Optical excitations in star-shaped fluorene molecules

Montgomery, Neil A. and Denis, Jean-Christophe and Schumacher, Stefan and Ruseckas, Arvydas and Skabara, Peter and Kanibolotsky, Alexander and Paterson, Martin J. and Galbraith, Ian and Turnbull, Graham A. and Samuel, Ifor D. W. (2011) Optical excitations in star-shaped fluorene molecules. Journal of Physical Chemistry A, 115 (14). pp. 2913-2919. ISSN 1089-5639

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Abstract

A detailed study of the low-energy optical transitions in two families of star-shaped molecules is presented. Both families have 3-fold rotational symmetry with oligofluorene arms attached to a central core. In one family, the core of the molecule is a rigid meta-linked truxene, while the other is a meta-linked benzene moiety. The low-energy transitions were studied both experimentally and using time-dependent density functional theory (TD-DFT). The optical transitions of these new star-shaped molecules were compared with corresponding linear oligofluorenes. Both families of star-shaped molecules showed higher absorption and fluorescence dipoles and photoluminescence quantum yields than straight chain oligofluorenes. TD-DFT calculations show that absorption takes place across the entire molecule, and after excited state relaxation, the emission results from a single arm. In both theory and experiment the transition dipole moments show an approximate n0.5 dependence on the number of fluorene units in each arm.