Understanding the impact of inter-molecular orientation on the optical properties 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, we present a systematic study of how various molecular crystal packing motifs affect the optical properties of a class of high-performance organic semiconductors: functionalized derivatives of fluorinated 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 4–7 molecules depending on the temperature. These delocalized Frenkel states were dominant across the full temperature range (78 K–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.

When considering the optimal molecular packing to realize charge multiplication in organic photovoltaic materials, subtle changes in intermolecular charge transfer (CT) coupling can strongly modulate singlet fission. To understand why certain packing arrangements are more conducive to charge multiplication by triplet pair (TT) formation, we measure the diffraction-limited transient absorption (TA) response from four single-crystal functionalized derivatives of fluorinated anthradithiophene: diF R-ADT (R = TES, TSBS, TDMS, TBDMS). diF TES-ADT and diF TDMS-ADT both exhibit 2D brickwork packing structures, diF TSBS-ADT adopts a 1D sandwich-herringbone packing structure, and diF TBDMS-ADT exhibits a 1D twisted-columnar packing structure. When brickwork or twisted-columnar single crystals are resonantly probed parallel to their charge transfer (CT)-axis projections, the TA signal is dominated by a rising component on the picosecond time scale (rate k TT), attributed to TT state population. When probed orthogonal to the CT-axis, we instead recover the falling TA kinetics of singlet state depletion at rate k A. The rising to falling rate ratio estimates the TT formation efficiency, εTT = k TT/k A relative to exciton self-trapping. εTT ranged from near 100% in diF TES-ADT to 84% in diF TDMS-ADT. Interestingly, diF TSBS-ADT crystals only manifest falling kinetics of CT-mediated self-trapping and singlet state depletion. Singlet fission is prohibitive in diF TSBS-ADT crystals owing to its lower symmetry sandwich-herringbone packing that leads to S1 to CT-state energy separation that is ∼3× larger than in other packings. Collectively, these results highlight optimal packing configurations that either enhance or completely suppress CT-mediated TT formation.

Abstract:We present an optical experiment on photonic crystals suitable for an advanced physics laboratory course or a senior capstone project. Photonic crystals are periodically ordered composite systems made of materials that have different dielectric constants, and can be arranged in one, two, or three dimensions. They are characterized by a bandgap that depends on the size, arrangement and dielectric constant of the microstructures that make up the crystal. In addition, the bandgap spectrally shifts with the angle of incident light. These observations are captured by the Bragg-Snell law. In this paper, we describe a vertical deposition method for growing photonic crystals from a water suspension of polystyrene microspheres, as well as a simple transmission experiment that students can perform using a USBspectrometer to explore the Bragg-Snell law.

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Single crystal excited state dynamics in functionalized anthradithiophene (ADT) derivatives were compared across four distinct packing morphologies. Using polarization-dependent transient absorption microscopy, morphology-dependent singlet fission was observed in only three of the four ADT derivatives.