1. MOLECULAR DYNAMICS Supramolecular structures consisting of donor-acceptor (D-A) pairs have attracted considerable interest in recent years due to their potential applications in molecular-scale optoelectronic devices. 1 A doubly linked porphyrin- fullerene dyad is studied by molecular dynamics (MD) simulations in polar and non-polar solvent with the solvent molecules included explicitly, as well as in vacuum. 2 ELECTRONIC STRUCTURE CALCULATIONS Optimization with DFT calculations of two representative vacuum MD conformations produces centre-to-centre distances that are within 0.07-5 % of the corresponding MD values of realistic environments. Figure 4. Graphical summary of the SE, DFT and MD results for conformation 1 and 2 (conf. 1 and conf. 2) and for the solvent conformations: the center-to-center distance (c-c) between the fullerene and porphyrin, the distances between the atoms N23 and C40 (N23- C40), N42 and C40 (N42-C40), N20 and C79 (N20-C79) and N62 and C79 (N62-C79). Figure 1. A 2- dimensional view of the DHD6ee dyad. Figure 2. Two conformations representing the cases most often found during the vacuum MD simulations at 300 K: (a) conformation 1 and (b) conformation 2 (the top views on the left and the side views on the right). (c) The distributions of different atom or atom group distances within the DHD6ee dyad obtained from three 50 ns MD runs in vacuum at 300 K: the distance between the centre of the fullerene and the centre of the four nitrogens of the porphyrin ring (c-c: solid line), the distance between atoms C40 and N23 (C40-N23), the distance between atoms C40 and N42 (C40-N42), the distance between atoms C79 and N20 (C79-N20) and the distance between atoms C79 and N62 (C79-N62). The codes c1 and c2 tell which of the peaks of the distributions correspond to the two conformations of (a) and (b), respectively. The GROMACS force field is applied within the GROMACS MD simulation package. (a) (b) (c) Figure 3. The distributions of different atom or atom group distances within the DHD6ee dyad obtained from 50 ns MD runs in (a) polar and (b) non-polar solvent at 300 K. (c) The top and side views of the representative structure in solvent. (a) (b) (c) References 1.V. Chukharev, N. V. Tkachenko, A. Efimov, D. M. Guldi, A. Hirsch, M. Scheloske and H. Lemmetyinen, J. Phys. Chem. B, 2004, 108, 16377. 2.K. Tappura, O. Cramariuc, T.L.J. Toivonen, T.I. Hukka and T.T. Rantala, Chem. Phys. Lett., 424, 2006, 156. 3.O. Cramariuc, Computational Characterization of Photoabsorption and Structure of Porphyrin- Fullerene Dyads, PhD Thesis, Tampere Univ. of Technology, Finland, 2006. Molecular dynamics simulations and quantum mechanical calculations of a doubly linked porphyrin-fullerene dyad O. Cramariuc, 1 K. Tappura, 3 T.I. Hukka, 2 H. Lemmetyinen 2 and T.T. Rantala 1 1 Institute of Physics and 2 Institute of Materials Chemistry, Tampere University of Technology, P.O.Box 692/541, FIN-33101, Finland. 3 VTT Information Technology, Microsensing, P.O.Box 12071, FIN-33101 Tampere, Finland. oana.cramariuc@tut.fi, kirsi.tappura@vtt.fi, Terttu.Hukka@tut.fi, helge.lemmetyinen@tut.fi, Tapio.Rantala@tut.fi Independent on the functional used one of the MD structure leads to a high energy conformer (HEC) while the other one to a lower energy conformer (LEC). Figure 5. Wavefunctions of HOMO and LUMO for the high- and low-energy conformations. The center-to-center (cc) distances are given in Å. Some delocalization of the wavefunction on both porphyrin and fullerene is seen for the more compact conformation. EXCITED STATES The first excited state of the dyad is studied by both DFT 2,3 and highly-correlated wavefunction based methods, i.e. TDDFT (LDA, GGA, B3LYP), CC2, CCS and CIS(D). The geometrical relaxation of the first excited state of HEC and LEC is studied by TD-DFT at the LDA level of approximation. 2 Figure 6. Schematic representations of the ground state and excited state (lowest and P1sC) potential energy curves of HEC and LEC. The decrease in energy of 0.16 eV and 0.14 eV for HEC and LEC, respectively, is accompanied by an increase of the cc distance of cca. 0.2 Å. The increase in the donor-acceptor distance can be explained by a shift of the positions of the ground and excited state potential energy surface (PES) minima relative to each other. The relative shifting of the PESs depicted in Figure 6 is schematically drawn based on a number of points (thick bullet points) obtained through a combination of several DFT and TD-DFT calculations. eV SVWNPBEB3LYPCCSCIS(D)CC2EXP LEC (5.8 Å) 1.121.141.602.743.27 1.8 HEC (6.9 Å) 0.480.521.062.673.23 Table 1. Excited state energies of HEC and LEC. The RI approximation is employed in all calculations, except for the hybrid-functional TDDFT calculation. Acknowledgments The authors acknowledge the Finnish IT Center for Science CSC for providing the computing resources and the National Graduate School of Materials Physics for the financial support.