Molecular Tailoring of New Thieno(bis)imide-Based Semiconductors for Single Layer Ambipolar Light Emitting Transistors Manuela Melucci,* ,† Laura Favaretto, † Massimo Zambianchi, † Margherita Durso, † Massimo Gazzano, † Alberto Zanelli, † Magda Monari, ‡ Maria G. Lobello, § Filippo De Angelis, § Viviana Biondo, ∥ Gianluca Generali, ∥ Stefano Troisi, ⊥ Wouter Koopman, ⊥ Stefano Toffanin, ⊥ Raffaella Capelli,* ,∥,⊥ and Michele Muccini* ,∥,⊥ † Istituto per la Sintesi Organica e la Fotoreattivita ̀ (CNR-ISOF), Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy ‡ Dipartimento di Chimica, ‘G. Ciamician’, via Selmi 2, 40138 Bologna, Italy § Istituto di Scienze e Tecnologie Molecolari (CNR-ISTM), Consiglio Nazionale delle Ricerche, via Elce di Sotto 8, 06123 Perugia, Italy ∥ E.T.C. s.r.l., via Gobetti 101, 40129 Bologna, Italy ⊥ Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy * S Supporting Information ABSTRACT: Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semi- conductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices. To this aim, the design of innovative molecular configurations combining effective electrical and optical properties in the solid state is highly desirable. Here, we investigate the effect of the insertion of a thieno(bis)imide (TBI) moiety as end group in highly performing unipolar oligothiophene semiconductors on the packing, electrical, and optoelectronic properties of the resulting materials. We show that, regardless of the HOMO−LUMO energy, orbital distribution, and molecular packing pattern, a TBI end moiety switches unipolar and nonemissive oligothiophene semiconductors to ambipolar and electroluminescent materials. Remarkably, the newly developed materials enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices. KEYWORDS: organic semiconductors, ambipolarity, electroluminescence, oligothiophenes, organic light emitting transistors ■ INTRODUCTION Small molecule organic semiconductors are currently a matter of intense research for use in charge transport-based devices spanning from field-effect transistors 1 to solar cells. 2 Despite the numerous structures realized so far (i.e., of different size, substitution, shape, etc.), 1d−f materials combining charge transport and efficient electroluminescence are still a synthetic challenge, 3a as these properties typically exclude one another in the solid state. 3b−e In particular, materials combining ambipolarity and electroluminescence would be of great interest to realize single layer molecular ambipolar light- emitting transistors (OLETs) 4 that are a new class of highly integrated organic devices, which combine the light emission capability with the switching function of an organic field effect transistor (OFET). 5 While the practical use of light emitting diodes, OLEDs, e.g., in active matrix electroluminescent displays, requires transistors to control their luminance, in OLETs the optoelectronic characteristics are intrinsically controlled without the use of any additional devices. Moreover, differently from OLEDs, in ambipolar OLETs the emitting zone can be placed within the transistor channel to be optically decoupled from the metal electrodes, which are one of the main sources of optical losses in electroluminescent devices. Although, multilayer geometries having several function-specific layers have been successfully developed, 4a,h,6 ambipolar OLETs based on a single semiconducting and emissive material are more attractive for both their simplified architecture and processing cost issues. Solution processed poly(9,9-dioctylfluorenealt-benzothiadia- zole) (F8BT) has recently led to high performance ambipolar single layer polymer OLETs. 7 However, molecular materials offer the advantages of higher structure and purity reproduci- bility from batch to batch, better control of the molecular order and packing in thin films, and higher field-effect charge Received: October 5, 2012 Revised: February 11, 2013 Published: February 11, 2013 Article pubs.acs.org/cm © 2013 American Chemical Society 668 dx.doi.org/10.1021/cm303224a | Chem. Mater. 2013, 25, 668−676