Organic molecules are being actively explored for use in logical devices, either as individual memory elements or as components embedded in small organic and polymeric materials. Conventional inorganic semiconductor devices are limited in terms of performance improvement owing to increased costs for device fabrication as well as physical limitations on minimum feature dimensions. Organic memory, however, is a possible substitute for both volatile and non-volatile memory devices. It has the advantages of facile tailoring through organic synthesis, simple device fabrication, even on flexible substrates, and very low power consumption. Volatile organic memory is expected to be applied towards dynamic random access memory (DRAM), which typically requires a data refresh every few milliseconds, while non-volatile organic memory can be applied to read-only memory (ROM) and flash-type memory.
As reported in Angewandte Chemie, the Forbes Group, in collaboration with researchers from North Carolina State University and Sungkyunkwan University in Korea, has developed the first known voltage-driven organic radical memory employing only one active layer of organic small molecules with both n- and p-type properties. Although the low ON–OFF ratio and switching cycles remained far from satisfactory in comparison with current silicon technology, this research on molecularly inherent radical memory provides new insight into the design of voltage-driven functional radical molecules.
The simultaneous, concerted transfer of electrons and protons-electron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. In a collaborative work, published in PNAS, members of the Forbes, Moran, Meyer, and Papanikolas groups report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures.
One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated H+-B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.