9176 Phys. Chem. Chem. Phys., 2012, 14, 9176–9184 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 9176–9184 Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene moleculesw Neil A. Montgomery, a Gordon J. Hedley, a Arvydas Ruseckas, a Jean-Christophe Denis, b Stefan Schumacher, c Alexander L. Kanibolotsky, d Peter J. Skabara, d Ian Galbraith, b Graham A. Turnbull a and Ifor D. W. Samuel* a Received 29th December 2011, Accepted 24th April 2012 DOI: 10.1039/c2cp24141b Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3–8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks. Introduction Branched conjugated molecules show attractive properties such as directional energy transfer, 1 enhanced two photon absorption 2–4 and high photoluminescence quantum yield in the solid state. 5,6 These properties make them useful in light harvesting systems for solar energy conversion, 1,7,8 fluorescence sensors, 9,10 organic light emitting diodes 11,12 and organic lasers. 6,13 In order to design efficient light-harvesting antennae which would absorb light and funnel energy to a desired location, it is important to understand exciton localisation and energy transfer. A number of experimental and theoretical studies investi- gating exciton localisation have been carried out on branched molecules with different chemical structures, observing their fluorescence 14,15 or transient absorption. 16 The depolarisation of fluorescence also provides complementary information about energy transfer, as it shows the change in orientation of the emission dipole from the absorption dipole. Molecules with a nitrogen core 14–18 show a very fast energy transfer between branches on a 30 fs time scale and subsequent localisation on one branch. 16 In contrast the molecules with benzene, 19–21 carbon and adamantine cores showed much slower depolarisation which implies that excitation localises quickly on one branch 20 and subsequently transfers to other branches via a Fo¨rster mechanism. 21 Recently there has been a lot of interest in a family of branched molecules with truxene-cores and arms consisting of fluorene oligomers. The truxene-cored molecules have been used in lasers 13,22 and field effect transistors. 23 Due to their desirable properties significant research has been performed to characterise their basic photophysical properties both experi- mentally and theoretically. 24–26 There has also been a small amount of work carried out on the fluorescence dynamics of the truxene molecules studying the picosecond planarisation process on the branches. 27 However there has yet to be a study of energy transfer and exciton localisation in truxene-cored molecules. In this paper we present such a study of ultrafast fluores- cence depolarisation and energy transfer in truxene-cored a Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, United Kingdom. E-mail: idws@st-andrews.ac.uk b Department of Physics, SUPA, School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh, EH14 4AS, United Kingdom c Physics Department and Center for Optoelectronics and Photonics Paderborn (CeOPP), Universita ¨t Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany d WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom w This article was submitted as part of a Themed Issue on ultrafast chemical dynamics. Other papers on this topic can be found in issue 18 of vol. 14 (2012). This issue can be found from the PCCP homepage: http://www.rsc.org/pccp PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by University of St Andrews Library on 09 October 2012 Published on 24 April 2012 on http://pubs.rsc.org | doi:10.1039/C2CP24141B View Online / Journal Homepage / Table of Contents for this issue