Rubrene: The Interplay between Intramolecular and Intermolecular Interactions Determines the Planarization of Its Tetracene Core in the Solid State

Abstract

Abstract.Rubrene is one of the most studied molecular semiconductors; its chemical structure consists of a tetracene backbone with four phenyl rings appended to the two central fused rings.Derivatization of these phenyl rings can lead to two very different solid-state molecular conformations and packings: One in which the tetracene core is planar and there exists substantive overlap among neighboring π-conjugated backbones; and another where the tetracene core is twisted and the overlap of neighboring π-conjugated backbones is completely disrupted.State-of-the-art electronic-structure calculations show for all isolated rubrene derivatives that the twisted conformation is more favorable (by -1.7 to -4.1 kcal mol -1 ), which is a consequence of energetically unfavorable exchange-repulsion interactions among the phenyl side groups.Calculations based on available crystallographic structures reveal that planar conformations of the tetracene core in the solid state result from intermolecular interactions that can be tuned through well-chosen functionalization of the phenyl side groups, and lead to improved intermolecular electronic couplings. Understanding the interplay of these intramolecular and intermolecular interactions provides insight into how to chemically modify rubrene and similar molecular semiconductors to improve the intrinsic materials electronic properties.

Stark Spectroscopy of Rubrene. I. Electroabsorption Spectroscopy and Molecular Parameters

“…32 The importance of intermolecular interaction for the stabilization of the planar structure was also indicated by a quantum chemical calculation. 8 For rubrene in solution, an experimental result of 13 C NMR spectroscopy was in agreement with the twisted structure. 33 In our experimental conditions, the contributions of molecular aggregates and intermolecular interaction to spectroscopic results are minor.…”

Energetic and Structural Insights into the Molecular and Supramolecular Properties of Rubrene

“…Rubrene holds the merit of being one of the known OSCs with the highest charge carrier mobilities: ultra‐pure single crystals of rubrene fabricated by vapor deposition can reach hole mobilities of about 40 cm 2 ⋅V −1 ⋅s −1 . While tetracene and pentacene are rigid molecules having an edge‐to‐face herringbone crystal packing, the four phenyl substituents in rubrene are relatively flexible and lead to a particular crystallographic arrangement that places neighboring tetracene units in an offset stacked π⋅⋅⋅π configuration . The bulky and flexible phenyl substituents in rubrene are expected to weaken intermolecular interactions and increase the vibrational reorganization energy; factors that are generally believed to contribute for a decrease of charge mobility in organic materials .…”

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

“…Thus, some of the features observed in a molecular solid can be traced back to the features observed in a free molecule25. The energy levels of a free rubrene molecule18192022 and the electronic band structure of the rubrene crystal326 have been theoretically calculated and experimentally confirmed by optical characterization methods for the cases of rubrene molecules dissolved in various solvents1820272829303132, rubrene thin films1823303233, and rubrene single crystals27282930313334. However, relatively little attention has been paid to the discussion on the role of conformational change in the rubrene molecular solid films fabricated from different growth conditions.…”

“…For a free rubrene molecule, the total energy of a twisted rubrene is smaller than that of a planar rubrene by about 163~210 meV based on the electronic structures calculation81718. Thus, in vapor phase or in thin amorphous films, rubrene molecules possess the low energy state of twisting conformation; however in highly crystalline films or single crystals, rubrene molecules adopt a planar conformation in which the energy required to planarize the backbone is compensated by the lattice energy81317181920212223. The rubrene molecular solids are formed by weak intermolecular interactions, and hence different growth methods can lead to different polymorphs24.…”