Simulation of chirally-induced excitonic optical effects in biscyanine dye with shiff base (optical and molecular struct
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Simulation of chirally-induced excitonic optical effects in biscyanine dye with shiff base (optical and molecular structure simulations) C. Andraud1, K.J. Plucinski, and I.V. Kityk2 Military University of Technology, Warsaw, POLAND 1 Ecole Normale Superieure de Lyon, Lyon, FRANCE 2 Institute of Physics, WSP Częstochowa, POLAND ABSTRACT For the first time simulations of chirally-induced excitons have been carried out, using molecular dynamic simulations and self-consistent quantum chemical calculations within the RHF [Restricted Hartree-Fock ] and DFT [Density Functional Theory] approaches. We found that the DFT approach gave substantially better agreement than the RHF. Comparison of theoretical and experimental results for biscyanine dye with a Shiff base showed a sufficiently good level of agreement. INTRODUCTION Since the discovery of chirally-induced optical effects [1], there have been various studies carried out on the optical properties of chiral molecules [2]. There are two main origins of chirality: the chirality caused by the aplanarity of the surrounding geometry [3] (‘traditional chirality’) and excitonic chirality [4-6], in which chirality is caused by coulombic interactions of several electric dipole moments in the molecule (or crystal) leading to a splitting of excited states. The latter phenomenon is called Davydov splitting, after A.S.Davydov, who first observed it in molecular crystals [7]. It has been used for detecting the biological activity of different chiral-like complexes [8-9]. Up till now, interpretation of optical properties has been based on non-empirical coupled oscillator and group polarizability approaches [10]. Molecular dynamics geometry optimizations have not normally been carried out, and for this reason, such a relevant parameter as molecular geometry has not been adequately taken into account. There have also been no investigations which focus on a self-consistent quantum chemical description of the phenomena observed. In Ref. 11, while calculations of rotation strengths were undertaken, using an SCF-CI MO method, electron correlation effects were taken into account only for the ground states, ignoring partially occupied states. In the present paper, we apply Restricted Hartree-Fock (RHF) and density functional theory (DFT) models to carry out structural geometry optimization and simulation of space dispersion optical effects. For our investigations, we chose the biscyanine dye molecule with a Shiff base (BDSB) (see Figure 1) [12]. The BDSB family has received considerable attention due to the possibility of varying cationic substitution, together with stereochemistry variation. These parameters determine biexciton coupling. An earlier semi-empirical theoretical study [12] established that circular dichroism (CD) and rotational strength (RS) are sensitive to the π-conjugation length, the distance between the chromophores (Endis), and to the inter-chromophore angle (Figure 1). Traditional methods for making quantum chemical calculations of the electronic properties of such molecul
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