Synthesis and optical properties of poly(¯uorene)-based alternating copolymers A. Charas a,b , N. Barbagallo b , J. Morgado b,* , L. Alca Âcer b a Instituto Tecnolo Âgico e Nuclear, 2686 Sacave Âm, Portugal b Instituto Superior Te Âcnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract The novel poly(9,9-bis(2-ethylhexyl)¯uorene-alt-2 0 ,5 0 -thiophene), and the similar poly(9,9-bis(2-ethylhexyl)¯uorene-alt-2 0 ,5 0 -thiophene- 1,1-dioxide) and poly(9,9-bis(2-ethylhexyl)¯uorene-alt-1 0 ,4 0 -phenylene) luminescent copolymers were synthesised by the Suzuki coupling reaction. Within this series, we show that an adequate choice of the unit combined with the ¯uorene monomer, effectively controls the energetic position of the frontier levels of the copolymer. This effect is evidenced by the different optical energy band gaps and different emission colours. Furthermore, the results of cyclic voltammetry (CV) studies show that the S,S-dioxide substitution on the thiophene ring increases both the ionisation potential (IP) and the electron af®nity (EA) in relation to the similar thiophene-based copolymer. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Poly¯uorenes; Copolymers; Suzuki coupling 1. Introduction Electroluminescent polymers are attracting considerable interest owing to their high potential for application as active layer in light emitting diodes (LEDs) [1]. From the synthetic point of view, polymers offer the possibility of tuning their emission colour and to change the energy position of the frontier levels, thereby allowing control of the charge injec- tion barriers at polymer±electrode and polymer±polymer interfaces. A strategy to achieve this goal is the preparation of copolymers, combining highly luminescent moieties, such as ¯uorene, with electroactive moieties. It was recently reported that the functionalisation of the thienyl sulphur to S,S-dioxide increases the electron af®nity (EA) of oligothio- phenes [2]. Results for LEDs based on blends of a poly- (¯uorene) and a S,S-dioxide-containing oligothiophene [3] con®rmed the expected reduction of the electron injection barrier from calcium cathodes. Instead of using the blending approach, we report, in this communication, on the synthesis and characterisation of a block copolymer combining 9,9- bis(2 0 -ethylhexyl)¯uorene with thiophene-S,S-dioxide (copolymer P3) (see Fig. 1). For comparison we have also prepared two other copolymers combining the same ¯uorene unit with phenylene (P1) and with thiophene (P2). 2. Results and discussion The copolymers were synthesised via palladium-cata- lysed Suzuki coupling of the appropriate dibrominated and boron ester monomers. The polymerisation was carried out in a re¯uxing tetrahydrofuran/aqueous potassium car- bonate solution (1 M) containing Pd(PPh 3 ) 4 as catalyst, for several days, in the dark. The copolymers were puri®ed by dissolving them in CHCl 3 and then poured in methanol. This process was repeated and the polymers dried under vacuum. Monomers and polymers were characterised by standard spectroscopic techniques and elemental analysis. The pre- sence of the thiophene-S,S-dioxide unit in P3 was con®rmed by IR (bands at 1310 and 1140 cm 1 ( n SO 2 )). Molecular weight of the polymers was estimated by gel permeation chromatography (GPC) of the tetrahydrofuran polymer solutions, against polystyrene standards. The obtained values for the number±average weight (M n ) are typical of the Suzuki type of polymerisation reaction: M n  12700, M w /M n  3:9 for P1; M n  14100, M w /M n  3:4 for P2 and M n  5300, M w /M n  2:7 for P3. Fig. 2(A) shows the optical absorption spectra of the three copolymers. The optical energy band gap (E g ), estimated from the absorption onset (see Table 1), decreases in the sequence P1, P2, P3. The wavelength at maximum photoluminescence, PL, increases in the same order, being 420 nm for P1, 505 nm for P2 and 575 nm for P3. The absolute PL ef®ciency has not been determined yet. However, P3 appears to be less Synthetic Metals 122 (2001) 23±25 * Corresponding author. Tel.: 351-21-841-8451; fax: 351-21-841-7675. E-mail address: jmorgado@gcsi.ist.utl.pt (J. Morgado). 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-6779(00)01328-X