III. Electronic Bistability and Molecular Devices for Data Storage

  • Business of molecular devices for data storage is developing nowadays incredibly fast. Different observables are used for design of switchable, 0-1-like responding materials. A molecule exhibiting nontrivial electronic bistability (“MNEMON”) has been theoretically designed over 15 years ago in The Quantum Chemistry Laboratory of The University of Warsaw (CHEM PHYS 85 (3): 451-460 1984). Linear chains of donor-acceptor complexes (with appropriate ionization potential values of donors, electron affinity values of acceptors, and with a proper donor/acceptor distance) have been predicted to exhibit the neutral-to-ionicĀ  transition (NIT). A nonadditivity of electronic excitations in the [DA]n chain is responsible for this bistability. Independently, a NIT has been discovered by Torrance et al. in organic materials (PHYS REV LETT 46 (4): 253-257 1981). Until nowadays, both pressure- and temperature-induced NIT has been detected for several organic DA complexes. The linear chain MNEMON molecule designed in Warsaw differs substantially from 3D extended structures known experimentally to exhibit NIT: the dipol moment is substantially different for NEU and ION forms of the former, while it is equal to zero for both forms of the latter. MNEMON also differs from quinone derivatives of Co, Fe and Mn exhibiting so-called valence tautomerism: D and A molecules are not covalently linked in the former, while a chemical D-A bond exists in the latter. Yet, all these interesting classes of molecular/extended systems have one common feature: there is an avoided crossing of electronic potential energy surfaces in these systems.
  • An interdisciplinary project has been initiated at The University of Warsaw, in order to synthesize MNEMON molecule. The search has fruited in the discovery of several interesting molecular architectures, including Ni(II) and Cu(II) complexes of tetraazatetraenemacrocyles (see paper10.pdf), and of an interesting TTF-based electron donor (TTF moelcules are hanged up on the helical oligopeptide). Search for the MNEMON molecule now continues in Warsaw.

Quite recently, we have predicted a novel family of compounds exhibiting a 0/1-type notrivially degenerated electronic ground state, based on intramolcular 2-electron reduction-oxidation process. These states do not show up disadvantages of NEU or ION states of MNEMON (particularly, the huge dipole moment of ION state does not appear in our compounds). We now try to (quantum-mechanically) tune up the properties of these species, so that to obtain ease of 0/1-switching while preserving the kinetic stability of both states (i.e. keeping large energy barrier for their interconversion). We also build up a strategy of a successful read/write processes for our novel molecular memory units. This is difficult task, but we never give up. The preliminary accounts of this work have been published (J MOLEC MODEL 11(4-5): 278-287 2005).