Molecular mechanics studies of thionin blue in zeolite mordenite Suraj Deore, Petra Simoncic, Alexandra Navrotsky * NEAT ORU and Thermochemistry Facility, University of California at Davis, Davis CA 95616, USA Received 3 January 2007; received in revised form 8 May 2007; accepted 10 May 2007 Available online 24 May 2007 Abstract The localization and arrangement of thionin blue (C 12 H 10 N 3 S + ) dye molecules in the zeolite mordenite framework is studied by molecular mechanics (MM) simulations. The computational results for dye molecule localization are compared to those from single crys- tal X-ray diffraction. Two low energy thionin orientations are observed. In both, the thionin molecules are inclined within the large 12-membered ring channels and indicate electrostatic interaction with the framework and with the extra-framework Na + ions. Molecule orientation and the determined SO (3.12 and 3.13 A ˚ ), NO (3.22 and 3.33 A ˚ ) and CO (3.45 and 3.41 A ˚ ) distances from the dye molecule to the channel wall are in reasonable agreement with the values found by single crystal X-ray diffraction (SO = 3.084 A ˚ , NO = 3.087 A ˚ and CO = 3.235 A ˚ ). Clustering of thionin molecules in the 12-membered ring channels is not observed. The calculated potential energies for these two low energy configurations are essentially the same (2236 kJ/mol) The global min- imum was confirmed by multiple simulation runs, using different starting orientations of the thionin molecules in the mordenite frame- work. Quench simulations were also performed to understand the energetics of diffusion of thionin molecules in mordenite. The calculated activation energy of diffusion, 12–15 kJ, is comparable to values reported in literature for molecular diffusion in zeolites. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Mordenite; Guest molecule; Molecular mechanics; Diffusion 1. Introduction Zeolites are aluminosilicate open frameworks forming channels and cavities with apertures up to 13 A ˚ in diame- ter. In addition to traditional zeolite applications [1–3], advanced applications of zeolites in photochemistry have emerged in the past decade: Host–guest systems built by photochromic, luminescent dyes intercalated into zeolite channels have remarkable optical properties such as aniso- tropic light absorption [4], luminescence [5], and fluores- cence [6] phenomena. These are caused by the preferred orientation of the transition-dipole moment of the dye mol- ecules [7]. The complex guest molecules find a defined arrangement within the zeolite framework channels result- ing in thermal and mechanical stabilization. Zeolite–dye host–guest systems allow various applications as microla- sers [8], optical switches [9], chemical sensors [10] or artifi- cial antenna systems [11]. Zeolites with various channel shapes and dimensions, such as zeolite L [12], AlPO 4 -5 [13], zeolite Y [14,15], and ZSM-5 [16] have been used suc- cessfully to form these host–guest systems. Due to this interest, research on zeolite host–guest sys- tems has generally focused on three areas: (1) incorpora- tion processes of dye molecules into zeolites, (2) investigation of optical phenomena, and (3) geometrical arrangement of the dye within the framework. Though data are available on the incorporation of dyes in zeolites and their reaction to optical stimuli, relatively little is known about the geometrical arrangement of these mole- cules in the zeolite frameworks. Knowledge of the geomet- rical arrangement of these dye molecules in the zeolite framework is of great importance in understanding the functionality of the host–guest systems and their thermody- namic and kinetic stability. Different microscopy (optical, fluorescence) [17,18], and spectroscopy (IR, Raman, UV– vis) [17,19], techniques have been used to understand the arrangement of guest molecules in the zeolite frameworks. 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.05.016 * Corresponding author. E-mail address: anavrotsky@ucdavis.edu (A. Navrotsky). www.elsevier.com/locate/micromeso Available online at www.sciencedirect.com Microporous and Mesoporous Materials 109 (2008) 342–349