Excimer formation of 6-(1-pyrenyl)hexyl-11(1-pyrenyl)undecanoate within an ionic liquid and cosolvent-modified ionic liquid mixture

The ring closing metathesis within an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), a 'green' solvent tetraethylene glycol (TEG), and their equimolar mixture is investigated through the intramolecular excimer formation of 6-(1-pyrenyl)hexyl-11-(1-pyrenyl)undecanoate [1]. The excimer formation efficiency correlates directly with the inverse of the bulk viscosity of the systems. A simplistic double exponential decay model based on Birk's scheme is enough to fit the excited state intensity decay of 1 in these liquids. This behavior is in stark contrast to that reported in organic solvents and in scCO2, where three exponentials are required to fit the fluorescence decay. The activation energies of the excimer formation (Ea), viscous flow (Ea,ηbulk), and microviscosity (Ea,ημ) were estimated from the temperature dependence. Ea and Ea,ημ are significantly higher for the equimolar mixture as compared to that for [bmim][PF6] or TEG, whereas Ea,ηbulk is close to the average. This highlights the important outcome that the cyclization dynamics scales with the microviscosity rather than the bulk viscosity of the systems investigated.