The Effect of Protonation on the Optical Properties of Conjugated Fluorene–Pyridine Copolymers

A series of conjugated alternating and statistical copolymers of fluorene and pyridine are synthesized and characterized. The compounds under investigation, namely poly[2,7-(9,9-dihexylfluorenyl)-alt-(2,6-pyridinyl)] and poly[2,7-(9,9-dihexylfluorenyl)-stat-(2,6-pyridinyl)], are prepared by a Suzuki-type cross-coupling reaction starting from commercially available materials utilizing the recently described catalyst trans-bis(dicyclohexylamine) palladiumdiacetate ("DAPCy"). The target compounds are investigated by nuclear magnetic resonance and infrared spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, thermal analysis, UV-Vis absorption spectroscopy, and luminescence measurements in solution and in the solid state. Special emphasis is placed on studying the consequences of protonation on the photophysical properties of the described materials. Achieving a concise microscopic understanding of the effects of protonation on the absorption and luminescence characteristics is crucial for potential applications in, e.g., optical sensors. To that aim, the results of the systematic studies of the photophysical properties are explained by quantum mechanical calculations.