Exciton Polaritons Reveal “Hidden” Populations in Functionalized Pentacene Films

Abstract

We present a systematic investigation of strong exciton− photon coupling in functionalized pentacene (TIPS-Pn)-based films in allmetal 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.

Exciton Polariton-Enhanced Photodimerization of Functionalized Tetracene

“…In this case, in C samples, only a subset of a general population of degrading molecules seen in T samples, which is more conducive to photodimerization, would be observed by measuring polariton properties of Figures 6 and 7. In our previous work on functionalized anthradithiophene-and pentacene-based polycrystalline films in cavities, 18,47 we have observed preferential cavity coupling to particular molecular populations (namely, to amorphous phases in the mixed-phase films with coexisting amorphous and crystalline domains). The TIPS-Tc films in the present study, however, are amorphous, and so a preferential cavity coupling to domains with a particular local nanoenvironment conducive for photodimerization would be harder to envision, although it cannot be excluded.…”

Controlling the Manifold of Polariton States Through Molecular Disorder

“…The key parameter that defines the strong-coupling regime is the coupling constant g. It sets the energy scale for light-matter interactions and is suggested to be pivotal in moderating or even suppressing electron-phonon coupling in systems with prominent vibronic structure. [58] In a discrete state model, g is directly linked to the Rabi splitting, which can be straightforwardly read off spectroscopic data, through [23,48,54]…”

Perspectives on weak interactions in complex materials at different length scales

“…168 However, development of a comprehensive understanding of the parameters that determine whether or not enhanced transport is realised in a given system is ongoing and will need to account for e.g. the role of disorder on carrier and polariton delocalisation under strong coupling conditions 169,170 and dark states. 171 Another promising approach may be to bestow charge on the highly delocalised polaritons by coupling not to charge-neutral excitons, but rather to polarons 172 or trions.…”