Carbon nanorings with inserted acenes: breaking symmetry in excited state dynamics

Abstract

Conjugated cycloparaphenylene rings have unique electronic properties being the smallest segments of carbon nanotubes. Their conjugated backbones support delocalized electronic excitations, which dynamics is strongly influenced by cyclic geometry. Besides being an excellent model system, the increasing research interest in these molecular family is driven by a diverse range of potential optoelectronic applications. Here we present a comparative theoretical study of the electronic and vibrational energy relaxation and redistribution in photoexcited cycloparaphenylene carbon nanorings with inserted naphthalene, anthracene, and tetracene units using non-adiabatic excited-state molecular dynamics simulations. Calculated excited state structures reflect modifications of optical selection rules and appearance of low-energy electronic states localized on the acenes due to gradual departure from a perfect circular symmetry. After photoexcitation, an ultrafast electronic energy relaxation to the lowest excited state is observed on the time scale of hundreds of femtoseconds in all molecules studied. However, the non-radiative relaxation rates to the lowest excited state decrease with an increase of the size of the acene unit. Concomitantly, the efficiency of the exciton trapping in the acene raises when moving from naphthalene to anthracene and to tetracene. Observed photoinduced dynamics is further analysed in details using induced molecular distortions, delocalization properties of participating electronic states and non-adiabatic coupling strengths. Our results provide not only detailed understanding of the internal conversion processes in the functionalized cycloparaphenylene systems considered, but also have a number of guiding insights into design of cyclic molecular systems for electronic and light-harvesting applications.