Note: This paper is part of the JCP Special Topic on Excitons: Energetics and Spatio Temporal Dynamics.

A multimode Brownian oscillator model is employed to investigate absorption line shapes of rubrene in solution and thin film. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using parameters obtained from fitting absorption spectra of rubrene, dynamics of singlet fission is explored by the Dirac-Frenkel time-dependent variation with multiple Davydov trial states. By comparing the absorption spectra between a conical intersection model and the multimode Brownian oscillator model, the optimal system-bath coupling parameter of η = 0.2 is determined, and ensuing population dynamics of the S1 state is found to concur with a previously proposed coherent singlet fission mechanism. We further simulate the two-dimensional spectra to reveal the wavepacket dynamics on both excited state and ground state potential energy surfaces. It is demonstrated that the wavepacket motion of tuning modes play a significant role in the evolution of two-dimensional spectra.

The understanding of its shape on the movement of microparticles and nanoparticles is crucial to the development of technologies of using these particles in drug delivery systems. The effect of shape on nanoparticles used in drug delivery, in particular, is a very active area of experimental investigation. Also, the determination of the drag force on nanoparticles of different shapes is very important in designing effective nanoparticle-mediated therapies. One of the common shapes of nanoparticles is rod. In this study we present a resolved discrete particle method (RDPM), which is also called the Direct Numerical Simulation (DNS), to investigate the effect of rod shapes on the drag force in a vicious fluid as compared to other particle shapes such as a sphere and a cone. These particles are assigned the same volume and placed in contact with the bottom wall in a simple shear flow. Their drag forces are computed numerically; it is found that the particle shape has a significant effect on the drag forces. In the case of a spherical particle, our results agree very well with the analytical results found in the literature. The drag force on a rod at different orientations and the motion of two rod-shaped particles of identical volume are in a shear flow are also examined. The motion of a rod-shaped particle and a cone-shaped particle in a shear flow at low Reynolds number is also compared.