In general terms, this thesis is about tuning coupled electronic and nuclear (or mechanical) dynamics in the nanoscale. With “tuning” we mean changing parameters to achieve a specific phenomenon or functionality. This is not a trivial task in this context, because the dynamics of the systems we consider depend nontrivially on the parameters. To be more concrete, we consider two systems which are “complimentary” in many aspects. We start by studying nonradiative decay of an electronic excitation in a minimal example from supramolecular chemistry: a molecular dimer. Each monomer in our model has two electronic states and the respective potential energy surfaces (PESs) are harmonic. Electronic de-excitation occurs in the monomeric level through well-localized regions in the nuclear space which we call ``NRD channels\'\'. The monomers interact via transition dipole-dipole interaction. The decay dynamics of the monomer are trivial due to its harmonic PESs and simple NRD channel. However, the dimer shows distorted and nontrivially coupled PESs conferring rather complex decay dynamics on it. Depending on the position of the NRD channel, we find that the NRD lifetime can exhibit a completely different dependence on the intermolecular-interaction strength. The extension to larger aggregates and the implications to the quantum yield of molecular systems will be discussed. Our findings suggest design principles for molecular systems where a specific fluorescence quantum yield is desired. The most part of this thesis is about a nanoscale rotor driven by charge tunneling. The rotor consists of electronic islands linked to a bearing via insulating arms. The islands can exchange electrons via tunneling with flanking electronic leads. An uniform electrostatic field brings about the coupling between electronic and mechanical degrees of freedom. Moreover, coupling to an environment lead to dissipation in the mechanical dynamics. In the literature one can identify two generic models of this type of rotor [1-3], which we refer to as “mean-field” and “stochastic” models in this thesis. In the mean-field model the system is described by a set of deterministic differential equations involving the average charge on the electronic islands, and therefore charge fluctuations are not taken into account. In the stochastic model the rotor is described by Fokker–Planck equations which fully take into account the charge fluctuations. We start by showing and comparing the dynamics of these models. The models show interesting phenomenology and predict useful functionality to the rotor. However, it is often unclear which assumptions are made upon the system when using these models. To clarify this matter we derived the models using the “orthodox” theory of single electron tunneling [4]. Next, we go on and propose experimental devices which can be described by these models. The parameter ranges accessible using these devices are estimated. Turning our attention back to functionality, we show how to introduce a preferred direction of rotation, which is useful in the context of motors. In the outlook we also show how to recast the system as a current rectifier. [1] A. Y. Smirnov, S. Savel’ev, L. G. Mourokh and F. Nori; Phys. Rev. E 78 031921 (2008). [2] A. Croy and A. Eisfeld; EPL (Europhysics Lett. 98 68004 (2012). [3] A. Smirnov, L. Murokh, S. Savel’ev and F. Nori; Bio-mimicking rotary nanomotors; volume 7364 (2009); doi:10.1117/12.821567; URL http://dx.doi.org/10.1117/12. 821567. [4] B. L. Altshuler, P. A. Lee and W. R. Webb; Mesoscopic phenomena in solids; volume 30; Elsevier (2012).

In general terms, this thesis is about tuning coupled electronic and nuclear (or mechanical) dynamics in the nanoscale. With “tuning” we mean changing parameters to achieve a specific phenomenon or functionality. This is not a trivial task in this context, because the dynamics of the systems we consider depend nontrivially on the parameters. To be more concrete, we consider two systems which are “complimentary” in many aspects. We start by studying nonradiative decay of an electronic excitation in a minimal example from supramolecular chemistry: a molecular dimer. Each monomer in our model has two electronic states and the respective potential energy surfaces (PESs) are harmonic. Electronic de-excitation occurs in the monomeric level through well-localized regions in the nuclear space which we call ``NRD channels\'\'. The monomers interact via transition dipole-dipole interaction. The decay dynamics of the monomer are trivial due to its harmonic PESs and simple NRD channel. However, the dimer shows distorted and nontrivially coupled PESs conferring rather complex decay dynamics on it. Depending on the position of the NRD channel, we find that the NRD lifetime can exhibit a completely different dependence on the intermolecular-interaction strength. The extension to larger aggregates and the implications to the quantum yield of molecular systems will be discussed. Our findings suggest design principles for molecular systems where a specific fluorescence quantum yield is desired. The most part of this thesis is about a nanoscale rotor driven by charge tunneling. The rotor consists of electronic islands linked to a bearing via insulating arms. The islands can exchange electrons via tunneling with flanking electronic leads. An uniform electrostatic field brings about the coupling between electronic and mechanical degrees of freedom. Moreover, coupling to an environment lead to dissipation in the mechanical dynamics. In the literature one can identify two generic models of this type of rotor [1-3], which we refer to as “mean-field” and “stochastic” models in this thesis. In the mean-field model the system is described by a set of deterministic differential equations involving the average charge on the electronic islands, and therefore charge fluctuations are not taken into account. In the stochastic model the rotor is described by Fokker–Planck equations which fully take into account the charge fluctuations. We start by showing and comparing the dynamics of these models. The models show interesting phenomenology and predict useful functionality to the rotor. However, it is often unclear which assumptions are made upon the system when using these models. To clarify this matter we derived the models using the “orthodox” theory of single electron tunneling [4]. Next, we go on and propose experimental devices which can be described by these models. The parameter ranges accessible using these devices are estimated. Turning our attention back to functionality, we show how to introduce a preferred direction of rotation, which is useful in the context of motors. In the outlook we also show how to recast the system as a current rectifier. [1] A. Y. Smirnov, S. Savel’ev, L. G. Mourokh and F. Nori; Phys. Rev. E 78 031921 (2008). [2] A. Croy and A. Eisfeld; EPL (Europhysics Lett. 98 68004 (2012). [3] A. Smirnov, L. Murokh, S. Savel’ev and F. Nori; Bio-mimicking rotary nanomotors; volume 7364 (2009); doi:10.1117/12.821567; URL http://dx.doi.org/10.1117/12. 821567. [4] B. L. Altshuler, P. A. Lee and W. R. Webb; Mesoscopic phenomena in solids; volume 30; Elsevier (2012).