The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14‐hexafluoropentacene (1) and the related 1,2,9,10,11‐pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X‐ray absorption spectroscopy data indicate that the single‐molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F⋅⋅⋅H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid‐state properties such as the exciton binding energy and the sublimation enthalpy.
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.
BN-substituted nanographene molecules are currently the focus of interest because the substitution of C–C units by isoelectronic and isosteric BN units is a straightforward way of changing the electronic properties of nanographenes. Another parameter influencing the electronic structure, orientation, and growth mode of nanographene molecules is the planarity of the molecules. The electronic structure, orientation, and film growth of the related molecules B3N3-hexa-peri-hexabenzocoronene (BN-HBC), B3N3-hexabenzotriphenylen (BN-HBP), and B3N3-hexabenzotriphenylen-2H (BN-HBP-2H) on Au(111) have been studied by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). XA spectra were simulated using time-dependent density functional theory (TDDFT). The calculation of C 1s excitation spectra allows the assignment of individual transitions and the examination of the degree of cross-linking between biphenyl units. It is shown that the planarity of the molecules distinctly affects the electronic structure, interface properties, as well as growth in thin films.
Contact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers to control the energy level alignment is demonstrated in many concept studies, mainly using (single crystalline) model substrates, the processability of electrodes and their robustness must also be considered in real devices. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics. The present study demonstrates that mono layers of F 4 TCNQ and F 6 TCNNQ provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well-defined monolayer that is further stabilized by additional charge transfer. Especially F 6 TCNNQ contact primer layers can also be prepared on oxidized silver electrodes yielding work functions of 5.5-5.6 eV, which can even withstand air exposure. Such contact primers show no interdiffusion into subsequently deposited layers of the prototypical p-type organic semiconductor pentacene, hence validating their use for organic electronic devices.
Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure‐property relationships for the partially regioselective fluorinated tetracenes 1,2,12‐trifluorotetracene, 1,2,10,12‐tetrafluorotetracene and 1,2,9,10,11‐pentafluorotetracene that were further compared with tetracene and perfluoro‐tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red‐shifted photoluminescence with larger lifetimes.
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