Electroluminescent poly(fluorene-co-thiophene-S,S-dioxide): synthesis, characterisation and structure–property relationships{ Mariacecilia Pasini, Silvia Destri,* William Porzio,* Chiara Botta and Umberto Giovanella Istituto per lo Studio delle Macromolecole del C.N.R., via E. Bassini 15, 20133 Milano, Italy. E-mail: s.destri@ismac.cnr.it; Fax: 139-02-70636400; Tel: 139-02-23699371 Received 9th September 2002, Accepted 12th February 2003 First published as an Advance Article on the web A new copolymer, constituted by the regular alternation of a fluorene derivative with a 1,1-dioxothiophene moiety, has been synthesised via Suzuki coupling. The molecular, thermal, structural, and photophysical characterisations have been performed. The material showed two dimensional order, enhanced by annealing; molecular modelling calculations afforded a consistent structural model accounting for the optical data. In comparison with both polyfluorenes and 1,1-dioxothiophene oligomers the unusual photoluminescence quantum yields observed in solution and in the solid state can be related to a significant contribution of internal conversion to the deactivation process in solution, and the formation of intra-chain hydrogen-bonds, due to a peculiar chain conformation, in the solid state. LED devices based on this new copolymer showed the highest efficiency compared with similar copolymers containing both fluorene and 1,1-dioxothiophene functionalities. Introduction In recent years poly(alkylfluorenes) have been the object of several studies due to their potential applications as blue emitters in light emitting diodes (LED); 1 unfortunately such emission is unstable over time, both in intensity and in colour, because of either the formation of polymeric chain aggregates 2a or electro-oxidation 2b during operation of the diode. To over- come this drawback several approaches have been pursued, among them the preparation of copolymers with other aromatic residues that are increasingly bulky, or involve a conjugation break ; . 3 Of the aromatic moieties, thiophene derivatives, which however shift the emission towards a lower energy, have been used. 4,5 On the other hand, it has been clearly observed that oligothiophenes or mixed thiophene–phenyl oligomers, when S,S oxidised, present an augmented photoluminescence in the solid state, thanks to their peculiar supramolecular organisation. 6 The introduction of alkylated 1,1-dioxothiophenes into a polyfluor- ene chain could solve, aside from the shift of the emission window, the problem of aggregate formation. Conversely, the presence of lateral substituents could also reduce the overall main chain planarity, limiting the bathochromic shift of the emission. The aim of this contribution is to evaluate the effect on the luminescence properties of the insertion of both S,S-dioxide functionality and two alkyl chains on a thiophene residue, as co-monomer with a fluorene moiety. As a matter of fact a very recent work 7 provides evidence that the presence of unsub- stituted thiophene rings between fluorene and 1,1-dioxo-3,4- dialkylthiophene moieties produces a progressive decrease of device luminance as a function of the number of inserted rings. In the following the synthesis and characterisation (molecular, thermal, structural, optical and electrical) of a new copolymer constituted by the alternation of a fluorene derivative and a 3,4-dialkyl-S,S-dioxothiophene is reported. Experimental Materials Fluorene (Aldrich), bromine (Merck), n-butyllithium (1.6 M in hexane), 1-bromohexane, 1-bromooctane, Na 2 CO 3 ,K 2 CO 3 , N-bromosuccinimide (NBS), 3-chloroperoxybenzoic acid (Aldrich), Pd(PPh 3 ) 2 Cl 2 (Aldrich) and Pd(PPh 3 ) 4 (Acros) were used as received. Poly(ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT/PSS) (Bayer), used in LED preparation, was filtered using a 0.45 mm nylon filter before spin coating. The solvents, tetrahydrofuran (THF), diethyl ether and toluene, were freshly distilled and dried following literature methods, while methyl alcohol, butyl alcohol and 2,4-pen- tanedione were used as received. 2-Isopropoxy-4,4,5,5-tetra- methyl-1,3,2-dioxaborolane obtained from Aldrich was stored under nitrogen. 2,5-Dibromo-3,4-dihexyl-1,1-dioxothiophene (1) was syn- thesised starting from commercial (Aldrich) tetrabromothio- phene, through reduction with zinc (Aldrich) and acetic acid (Aldrich) to 3,4-dibromothiophene which was processed according to ref. 8: this was alkylated via Ni catalysed cross- coupling with hexylmagnesium bromide, subsequently bro- minated with NBS in DMF and finally 1,1 oxidised with 3-chloroperoxybenzoic acid. For the preparation of 2,7- dibromo-9,9-dioctylfluorene we followed the procedure described in ref. 9. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-di- octylfluorene (2). To a solution of 1 g (1.83 6 10 23 mol) of 2,7- dibromo-9,9-dioctylfluorene in dry THF (14 ml) at 278 uC was added dropwise 1.6 ml (4 6 10 23 mol) of n-butyllithium (2.5 M in hexane). The pink–orange mixture was stirred at this temperature for 10 min, warmed at 0 uC for 7 min and cooled again at 278 uC for 20 min. 2.1 ml of 2-isopropoxy-4,4,5,5- tetramethyl-1,3,2-dioxaborolane (1.92 g, 0.01 mol) was added rapidly by a syringe, and the resulting solution was stirred at 278 uC for 6 h. In that time the colour changed to orange, then the bath was allowed to reach room temperature very slowly under stirring for 24 h. The pale yellow mixture was poured into water and extracted with ether. The organic layer was JMC: b208742a.3d 25/2/03 17:54:41 Rev 6.06e/W (Aug 31 2000) (gamma) The Charlesworth Group, Hudds 01484 517077 {Electronic supplementary information (ESI) available: crystal struc- ture of poly(fluorene-co-thiophene-S,S-dioxide) and comparison of its absorption and luminescence spectra. See http://www.rsc.org/suppdata/ jm/b2/b208742a/ DOI: 10.1039/b208742a J. Mater. Chem., 2003, 13, 1–8 1 This journal is # The Royal Society of Chemistry 2003