Behavior of Pyrene End-Labeled Poly(dimethylsiloxane) Polymer Tails in Mixtures of 1-Butyl-3-methylimidazolium Bis(trifluoromethyl)sulfonylimide and Toluene

Abstract

We report on the steady-state and time-resolved fluorescence of a linear poly(dimethylsiloxane) polymer that is end-labeled with pyrene (Py-PDMS-Py, Mn = 3100, Mw/Mn = 1.07) when it is dissolved at low concentration in an ionic liquid,1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ([C4mim][Tf2N]), as a function of temperature and added cosolvent (toluene). Toluene is a good solvent for PDMS. The Py-PDMS-Py behavior in [C4mim][Tf2N]/toluene is reminiscent, in part, of its behavior in liquid MeOH, a poor PDMS solvent. The Py residues are surrounded by a PDMS-rich microenvironment at all toluene loadings (0−50 vol %). No detectable excimer-like emission is seen in pure [C4mim][Tf2N] or [C4mim][Tf2N]/toluene mixtures until 50 vol % toluene. The Py-PDMS-Py ground state is heterogeneous, containing monomers and preformed dimers prior to photoexcitation. The observed excimer-like emission does not arise from a dynamic excimer. The Py-PDMS-Py time-resolved intensity decay data reveal the presence of at least four emitting species. In the [C4mim][Tf2N]/50% toluene mixture the species are (i) a Py monomer in a PDMS-rich microenvironment, (ii) a Py monomer in a microenvironment between pure toluene and pure [C4mim][Tf2N], (iii) a static ground-state dimer that has a geometry akin to a classical dynamically formed excimer, and (iv) a ground-state dimer that has the Py residues misaligned in comparison to (iii). Steady-state fluorescence anisotropy experiments are not consistent with the formation of rigid aggregates (i.e., (Py-PDMS-Py)n); results are consistent with Py residues and PDMS segments of individual Py-PDMS-Py molecules reorienting in a correlated manner.