Session: Inorganic Spectroscopy:

Previous work from our lab had successfully utilized [Cu(I)(dmp)₂] (dmp = 2,9-dimethyl-1,10-phenanthroline) as an optical probe for sensing the viscosity of different polymers. The greater viscosity of the polymer was found to be correlated to an increase in the lifetime of the excited state of the complex. As the geometry of [Cu(I)(dmp)₂] went from tetrahedral in the ground state to square planar like in the excited state (jahn-teller distortion), the observed lifetime increase was proposed due to restricted rotation. However, it is a challenge to precisely monitor the in-situ viscosity of polymers in real-time. It get’s harder to measure them in confined environments where traditional rheological methods can’t be utilized. In this study, [Cu(I)(diptmp)₂] (diptmp = 2,9-diisopropyl-2,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe was used in various adhesives during curing process (viscosity increases) for real time sensing of viscosity via changes in luminescence. We found that the emission lifetime of the triplet metal to ligand charge transfer (³MLCT) state of [Cu(I)(diptmp)₂] in epoxy glue increased exponentially in real-time similar to viscosity values obtained from oscillatory rheology while curing. We also tested for quenching issues by doing reversibility verification. This was done by monitoring the real time viscosity by swelling (viscosity and lifetime decrease) and by unswelling (viscosity and lifetime increase) the polymer film. We even measured viscosity in opaque samples (advantage for emission and high quantum yield) using Gorilla Glue, which depicted the same correlation. [Ru(II)(bpy)₃] (bpy = bipyridine) as a standard was also used to monitor lifetime changes. [Cu(I)(diptmp)₂] depicted higher emission lifetime and was more ubiquitous as a real-time viscosity sensor.

Dr. Alexis D. Ostrowski

Associate Professor, Bowling Green State University

Appearances