Exciton Polaritons Reveal “Hidden” Populations in Functionalized Pentacene Films

We present a systematic investigation of strong exciton–photon coupling in functionalized pentacene (TIPS-Pn)-based films in all-metal cavities, depending on molecular concentration and film morphology. Rabi splittings of up to 270 meV are observed, with the highest values achieved in pristine amorphous TIPS-Pn films. The exciton–photon interaction strength for the lowest-energy (0–0) excited state scaled with the square root of the molecular density, which was independent of whether the long-range molecular order were present in films. The molecular populations in the disordered regions of the films coupled to the cavity most strongly in all films, including pristine crystalline films. Such populations, with molecular configurations favoring interaction with the cavity electromagnetic field, were not readily identifiable in optical absorption spectra of bare (i.e., not coupled to the cavity) films, which highlights the capability of polariton spectroscopy to reveal these molecular ensembles, which are “hidden” in polycrystalline films. The linear scaling of the exciton–photon interaction strength with the square root of the oscillator strengths was observed in dilute TIPS-Pn:PMMA films but not in pristine TIPS-Pn films, either amorphous or crystalline. In particular, in pristine films, the exciton–photon interaction strength for the vibronic (0–m, m > 0) excitons was higher than expected based on the oscillator strengths extracted from the optical spectra of bare films, which was attributed to enhanced exciton delocalization facilitated by the 2D brickwork motif of TIPS-Pn. Similar observations were made in functionalized anthradithiophene (diF TES-ADT) films (also exhibiting a 2D brickwork packing motif) but not in functionalized tetracene (TIPS-Tc) films, which suggests that the underlying mechanisms rely on short-range intermolecular interactions determined by the molecular packing motif and resulting nanomorphology.