DOI: 10.1002/chem.200800802 Rubrenes: Planar and Twisted Abhimanyu S. Paraskar, [a] A. Ravikumar Reddy, [a] Asit Patra, [a] Yair H. Wijsboom, [a] Ori Gidron, [a] Linda J. W. Shimon, [b] Gregory Leitus, [b] and Michael Bendikov* [a] Introduction Rubrene (5,6,11,12-tetraphenyltetracene; 1) has been known since the beginning of the last century. [1] The electrolumines- cence and chemiluminescence of 1 were well studied in the 1960s and it is a classic example of a material with excellent electrochemiluminescent properties. [2] Nowadays, compound 1 is used as a dopant and a photosensitizer in organic light- emitting diodes (OLEDs). Recently, compound 1 was inves- tigated as an active semiconductor in organic field-effect transistors (OFETs). [3] An exceptionally high field-effect mobility of up to 15 cm 2 V À1 s À1 was measured for single crys- tals of 1, [4] and later, a contact-free intrinsic mobility of 40 cm 2 V À1 s À1 was reported for 1. [5, 6] Surprisingly, despite its very high mobility in a single crystal, compound 1 shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. [7, 8] To overcome this problem, thin films based on crystalline mixtures of 5,12-diphenylan- thracene and 1 were prepared, with these reported to yield a hole mobility of up to 0.7 cm 2 V À1 s À1 , [8] and transistor arrays based on single crystals of 1 [9] were also fabricat- ed. [10, 11] Apparently, good crystal packing is responsible for the ob- served high mobility, [12] and indeed, examination of the crys- tal structure of 1 [13] shows that it has p–p stacking in one di- rection and a herringbone motif in another direction. The molecular structure of 1 consists of an absolutely planar tet- racene backbone in the solid state, [13] although very similar molecules were found to be twisted. [14, 15] In the X-ray struc- ture of rubrene, significant steric repulsion between the phenyl rings shifts them above and below the tetracene plane forming a dihedral angle of 258. The tetracene core is still practically planar, thus steric repulsion between the phenyl rings is balanced by the rigidity of the tetracene core. Even large twisting of the acene backbone does not change the HOMO–LUMO gap of acenes significantly. [16] On the other hand, the energy required for twisting acenes by 10–208 is only a few kcal mol À1 , however, it increases to 40 kcal mol À1 for twisting more than 808. [16, 17] Recently, the supramolecular self-assembly of 1 on an AuACHTUNGTRENNUNG(111) surface was studied by STM and other methods. [18] Abstract: Surprisingly, despite its very high mobility in a single crystal, ru- brene shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. We syn- thesized several rubrene analogues with electron-withdrawing and elec- tron-donating substituents and found that most of the substituted rubrenes are not planar in the solid state. More- over, we conclude (based on experi- mental and calculated data) that even parent rubrene is not planar in solution and in thin films. This discovery ex- plains why high mobility is reported in rubrene single crystals, but rubrene shows very low field-effect mobility in thin films. The substituted rubrenes ob- tained in this work have significantly better solubility than parent rubrene and some even form films and not crys- tals after evaporation of the solvent. Thus, substituted rubrenes are promis- ing materials for organic light-emitting diode (OLED) applications. Keywords: field-effect transistors · materials science · organic electron- ic materials · rubrenes · substituent effects [a] Dr. A. S. Paraskar, # Dr. A. R. Reddy, # Dr. A. Patra, # Y. H. Wijsboom, O. Gidron, Dr. M. Bendikov Department of Organic Chemistry Weizmann Institute of Science Rehovot 76100 (Israel) Fax: (+ 972) 8934-4142 E-mail : michael.bendikov@weizmann.ac.il [b] Dr. L. J. W. Shimon, Dr. G. Leitus Chemical Research Support Unit Weizmann Institute of Science Rehovot 76100 (Israel) [ # ] These authors contributed equally to the work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200800802. Chem. Eur. J. 2008, 14, 10639 – 10647  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10639 FULL PAPER