Semi-Empirical Calculations and Measurements of Modified Carbocyanines Optical Properties
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SEMI-EMPIRICAL CALCULATIONS AND MEASUREMENTS OF MODIFIED CARBOCYANINES OPTICAL PROPERTIES Daniel G. McLean*, Paul Dayl, Zhiqiang Wangl, Nansheng Tangt, Weijie Sul, Donna M. Brandelik*, Jiaoshi Si§, and Ruth Pachter, Materials Directorate, Wright Laboratory, WL/MLPJ. Wright Patterson AFB, Ohio 45433-7702 *Science Applications Int'l Corp., Dayton, Ohio, tNational Research Council, §Department of Polymer Science, Akron University, Akron, Ohio, JIndependent Consultant ABSTRACT We report semi-empirical calculations of modified 3,3'-diethylthiacyanine iodide (DTC I), 3,3'-diethylthiacarbocyanine iodide (DTC3) and 3,3'-diethylthiadicarbocyanine iodide (DTC5), particularly with halogen substitution at the meso position in the polymethine bridge. Primarily we investigate geometrical changes and infer spectral trends from the molecular orbital levels. The semi-empirical calculations for the unsubstituted DTC3 agree well with an ab initio Hartree-Fock result. In all molecules multiple conformations are examined. Significant changes in dipole moments are noted between the cis and trans forms. Calculated electronic spectra at the CI singles level are compared to the measured spectra with reasonable agreement. Halogenation effects show a rotation of the benzthiazole groups out of planarity. We draw conclusions about consequent effects on solubility and excited state spectral changes. INTRODUCTION The family of thiacyanine dyes, first synthesized in the late 1800's[l], have been extensively studied as model compounds for the understanding of cyanine chromophores[2-4]. In addition, these dyes have a low energy HOMO-LUMO gap, Eg, coupled with a large HOMO-LUMO+ I gap. This energy level structure gives rise to a large window in the visible spectrum between the first and second transitions with a low but real transition probability. Indeed, this low transition probability is a major characteristic required for the development of reverse saturable absorption (RSA) materials. Moreover, Eg of these materials can be tuned over 100's of nm by the addition of more vinylene units[5]. Good RSA properties also include high triplet quantum efficiency. Although these dyes in general have a low triplet quantum efficiency, near unity quantum efficiencies have been reported in an iodine substituted dye at the meso position [6]. In this paper we report preliminary results of a study of halogenation at the meso position on the thiacyanines having 1, 3, and 5 carbon linkages. METHODS Computational details The semi-empirical geometry calculations were done using MOPAC93 [7] with the PM3 Hamiltonian. The cis and trans forms are studied with additional attention given to the C, and C 2 variants of the cis form, and to variations of the ethyl dihedral angle with no symmetry imposed. The cis structures were also minimized in the presence of a solvent dielectric using the MOPAC version of the self-consistent reaction field (SCRF) formalism COSMO [8] for the solvent dimethyl sulfoxide (DMSO) with 30 geometrical segments per molecule. All convergence criteria
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