Organic semiconductors have attracted attention due to their low cost, enhanced processability, and tunable properties. Charge carrier mobilities exceeding 10 cm2/Vs in organic thin-film transistors and power conversion efficiencies over 10% in organic solar cells have been demonstrated in several systems. Another area that has seen dramatic progress is in organic polaritonic devices relying on strong exciton-photon coupling in organic microcavities or on plasmonic nanostructures. The hybridized light-matter states formed in these devices have opened up a host of new and exciting phenomena to explore, such as low-threshold polariton lasing and polariton Bose-Einstein condensation. While these exotic phenomena are being investigated, there are still unanswered questions pertaining to the basic physics of organic polaritons, particularly in crystalline media. One such question involves the dependence of the exciton-photon coupling—enabled by the organic crystal placement in a cavity (or on a plasmonic surface)—on the intermolecular coupling in crystal, which determines exciton physics for “bare” (cavity-free) crystals. We present a systematic study of intermolecular interactions and their effect on photophysics and coupling to cavity/plasmon modes in high-performance functionalized anthradithiophene (ADT) derivatives, depending on the molecular packing.
The fluorinated ADT derivatives (diF R-ADT) with varying side groups R, which we chose as model systems, exhibit identical optical properties in solution, but considerably different properties in the solid state owing to the side groups R controlling the molecular packing and, thus, exciton dynamics. In order to quantify intermolecular interactions, we explored in detail temperature- and polarization-dependent optical absorption and photoluminescence properties of single crystals of several diF R-ADT derivatives, which were analyzed using Spano’s model of molecular aggregate spectra. Strongly anisotropic characteristics and temperature-dependent exciton dynamics were observed, which depended on the molecular packing. The polarization dependence was well described in the framework of switching between the J-aggregate- and H-aggregate-dominated behavior depending on the polarization of light with respect to the crystal axes. The prominence of such behavior could be controlled with a choice of the side group R.
The changes in the exciton dynamics that occurred when diF R-ADT crystals were placed in optical cavities and on plasmonic nanostructured substrates were then studied in detail. Coupling between the cavity (plasmon) modes and excitons was observed, depending on the R group, polarization, and cavity detuning. The properties of light-matter hybrid states were then correlated with the exciton dynamics in “bare” crystals and J/H-aggregate switching behavior in particular.