4197with the condition ensure memory retention at modest X while permitting fast Aviram has discussed the theoretical possibility of constructing logic gates by suitable molecular switch circuits. It is of interest to note the restraints on the molecular parameters required to match specifications of current chip technology. In the absence of an applied field (F, = 0) the memory retention time 7mem is given by T,~,,, = 70 exp(X/4kBT): the ratio of the memory retention time to the minimum switching time 70 depends sensitively on both the electronic coupling J and the reorganization energy.For current dynamic RAM operation values of the data retention and switching times 7mem and 70 on the order of 8 ms and 70 ns, respectively, with ratio IOs, are typical. At 300 K, the molecular requirements to match this are J = 0.3 X IO4 eV and X = 0.8 eV. For static or ECL RAM operation, typical specifications are z,,,~,,, = IO yr (3 X IO7 s), with 70 in the range 2-25 ns. At the molecular level at 300 K these would be matched by J = 2.4-0.67 X eV and X = 2.5-2.4 eV, respectively: these molecular specifications for J and X are reasonably and easily met by suitable bridge design, but very high critical field strengths are required for fast switching times because of the large value of A. Lowering the temperature assists the operation of these devices by slightly reducing 7,, and dramatically increasing 7,,,,,. In an alternative architecture, logic gates could be constructed from combinations of these "molecular gates" which would, like modern electronic gates, contain high feedback loops. These feedback loops would switching times. For the isolated model system considered above, only the internal contribution to the reorganization energy has been explicitly considered. If the system is transferred to an ionic or ionizing environment, however, coupling to ionic or solvent modes will make a large contribution to A,,,: for the anthracene/anthracene-pair at close contact distance in acetonitrile, for example, this is estimated to be 1 eV,'O very much larger than that estimated for coupling to internal modes. On the other hand, in one of the configurations envisaged by Aviram for molecules of the type of Figure 2, the molecules are linked into a presumably metallic grid: in this case, most of the contribution to X will still arise from coupling to internal modes. Calculation of the relevant internal coupling constants g, would be required in order to estimate the switching and retention times for these proposed systems. It should also be noted that the effective two-level Hamiltonian of the type considered above is only appropriate in the case that there are no resonances or near resonances with bridge states: where this is not the case, the critical field and dynamical field parameters will have a less simple relationship to geometry and reorganization energy.8
Abstract:The vibronic coupling model we described in a recent paper is applied to the Creutz-Taube (C-T) ion, (wpyrazine)bis(pentaammineruthenium)(5+). We present calculation...