Dual-Mode Optical Molecular Switching Systems for Organic Memories
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DUAL-MODE OPTICAL MOLECULAR SWITCHING SYSTEMS FOR ORGANIC MEMORIES D.S. Galvão1, S.F. Braga1, P.M.V.B. Barone and S.O. Dantas Departamento de Física, Universidade Federal de Juiz de Fora – UFJF, Juiz de Fora, Minas Gerais, CEP 36036-330, Brazil 1 Instituto de Física, Universidade Estadual de Campinas – UNICAMP, Campinas, São Paulo, CEP 13083-970, CP 6165, Brazil ABSTRACT The synthesis of dual-mode optical molecular switching systems has been recently achieved. These systems were based on chiral helical-shaped alkenes in which the chirality can be reversibly modulated by light. In this work we report a theoretical study on the geometric and spectroscopic properties of these structures using the well-known semi-empirical methods PM3 (Parametric Method 3) and ZINDO/S-CI (Zerner’s Intermediate Neglect of Differential Overlap Spectroscopic - Configuration Interaction). Our results show that there are two stable conformers very close in energy for each possible molecular helicity presenting a barrier of ~40 kcal/mol for bond rotation along the main molecular axis. Under protonation these barriers increase significantly and might explain why the protonation leads to the blocking of the switching process. We propose a scheme for the switching mechanism based on charge transfer and conformational changes during the isomer interconversion. INTRODUCTION There is a great interest in molecular switching processes in part due to the possibility of optical information storage at molecular level [1]. The design of functional devices at molecular level has been object of intense theoretical and experimental research in the last years [2-12]. Among these systems, photochemically switchable bistable organic molecules have a special appeal due to their possible technological applications in reversible optical data storage and optical computing. In order to be of practical application those molecules need to fulfill some requirements as [7, 10]: thermal isomeric stability, possibility of many repeatable cycles (read and write processes, for instance) without loss of activity, ready detectability of two different isomers and retention of all these properties when the molecular structures are integrated into macromolecular systems. The bistability is very fundamental and it might be based on various molecular properties such as electron transfer, isomerizations and photocyclizations whereas light, heat, chemical reactions, magnetic and electric fields, etc. trigger the process. Progress along these lines in order to obtain feasible systems has been made in the last years [1-3], in particular by Feringa and collaborators [4-10]. Feringa et al. [6, 8, 9] have reported the synthesis of dual mode optical molecular switching based on chiral helical-shaped alkenes in which chirality, fluorescence and UV/VIS spectra can be reversibly modulated by light (Figure 1). More recently [7] the same group has reported the
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