www.afm-journal.de FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com 1. Introduction Polymorphism in organic crystals, namely the existence of different crys- talline forms for the same substance, is a phenomenon of considerable funda- mental and technological relevance. [1] The intermolecular interactions and the mechanisms leading to different crystal arrangements are matter of intense studies aimed to achieve the control and prediction of solid-state structures. [2] Polymorphism has acquired great impor- tance for organic solids of pharmaceu- tical interest, since the solid-state packing may affect biological activity or other fea- tures such as rate of dissolution or shelf life. [3] In conjugated materials, which are of major importance in nanotech- nologies, solid-state molecular packing plays a crucial role in controlling the Polymorphism in Crystalline Microfibers of Achiral Octithiophene: The Effect on Charge Transport, Supramolecular Chirality and Optical Properties Francesca Di Maria,* Eduardo Fabiano, Denis Gentili, Mariano Biasiucci, Tommaso Salzillo, Giacomo Bergamini, Massimo Gazzano, Alberto Zanelli, Aldo Brillante, Massimiliano Cavallini, Fabio Della Sala, Giuseppe Gigli, and Giovanna Barbarella* Polymorphic crystalline microfibers from an achiral octithiophene with one S-hexyl substituent per ring are separately and reproducibly grown with the same characteristics on various solid surfaces, including the interdigi- tated electrodes/SiO 2 surface of a bottomcontact field-effect transistor. The arrangement of the same molecule in two diverse supramolecular structures leads to markedly different electronic, optical, and charge mobility proper- ties. The microfibers—straight and yellow emitting or helical and red emit- ting—exhibit p-type charge transport characteristics, with the yellow ones showing a charge mobility and on/off current ratio of one and three orders of magnitude, respectively, greater than the red ones. Both forms show circular dichroism signals with significant differences from one form to the other. DFT calculations show that the octithiophene exists in two different quasi- equienergetic conformations aggregating with diverse orientations of the substituents. This result suggests that the observed polymorphism is confor- mational in nature. The self-assembly, driven by sulfur–sulfur non-bonding interactions, amplifies the small property differences between conformers, leading to quite different bulk properties. DOI: 10.1002/adfm.201400534 Dr. F. Di Maria, Dr. M. Gazzano, Dr. A. Zanelli, Dr. G. Barbarella ISOF, MIST.E-R, Mediteknology Consiglio Nazionale Ricerche Via P. Gobetti 101, I-40129, Bologna, Italy E-mail: francesca.dimaria@isof.cnr.it; giovanna.barbarella@isof.cnr.it Dr. E. Fabiano, Dr. F. Della Sala NNL CNR-Istituto Nanoscienze Via Arnesano, I-73100, Lecce, Italy Dr. D. Gentili, Dr. M. Cavallini ISMN, Consiglio Nazionale Ricerche Via P. Gobetti 101, I-40129, Bologna, Italy Dr. M. Biasiucci NNL-CNR Nanoscience Institute c/o Dpt, Fisica Ed. G. Marconi Center for Life, NanoScience La Sapienza University and IIT Viale Regina Elena 295, I-00161, Roma, Italy T. Salzillo, Prof. A. Brillante Dpt of Industrial Chemistry Toso Montanari University of Bologna Viale Risorgimento 4, I-40136, Bologna, Italy Dr. G. Bergamini Dpt of Chemistry Giacomo Ciamician University of Bologna Via Selmi 2, I-40126, Bologna, Italy Prof. G. Gigli Dpt Ingegneria Innovazione Università del Salento Via Arnesano, I-73100, Lecce, Italy Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201400534