Anisotropic Triplet Exciton Diffusion in Crystalline Functionalized Pentacene

Abstract

Exciton multiplication through singlet fission (SF) offers scope for next-generation photovoltaic devices to exceed the Shockley–Queisser limit. Organic SF chromophores typically exhibit significant structural anisotropy in their crystal packing, which can impact exciton transport and influence the design of SF-enhanced devices. An improved understanding of the link between structural anisotropy and exciton diffusion is therefore crucial for developing SF-based photovoltaics. Here, we use femtosecond transient absorption spectroscopy to quantify the anisotropic triplet mobility in 6,13-(triisopropylsilylethynyl)­pentacene (TIPS-Pn), a prototypical SF chromophore. Bimolecular triplet–triplet annihilation (TTA) in crystalline TIPS-Pn is well-described by a kinetic model that assumes isotropic, three-dimensional triplet exciton diffusion, but with best-fit parameters that do not correspond to any physical parameters of the material. Kinetic models that assume either one-dimensional or anisotropic three-dimensional exciton diffusion describe the annihilation equally well but yield more physically realistic fit parameters, suggesting that triplet diffusion on the subnanosecond time scale occurs mostly along a single axis of the material. These findings highlight the need to treat parameters obtained from fits of experimental data with models of isotropic diffusion with caution for systems with anisotropic packing such as TIPS-Pn. Diffusion coefficients calculated by density functional theory predict that triplet exciton diffusion occurs predominantly along the crystallographic a-axis, with migration in any other direction through the crystal slower by over an order of magnitude. This anisotropic diffusion suggests that fast, directional exciton transport in layers or films of TIPS-Pn may be achieved by control of the chromophore morphology.

Diphenyldihydropentalenediones: Potential Singlet Fission Materials with High Triplet Energy Levels

“…[50] [h] In crystals. [32] X-Ray Crystallographic Analysis. Because SF is a phenomenon that requires close intermolecular interactions, it ideally takes place in condensed media such as the crystalline state.…”

“…1) has a relatively large DEST of ca. 1.1 eV [32] owing to the existence of significant spatial HOMO-LUMO overlap. One strategy used to increase DEST (and ET) involves incorporation of a linearly-conjugated polyene system, like those present in conventional SF materials including carotenes [20] [21] and arylated polyenes such as trans,trans,trans-1,6diphenylhexatriene (HT, Fig.…”

Nanoparticle–Size–Dependent Singlet Fission and Exciton Dynamics in Amorphous TIPS-Pentacene

“…Aqueous suspensions of amorphous-phase TIPS-Pn NPs were prepared by re-precipitation from THF solution into water, as has been previously reported by our group and others. 18,19,21,[23][24][25] Multiple parameters in the preparation procedure were carefully tuned to achieve the broad range of NP sizes studied in this work. However, the overall procedure remained similar for the preparation of each NP sample, and so here we describe this procedure in general terms, with…”

“…Aqueous dispersions of TIPS-Pn nanoparticles (NPs) are commonly used for spectroscopic characterization of SF and associated exciton dynamics. 18,21,[23][24][25] Study of NP dispersions allows solid-state photophysical processes to be probed while using liquid-phase samples, which minimizes experimental issues associated with the study of thin films and other solidphase media such as photo-bleaching or sample opacity. Such dispersions are generally prepared through the "re-precipitation" or "flash precipitation" method first reported by Matsuda and co-workers.…”

Excimer Diffusivity in 9,10-Bis(phenylethynyl)anthracene Assemblies on Anodic Aluminum Oxide Membranes

“…Synthesis of 9-Bromo-10-phenylethynylanthracene (2) Br Br + Br CuI, Pd(PPh 3 ) 4 diisopropylamine, toluene…”