The photophysics of extended systems like conjugated polymers or molecular aggregates is characterized on the one hand by the properties of the molecular building blocks and on the other hand by the delocalized nature of the electronic excitations, i.e., the formation of excitonic states. The dynamical phenomena induced by photoexcitation therefore involve an interplay of site-site interactions entailing excitation energy transfer, and vibronic (electron-phonon) coupling which typically leads to ultrafast internal conversion processes. We propose here a molecular-level, quantum-dynamical approach as exemplified by our recent study of exciton dissociation at interfaces of semiconducting polymer phases (so-called heterojunctions) [1]. This study combines a vibronic coupling model parametrized for the three most relevant electronic states and 20-30 phonon modes, with accurate multiconfigurational quantum dynamics calculations using the MCTDH method and a Gaussian-based variant thereof (G-MCTDH) [2]. In addition, we employ recently developed transformation techniques [1,3] by which a relevant set of effective modes is constructed which account for the short-time dynamics in high-dimensional systems involving conical intersection topologies. For the semiconducting polymer systems under study, which typically involve high- vs. low-frequency phonon bands, this analysis leads to a mechanistic picture showing that the dynamical interplay between the two types of phonon modes is crucial for the ultrafast dissociation of the photogenerated exciton state. A perspective is given on the effect of averaging over ensembles of interface structures, on the role of coherence, and on the extension of the analysis to finite temperatures. [1] H. Tamura, J. G. S. Ramon, E. R. Bittner, and I. Burghardt, Phys. Rev. Lett. 100, 107402 (2008), J. Phys. Chem. B, 112, 495 (2008). [2] G. A. Worth, H.-D. Meyer, H. Koeppel, L. S. Cederbaum, and I. Burghardt, Int. Rev. Phys. Chem., 27, 569 (2008); I. Burghardt, K. Giri, and G. A. Worth, J. Chem. Phys., 129, 174104 (2008). [3] L. S. Cederbaum, E. Gindensperger and I. Burghardt, Phys. Rev. Lett., 94, 113003 (2005).