Franco Cacialli: "Threaded molecular wires as model conjugated polymers with controlled interstrand interactions"

Franco Cacialli
Department of Physics and Astronomy, and London Centre for Nanotechnology, University College London, UK

Threaded molecular wires as model conjugated polymers with controlled interstrand interactions

Threaded molecular wires made with conjugated-polymers-based polyrotaxanes offer an example of a "bottom-up" approach to electroluminescent nanostructures.¹ This class of materials is engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4’-diphenylene vinylene) or poly(9,9’-fluorene) through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects, that red-shift and partially quench the luminescence. Such a supramolecular approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport and thus electroluminescence (EL). The reduced tendency for polymer chains to aggregate shows in both solid-state films and in solution (as probed by fluorescence decay dynamics) and allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning-force microscopy.² Control of the threading ratio is possible, thereby resulting in fine tuning of the excitonic vs. aggregate contribution to the luminescence, as well as of the electro- and photo-luminescence efficiency.³ A particularly intriguing possibility afforded by supramolecular and nanoscale encapsulation of these soluble semiconductors is the suppression of energy transfer which enables both fabrication of white-emitting LEDs,⁴ and achievement of unprecedentedly broad gain bands, in "conjugated" blends of different semiconductors, with potential application to broad-band amplifiers and multi-colour lasers.⁵ Water solubility of rotaxanes carrying un-susbtituted cyclodextrins also enables their incorporation into stretchable matrices and thus strong polarisation of absorption and emission from such films.⁶

1. F. Cacialli, J.S. Wilson, J. J. Michels, C. Daniel, C. Silva, R. H. Friend, N. Severin, P. Samorì, J. P. Rabe, M. J. O'Connell, P. N. Taylor, H. L. Anderson. "Cyclodextrin-threaded conjugated polyrotaxanes as electroluminescent insulated molecular wires with reduced interstrand interactions". Nature Materials. 1, 160-164 (2002).
2. J.S. Wilson, M.J. Frampton, J. J. Michels, L. Sardone, G. Marletta, R. H. Friend, P. Samorì, H. L. Anderson, F. Cacialli. "Supramolecular complexes of conjugated polyelectrolytes with poly(ethylene oxide): multifunctional luminescent semiconductors exhibiting electronic and ionic transport". Adv. Mat. 17, 2659–2663 (2005).
3. S Brovelli, G. Latini, M. J. Frampton, S. O. McDonnel, F. Oddy, O. Fenwick, H. L. Anderson and F. Cacialli. "Enhanced electroluminescence of threaded molecular wires via fine tuning of their threading ratio". Nano Letters 8, 4546-4551 (2008).
4. S. Brovelli, F. Meinardi, G. Winroth, O. Fenwick, G. Sforazzini, M. J. Frampton, L. Zalewski, J. A. Levitt, F. Marinello, P. Schiavuta, K. Suhling, H. L. Anderson, and F. Cacialli. "White electroluminescence by control of resonant energy-transfer in organic-soluble polyrotaxane:polyfluorene blends". Adv. Funct. Mat. 20, 272-280 (2010).
5. S. Brovelli, T. Virgili, M.M. Mroz, G. Sforazzini, A. Paleari, H.L. Anderson, G. Lanzani, and F. Cacialli. "Ultra-broad optical gain and two-colour amplified spontaneous emission in binary blends of insulated molecular wires". Adv. Mat. 22, 3690-3694 (2010).
6. F. Di Stasio, P. Korniychuk, S. Brovelli, P. Uznanski, S. O. McDonnel, G. Winroth, H. L. Anderson, A. Tracz, F. Cacialli. "Highly-polarized emission from oriented films incorporating water-soluble conjugated polymers in a polyvinyl alcohol matrix". Adv. Mat. 23, 1855-1859 (2011).