Internal Conversion Cascade in a Carbon Nanobelt: A Multiconfigurational Quantum Dynamical Study.

Carbon nanobelts feature intriguing photophysical properties, due to their high symmetry and structural rigidity. Here, we consider a (6,6) armchair carbon nanobelt, i.e., the very first carbon nanobelt to be synthesized [Povie et al., , 356, 172] and characterize the internal conversion dynamics using multiconfigurational quantum dynamics via the multi-layer multiconfiguration time-dependent Hartree (ML-MCTDH) method. A symmetry-adapted linear vibronic coupling Hamiltonian for 26 electronic states and 210 vibrational modes is employed. Electronic excitations are found to decay through a dense manifold of excited states, which interact via multiple conical intersections, while inducing minimal geometry change. It is shown that a rapid coherent decay, exhibiting a nonvanishing quantum flux on a time scale of less than 50 fs, transitions toward a slower, decoherent decay at longer times. As previously suggested in the literature, electronic relaxation is hindered by phonon bottlenecks such that a stepwise internal conversion cascade is observed. The computed vibronic absorption spectrum is shown to be in good agreement with the experimental spectrum.