High photoluminescence quantum yield due to intramolecular energy transfer in the Super Yellow conjugated copolymer E.W. Snedden * , L.A. Cury 1 , K.N. Bourdakos, A.P. Monkman Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK article info Article history: Received 27 January 2010 In final form 10 March 2010 Available online 15 March 2010 abstract Steady-state and time-resolved photoluminescence have been used to investigate the photophysical properties of the poly (para-phenylene vinylene) copolymer Super Yellow. Analysis of the steady-state photoluminescence at different temperatures enabled an estimate of the average conjugation length in spin-coated solid thin films to be made. Analysis of time-resolved photoluminescence measurements revealed matching exponential behaviour characteristic of exciton energy transfer at early times from high energy copolymer subunits to low energy subunits. The photoluminescence quantum yield was measured and it was proposed that its relatively high value was directly related to this intramolecular process. Ó 2010 Elsevier B.V. All rights reserved. Soluble poly (para-phenylene vinylene) copolymer Super Yel- low (SY) prepared via GILCH polymerization [1] has been mainly employed as the light-emitting polymer in electro-optical devices due to its outstanding properties involving efficiency, shelf life- time, operational lifetime, and performance at elevated tempera- ture [2–6]. Beyond the device applications few studies have been made in order to explore the photophysics of the SY copolymer. The correlation of the morphological and optical properties of blends of SY and poly (ethylene oxide) [7] has been studied with the main issue of investigating the single-molecule spectroscopy, where a remarkable sensitivity to the physical and chemical envi- ronment of SY molecules has been probed by the SY fluorescence spectrum. Quenching of the photoluminescence (PL) intensity of a SY emitting layer from a polymeric light-emitting device (PLED) has been observed from time dynamic measurements with increasing the electric field applied to the PLED contacts [8]. The main conclusions in Ref. [8] shown that photoexcitation in SY pri- marily results in intrachain excitons with an activation energy of 0.6 eV, and that mobile charge carriers are only created later, by field-induced ionization of this exciton that happens by tunneling through the potential barrier of its binding potential. To the best of our knowledge there are no studies exploring the photophysical properties of SY in solution or in a single SY thin film. From tem- perature dependent steady-state photoluminescence measure- ments we have estimated the conformational energy and the average conjugation length in thin films of SY. The overall features and behaviour of the PL spectra as a function of temperature corre- spond to the relatively high conformational energy and the rela- tively short conjugation length. Those characteristics corroborate the presence of tolane-bisbenzyl moiety (TBB) defects in the SY backbone [1]. Thus, we address this Letter to undertake in depth a study of the photophysical properties of SY solutions and films concentrating on relaxation of the intrachain excitons in order to correlate the results of both steady-state and time-resolved spec- troscopy to the high photoluminescence quantum yield (PLQY) ob- served in this material. The poly (para-phenylene vinylene) copolymer Super Yellow solution was produced by dissolving 12.2 mg of SY in 4.0 ml of q-xylene and pre-heated in a water bath at 80 °C for 30 min before lefting to stir for more than 24 h. The well dissolved final solution after stirring undergoes gelation, which was observed to occur at around room temperature. The gel was heated in a water bath at 80 °C up to reobtain a liquid solution. Thin films were then fabricated by spin-coating on glass substrates at 2000 rpm and heated again at 70 °C in a hot-plate for 30 min to further remove the q-xylene. Although the gelation of the solution indicates high chain regularity, the fact we have observed it to occur at around room temperature would correspond to a level of TBB defects of about 2%, according to Ref. [1]. For time-resolved photolumines- cence measurements the base SY solution was diluted down to a concentration of approximately 0.001 mg/ml in order to reduce the intensity of emission and prevent saturation of the detection equipment. The steady-state PL measurements at low temperatures were made using a displex He cryostat coupled to a Jobin Yvon Horiba Fluorolog fluorimeter. The absorption measurements were per- formed in a Shimadzu model UV3600 spectrophotometer. The PLQY = (17 ± 1)% was measured for the solid thin film using a PTFE-coated integrating sphere (Lab Sphere) mounted into the 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.03.030 * Corresponding author. Fax: +44 0191 3345823. E-mail address: E.W.Snedden@durham.ac.uk (E.W. Snedden). 1 Also from Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Minas Gerais, Brazil. Chemical Physics Letters 490 (2010) 76–79 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett