Synthetic Metals 139 (2003) 827–830 Investigation of luminescence in new blue-emitting poly(2 ′ ,5 ′ -dioctyloxy-4,4 ′ ,4 ′′ -terphenylenevinylene)s T. Cassano a,∗ , R. Tommasi a,b , F. Babudri c,d , A. Cardone c , G.M. Farinola d , F. Naso c a Department of Physics and INFM, University of Bari, Via Amendola 173, 70126 Bari, Italy b DETO, University of Bari, P.zza G. Cesare 11, 70124 Bari, Italy c CNR ICCOM—Sez. di Bari, University of Bari, Via Amendola 173, 70126 Bari, Italy d Chemistry Department, University of Bari, Via Amendola 173, 70126 Bari, Italy Abstract The dependence of the linear optical properties of some soluble poly(terphenylenevinylene)s on the relative amount of cis- and trans-vinylene units has been investigated. A regular blue-shift of the absorption spectra in solution has been observed upon the ad- dition of cis double bonds. Luminescence spectra show a vibronic structure which accounts for the reduced inter-ring torsion in the excited state with respect to the ground state. Moreover, an increased electron–phonon coupling for cis-rich polymers has been observed. © 2003 Elsevier Science B.V. All rights reserved. PACS: 42.70.Jk; 78.20.-e; 78.40.Me; 78.55.-m Keywords: Polymers; Steric effects; Luminescence; Optical properties 1. Introduction The ability to produce large area displays from eas- ily processable semiconducting conjugated polymers has driven much of the recent research in the area of polymeric organic light-emitting diodes (OLEDs). Multicolor display applications require at least three basic colors: red, green and blue. Conjugated polymers are materials particularly suitable for this purpose because their emission can be tuned by modifying the polymer backbone and/or the side groups. In particular, for blue emission polymers are required to have wide energy gaps. This property can be obtained by decreasing the degree of electronic conjugation within the backbone. Although many polymeric materials exhibit this behavior, bright and stable blue emission is difficult to ob- tain. Indeed, emission in polymers occurs from the more conjugated segments, therefore, the luminescence spectrum is always red-shifted with respect to absorption. A possible route to decrease conjugation in polymers consists in intro- ducing cis-vinylene units in the main chain [1]. This kind of linkage geometry can be regarded as an induced disor- der which decreases the electron delocalization within the ∗ Corresponding author. Tel.: +39-0805-442379; fax: +39-0805-442219. E-mail address: cassano@fisica.uniba.it (T. Cassano). polymer backbone [2]. In addition, if the copolymers made of the mixture of monomers containing either cis or trans bonds are considered it is possible, in principle, to tune the emission wavelength in a specific range by changing the cis/trans ratio. 2. Experiment Three poly(2 ′ ,5 ′ -dioctyloxy-4,4 ′ ,4 ′′ -terphenyleneviny- lene)s’, differing for the relative amount of cis/trans double bonds have been synthesized via the Suzuki cross-coupling reaction [3,4] 1 (the schemes of polymers A and C are re- ported in Fig. 1 as inset; polymer B is obtained by the com- bination of monomers constituting polymers A and C). The cis-vinylene/trans-vinylene ratios are 0/100 for polymer A, 20/80 for polymer B, and 100/0 for polymer C, respectively. The UV-Vis absorption spectra of diluted solutions have been recorded using a Varian-Cary 3 spectrophotometer. The emissive properties of samples A–C have also been investigated by measuring photoluminescence (PL) spec- tra both in solutions and thin films. Due to the relatively low emission quantum yield of some of the investigated polymers the concentration of solutions was kept high 1 The synthesis of these polymers will be reported elsewhere. 0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00270-4