Rubrene (5,6,11,12-tetraphenyltetracene) is a polyacene material that has been well studied throughout its nearly one-hundred year history. Originally found fascinating for its luminescent properties, it has emerged at the forefront for organic electronics due to its particularly high charge carrier mobility for an organic crystal. Despite great interest and its explosion in the literature over the past two decades, the commercial synthesis of rubrene has remained relatively unchanged since its initial discovery in 1926. Several recent studies have reported alternate routes to the rubrene structure with substitutions on the peripheral­ aromatic rings and tetracene core. Substituting in this manner has the potential to improve upon rubrene’s electronic properties. We review the various routes to rubrene and its derivatives and provide a brief overview of the solid-state library available for study. The information gained by comparing the solid-state properties between derivatives offers insight into unpredictable crystallization and polymorphism – complicated issues – which have hindered research into materials applications­ of rubrene. We hope that these insights inspire work in application­-driven synthetic chemistry for future rubrene derivatives.1 Introduction2 Synthesis2.1 Traditional Rubrene Synthesis2.1.1 Recent Applications2.2 Multi-Step Synthesis2.2.1 Historical Routes2.2.2 Diels–Alder Approaches2.2.3 Cross-Coupling Approaches2.2.4 Comparative Synthesis of Perfluororubrene3 Crystal Engineering4 Conclusions and Outlook

Femtosecond stimulated Raman spectroscopy (FSRS) is a chemically specific vibrational technique that has the ability to follow structural dynamics during photoinduced processes such as charge transfer on the ultrafast timescale. FSRS has a strong background in following structural dynamics and elucidating chemical mechanisms; however, its use with solid-state materials has been limited. As photovoltaic and electronic devices rely on solid-state materials, having the ability to track the evolving dynamics during their charge transfer and transport processes is crucial. Following the structural dynamics in these solid-state materials will lead to the identification of specific chemical structures responsible for various photoinduced charge transfer reactions, leading to a greater understanding of the structure–function relationships needed to improve upon current technologies. Isolating the specific nuclear motions and molecular structures that drive a desired physical process will provide a chemical blueprint, leading to the rational design and fabrication of efficient electronic and photovoltaic devices. In this perspective, we discuss technical challenges and experimental developments that have facilitated the use of FSRS with solid-state samples, explore previous studies that have identified structure–function relationships in charge transfer reactions, and analyze the future developments that will broaden and advance the field.

Polymorphism is an issue troubling numerous scientific fields. A phenomenon where molecules can arrange in different orientations in a crystal lattice, polymorphism in the field of organic photovoltaic materials can dramatically change electronic properties of these materials. Rubrene is a benchmark photovoltaic material showing high carrier mobility in only one of its three polymorphs. To use rubrene in devices, it is important to quantify the polymorph distribution arising from a particular crystal growth method. However, current methods for characterizing polymorphism are either destructive or inefficient for batch scale characterization. Lattice phonon Raman spectroscopy has the ability to distinguish between polymorphs based on low frequency intermolecular vibrations. We present here the addition of microscopy to lattice phonon Raman spectroscopy, which allows us to not only characterize polymorphs efficiently and nondestructively through Raman spectroscopy but also concurrently gain information on the size and morphology of the polymorphs. We provide examples for how this technique can be used to perform large, batch scale polymorph characterization for crystals grown from solution and physical vapor transport. We end with a case study showing how Raman microscopy can be used to efficiently optimize a green crystal growth method, selecting for large orthorhombic crystals desired for rubrene electronic device applications.

We report the first examples of mixed cocrystallization in rubrene derivatives. We grew crystals with differing ratios of two previously characterized rubrene derivatives. When crystallized individually, each rubrene derivative packs with a twisted tetracene core in the solid state. In both examples of mixed cocrystallization, the tetracene cores are planar, providing some of the first experimental evidence that intermolecular interactions are a major driving factor for planarization of the tetracene core. Additionally, a larger ratio of the methylated rubrene derivative resulted in a mixed cocrystal displaying a herringbone crystal formation, which is optimal for applications in optoelectronics.