Excited State Conformations of Bridged and Unbridged Pyrene Excimers

Abstract

AbstractPyrene is a prime example of molecules that form dimers in the excited state. These excimers give rise to a highly distinguishable, red‐shifted emission band which is of major importance in applications such as bio‐labeling, light‐harvesting or as organic sensors. Understanding this excimer fluorescence on a molecular level is therefore of crucial importance. We systematically investigate excited state conformations of pyrene dimers, using quantum chemical calculations and geometry optimizations. Furthermore, we study the effects of a bridging alkyl chain between the pyrene units on the conformation of the excimers. We report several conformations that were previously unknown. While fluorescence from the often discussed fully stacked excimer structures is symmetry forbidden, the newly found molecular arrangements where the pyrene units are rotated against each other are emissive in the experimentally observed range. These minima can be a contributing factor to the observed broadened fluorescence band of pyrene excimers.