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Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Tayi, Alok S.,Shveyd, Alexander K.,Sue, Andrew C.-H.,Szarko, Jodi M.,Rolczynski, Brian S.,Cao, Dennis,Kennedy, T. Jackson,Sarjeant, Amy A.,Stern, Charlotte L.,Paxton, Walter F.,Wu, Wei,Dey, Sanjeev K. Nature Publishing Group, a division of Macmillan P 2012 Nature Vol.488 No.7412
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Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.
2
Radically Enhanced Molecular Switches

Fahrenbach, Albert C.,Zhu, Zhixue,Cao, Dennis,Liu, Wei-Guang,Li, Hao,Dey, Sanjeev K.,Basu, Subhadeep,Trabolsi, Ali,Botros, Youssry Y.,Goddard, William A.,Stoddart, J. Fraser American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.39
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The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4′-bipyridinium (BIPY2+) unit, is described. The BIPY2+ unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT4+ ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT4+ rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY2+ unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable [2]rotaxanes and [2]catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY•+ units in the CBPQT2(•+) ring is oxidized to the BIPY2+ dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the [2]catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable [2]rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.Graphic Abstract <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-39/ja306044r/production/images/medium/ja-2012-06044r_0014.gif'><A href='http://pubs.acs.org/doi/suppl/10.1021/ja306044r'>ACS Electronic Supporting Info</A>
3
Tetrathiafulvalene Hetero Radical Cation Dimerization in a Redox-Active [2]Catenane

Wang, Cheng,Dyar, Scott M.,Cao, Dennis,Fahrenbach, Albert C.,Horwitz, Noah,Colvin, Michael T.,Carmieli, Raanan,Stern, Charlotte L.,Dey, Sanjeev K.,Wasielewski, Michael R.,Stoddart, J. Fraser American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.46
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The electronic properties of tetrathiafulvalene (TTF) can be tuned by attaching electron-donating or electron-withdrawing substituents. An electron-rich macrocyclic polyether containing two TTF units of different constitutions, namely 4,4′-bis(hydroxymethyl)tetrathiafulvalene (OTTFO) and 4,4′-bisthiotetrathiafulvalene (STTFS), has been synthesized. On two-electron oxidation, a hetero radical dimer is formed between OTTFO•+ and STTFS•+. The redox behavior of the macrocyclic polyether has been investigated by electrochemical techniques and UV–vis and electron paramagnetic resonance (EPR) spectroscopies. The [2]catenane in which the macrocyclic polyether is mechanically interlocked with the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring has also been prepared using template-directed protocols. In the case of the [2]catenane, the formation of the TTF hetero radical dimer is prevented sterically by the CBPQT4+ ring. After a one-electron oxidation, a 70:30 ratio of OTTFO•+ to STTFS•+ is present at equilibrium, and, as a result, two translational isomers of the [2]catenane associated with these electronically different isomeric states transpire. EPR titration spectroscopy and simulations reveal that the radical states of the two constitutionally different TTF units in the [2]catenane still experience long-range electronic intramolecular coupling interactions, despite the presence of the CBPQT4+ ring, when one or both of them are oxidized to the radical cationic state. These findings in the case of both the free macrocyclic polyether and the [2]catenane have led to a deeper fundamental understanding of the mechanism of radical cation dimer formation between constitutionally different TTF units.Graphic Abstract <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-46/ja307577t/production/images/medium/ja-2012-07577t_0013.gif'><A href='http://pubs.acs.org/doi/suppl/10.1021/ja307577t'>ACS Electronic Supporting Info</A>

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