Graduate Thesis Or Dissertation
 

Exciton and Polariton Nature in Organic Crystals

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  • Understanding the impact of inter-molecular orientation on the optical proper-ties of organic semiconductors is important for designing next-generation organic (opto)electronic and photonic devices. However, fundamental aspects of how various features of molecular packing in crystalline systems determine the nature and dynamics of excitons have been a subject of debate. Toward this end, this work presents a systematic study of how various molecular crystal packing motifs affect the optical properties of a class of high-performance organic semiconductors: functionalized derivatives offluorinated anthradithiophene. The absorptive and emissive species present in three such derivatives (exhibiting “brickwork”, “twisted-columnar”, and “sandwich-herringbone” motifs, controlled by the side group R) were analyzed both in solution and in single crystals, using various modalities of optical and photoluminescence spectroscopy, revealing the nature of these excited states. In solution, in the emission band, two states were identified: a Franck-Condon state present at all concentrations and an excimer that emerged at higher concentrations. In single crystal systems, together with ab initio calculations, it was found in the absorptive band that Frenkel and Charge Transfer (CT) excitons mixed due to nonvanishing CT integrals in all derivatives, but the amount of admixture and exciton delocalization depended on the packing, with the “sandwich-herringbone” packing motif least conducive to delocalization. Three emissive species in the crystal phase were also identified: Frenkel excitons, entangled triplet pairs ¹(TT) (which are precursors to forming free triplet states via singlet fission), and self-trapped excitons (STEs, similar in origin to excimers present in concentrated solution). The “twisted-columnar” packing motif was most conducive to the formation of Frenkel excitons delocalized over 4 to 7 molecules depending on the temperature. These delocalized Frenkel states were dominant across the full temperature range (78 K to 293 K), though at lower temperatures, the entangled triplet states and STEs were present. In the derivative with the “brickwork” packing, all three emissive species were observed across the full temperature range and, most notably, the ¹(TT) state was present at room temperature. Finally, the derivative with the “sandwich-herringbone” packing exhibited localized Frenkel excitons and had a strong propensity for self-trapped exciton formation even at higher temperatures. In this derivative, no formation of the ¹(TT) state was observed. The temperature-dependent dynamics of these emissive states are reported, as well as their origin in fundamental inter-molecular interactions. In addition this this, this work also reports on strong exciton-photon coupling in all-metal microcavities containing functionalized anthradithiophene (ADT) in host poly(methyl methacrylate) (PMMA) matrices for a wide range of ADT concentrations. Angle-resolved reflectance of polycrystalline films revealed Rabi splittings up to 340 meV . Angle-resolved photoluminescence in films with low ADT concentrations (dominated by “isolated” ADT molecules) showed Rabi splittings which scaled with the square root of oscillator strength. When “aggregated” and “isolated” ADT molecules coexisted in film, cavities preferentially coupled to “isolated” molecules due to an anisotropic distribution of aggregates. Finally, this work also discusses strong exciton-photon coupling for several other closely related materials: TIPS–Pentacene and TIPS–Tetracene, both of interest because of their unique singlet fission proper-ties. Reported here are Rabi splittings as high as 276 meV in TIPS-Pentacene films and 303 meV in TIPS-Tetracene films. As solution-processable high-performance or-ganic semiconductors, the family of functionalized (thio)acenes: ADT, Pentacene and Tetracene show promise as (opto)electronic polaritonic materials.
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