Functionalized Oligothiophenes for Optoelectronic Applications: 3′,4′,3′′′,4′′′-Tetra [(methoxycarbonyl)methyl]-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquithiophene and Related Polymers W. Porzio,* ,² S. Destri, ² U. Giovanella, ² S. V. Meille, ‡ G. Raos, ‡ R. Consonni, ² and G. Zotti § Istituto per lo Studio delle Macromolecole del C.N.R., Via E. Bassini 15, 20133 Milano, Italy, Dipartimento di Chimica, Materiali ed Ingegneria Chimica “G. Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy, and Istituto per l’Energetica e le Interfasi del C.N.R., corso Stati Uniti 4, 35127 PadoVa, Italy ReceiVed July 14, 2004. ReVised Manuscript ReceiVed September 29, 2004 We present the optical properties and LED performance of single-layer devices of a tetra-ester of R-quinquithiophene (TET5) and of two related polymers (PTET5 and PDET3) and discuss them in connection with the crystal structure features of this oligomer. The solution photoluminescence quantum yield (PLQY) of TET5 is smaller than that of the corresponding unsubstituted oligomer, while its value in the solid state is appreciable. Molecular packing, consisting of a sequence of molecular stacks linked by relatively strong polar hydrogen-bond-like interactions, favors PL quenching and hence accounts for the limited quantum efficiency of LED devices built by a single-layer film displaying substantial order (η ext g 5‚10 -3 %). Films of the corresponding PTET5 polymer are amorphous, morphologically homogeneous, and behave differently, with LED devices showing over 500 cd/m 2 at 15 V. The probable reason for the difference between TET5 and PTET5 is to be found in molecular aggregation and orientation with respect to the substrate, implying that PLQY in this class of materials is substantially influenced by self-assembly. This is confirmed by the poor efficiency of the PDET3 polymer, the films of which are substantially more ordered than those of PTET5. Introduction The ability to synthesize a variety of functionalized conjugated oligomers and polymers based on thiophene residues, particularly those carrying heteroatoms in the side chains, has greatly advanced in recent years. We have also witnessed significant progress in our understanding of the correlation between solid-state aggregation and optoelectronic properties in molecular crystals and macromolecules. 1,2 Fine- tuning of the material properties has been shown to be crucial for the development and optimization of optoelectronic devices such as electroluminescent diodes (LED), field effect transistors (FET), optical waveguides, etc. 1,2 In fact, the performance of a particular compound is strongly dependent on its molecular organization in the actual device prototype. 3-5 For example, proper working of an LED demands a balance between molecular separation, avoiding PL quenching, and a good charge transfer, i.e., appreciable mobility favoring exciton recombination. 3 In this respect, relevant results have been recently reported concerning oligomeric or polymeric thiophene derivatives. 6-10 The preparation of thiophene- fluorene and of other copolymers has significantly widened the choice of materials, allowing better tuning of the usable window. 11-13 Another important and stimulating advance is the development of devices exploiting phosphorescence, which maximizes the quantum yield, both in polymers 3 and in transition metal complexes. 14 * Corresponding author. E-mail: w.porzio@ismac.cnr.it. ² Istituto per lo Studio delle Macromolecole del C.N.R. ‡ Politecnico di Milano. § Istituto per l’Energetica e le Interfasi del C.N.R. (1) Ziegler, C. In Handbook of Organic ConductiVe Molecules and Polymers; da Nalwa, H. S., Ed.; J. Wiley & Sons: Chichester, 1997; Vol. 3, p 677. (2) Bolognesi, A.; Porzio, W.; Provasoli, A.; Botta, C.; Sozzani, P.; Comotti, A.; Simonutti, P. Macromol. Chem. Phys. 2001, 202, 2586, and references therein. (3) Kohler, A.; Wilson, J. S.; Friend, R. H. AdV. Mater. 2002, 14, 701, and references therein. (4) Destri, S.; Pasini, M.; Botta, C.; Porzio, W.; Bertini, F.; Marchio `, L. J. Mater. Chem. 2002, 12, 924. (5) Tonzola, C. J.; Maksudul, M. A.; Kaminsky, W.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13548. (6) Antolini, L.; Tedesco, E.; Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi, M.; Casarini D.; Gigli, G.; Cingolani, R. J. Am. Chem. Soc. 2000, 125, 9006. (7) Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi, M.; Bongini, A.; Arbizzani, C.; Mastragostino, M.; Anni, M.; Gigli, G.; Cingolati, R. J. Am. Chem. Soc. 2000, 122, 11978. (8) Barta, P.; Cacialli, F.; Friend, R. H.; Zago ` rska, M. J. Appl. Phys. 1998, 84, 6279. (9) Liu, B.; Yu, W.; Lai, Y.; Huang, W. Macromolecules 2000, 33, 8945. (10) Donat-Bouillud, A.; Le ´vesque, I.; Tau, Y.; D’Iorio, M.; Beaupre `, S.; Blondine, P.; Ranger, M.; Bouchard, J.; Leclerc, M. Chem. Mater. 2000, 12, 1931. (11) Bellete ˆte, M.; Morin, J. F.; Beaupre ´, S.; Ranger, M.; Leclerc, M.; Durocher, G. Macromolecules 2001, 34, 2288. (12) Liu, M. S.; Jiang, X.; Herguth, P.; Jen, A. K.-Y. Chem. Mater. 2001, 13, 3820. (13) Vamvounis, G.; Holdcroft, S. AdV. Mater. 2004, 16, 716. (14) Gong, X.; Ostrowski, J. C.; Moses, D.; Bazan, G. C.; Heeger, A. J. AdV. Funct. Mater. 2003, 13, 439. 242 Chem. Mater. 2005, 17, 242-249 10.1021/cm048850r CCC: $30.25 © 2005 American Chemical Society Published on Web 12/22/2004