2 the decomposition product remains unidentified. The decomposition of 1 in particular was followed using ultrafast transient absorption spectroscopy, investigating the dynamics of the biquinoxen system after photoexcitation. Given the redox activity of biquinoxens, additionally the oxidation of the compounds was investigated using (spectro)electrochemistry.