Perfluorinated Acenes: Crystalline Phases, Polymorph-Selective Growth, and Optoelectronic Properties

Abstract

The emerging field of organic electronics triggers the search for new molecular semiconductors and fluorophores. Since optoelectronic properties of molecular solids also depend crucially on molecular packing motifs, it is not sufficient to only consider single-molecule properties, but also to gain precise knowledge of the crystalline phases to understand structure−property relationships. Here, we analyze and compare structural and optoelectronic properties of perfluorinated acenes ranging from perfluoronaphthalene (PFN) via perfluoroanthracene (PFA) and perfluorotetracene (PFT) to perfluoropentacene (PFP), while previous studies, due to the lack of availability of PFA and PFT, were limited to PFN and PFP, which exhibit rather different crystal structures. Applying various crystallization techniques such as gradient sublimation, solution growth, and liquid-assisted organic molecular beam deposition, we identified different crystalline phases that allow closing this structural gap. A comparison of all known phases reveals clear trends in the molecular packing motifs, which are rationalized by corresponding Hirshfeld analyses. Using UV/vis and X-ray absorption spectroscopy, also optoelectronic solid state properties of the various compounds were analyzed, and from a comparison with the corresponding solution spectra, the exciton binding energies are estimated. In addition, we demonstrate that like PFP PFT also exhibits a π-stacked polymorph in films grown on graphene, where molecules are flat lying and adopt a slip-stacked packing. This interface-mediated phase is found to be stable only for PFP and PFT, while it is unstable for the smaller PFA. Finally, we demonstrate for the case of PFT that the different packing motifs occurring in the various phases have a strong influence on the photoluminescence spectra.

Regioselective Fluorination of Acenes: Tailoring of Molecular Electronic Levels and Solid‐State Properties

“…Interestingly, for fluorinated tetracenes this quantity, which is related to the exciton binding energy, is substantially smaller than for unsubstituted tetracene. A similar trend has also been observed for other perfluorinated acenes [11c] . This difference can be explained by the different molecular packing motifs herringbone vs. planar stacking, which are found also in a previous study on substituted pentacenes to affect the exciton binding [8d] …”

“…While a perfect cofacial π‐stacking of acenes as well as (partially) fluorinated acenes is repulsive, the F 4 TET and F 5 TET molecules are slip‐stacked with vertical distance of their aromatic planes of about 3.4 Å, a value that is also found in the bulk structures of perfluorinated acenes [11] . This value is somewhat larger than the closed stacking that was found for partially fluorinated hexabenzocoronone as well as the substrate stabilized π‐stacked phase of perfluoropentacene or PFTET formed in films grown on graphite or graphene [11c,19c,25] …”

“…For tetracene, however, impurities appeared in the powder spectrum of the raw material, which is why we recorded the optical spectra on single‐crystals as described in the Supporting Information. The results are further compared with optical properties of PFTET crystals of the α‐phase studied previously [11c] …”

“…Notably the solution absorption spectrum of PFTET reveals an additional distinct absorption band at 2.95 eV (cf. Figure 5e), which was identified as the HOMO‐LUMO+1 transition, [11c] while corresponding transitions of non‐ or partially‐fluorinated tetracenes occur at higher energies that are outside the probed energy region.…”

Template and Temperature Controlled Polymorph Formation in Squaraine Thin Films

“…3,4 In addition, fine-tuning can be obtained by selecting a specific polymorph through the processing conditions. [5][6][7][8] The interest in selective polymorph growth of organic molecules is widespread, since the polymorph choice can improve opto-electronic device performance, 9,10 or decide about the bio-functionality of for instance active drug ingredients. [11][12][13][14] The dihydroxy anilino squaraine SQIB (2,4-bis[4-(N,N -diisobutylamino)-2,6dihydroxyphenyl]squaraine), as sketched in Figure 1(a), is a prototypical quadrupolar donor-acceptor-donor type semiconductor compound.…”