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Azobenzene photoisomerization in liquid crystal mesophases

Project ALC
Research Area Materials Science
Principal Investigator(s) Prof. Claudio Zannoni
Institution(s)
  • Università di Bologna e INSTM, Italy

Abstract

Azobenzene (AB) molecule is the prototype of a class of materials whose shape and properties can be rapidly modified, upon illumination, thanks to a trans-cis interconversion. This photo-responsive process is key to a number of applications, in which either the mechanical effects generated by the conformational change or the changes in physical properties are exploited [1-3]; however the actual conversion mechanism in condensed phases is still largely unknown. It is particularly important to clarify the mechanism when AB is dissolved in a low molar mass or elastomeric liquid crystal [2,3], since their order and molecular correlation amplify this relatively weak molecular effect into mesoscopic or even macroscopically observable opto-mechanical effects [2]. We have recently developed a molecular dynamics based methodology suitable for the problem [5] and we propose to apply it to provide the first attack on this important problem. More in detail, the project is aimed at extending a recent simulation study of azobenzene isomerization in vacuum and in various solvents [3] to the more complex environment provided by liquid crystalline phases, namely the nematic and smectic phase of the well known mesogen 8CB (4-n- octyl-4’-cyano biphenyl), whose properties have already been well reproduced by atomistic simulations [5]. We plan to use HPC- parallel molecular dynamics (MD) simulations and classical force fields to describe both the ground state of azobenzene and 8CB, and the excited state of azobenzene.
The expected results are twofold: on one hand we aim to predict the shift of transition temperatures between the different phases of the LC solvent induced by the two isomers of azobenzene solute observed experimentally [6]; on the other we will assess the effect of the anisotropy of the medium and of the smectic formation on the isomerization extent, on the quantum yield and on the photoisomerization mechanism.

[1] Zhao, Y.; Ikeda, T. Smart Light-Responsive Materials. Azobenzene Containing Polymers and Liquid Crystals; Wiley Interscience, 2009.
[2] T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873; H. Finkelmann, E. Nishikawa, G. G. Pereira, M. Warner, Phys. Rev.Lett. 2001, 87, 15501.
[3] Lansac, Y.; Glaser, M.A.; Clark, N.A.; Lavrentovich, O.D. Nature 1999, 398, 54.
[4] G. Tiberio, L. Muccioli, R. Berardi, C. Zannoni, ChemPhysChem , 2010, in press.
[5] (a) G. Tiberio, L. Muccioli, R. Berardi, C. Zannoni, ChemPhysChem 2009, 10, 125; (b) M. F. Palermo, A. Pizzirusso, L. Muccioli, C. Zannoni, in preparation
[6] I. Vecchi, A. Arcioni, C. Bacchiocchi, G. Tiberio, P. Zanirato, C. Zannoni, J. Phys. Chem. B 2007, 111, 3355

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