Embrace the darkness
From singlet fission to exciton-polaritons
Andrew J Musser
Cornell University, Department of Chemistry & Chemical Biology
Polaritons are increasingly touted as a promising tool to ‘rewrite’ the functional behavior of molecular systems. Polaritons are mixed states formed from the hybridization of molecular transition dipoles with a confined electromagnetic field. Originally the purview of ultra-cold physics, when this concept is applied to molecular vibrational or electronic absorption transitions polaritons can be attained at room temperature. Studies over the last decade have revealed a host of weird and wonderful effects that result, from enhanced energy transport and charge carrier mobility to changes in the selectivity of chemical reactions – all by simply enclosing the materials between a pair of mirrors. Yet, while the field has become better and better at identifying exciting polaritonic phenomena, we lack a fundamental understanding of their underlying mechanisms. The temptation is strong to explain their exotic behavior in terms of the bright, strongly coupled states that we can easily observe. However, the bright polariton states are not alone. When we peer into optical cavities with ultrafast spectroscopy, we see that they are accompanied by a host of dark states that can dominate the photophysical response. An improved model, then, frames their photophysics in terms of an interplay between bright and dark states. But to make matters worse, we find that most systems studied today don’t even fit this neat bright-dark dichotomy. The ‘grey’ states in these materials mix the properties of both manifolds, whether due to disorder or higher-order state couplings that are frequently overlooked. The challenge of dealing with these bright, dark, and grey states is often treated as a unique problem for polariton science. Yet when we look deeper, we find compelling parallels in the intrinsic photophysics of organic semiconductors. Our results from simple molecular dimers to complex thin-film microcavities force us to reevaluate our basic pictures of molecular photophysics and present new opportunities for materials design, from the optical generation of entangled spins to polaritonic structures with orders-of-magnitude enhanced donor-acceptor transfer.
