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

Abstract

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 (E_a), viscous flow (E_a,ηbulk), and microviscosity (E_a,ημ) were estimated from the temperature dependence. E_a and E_a,ημ are significantly higher for the equimolar mixture as compared to that for [bmim][PF₆] or TEG, whereas E_a,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.