Self-trapping and excited state absorption in fluorene homo-polymer and copolymers with benzothiadiazole and tri-phenylamine.

Jean-Christophe Denis,Gordon J Hedley,Graham A Turnbull,Arvydas Ruseckas,Martin J Paterson,Ifor D W Samuel,Andrew B Matheson,Ian Galbraith

doi:10.1039/c6cp02059c

Abstract

Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions. The molecules investigated are a fluorene pentamer, polyfluorene F8, the alternating F8 copolymer with benzothiadiazole F8BT, and two blue-emitting random copolymers F8PFB and F8TFB. Calculated and measured spectra show qualitatively comparable results. The ESA cross-section of co-polymers at its maximum is about three times lower than that of F8. The ESA spectra are found to change little upon structural relaxation of the excited state, or change in the order of sub-units in a co-polymer, for all studied molecules. In all these molecules, the strongest ESA transition is found to arise from the same electronic process, exhibiting a reversal of the charge parity. In addition, F8PFB and F8TFB are found to possess almost identical electronic behaviour.

Highlights

Semiconducting polymers, reported in 1963,1 have been widely studied since the late eighties, following the successful demonstrations of the functional organic transistor in 19862 and the organic light-emitting diode in 1987.3 Since organic semiconductors have been proposed as the generation materials, which would provide cheaper, more tuneable, flexible and more environmentally-friendly electronics for displays, lighting and solar energy harvesting applications
The molecules we investigated are polyfluorenes (Fig. 1(a)), because they are prototypical conjugated polymers for organic light-emitting devices (OLEDs),[12] and some fluorene copolymers with tri-phenylamine which have been developed to tune both their emission and functionality e.g. electron or hole transport
Our experiments reveal strong excited state absorption (ESA) transitions in fluorene copolymers in the spectral region of 0.8–1.3 eV which has not been accessed in previous studies.[14,15,16]

Summary

Introduction

Semiconducting polymers, reported in 1963,1 have been widely studied since the late eighties, following the successful demonstrations of the functional organic transistor in 19862 and the organic light-emitting diode in 1987.3 Since organic semiconductors have been proposed as the generation materials, which would provide cheaper, more tuneable, flexible and more environmentally-friendly electronics for displays, lighting and solar energy harvesting applications. The molecules we investigated are polyfluorenes (Fig. 1(a)), because they are prototypical conjugated polymers for organic light-emitting devices (OLEDs),[12] and some fluorene copolymers with tri-phenylamine which have been developed to tune both their emission and functionality e.g. electron or hole transport. One class of such emitters are based on fluorene copolymers with tri-phenylamine. They will be denoted as F8PFB and F8TFB in this work, and are shown in Fig. 1(b) and (c).

Objectives

Methods

Results

Conclusion