Optical evidence for intermolecular coupling in mixed films of pentacene and perfluoropentacene

Abstract

We present optical absorption spectra of mixed films of pentacene (PEN) and perfluoropentacene (PFP) grown on SiO 2 . We investigated the influence of intermolecular coupling between PEN and PFP on the optical spectra by analyzing samples with five different mixing ratios of PFP:PEN with variable angle spectroscopic ellipsometry and differential reflectance spectroscopy. The data show how the spectral shape is influenced by changes in the volume ratio of the two components. By comparison with the pure film spectra an attempt is made to distinguish transitions due to intermolecular coupling between PEN and PFP from transitions caused by interactions of PEN (PFP) with other molecules of the same type. We observe a new transition at 1.6 eV which is not found in the pure film spectra and which we assign to the coupling of PFP and PEN.

Organic–Organic Heterostructures: Concepts and Applications

“…[123] Importantly, upon mixing PEN and PFP, in optical absorption spectra a new transition arises at 1.6 eV (Figure 7 b), which is not present in either of the spectra of the pure materials and indeed also below both of the respective gaps. [121] Detailed analysis shows that the absorption spectrum of PEN:PFP cannot be explained by a superposition of the spectra of the pure substances. This and in particular the new transition are taken as evidence for coupling between the two components, which is another hint that both compounds form a molecular complex.…”

“…[121] are presumably related to possible coupling on a molecular, that is, local scale. This has been observed in several systems and is considered strong evidence for intermixing on a local scale, or, in turn, the structural intermixing is considered a precondition for efficient optical coupling.…”

“…These compounds are sterically compatible, since both are derived from the same molecular structure. The system PEN:PFP was studied structurally [118,119] as well as spectroscopically [120,121] and was shown to exhibit molecular complex formation. With grazing incidence X-ray diffraction (GIXD) data from different mixing ratios (Figure 7 a), it is shown that PEN:PFP forms its own structure with 1:1 stoichiometry.…”

Structure formation in perfluoropentacene:diindenoperylene blends and its impact on transient effects in the optical properties studied in real-time during growth

“…[21][22][23][24] Apart from the film structure, also intermolecular coupling can significantly affect the optical properties of the mixed system either due to the differences in the film structure compared with the pure films, which results in changes in the intermolecular interactions between molecules of the same compound, 25 or due to a a) Electronic mail: alexander.gerlach@uni-tuebingen.de possible charge transfer between molecules of the two different compounds. [26][27][28][29][30][31] Pentacene (PEN, C 22 H 14 ) and diindenoperylene (DIP, C 32 H 16 ) are two of the most prominent small-molecule organic semiconductors, 13,[33][34][35][36] which exhibit an anomalous ordering in PEN:DIP blends with a smectic-C-like structure. 11 The perfluorinated derivative of PEN (PFP, C 22 F 14 ) has recently attracted significant interest in pure films as well as in blends with PEN.…”

Impact of molecular tilt angle on the absorption spectra of pentacene:perfluoropentacene blends

“…However, for complex systems such as blends consisting of donor‐acceptor pairs with strong intermolecular interactions, they are not yet understood. Apart from possible charge‐transfer processes, the short and long range order within the film and the molecular arrangement (including the molecular tilt angle) are known to affect the optical properties 3–10. For a better understanding of these effects, experiments elucidating the molecular arrangement within the film are mandatory, such as near‐edge X‐ray absorption fine‐structure spectroscopy (NEXAFS) 11–14, which is a well‐established technique to investigate the ensemble averaged molecular tilt angle in thin films.…”