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We propose a feasibility demonstration of an unprecedented concept: preparation of regular two-dimensional arrays of artificial surface-mounted dipolar molecular rotors and control of their coherent motion by the application of an outside electric field. The proposal involves a highly interdisciplinary endeavor, which requires experience in synthesis (preparation of molecular rotors), surface chemistry (assembly of rotors into arrays on surfaces), surface spectroscopy and scanning microscopy (characterization of rotor arrays on surfaces), and theory (modeling of rotor dynamics). The principal investigator is presently actively working and publishing in all of these subdisciplines.
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The Michl group is engaged in several research efforts. (i) The largest is an attempt to develop methods for the preparation of regular (preferably triangular) two-dimensional arrays of interacting dipolar molecular rotors with a ferroelectric ground state. (ii) Another is an attempt to develop procedures for covering the surface of gold and possibly other metals with alkyl and other organic groups attached directly, without intermediate linkers such as the usual sulfur atoms. Both of these projects are financed by an ERC grant. Smaller projects deal with (iii) the exploration of the properties of new extremely strong oxidants, (iv) examination of new reactions of 12-vertex carboranes, and (v) search for new chromophores suitable for singet fission.
ad (i)
To our knowledge, arrays of molecular rotors have never been prepared before. Two approaches are under investigation. In one, synthetic rotors consisting of a dipolar rotator connected to a molecular stopper and a long shaft are allowed to insert into a set of hollow channels present naturally in crystal surfaces of certain materials such as tris(o-phenylenedioxy)cyclotriphosphazine (TPP, Figure 1). Several possible resulting structures are shown schematically in Figure 2, and a surface inclusion that has actually been synthesized is displayed in Figure 3. The structures of the inclusion compounds are investigated by a combination of tools that include X-ray diffraction, dielectric spectroscopy, and solid-state NMR, performed in collaborating laboratories in Boulder, Colorado, femtosecond resolved fluorescence anisotropy, scanning microscopy, and others. Although we have succeeded in producing rotors with very low barriers to rotation (down to 5.7 kJ/mol) and arranged them in fairly regular arrays, we have not yet produced an artificial ferroelectric surface.
ad (ii)
We have made and patented the discovery that alkylstannanes produce sturdy self-limiting monolayers on the surface of gold, and have since extended the technique to other organometallics. Although the mode of adhesion of the organic moieties to the metal surface was not clear at first, it has now become obvious that the attachment is through carbon-metal bonds. We have found ways to remove all other constituents from the monolayers and end up with alkylated pure gold surfaces, which may be useful in nanoelectronics. The techniques used for surface characterization in our laboratory are FTIR spectroscopy, ellipsometry, contact angle measurement, scanning microscopy, and electrochemistry, and in collaboration with other laboratories we also use X-ray and UV photoelectron spectroscopy.
ad (iii)
We have built laboratory facilities suitable for work with elemental fluorine and liquid HF and use them for the synthesis of fluorinated and trifluoromethylated carborane anions and radicals. These reversible redox couples have very positive potentials, making the radicals the strongest known neutral oxidants, and we are exploring their chemistry. Such redox couples promise to be useful in high-voltage batteries.
ad (iv)
We are exploring new chemical reactions of icosahedral carborane anions with the ultimate goal of finding ways to produce new types of conducting polymers. In the process, mechanistic understanding of the behavior of this unique class of structures is building up.
ad (v)
New types of structures are being designed for singlet fission, a process that could greatly improve the efficiency of solar cells. Quantum chemical calculations are used to guide the search.
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