  Exciton-polaritons in organic materials are hybrid states that result from
the strong interaction of photons and the bound excitons that these materials
host. Organic polaritons hold great interest for optoelectronic applications,
however progress towards this end has been impeded by the lack of a first
principles approach that quantifies light-matter interactions in these systems,
and which would allow the formulation of molecular design rules. Here we
develop such a first principles approach, quantifying light-matter
interactions. We exemplify our approach by studying variants of the conjugated
polymer polydiacetylene, and we show that a large polymer conjugation length is
critical towards strong exciton-photon coupling, hence underlying the
importance of pure structures without static disorder. By comparing to our
experimental reflectivity measurements, we show that the coupling of excitons
to vibrations, manifested by phonon side bands in the absorption, has a strong
impact on the magnitude of light-matter coupling over a range of frequencies.
Our approach opens the way towards a deeper understanding of polaritons in
organic materials, and we highlight that a quantitatively accurate calculation
of the exciton-photon interaction would require accounting for all sources of
disorder self-consistently.
