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Long acenes beyond pentacene are of considerable interest for organic electronics because their extended π-conjugation is associated with reduced energy gaps and enhanced charge carrier mobilities. At the same time, their pronounced reactivity and increasing open-shell character make their preparation and characterization increasingly challenging [1,2]. In this contribution, heptacene is used as a long-acene system to investigate how fluorination can be employed as a chemically controlled route to tune molecular electronic structure and interface properties.
In the gas phase, fluorination induces a stabilization of the molecular orbital manifold due to the strong electron-withdrawing character of fluorine substituents, leading to an increase in electron affinity and a shift of the electronic spectrum toward lower energies [3]. This effect is directly reflected in core-level spectroscopy, where calculated C 1s binding energies shift toward higher values and exhibit a clear separation between chemically distinct C–F and C–C environments [3]. These gas-phase trends establish a well-defined reference for interpreting fluorination-induced modifications at interfaces.
We then consider adsorption on Ag(110) and Cu(110), two substrates with markedly different interaction strengths [4]. On Ag(110), where molecule–substrate coupling is comparatively weak, fluorination changes the balance between push-back and molecular dipole effects and reverses the sign of the work-function shift. On Cu(110), by contrast, stronger hybridization and charge redistribution dominate the interface electronic structure. The resulting changes are analyzed through the projected density of states (MOPDOS), which reveals how fluorination modifies frontier-orbital alignment and hybridization with the substrate.
These effects are further reflected in the simulated X-ray photoelectron spectra (XPS), which connect chemical substitution, charge transfer, and screening to observable C1s line-shape changes. Finally, we discuss C K-edge X-ray absorption spectroscopy (XAS/NEXAFS) as a natural extension of this framework. In combination with XPS, C K-edge XAS provides direct information on the unoccupied π* states and their evolution under fluorination and substrate interaction. Altogether, this study shows that fluorination is a powerful handle for tailoring vacuum-level alignment, hybridization, and spectroscopic fingerprints in acene-based interfaces.
References :
[1] Miyazaki T, Watanabe M, Matsushima T, et.al . Heptacene: Synthesis and Its Hole‐Transfer Property in Stable Thin Films. Chemistry–A European Journal. 2021 Jul 21;27(41):10677-84.
[2] Han J, Liu X, Li Y, Lou Z, Yi M, Kong H, Luo J. New synthetic approaches for hexacene and its application in thin-film transistors. Organic Chemistry Frontiers. 2019;6(16):2839-43.
[3] Bischof D, Radiev Y, Tripp MW, et. al. Chemical Doping by Fluorination and Its Impact on All Energy Levels of π-Conjugated Systems. The Journal of Physical Chemistry Letters. 2023 Mar 6;14(10):2551-7.
[4] Sättele MS, Windischbacher A,et al. Hexacene on Cu (110) and Ag (110): influence of the substrate on molecular orientation and interfacial charge transfer. The Journal of Physical Chemistry C. 2022 Mar 7;126(10):5036-45.