Karl J. Thorley

Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction.

On the impact of isomer structure and packing disorder in thienoacene organic semiconductors

Many high performing organic semiconductor materials contain heteroaromatic rings in order to control the molecular packing and material electronic properties. Here we use a combination of density functional theory and symmetry-adapted perturbation theory calculations to explore the intermolecular noncovalent interactions, which guide solid-state molecular packing, and electronic couplings in a series of benzodithiophene-based dimer models. A novel concept, termed the disordermer, is introduced to delineate how the reduced molecular symmetry of benzodithiophene, when compared to the more highly symmetric anthracene molecule, can present intermolecular isomerism in the solid state that results in a wide range of available molecular packing arrangements that in turn influence the magnitudes of the electronic couplings. The insight developed through the investigation of these disordermers is demonstrated to hold important implications in the design of new generations of organic semiconductor materials.

Mapping the configuration dependence of electronic coupling in organic semiconductors

The ability to effectively transport charge carriers is often a key determinant concerning the deployment of materials derived from π-conjugated molecules and polymers in (opto)electronic applications. Theoretical models to evaluate charge-carrier transport parameters across a range of organic materials often work under the approximation of evaluating the intermolecular electronic couplings for supermolecular complexes (i.e. dimers) in the neutral state. Here, we investigate how the explicit inclusion of the nature of the charged state (i.e. both the neutral and radical-cation states) impacts the assessment of the intermolecular electronic couplings, and how considerations of the density functionals often used to determine these couplings effect the computed magnitude. From a materials perspective, we explore the role that the dimer configuration plays in determining the magnitudes of the electronic couplings for oligoacenes. The results suggest that appropriate consideration of translational alignment along the long and short acene axes, even in configurations with near perpendicular edge-to-face interactions, can lead to molecular packing arrangements in the solid state with large electronic couplings. These results give insight into ways to fine tune solid-state molecular packing to ensure the highest possible electronic couplings.

The effect of regioisomerism on the crystal packing and device performance of desymmetrized anthradithiophenes

Anthradithiophenes (ADTs) are typically synthesized as inseparable mixtures of regioisomers. In this paper, we describe the synthesis of desymmetrized anthradithiophenes containing one trialkylsilylethyne solubilizing group, which allowed chromatographic separation of the three resulting isomers. Cyclic voltammograms, as well as absorption and emission spectra for all isomers, were nearly identical. However, X-ray crystallography revealed that the positions of the sulfur atoms in each isomer strongly influence crystal packing, corroborating calculations that show the S–π interaction to be less stabilizing than the C–H–π interaction. Isomer 3c packs in a pseudo 1-D fashion while isomers 3a and 3b pack as isolated π-stacked pairs. Isomer 3c shows a field-effect mobility four orders of magnitude higher than isomers 3a and 3b, presumably due to this difference in packing motif.

Understanding the Crystal Packing and Organic Thin-Film Transistor Performance in Isomeric Guest–Host Systems

In order to understand how additives influence the structure and electrical properties of active layers in thin-film devices, a compositionally identical but structurally different guest-host system based on the syn and anti isomers of triethylsilylethynyl anthradithiophene (TES ADT) is systematically explored. The mobility of organic thin-film transistors (OTFTs) comprising anti TES ADT drops with the addition of only 0.01% of the syn isomer and is pinned at the mobility of OTFTs having pure syn isomer after the addition of only 10% of the isomer. As the syn isomer fraction increases, intermolecular repulsion increases, resulting in a decrease in the unit-cell density and concomitant disordering of the charge-transport pathway. This molecular disorder leads to an increase in charge trapping, causing the mobility of OTFTs to drop with increasing syn-isomer concentration. Since charge transport is sensitive to even minute fractions of molecular disorder, this work emphasizes the importance of prioritizing structural compatibility when choosing material pairs for guest-host systems.

Site-selective measurement of coupled spin pairs in an organic semiconductor

Significance Pairs of spins in molecular materials have attracted significant interest as intermediates in photovoltaic devices and light-emitting diodes. However, isolating the local spin and electronic environments of such intermediates has proved challenging due to the complex structures in which they reside. Here we show how exchange coupling can be used to select and characterize multiple coexisting pairs, enabling joint measurement of their exchange interactions and optical profiles. We apply this to spin-1 pairs formed by photon absorption whose coupling gives rise to total-spin S=0,1 and 2-pair configurations with drastically different properties. This presents a way of identifying the molecular conformations involved in spin-pair processes and generating design rules for more effective use of interacting spins. From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet (S=0) and dark triplet quintet (S=1,2) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3–5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.

Research data supporting “Strongly exchange-coupled triplet pairs in an organic semiconductor”

Text files of (1) transient electron spin resonance (ESR) of TIPS-Tetracene as a function of temperature taken at X-band frequencies (~9.6 GHz) in a home-build transient ESR spectrometer, time slices are taken at 300 ns after laser flash in a TIPS-Tetracene film (2) Rabi nutation time traces of Hahn echo intensity as a function of microwave pulse length taken on a Bruker E580 spectrometer configured for X-band using a dielectric resonator for a TIPS-Tetracene film and macrocrystalline sample (3) Hahn echo decay time traces showing the decay of the Hahn echo as a function of delay between a 90 degree and 180 degree microwave pulse for a TIPS-Tetracene film and macrocrystalline sample. (4) Transient ESR intensity time traces taken at 10K in a TIPS-Tetracene film (5) metadata file with any necessary information to understand what each file corresponds to. Details of the experimental setup and corresponding figures can be found in the corresponding publication.